General Radar Principles
General Radar Principles
INTRODUCTION 1-4 Visual detection methods and limitations 1-4 Radar overcomes visual limitations 1-5 Derivation of word "radar" 1-5 Information given by radar 1-5 Importance of radar 1-6 Battle of Britain Battle of Midway Navy types of radar 1-6 Search Fire-control Special Navy letter system 1-8 Radars on board ship 1-8 Importance of radarman 1-8 Security 1-9 BASIC PRINCIPLES OF RADAR 1-9 Definitions 1-9 Frequency code 1-11 Equivalent measurements 1-11 Principle of pulse reflection 1-11 Bearing determination 1-12 Range determination by sound 1-12 Range determination by radar 1-13 Summary 1-13 MAIN PARTS OF A RADAR SYSTEM 1-13 The transmitter 1-14 Wave length or frequency Duration and power of pulse Pulse rate The antenna system 1-16 Transmission line Antenna: an emitter of radio waves Non-directional antenna Directional antenna
How does radar determine bearing? 1-20 Concept of lobe "E" units Maximum echo method Accuracy consideration Minimum echo method Lobe switching Minor lobes The receiver 1-28 Signal amplification The indicator 1-28 Basic electron theory Structure of electrostatic cathode-ray tube Concept of sweep and time base Electromagnetic cathode-ray tube Echo indication by deflection and by intensity method Standard C.R.T. controls Development of trace by the electron beam Continuous and discontinuous sweeps Calibration 1-37 Internal calibration External calibration Types of scopes 1-38 The "A" scope The "J" scope The "R" scope The PPI scope The "B" scope The "H" scope Summary The modulation generator 1-42 The duplexer 1-44 Summary of one cycle of radar operation 1-44 GENERAL RADAR CHARACTERISTICS 1-47 Air-search radar 1-47 Surface-search radar 1-47 Fire-control radar 1-47 FACTORS AFFECTING RADAR RANGE 1-47 Maximum range factors 1-47 Minimum range factors 1-47
HOW DOES RADAR DETERMINE ALTITUDE 1-47 Position angle and range method 1-47 Air-search radar for altitude determination 1-48 Addition and cancellation of radio waves 1-49 Fade chart 1-50 SPECIAL USES OF RADAR 1-54 Navigation 1-54 Spotting 1-54 Direction finding 1-55 Fire control 1-55 Fighter direction 1-55 FUTURE OF RADAR 1-55
The primary purpose of the Navy is to "destroy the enemy," and all the Navy's activities exist only for this purpose. Only those pieces of equipment which will enable it to do this effectively are considered as being of any value.
Closely related to this primary purpose of the Navy is the secondary purpose: self preservation. The ship that fights the most effectively has the best chance of coming safely into home port again. A poorly fought ship may never have another chance to fire its guns. Sunken ships do not shoot.
You, as a radar operator, may be wondering how radar helps the ship fire the guns and come safely home again. The purpose of this hook is to clear up that question. Radar performs the old task of finding the enemy but uses new methods. If you understand how this task was performed before the development of radar, it will help you to realize the superiority of radar over any device formerly employed.
Visual detection methods and limitations. Ships have used visual lookouts since the early days of sail. A lookout, however, cannot see through fog or smoke or darkness for any great distance, and even on a clear day cannot see far beyond the horizon. Such limitations of visual lookouts have hampered ships for centuries-and still do, unfortunately.
Navy men of the past realized that when an enemy vessel appeared over the horizon or out of a fog bank too little time remained to prepare for battle. So when in search of the enemy, they sent out pickets, a line of the faster, smaller ships in the direction from which the enemy was expected. Then, when the ship farthest ahead saw the enemy rise above his horizon, it signaled the next vessel, and thus the word was passed.
But the picket system did not help much in bad weather or at night. The black of night or a curtain of fog still could hide an enemy's approach. After radio came into general use, pickets or patrol vessels could relay the information beyond the horizon at night or through fog. But even so, the visual lookout of the patrol vessel was handicapped by his limited field of view.
Even if the presence of an enemy were known, battles could not be waged very successfully on dark
Figure 1-1. Ship to ship communication before radar.
nights because of the difficulty of locating the target. Firing guns blindly in the dark is not effective. Besides being a waste of ammunition it can do harm by betraying your position to the enemy.
Figure 1-2. Indiscriminate firing betrays your position.
Radar overcomes visual limitations.
Radar, generally speaking, can reach out beyond the visual horizon. It can detect through darkness, fog, and smoke as well as through sunshine. You no longer have to wait until the enemy appears over the horizon before you know several facts about him-his presence, number, size, course, and speed. You can be preparing a plan of action before the enemy even knows where you are. Thus, radar enables your ship to "shoot, the guns," even in the dark, and to make hits with the first or second salvo.
Remember that for all practical purposes radar is not affected by visual limitations. It can detect equally well through darkness and smoke, and almost as well through fog. True, it does have a maximum range, a radar horizon, but this is usually well beyond the visual horizon. The wider radar horizon gives you earlier warning of the approach of the enemy, affording you precious minutes in which to prepare for battle. Moreover, radar is more than a walking stick for groping in the dark. It actually gives valuable information as to identity, size, and location of objects, which, without it, would be undetected because of distance or poor visibility.
Derivation of word "RADAR".
Let us digress for a minute to study the derivation of the word radar. We know that radar uses radio techniques, hence the first two letters RA. We know that it is used in detection and this gives us the letter D. In addition to detection, radar is useful in giving the range of an object. This then gives the last two letters AR, A for and, and R for ranging. If we combine them we have:
RA Radio equipment for D Detection A And R Ranging
Information given by radar.
Up to this point, we have discussed the function of radar without mentioning just how it operates, other than remarking that it uses radio methods and techniques. Now let us consider what information it furnishes, and how it functions.
Radar gives the following information:
- Presence of an object.
- Position angle (angle of elevation) or altitude.
Radar operation consists of sending out a series of radio frequency (R.F.) pulses from a high power ultra-high-frequency radio transmitter. These pulses are directed into a beam by a directional antenna. When this beam strikes an object in its path, most the R.F. energy will go around the obstruction, and a small amount, depending on the size of the object, will be reflected toward the sending antenna, the transmitter position there is a highly sensitive receiver which will receive or detect the small amount returning R.F. energy. From the receiver the
returning pulse goes to the indicator where it can be observed.
In radar the reflected R.F. energy is called an echo. The presence of an object is indicated by the echo appearing on the indicator.
Since the echo returns to the antenna when it is pointed at the object, it can be said that the object must have the same direction, or bearing, as the antenna.
The range to an object can be determined by measuring the time it takes for the pulse to go out from the transmitter to the object and return. To avoid confusion, enough time is provided between pulses to allow an echo to return from the greatest distance at which radar can be expected to function.
Position angle is the angle above the horizontal at which a plane may be seen. Since protection against enemy aircraft is of great importance, some radars have been adapted to give this position angle as well as range and bearing. This has been accomplished in several Navy sets with sufficient dependability to permit full radar control of AA batteries.
As an operator you can learn to get the information previously mentioned with relatively few hours practice. A superior operator, however, can gain far more information than this. He can determine the composition of the target, including the number and type of units involved. With experience and a reasonable amount of practice you will soon be able to recognize the difference in appearance of the blip (radar indication) representing a surface vessel and that representing an aircraft. Presently, you will be noting differences in blips caused by large ships as compared with those caused by small ones, and can estimate the size of the ship from the size of the blip and the way it behaves. You will also be able to estimate the number of planes or ships producing blips on your screen.
Remember that you will learn how to do these things only by keeping your eyes open and actually trying to learn. The extra information gained can be of vital importance to your ship. You should not be disappointed because you are unable to establish such data the first time that you stand a radar watch, since this ability comes only from skill and familiarity with your set. However, you cannot use inexperience as an excuse for laxity. Excuses will not save your ship; your ability and experience can.
Importance of Radar.
It is imperative that you as a U. S. Navy radar operator realize the importance of this super-weapon which you are about to master. The importance of the role which radar is playing in the present war can best be set forth by relating actual instances in which it proved beyond a doubt its superior merit.
Battle of Britain. The Nazi Luftwaffe, intent on bombing England, was itself defeated in part through the use of radar. With it, the British beamed directly on Germany and occupied Europe and saw the enemy planes shortly after they rose from the ground. As the huge armadas approached, the RAF, at that time vastly outnumbered, was always at the right place at the right time to intercept them.
On a Sunday evening, January 17, 1943, the then mighty Luftwaffe appeared in force over London in reprisal for RAF raids on Berlin. There was a bright moon and everything seemed to be in their favor. Much to their surprise, however, the searchlights, which previously on similar occasions had scanned the skies in futile search action, now followed the planes with unerring precision. Radar was not only directing the few planes of the RAF to the right spot at the right time, but aiming the searchlights and guns as well.
Without radar, the Air Ministry has said, the Battle of Britain, one of the greatest decisive battles of all history, would have been lost. Radar helped turn the tide of the war. In a sense, it probably saved our entire civilization.
Battle of Midway. Another illustration of radar's effectiveness occurred in the Battle of Midway. A large Japanese force was approaching the island, presumably to attempt occupation. At the island there were several squadrons of land-based bombers. Near by was the carrier Yorktown. Without radar a continuous patrol depending on visible detection would have been necessary. With radar, the enemy was detected while still about a hundred miles distant, and his course plotted. The Japanese were first allowed to close in (thus saving fuel); then the carrier planes and the land-based bombers were sent to the attack. Directed by the large radar stations on the carrier and on Midway, our pilots went straight for their targets. Thus our force, though considerably outnumbered, was able to disperse and defeat the larger Japanese force.
Navy types of radar.
The fundamental principles of all radar sets are alike. However, radar lends itself to many different uses. Each use requires a different application of these principles. In this section, the different types of
Navy radar will be discussed. Every radar operator is to some extent a specialist on certain types of equipment. It is important that he should know his own apparatus particularly well.
Search. Due to the great speeds with which enemy aircraft make their approach, need for detecting them in ample time has become only too evident. A large portion of shipboard radar has been designed for just this purpose. It picks up targets far at sea, giving our own ships sufficient time to prepare for immediate action. Reports from this radar are given at regular intervals to the combat information center (covered later in this book). This makes possible the rapid and accurate plotting of the enemy's course and speed. It is a most valuable aid in sending our own planes aloft to intercept the enemy.
There is also a need for detecting surface targets and securing knowledge of their movements. For this purpose every ship in the U. S. Navy has some type of surface search radar aboard. This equipment is not only invaluable for the location of the enemy task force, but is equally important in the location of surfaced submarines (it can detect even their periscopes), or for obtaining positions of ships in convoy.
Both surface and aircraft search radar are fundamentally alike. However, since each has a different task to perform, special consideration was given to the design of their respective antennas. In the days following the birth of this revolutionary weapon, a group of technical experts devoted a considerable amount of time to the development of the antenna, realizing that therein lay the means of obtaining more accurate target information. For instance, it was found that surface search radar, in order to do its job efficiently and effectively, requires an antenna beam width comparatively narrow in a horizontal plane, yet large enough in the vertical plane to compensate for the roll of the ship. On the other hand, long wave aircraft search radar must have a beam that is wide in the vertical plane because the aircraft targets may be at any angle with respect to the horizon. The horizontal beam width is also large because of a necessary compromise between the need for a narrow beam width, and at the same time a reasonably small antenna.
Engineers and designers were also agreed that aircraft search equipment should have a greater range capability. This was necessary because of the rapid approach of the enemy.
Fire control. During the first year of the war, radar was widely employed as a searching device. Even at that time there were those who thought it could be used equally well in controlling the fire of the ship's guns. Before long, a fire control radar was produced: radar that not only gave the direction and distance of the enemy, but also aimed the guns. This particular type of radar has been used extensively and effectively in the vast Pacific in night operations against the Japanese fleet, and it did an equally fine job in silencing the shore batteries at Casablanca.
Shipborne fire control radar, like search gear, is divided into two general classes, namely, anti-ship and anti-aircraft. Here again, the main difference lies in the antenna and beam it emits. An anti-ship fire control antenna must provide a beam which is extremely narrow in the horizontal plane in order to obtain sharp bearing accuracy. This beam is made somewhat wider in the vertical to allow for the roll and pitch of own ship. Otherwise, the beam might go over or fall short of the target. An anti-aircraft
Figure 1-4. Disadvantage of a narrow beam.
fire control antenna must provide a beam that is very narrow in both vertical and horizontal planes; it must also be made so that it will elevate from horizon to zenith (0° to 90°) as well as in any direction on the horizon.
Special. As you progress in your study of radar, you will encounter equipment that is entirely portable and can easily be moved about from one location to another. This apparatus is especially valuable when a beachhead has been established, and it is used to warn against both aircraft and surface targets.
IFF (Identification, Friend or Foe) equipment is a part of the radar in use today. This equipment, rather than being an actual radar, is an aid to radar. It has its own transmitter and receiver and answers the all important question as to whether the target is enemy or friendly.
Navy letter system.
In conclusion, it is important that you learn a little about your Navy's way of naming the gear with which you will soon work. There are many radars in the Fleet, each doing its own particular job, each having its own particular name.
First, there are two large divisions of radar, those used for searching action, and those used for fire control. To distinguish them, their first letter is always "S" if the instrument is for search, and "F" if for fire control.
The second letter in the designation of search radars is usually an indication of the model of one particular type. For instance, a model SC radar is a search radar and is older than an SK. An SO model, on the other hand, is more recent than an SK, all, however, are used for search. When a modification is developed, the Navy uses numbers to designate the new model; i.e., an SC-2 is a modification of an SC.
All models and types of lire-control radar are now named by the Mark system. This system is employed by the Bureau of Ordnance and is used in connection with all gunnery equipment. Some of the earlier models of fire-control equipment were also known by the letter system used on search gear. For example, the Mark-3 radar was also referred to as FC; the Mark-4 as the FD; the Mark-8 as the FH, etc. Later fire-control models are known only by their Mark number such as the Mark-9, Mark-12, Mark-19, etc.
The identification of airborne radar follows the same general rule; i.e., in the case of ASE, "A" is for aircraft, "S" for search, and "E" for the model of the equipment. Other examples of identification letters are the ASB, ASC, ASD, ASG, etc.
Usually, all types of recognition (IFF radar) equipment used with radar, both airborne and shipborne, have the letter B in their designation. Examples of these are the BL or BK models. Following this same system, the airborne model becomes the ABK. The combinations of IFF units are designated Mark IFF radars; i.e., Mark 3 IFF, Mark 4 IFF, etc.
Radars on board ship.
It should be mentioned that there are certain natural combinations of search gear from the standpoint of the functions the sets serve and the ships which carry them. The SG (surface search) and SC or 5K (air search) always go together. These sets are designed for combatant ships of DD size or larger. The SL (surface search) and SA (air search) also go together. These sets are designed for ships of the DE class. Another group of sets consists of the SO and SF (the SL can also be included). These sets are all for surface search and one is used on small ships and auxiliaries, such as PT's, PC's, SC's, AK's. and AT's, which do not carry an air search radar. You will be expected to become an expert operator of one of these combinations.
Importance of the radarman.
Since radar does let you "look" through fog and smoke and darkness, you find that it is a great help not only for detecting the enemy, but also for locating your own vessels and for warning you of nearby rocks, islands, icebergs, and similar objects. It is most important for the safety of your ship that you have this information. Of course, if your radar is operated carelessly, you cannot expect it to give good results. If you fail to do your task well, fail to notice instantly the indication of an enemy ship or of a rock, you may well be responsible for the sinking of your own ship. Radar is capable of performing its task well, but only if there is an efficient, alert operator at the controls. Yours is indeed a big responsibility, and the safety of your ship and shipmates depends on how well you do your job. Keep this fact before you every minute you are on duty.
You as a radar operator, are the first aboard to know of the enemy's presence, his strength, and his precise
location. Before your Captain can begin to maneuver the ship, before the gunnery officer can give the command to fire, you must pass on your radar information to them.
The information you receive is of no use whatsoever if you fail to relay to the proper place the information your radar gives you. This equipment is not installed merely for the purpose of satisfying your curiosity. Remember that information, in order to be used effectively, must be received by those with authority to act on it. Your duty is to see that those in command get the information they need. They are depending on you. Remember this: what you tell them or fail to tell them may determine the fate of your ship.
The more familiar you become with your equipment, the more you will realize the importance of keeping what you know to yourself. You are being entrusted with a vital military secret when you learn about radar. It is imperative that you keep it a secret. Stop and consider what radar does for you in guarding your ship and in helping to protect your life and the lives of your shipmates. This miracle weapon is largely on our side. That is where it will remain if security functions. Keep what you learn to yourself!
BASIC PRINCIPLES OF RADAR
Before beginning with the basic principles of radar there are a few terms, symbols, and abbreviations that you should learn in order to help you understand the material that follows.
The word cycle is familiar to most of us, occurring in such expressions as "vicious cycle," "cycle of prosperity," "cycle of life," etc.
The cycle is one complete series of events at the end of which conditions are back at the starting point. Beginning with any particular phase or condition, one cycle is completed as soon as it starts repeating itself.
Frequency means only how often something is done, as, for example, the frequency of eating, which should be three times a day. The frequency of your heartbeat is about 72 beats per minute. In general, frequency indicates the number of times something occurs in a certain period of time. Frequency in radio is the number of cycles per second, or the rate at which the cycles occur.
Since these cycles occur regularly, there must be a definite time required for each cycle. This time is known as the period. Remember that it is a measure of time required for one cycle to occur. Hence it is reasonable to expect the time for each cycle to decrease when the number of cycles per second increases.
If one second is divided into one hundred equal parts, each part will be one one-hundredth of a second long. The period is 1/100-second when the frequency is 100 c.p.s. (cycles per second). Now, note this carefully: The period is the same as one divided by the frequency.
Another term that will help to make our discussion simpler is wave length. This is the actual distance traveled by the energy while it is completing one cycle.
You are familiar with the fact that as long as you travel at a definite speed, you can cover a distance proportional to the time. In other words, if you travel at a speed of 30 miles an hour for one hour you go 30 miles; if you travel for one-half hour, you go only half as far; if you travel fur two hours, you go twice as far, etc. The distance increased with the time or, as expressed before, distance is proportional to the time. The distance you can go in a certain time is equal to the speed, or velocity, multiplied by the time. Radio energy too, can travel a certain distance in a given period (i.e., in a definite amount of rime). This distance is called the wave length. It is a measure of length, just as feet and inches and yards and miles are measures of length. Using the rule: distance equals velocity multiplied by time, you find that
wave length (distance) = velocity (speed) × period (time).
Usually, you will not know the period, but this information is not necessary for finding the frequency or wave length. Therefore, a formula which gives the wave length when the period is not known will be better for our purpose. Remember that the period equals 1 divided by the frequency: P = 1/f. Substituting 1/f for its equal P the new formula will read:
wave length = velocity × 1/frequency, or WL = V/F
this is the usual formula used in calculating the wave length.
Radar energy travels at a velocity (or speed), of 162,000 nautical miles a second. If frequency is 2,000 cycles per second, what is the period, and what, then, is the wave length?
Frequency (f) = 2,000
Therefore the period 1/f = 1/2,000 second = 1,000,000/2,000 microseconds = 500 microseconds.
Abbreviation or symbol Meaning Example or equivalent Kilo Prefix indicating 1,000 10 Kilocycles = 10,000 cycles Mega Prefix indicating 1,000,000 10 Megacycles = 10,000,000 cycles Micro (μ) Prefix indicating one millionth 10μ sec = 10 microseconds=10/1,000,000 sec. Milli Prefix indicating one thousandth 1 ma = 1 millampere = 1/1,000 ampere V,S Velocity, Speed V,S,C Speed of light or radio waves 300,000,000 meters per sec.
162,000 nautical miles per sec.
˜ Cycle per second 100˜ = 100 cycles per sec. ° Degree 360° = 360 degrees F,f Frequency λ (lambda) Wave length
Note: 1 second = 1,000,000 microseconds.
The wave length = velocity/frequency.
162,000/2,000 = 81 miles.
Thus you see that the energy travels 81 miles while it goes through one cycle.
You have learned that radar may be used to determine the range and bearing of a target. Range is the distance of the target from you. Bearing is the direction of the target.
The table at the bottom of page 1-10 gives the most common symbols and abbreviations which you may encounter in reading the various radar publications,
Radar equipment operates on many frequencies. some of which are just being explored today. To safeguard this system, operating frequencies or wavelengths are classified and described only by the following code:
Frequency in megacycles Code 0 to 300 P 300 to 1,500 L 1,500 to 5,000 S 5,000 to 10,000 X 10,000 and above K
1 inch = 2.54 centimeters 1 yard = 36 inches 1 meter = 100 centimeters 1 meter = 39.37 inches 1 meter = 1.09 yards 1 nautical mile = 2,000 yards (approx.) 1 statute mile = 1,760 yards
Principle of pulse reflection.
Now that you know something of the importance of radar, its various types and their uses, and have built up a vocabulary of terms frequently used, let us consider next just how this equipment functions in securing information.
If you understand the principle of sound echoes you have mastered one of the basic understandings of radar. Suppose that you are in a canyon and that some distance from you is one of the walls of this canyon. You shout loudly-then wait. What happens? The shout comes back in the form of an echo. Why? It is simply that the sound wave from your voice travel through the air, hit the wall of the canyon, and bounce back. You hear the reflected sound wave. If you want to hear it distinctly you do not shout continuously, but utter one brief sound and then maintain silence until the echo returns.
By shouting for a short interval and then waiting, it is possible for you to shout loudly again the next time. In other words, you are sending out pulses of energy of short time duration, thereby making it possible for you to shout at maximum strength without straining your voice.
This is the basic principle of echo ranging: sending out brief pulses of energy and measuring the time it takes them to return.
SENDING ENERGY TIME TO RETURN
Figure 1-7 Timing echoes.
Radar also works on the principle of pulse reflection. A strong pulse of radio energy is sent out into space from the radar. If there is a target such as a ship or aircraft in the path of this radio energy, some of the radio waves upon striking the target rebound just as sound waves do, and produce an echo. This echo which is not heard, but seen on a special device called the cathode-ray tube, is called a blip or a pip.
In radar we deal not with sound waves, but with radio waves. In shouting, the sound waves are produced by your vocal cords. In radar the radio waves are produced by a unit called the transmitter. This radar transmitter is turned on for a very short period, so short that the time is measured in microseconds (millionths of a second). This short time during which the radar is sending out radio energy is called the pulse width, or pulse duration. During this short time the transmitter produces a maximum amount of energy.
After the transmitter is turned off, there is a definite amount of time (measured in microseconds) before it is turned on again. This is called the rest period. It is during this rest period that the echo returns, if there is a target in the path of the radio waves. This rest period must be long enough to allow time for an echo to return before another pulse of energy is sent out. Accordingly, we have the pulse width during which the strong radio signal is sent out into space, and the rest period during which time the echo returns. The transmitter is also turned on a certain number of times a second, thus setting up a pulse repetition rate.
Suppose that there are several large cliffs or walls in different directions from you in a canyon in which you are shouting, and that you desire to receive an echo from one certain cliff. By shouting and sending the sound waves in all directions, you have no way of telling which wall is sending the echo back to you; perhaps they are all sending back a small amount of the energy. By placing a megaphone to your mouth, or by cupping your hands around your mouth, it is possible to direct most of the sound waves in any desired direction. If you desire to receive an echo from one certain wall or cliff, you point the megaphone in the direction of that cliff and shout into it. The sound waves are concentrated into a narrow beam, go out, hit the cliff, and are reflected back from the same direction. Since you know the direction in which you have sent the wave, you know the direction from which the reflected waves come.
In radar the radio waves are concentrated into a narrow beam by a special antenna which will be described later. This narrow beam is much the same as the concentrated beam of light a searchlight sends out. Just as you can point a megaphone or searchlight, you are also able to point the antenna and direct the narrow beam of radio waves in any desired direction. If now you receive an echo while the antenna is pointing in a certain direction, you know the target is in that direction. Radar works the same at night or during bad weather as it does in daytime or during good weather. Accordingly, it is possible to detect a target with radar when it is impossible to see it with optical equipment. By turning the antenna through a complete circle or 360° around you, it is possible to detect any target. You will know the direction or bearing of this target by knowing the direction the antenna is facing.
Range determination by sound.
Detecting the target and knowing its bearing are not enough. You must also know how far it is from you, or the range of the target. By knowing both the bearing and range of a target, you locate the target exactly.
The following analogy will help to make the concept of range clear. If a stone is dropped into a pool, a small wave will start out from where the stone hit. This wave spreads out in a circle in all directions. If there is a pole or piling in the pool a short distance from where the stone is dropped, the wave going out will hit the piling and a reflected wave will start back. Assume for the sake of explanation that the wave is traveling at a speed of one foot per second through the water. If you start a stop watch when the stone hits the water and note how many seconds elapse before the reflected wave returns to the starting point, you can easily tell how far away the piling is. For example, if it takes eight seconds for the wave to go out and return, then the distance traveled is eight feet. The range, which is the distance out to the piling, will be one-half of the total distance, or four feet.
A special stop watch could he devised with the face marked off in feet instead of seconds. The dial would read distance instead of time. For example, in place of one, two, three seconds, and so on, the face would read one, two, three feet, etc. Better yet, since we are interested only in the distance to the target, the face
of the watch could read one-half foot in place of one second, one foot in place of two seconds, etc. Thus, at the instant the wave returned to the starting place, you could either note where the hand of the stop watch was, or you could stop the watch and read the exact range out to the piling.
What you actually do is measure the elapsed time from the instant the stone hits the water until the reflected wave returns to the starting place. Knowing the speed of the moving wave, you multiply the time by the speed. in order to compute the distance. As range is only half the distance traveled, you divide the distance by two and obtain the range. Put in the form of a formula, R = (s × t)/2 where R is range, s is speed, and t is time.
Let us go back to the example of sound echoing in a canyon. Suppose you want to know how far away that cliff (i.e., its range) which sent back the echo is. You need to know that the speed of sound is approximately 1,100 feet per second, and you also need a watch, or better still, a stopwatch. It is now an easy matter to obtain the range. If, for instance, four seconds pass from the time you shout in this canyon until you receive the echo, you know that the distance traveled by the sound waves out to the cliff and back is 4 × 1,100 feet or 4,400 feet. You know that range is one-half the distance out and back: therefore the range is (4 × 1,000)/2 or 2,200 feet. Since you divided by two, why not divide by two at the start, and make a statement that for sound, range equals 550 feet multiplied by time? Thus, it would be 550 × or 2,200 feet.
On your stop watch, you could make a special face to measure range in terms of echoes. Since for sound one second is equal to 550 feet range, in place of one second on the watch have 550 feet, in place of two seconds have 1,100 feet, etc. Thus, whenever the echo returns, it is possible to note where the second hand is at that instant on the face of the watch and to read directly the range of the target.
Range determination by radar.
How does all this fit in with radar? Let us next see how see are able to tell how far away the target is, or how to obtain the range with radar.
Light travels so fast that it is almost instantaneous. Radio waves, electricity, and light, all travel at about the same speed, which is 186,000 land miles per second, or almost seven and one-half times around the earth in one second. Expressing this in nautical miles (a nautical mile is approximately 2,000 yards): radio waves travel 162,000 nautical miles in one second, or 300,000,000 meters in one second. Knowing the speed of radio waves, it is possible to obtain the range of a target with radar by the same method as that used in the case of sound.
For example, suppose that the transmitter sent out a short pulse of radio energy and that the reflected wave was received after 1,000 microseconds. The distance traveled out to the target and back is 1,000/*1,000,000 × 162,000 = 162 nautical miles. Referring to the sound analogy, remember that in order to compute the range, the total distance out and back must he divided by two. So also with radar; the total distance must be divided by two. In this case range is 162/2 or 81 nautical miles.
However, the speed of radio waves is much greater than that of sound. Therefore, an ordinary stop watch cannot be used. A special timing device is needed to measure such small time intervals as microseconds. This special timing device is called the cathode-ray tube, and on it there is a special time base which takes the place of a second hand on the stop watch. Instead of being marked off in divisions of one, two, three microseconds, etc., this time base can be marked off directly in miles or yards of range.
By sending out a very short pulse of energy from a high powered transmitter, and receiving the echo which is called the pip, you have detected a target. By knowing the direction the antenna is facing, you know the direction or bearing of the target. By measuring the time it takes the wave to go out to a target and return, you have a means of obtaining the range of the target. If you know the bearing and range of a target, you then know the exact position of this target at any instant. In good weather or bad, in daytime or at night, whether surface craft or aircraft, you are able to establish the desired data. But these are only some of the many things you are able to tell about the target. In the pages which follow you will learn about other information which a good operator can get through the medium of radar.
MAIN PARTS OF A RADAR SYSTEM
In the preceding section you have learned about the principles of radar. This section will deal with
* The 1,000 microseconds must be divided by 1,000,000 to give seconds.
the main parts of a radar set. If you are familiar with the several units that go to make up the radar set you operate, the chances are that you will be able to operate it more skillfully and intelligently. In studying the derivation of the word radar you found that the first two letters ra came from the word radio. You know that in radio there must be a transmitter and antenna system to send our the program; to reproduce the program there must be an antenna system, receiver, and loudspeaker. Likewise in radar the main parts are the transmitter, antenna system, receiver, and indicator which functions in radar in the same manner that the loudspeaker functions in radio.
Wave length or frequency. To make radio detection and ranging possible, it is necessary at the outset to send out a pulse of radio waves. It is the function of the transmitter to generate the pulse. Since the transmitter creates the pulse, it is the source of the radio energy. There are well defined differences between the radar transmitter and the radio transmitter that operates in the radio station. One of the main differences is in wave bands on which the two types of transmitters operate. Radio stations operate on either the broadcast or the short wave bands, but radar uses the ultra (very) short waves that were previously used only for experimental purposes. Another difference is that radio broadcast transmission is continuous while radar transmission is intermittent.
Duration and power of pulse. An important point to remember about the compact radar transmitter is its ability to send out pulses of radio energy as powerful as, and in many cases more powerful, than the transmissions from the biggest broadcast transmitters. High power is necessary in radio transmissions to carry the broadcasts to the listeners at distant points. In radar very strong pulses must be emitted in order to get back even a small echo or reflection from the waves striking the target while the rest of the waves continue into space until they die out. Unfortunately, only a minute amount of the energy of the pulse sent out is bounced back. In generating the strong pulses that are needed, high voltages, dangerous to life, are required. Everything possible, however, has been done to make the equipment safe for the operator, so long as certain safety precautions are observed.
In the section on the principles of radar, the point was stressed that short duration pulses rather than continuous transmission were used in order to provide time for the echo to return. Continuous operation would drown out all reflections or echoes. The actual time the transmitter is sending out radio waves is measured in very small units of time, micro (millionth) seconds. When you read the expression, pulse width, banish any and all thought of expressing this value in feet, inches, or meters, because the pulse width is a measurement of the time the transmitter is working. At first, it may be bewildering to consider time reckoned in such small denominations, but later on it will serve as a constant reminder of the importance of speed in all phases of radar.
One advantage that results from the transmitter's working only in brief periods is the long rest or idle period during which sufficient electrical energy can be stored up to provide the extremely high power necessary for the next pulse. Thus the overall or average power output of the transmitter is low, and within the transmitter's capacity. During the brief moment of transmission, considerable heat is generated by the tubes. During the rest period the tubes cool. Should the transmitter be allowed to operate continuously at such high power the unit would be badly damaged or even destroyed by the intense heat. Motor-driven fans or blowers circulate cool air inside the cabinet of the radar set to aid in keeping the temperatures at safe operating levels. The length of the rest period (expressed in microseconds) is dependent upon the pulse interval (the time between the beginning of one pulse and the start of the next one) and the pulse width (the working time).
Pulse rate. The keyer unit performs the task of pulsing the transmitter. The keyer, which will be described later in this section, does exactly what its name implies, it keys the transmitter, turning it on for an instantaneous surge of radio energy for a few microseconds (or even a fraction of a microsecond); then, after the pulse, it turns it off for a comparatively long time until the next pulse. It is during this period while the transmitter is resting that the echoes return from any object that was in the path of the outgoing pulse. The rate at which the keyer pulses the transmitter is the pulse rate or pulse repetition rate, which simply means the number of times the transmitter is sending out a pulse of radio waves each second. The keyer operates at a constant rate, spacing the pulses so that the interval between any two is always the same. The length of time of
Figure 1-8. Block diagram of a typical radar system.
the pulse interval is a direct result of the pulse rate. For example, if a transmitter sends 50 pulses at regularly spaced intervals in one second's time, the repetition rate of the unit would be termed 50 cycles per second (a cycle being one complete operation). To calculate the interval between each pulse, divide one second (one million microseconds) by the pulse rate. 1,000,000/50 = 20,000 microseconds-the pulse interval. (Assume a pulse width of 10 microseconds.) 20,000 microseconds - 10 microseconds = 19,900 microseconds, the rest period. Note the extremely long rest period!
The antenna system.
The antenna system is one of the most important parts of the radar equipment since it radiates the radio frequency energy into space and receives the reflected energy or echo. The antenna system is made up of two parts: (1) the transmission line, and (2) the antenna.
Transmission line. The purpose of the transmission line is to carry the high frequency energy from the transmitter to the antenna, and to carry the reflected energy from the antenna to the receiver. This transfer of energy must he done with a minimum of loss.
Two types of transmission lines are in general use in radar equipment: the concentric or coaxial line, and the hollow wave guide. The coaxial line consist of one conductor inside another. Both conductors may be tubular, or the outside one may be a hollow tube and the inside a solid conductor. Since the inner conductor must be exactly in the center of the outer conductor, a great number of insulating disks are required. Moisture on these insulators may cause a flash-over or breakdown between the two conductors so it is necessary to keep the line filled with nitrogen or dry air at a pressure of about five pounds per square inch. The wave guide is a hollow metal pipe, the cross section may be rectangular or circular. The rectangular wave guide is most commonly used today.
Now the question arises, "Why have two types of transmission lines?" The reason for this is that coaxial lines are more suitable for radars operating below 3,000,000,000 cycles while wave guides are better for radars at and above 3,000,000,000 cycles.
When comparing the coaxial line with the wave guide, the following advantages seem to favor the wave guide: (1) construction is simpler. (2) losses are lower, and (3) power capacity is greater.
The wave guide's principal disadvantage is that unless the frequency is very high, the size of the pipe must be unreasonably large.
Antenna: an emitter of radio waves. In order to utilize the radio energy created by the transmitter pulse, it must be converted into radio waves, which can be shot out to strike any object in the outgoing path. The antenna functions as the converter of the radio energy into radio waves which can be radiated in any desired direction into the atmosphere. To better illustrate this fact, consider the situation resulting when a flashlight is supplied with new batteries but lacks a bulb. Even though the necessary power is available, the means of using it are lacking. As soon as you insert a bulb in the socket and close the switch, the battery energy is put to work and its conversion to light waves results in a beam of light. The transmitter can be likened to the battery-filled flashlight, for it, too, is a source of stored energy-radio energy. The transmitter is inoperative without an antenna for the same reason that the flashlight is inoperative without a bulb. The antenna converts the pulse of radio energy from the transmitter into radio waves, just as the bulb converts the battery energy to light waves.
The fundamental element from which most antennas are built is the half-wave antenna, or dipole. This is a metal rod or wire one-half wave length long. Since the velocity of radio waves is constant, 300,000,000 meters per second, the length of a
Figure 1-9 Coaxial. Wave-guide.
dipole gets shorter as the frequency increases. This is another interpretation for the formula; wave length equals the velocity of a radio wave divided by the frequency of a radio wave. Therefore, a given antenna works best at only one frequency.
Non-directional antenna. A single dipole will send its energy out in all directions around itself. The greatest amount of power goes directly outward at right angles to the length of the rod with decreasing amounts of energy out in other directions except in the direction of the rod, where no energy is sent out. Consequently, the side view of the lobe looks like a pair of circles touching the dipole, as shown in figure 1-11.
Figure 1-11. Dipole and radiation pattern.
A target at right angles to this dipole would give a strong echo, while one in the actual direction of the dipole would give a very weak echo (theoretically no echo). One of these dipoles mounted vertically would send a strong signal toward anything around it and on its level. It would give a weaker signal to anything above it. Some of our sets use antennas like this so that any target around them, regardless of its bearing, can be detected. Because the radio waves are going out in all directions, bearing cannot be indicated and only range can be determined.
Directional antenna. A reflector can be used with a radar antenna to make it unidirectional, that is, to cause all the waves to leave the antenna in one direction rather than in two directions. This reflector serves the same purpose, and functions in the same manner as the reflector used in a flashlight. The four most popular types of reflectors are: (1) the flat or "bedspring" type, (2) paraboloidal or "dish pan", (3) parabolic or "barrel stave", and (4) semi-parabolic.
The type of radar antenna using a number of dipoles, called a curtain array, or bedspring array, has a metal screen reflector. The dipoles are a definite distance forward of the screen. The more dipoles used, the sharper the beam. However, the energy is strongest in a direction approximately at right angles to the screen. The metal reflector is perforated to reduce weight and wind resistance, or it may be merely a wire mesh.
If the antenna is wide, you can expect a narrow horizontal lobe because there will be several dipoles horizontally. If there are many dipoles vertically, the vertical lobe will be sharp. Of course, the physical size (in feet and inches) of any particular lobe width will depend on the wave length. When comparing antenna sizes, be sure to measure them in square wave lengths.
Figure 1-12. Wave lengths.
As an example, if you have an antenna like the one shown in figure 1-12 you can measure the area easily by noting that each rod is one-half wave length long, and that each row of rods is one-half wave length away from the next. This antenna, then, is three wave lengths broad, one wave length high, and has an area of three square wave lengths.
The antenna in figure 1-13, though physically smaller, is four wave lengths wide, two wave lengths high, and has an area of eight square wave lengths-almost three times as great as the previous example.
Figure 1-13. Wave lengths.
As a general rule, a sharp vertical lobe is not desirable for search sets because the lobe might shoot over the target and miss it as the ship rolls. If this happened, an echo would he reflected from the target only occasionally, instead of almost every time the
antenna completes a rotation. To avoid this, only a few dipoles are stacked vertically. Remember that the greater the horizontal dimension of the antenna (when measured in wave lengths), the sharper the horizontal lobe; the greater the vertical dimension of the antenna, the sharper the vertical lobe.
Fire-control sets must give accurate bearings, and (if used to control antiaircraft fire) accurate position angles. By reducing the wave length (increasing the frequency), the antenna necessary for this bearing accuracy can be reduced until its size is physically practical. If, also, the reflector is bent into a semi-parabola, the sharpness of the vertical lobe can he increased. From the side, this antenna has the appearance of a "V" with a rounded bottom, the "V" being on its side with the wide opening toward the target.
This antenna is not entirely satisfactory for antiaircraft fire control. Being a wide antenna (in wave lengths), it gives satisfactory bearing accuracy, but lacks sufficient position angle accuracy for AA gun laying. To increase this accuracy, a modification has been made. In effect, two of these semi-parabolic antennas have been fastened together, one atop the other. In this way, the sharpness of the vertical lobe has been increased. The antenna shape is shown in figure 1-14
As you go to higher frequencies, dipoles and curtains are of less concern. Reflectors of the type used with searchlights can be employed. This is due to the fact that radio waves at the higher radar frequencies behave much like light.
Paraboloidal reflectors, bowl-shaped, are usually called "dishpan" reflectors, or "spinners". They are used for surface-search and some air-search and fire-control sets. Their hearing accuracy depends on the diameter of the spinner measured in wave lengths (the unit of measure used for other types of antennas).
Since this type of antenna produces a "pencil" beam, the beam is as narrow in the vertical plane as it is in the horizontal, which for some types of radar necessitates a helical search (a spiral search, changing the position angle as well as the bearing angle).
On some sets, a sharp, vertical lobe is a definite disadvantage. If a large spinner is used (for good bearing accuracy), the vertical lobe may be too narrow. To increase the vertical beam width, the rep and the bottom parts of the spinner are cut off, which reduces the vertical size of the spinner without reducing the
Figure 1-14. Mark 4 antenna.
width, thus increasing the width of the vertical lobe without affecting the horizontal beam width. This type is called the parabolic reflector and, since it resembles a barrel stave, is also known as a barrel slate reflector. Many of our surface search radar sets use this antenna.
Another type of antenna involves an entirely different principle. Tapered plastic rods about three feet long are used, and the energy comes out along the full length of a given rod. A rod by itself will produce a beam about 30° wide. By placing 14 of them side by side, the beam is narrowed to 2°. Three vertical rows are used to narrow the beam to 6° in the vertical plane. When these rods are energized at different times, the lobe goes out to one side-toward the side which received the energy last. Constantly changing the amount of delay causes the lobe to move steadily from 15° to the left of the center line to 15° to the right in 1/10 of a second. The great advantage of this system is that 30° can be seen at once, with the accuracy of a 2° beam (similar to television scanning). This permits accurate spotting in both range and deflection. Such an antenna, however, is extremely heavy and complicated.
Figure 1-15. Parabolic or barrel stave antenna.
The foregoing discussion of antennas has avoided technical treatment of the subject because for purposes of this handbook only general ideas are needed. It will he helpful to keep the following in mind:
The antenna size, in wave lengths, determines the size and shape of the lobe.
A wide antenna gives a narrow, sharp horizontal lobe, while a narrow antenna produces a broad horizontal lobe and poor bearing accuracy.
Use of high frequencies permits use of parabolic reflectors, which give narrow beam widths, presenting better hearing accuracy.
The wave length is the unit used in rating antenna size.
How does radar determine bearing?
You have already learned how radar detects the presence of a target and determines range. In this section you will discover just how it establishes the direction, or bearing and position angle.
When you shout in the direction of a cliff or big building you hear an echo, but when there is no cliff or building to reflect the energy there is no echo. If the radar energy, like sound, is sent out in one general direction, you can tell approximately the direction of an object by simply observing the direction from which the echo returns. By knowing this direction, you know the target's direction since it is the same.
When you shout, the sound as you know, does not go only straight out, but can also he heard to either side of the direction you are facing. The degree to which the energy is scattered will determine how accurately you can judge from the echo whether or not you are facing the cliff. If the sound energy is scattered over a wide angle, perhaps you can receive an echo when you are facing, and shouting, in a direction far off to one side of the target. A small amount of the energy goes off in the direction of the target, and will he reflected as an echo, but it will be a weak echo.
You are probably wondering now how you can tell whether you are getting the echo back from the direction you are facing, rather than from a direction off to one side? The answer to this is that you get the largest echo when you are facing the target directly.
Concept of lobe. Your radar set sends out its energy like sound in a general direction, but with some scattering. You receive radar echoes from a target even if you are not pointing the antenna in the exact direction thereof. But, as in the case of sound, you get the strongest echo when you are directly on the target.
Since you get the strongest echo from an object when the antenna is pointed directly at it, you can reasonably expect to locate a small target in that direction more easily than in any other direction. Likewise, you can detect an echo from a ship at a greater range in the direction the antenna is pointing than in any other direction. It is logical to expect that a ship will reflect a stronger echo when it is only slightly off the antenna hearing than when it is considerably
off the antenna hearing for if a ship lies to one side of the antenna bearing it cannot give as strong an echo as one directly on the antenna bearing, unless it happens to be nearer or larger.
In fact, if a ship sails around your antenna hearing in such a way as to always produce the same strength echo, it will follow the path shown in figure 1-16. This shape is called a lobe, and is actually a representation of the range at which you can get an echo of any particular size in any direction from the antenna. It represents approximately the amount of power sent out in any direction. Figure 1-19 shows how great a change in echo height results simply from turning the antenna around, sweeping the lobe past the target.
"E" units. Referring to the actual height of the echo in inches is of little value because you can increase or decrease the height of the echo at will merely by varying the gain control. Even a weak signal can easily be "blown up" in this manner until it approaches saturation, but this increase may not help because of the corresponding increase in grass height.
Grass is a disturbance on the cathode, ray tube (C.R.T.) screen, caused by noise in the tubes, static, etc. It shows up as a fuzzy, jumpy fur along the time base on "A" and "R" scopes, and as many small, bright spots (sometimes called snow) on the B and PPI scopes. It is always present to some degree, and pips smaller than the grass are very difficult to find. The grass seems to wave just as actual grass does, and may cover the pip. The height of this grass can be controlled by the receiver sensitivity (or gain) control low sensitivity means low grass height and high sensitivity means a large amount of grass. You know that the pip height can he made larger by increasing the gain, and smaller by reducing the gain. Since both the pip height and the grass height vary together, the pip size can he compared to the height of the grass for this discussion. The comparison is made in units.
The "E" unit system of discussing echo strength is standard in the Navy. It is a convenient system to use; by providing common terms in which to discuss the strength of echoes confusion and misunderstanding are reduced. A small pip about the same height as the grass is an E-1 echo. It will always he difficult to detect, and only a wide-awake operator will notice it. An echo of this strength will probably he overlooked on a PPI scope by even the very best operator. That is the main reason that an operator should not devote all his attention to the PPI scope.
The complete E system includes five values: E-1, E-2, E-3, E-4, and E-5. E-1, as is indicated above, includes those signals whose ratio of signal to noise, or of pip height to grass height is one to one (i.e., the pip and grass are the same height). An E-2 echo is an echo whose pip is twice as high as the grass. An E-3 echo is one producing a pip four times as high as the grass. If the pip is eight times as tall as the grass, we say we have an E-4 echo. An exceptionally great echo reaching saturation, or 16 times, or more the height of the grass, is spoken of as an E-5 echo.
An E-1 echo is very weak, an E-2 echo is weak, and F-3 echo is good, and E-4 echo is strong, and an E-5 echo is very strong. The F-5 echoes are the echoes we get from large, nearby targets.
The E-number system enables you to report echo strength in a definite way. You can also use the E-number system for labeling lobes which, as you know, are representations of the range at which you can get an echo of any particular size in any direction from the antenna. If lobes were drawn to represent all echoes for a particular size target from E-1 (very weak) to E-5 (very strong) they would appear as
shown in figure 18. This picture is only for one particular size of target. A larger target would have lobes that extend out farther, while a smaller target would not have any lobes as far out as the E-1 lobe in our illustration. What may he an E-1 lobe for a destroyer might be an E-3 lobe for a battleship. Therefore, you could draw a series of the same picture for different types of ships.
The closer the ship is to you, the larger the blip produced, provided that it stays in any one direction from the lobe center. Also, for any range, the size of the returned echo (and thus the blip) is smaller as the direction of the target gets farther and farther from the direction of the lobe. As the target ship steams toward the DL. (direction of the lobe) the opposite will happen. The blip will get bigger and bigger, reaching its greatest size when the antenna is pointing the lobe directly at the ship.
Maximum echo method. Knowing the D.L., you have a way of telling just where the ship is. Simply turning the antenna until you get the largest blip or the brightest spot, will give you the direction of the target.
Now, let us repeat the procedure in a brief, summarized form. As the target comes steaming in, keeping always at a certain angle with the D.L., the signal
Figure 1-17. "E" unit system for measuring echo.
gets bigger and bigger. (The item of fades which might enter here will he explained later.) The target is changing range without changing its direction from the D.L. This is exactly what is to he expected, because, going hack to the analogy of sound echoes, you know that the nearer you are to the cliff, the louder the echo you will hear. Similarly, the nearer a radar target, the greater the echo received by the radar set.
You cannot affect the range of the target, and hence are unable to do much to increase the height of the blip in this way, while keeping the target on a certain angle with the DL., but if you swing the antenna around a larger echo might come hack to you. lithe target steams along, keeping at the same range, but moving nearer and nearer the D.L., you know that an increasing amount of the energy is hitting it, resulting in a bigger and bigger echo. Conversely, as soon as it begins to move away from the D.L., the echo begins, to grow weaker. As the target crosses, so that its direction coincides with the D.L., the biggest echo possible from that target and at that particular range is received. This is very convenient, because, as we have said before, you can determine the direction of the target by just turning your antenna around until you get the biggest blip or brightest spot. You know where you have "aimed" the lobe, so if you set the antenna for the maximum echo, you know the direction of the target.
This method of setting the antenna (which determines the direction of the lobe), to find the bearing of a target is called the maximum echo method, since you read the direction from which you get the maximum echo. It is the easiest, and consequently the most frequently used method.
Figure 1-18. Lobe and corresponding echo height.
Figure 1-19. Determining correct bearing by echo height.
Accuracy consideration. Let us look at the picture again, noticing especially the nose of the lobe, which is rather flat. That flatness is significant to a radar operator, because it causes greater difficulty in getting accurate bearings. Perhaps you are wondering why this should be the case. To begin with, in this case accuracy depends on effecting a big change in what you are looking at, through just a small change in what you are adjusting. If the blip height is what you are looking at (or what you are using to determine when you are on the target), and you are training the lobe in the direction of the target, you can get the bearing of the target accurately if just a small change in setting the antenna gives you a big change in blip height. When you are on the target, you will know it is time to read your bearing indicator. You will then know that the target is on this bearing.
Minimum echo method. Look at the lobe again. Where can you get a bigger change in blip height than near the D.L.? The biggest change you can get will be right at the edge of the lobe. For this sketch, a ship at "A" would be on the barely pick-up, or the minimum echo line of the lobe. It would give you an E-1 echo. If you swing the lobe around toward the direction of the target (the ship), the
echo will get larger, naturally, but it will increase in size quite rapidly! In fact, for the sketch shown, the echo strength would change perhaps five times as much in the first degree of lobe trace here near the edge as it would for an equal swing in lobe direction near the center of the lobe. It changes in strength more per degree of lobe train right at the edge than it does anywhere else in the lobe. You can tell when a target is in the edge of the lobe five times as easily (from this sketch) as you can tell when it is right at the center of the lobe. That means that you can set your lobe so that the target is in the edge much more accurately than you can for the maximum echo.
However, you find one difficulty: you can read the hearing of the lobe (or the antenna), but not the bearing of the target. Since you want the direction of the target, you are interested in the antenna direction only if it can give you this information. In your present situation, it is obvious that you do not know the difference in direction of the antenna and the target. Consequently when the target is in one edge of the lobe, you cannot find the direction of the target simply by knowing the direction of the antenna. You need to know something more.
If you can find the angle between the antenna direction and the direction of the target when it is in the edge, you can either add or subtract this angle from the bearing of the antenna and arrive at the desired answer.
You can set your antenna so that the target is accurately in one edge of the lobe and then in the other edge, getting two bearings: one larger than the bearing of the target by a certain (unknown) amount, and the other smaller than the bearing of the target by the same amount. You now know that the bearing of the antenna is halfway between these minimum echo bearing readings. Average these two bearing readings and you have the accurate bearing of the target. This is accurate because you determined the two minimum echo bearings several times as accurately as you could have found the bearing corresponding to the maximum echo.
Lobe switching. So far, you have found two ways of setting your antenna (to direct the lobe), to find the bearing of the target: the maximum echo setting and the minimum echo settings. The minimum echo setting gives you the bigger change in echo size for each degree change in antenna train; hence it is the more precise. However, there is an even more accurate way of setting your antenna, and for two reasons: first, because the side of the lobe is used instead of the blunt end, consequently the size of the echo is extremely sensitive to any small change in antenna train, and second, because an improved method of indication is used, based on the comparison of two pips heights (it is easier to judge when two pips are the same size than to judge when a single pip is at maximum size). This is the procedure called lobe switching.
If you have an object that increases in height while another object decreases, the difference in their comparative heights will change twice as fast as the height of either one. If two echoes work together in this way to show when you are on the target (one going up and the other down when you get off the target), you can get the antenna set in the target direction just twice as easily as you could by looking at the change in only one echo.
Figure 1-21. Angle between antenna and target direction.
You could create such a situation in this way:
Direct the energy out to the port side of the antenna direction and get an echo hack. The size of this echo will depend, of course, on the part of the lobe in which the target appears (see fig. 1-22). With this sketch, the echo would be about an E-2 echo.
Figure 1-22. Lobe to port.
Then direct the energy out to the starboard side of the antenna direction to see how big that echo is. Of course, the echo size depends on its position in this lobe (see fig. 1-23).
Figure 1-23. Lobe to starboard.
In comparing the size of these echoes you found that they were not the same. Why? Simply because the target is nearer to the center of one lobe than it is to the other. The target is nearer the edge of the port lobe (in our example), so you get an E-2 echo back from the port lobe and an E-3 echo from the starboard lobe. Now, if you turn the antenna toward the direction of the target, you will be increasing the echo size of the smaller echo, and decreasing the size of the larger echo, with the difference in height changing to ice as much as either echo height. When you have the target exactly halfway between the two lobes the echoes are matched in height: about three-quarters of an inch high in our example. Each echo has changed perhaps one-quarter of an inch, but the difference changed one-half inch, or twice as much.
Several things must be kept in mind in lobe switching. One point is that you send the energy out on only one direction at one time. You send a few pulses out to one side of the antenna direction, and then an equal number of pulses out to the other side of the antenna bearing. You are sending the energy out one way or the other; not both at the same time. Another is that you need to separate the echoes so that you can compare them easily. You can make the blips from the starboard lobe show up a little to one side of the blips from the port side (fig. 1-24), so that the blips will appear side by side. It is vitally important that all the echoes returning from pulses sent out to port appear at the same definite position, and that all those returning from starboard pulses appear at another precise position.
Notice that you do not change the direction of the antenna between these pulses: you simply switch the lobes from one side to the other. When you change the antenna hearing, you change the direction of the lobes as well, maintaining their position a certain number of degrees from the antenna bearing. When you get the blips matched you know that the hearing of the antenna is very close to the desired bearing of the target, but you realize that the target is not in the center of either lobe. Since this is true the echo height is smaller than it would be if the target happened to be centered in either lobe.
The lobe switching method is used in some of our fire-control sets, where bearing accuracy is absolutely vital. When bearing accuracy is as important as in the case of fire-control gear, you usually have a separate scope on which to match pip heights. You can find details of the actual method employed in Part 4 of this book, and in the instruction books furnished by the manufacturers of the various sets. On radars such as the SJ or SA, in which cases the precise bearing is not as important as it is in a fire-control set, we use
the conventional range scope ("A" scope) to show the rips to be matched.
You have found that you cannot read the bearing of the target directly, but only the bearing of the antenna. But if you know when your antenna is bearing directly on the target, you can read the bearing of the antenna as the bearing of the target. If the antenna is off the target slightly when you read, you naturally get a bearing that is incorrect. But, if you can tell when you are off the target, you can tell when you are on the target. The simpler the method for finding when you are off, the mote accurately you can read the bearing of the target. The bigger the change produced in whatever you are looking at (the blip height, for instance), with a small change in antenna bearing, the greater the accuracy will be. The maximum echo method is the least accurate of the three methods you have studied here because the change in echo strength is small per degree of antenna train. The minimum echo method is more accurate than the former, but it takes more time, and so it is not used very often. The lobe switching method is the most accurate method in general use. It gives a big change in height difference for a small change in antenna bearing.
Minor lobes. So far, in our discussion of lobes, antennas and bearings, we have assumed that most of the energy goes in one direction and that the amount drops off rapidly as we move away from the direction of the lobe. Unfortunately, this is not always true. Sometimes there will be a considerable amount of energy which has a definite direction different from the direction of the main lobe.
When enough energy goes our to form a distinct lobe like this, we say that we have minor lobes, or side lobes (see fig. 1-25).
What does this mean to you as an operator? The primary thing is that you can pick up a target in a side lobe and report it as being in the main (or major) direction of the antenna assuming that the antenna is trained in the direction of the target. If you have a target in a side lobe, however, and are unaware that it is there, you might think that your antenna is bearing directly on the target, when such is not the case at all, and any bearing reading that you may take will be incorrect. If you have side lobes, the assumption that you can pick up targets only when your antenna bears on them is wrong.
For example, let us suppose that you have side lobes 60° to each side of the main lobe. If your target actually bears 135°, you might read a bearing of 195° or of 075° if you have it in one of the side lobes. That is evident, for your antenna is actually directed 60° to one side or the other of the target.
You should remember that it is the bearing of the main lobe you read on the bearing indicators, and not the bearing of side lobes.
These side lobes occur to some extent with all types of antennas. They are generally most noticeable and troublesome with a curtain array. They occur because the dimensions of our antennas are only a small number of wave lengths. We cannot escape them entirely with our present radar techniques; we must recognize their presence, and be careful to avoid errors resulting from them.
Although these minor lobes may cause errors in establishing bearing, they will not result in incorrect range readings. How to recognize pips from side lobes is discussed in Part 3 under Composition."
To all intents and purposes the radar receiver bears a close resemblance to an ordinary radio receiver. Of course, the two sets differ in frequencies of operation, for the receiver must be tuned to the same spot in the wave band as its associated transmitter, and as earlier emphasized, the bands used by radio and radar are widely separated.
Signal amplification. The receiver used in radar must be very sensitive so as to operate on weak echoes. The power represented in the echo would be of little value if it were not built up in some way. This reinforcing or strengthening action takes place in the receiver, and is called amplification. It is the amplifier that builds up the weak radio signals into energy that is finally converted into sound issuing from the loud speaker of a radio set.
In the radar receiver the echo is amplified or increased by similar amplifiers. A manually operated gain control enables you to control the amount that theecho is built up. A relatively strong echo would require less gain or increase, while a weaker signal would require more gain. Actually, the gain control performs the same task as the volume control on the home set. The reinforced or amplified echo is converted into signal energy, but this energy is not fed to a loud speaker or head phones, since you do nor wish to hear a radar echo. It is your desire to see the signal and to derive the information it represents.
The new signal, having been made much stronger and lowered in frequency, is now one that you can use. So far, however, you have nothing to indicate your receiver output. For this purpose, you must connect the receiver to a cathode-ray tube, or INDICATOR, which will indicate the return of the echo. The speaker on your home radio set corresponds to the indicator.
In radar, extremely small divisions of time are measured. The unit is the microsecond, a millionth of a second. Obviously, no ordinary time-measuring device will serve this purpose. However, it has been found that a device used for many years by television people fits radar's demands well. This is called a cathode-ray tube, or the scope. Learning about some construction features of the C.R.T. (cathode-ray tube) will help you to understand how it is used to measure time. Basic electron theory. Before discussing the functions of the various parts of the C.R.T. you should know some fundamentals of electron theory with particular reference to how an electron beam is used to measure time.
Scientists tell us that everything is made up of atoms. which are extremely small particles of matter, Each
Figure 1-25. Main, minor, and back lobes.
of there atoms has from one to 92 electrons circulating around the center in much the same way as the planets rotate around the sun, except that in the case of atoms the units are billions and billions of times smaller. These electrons continue circulating about the center of the atom until shaken loose by some great impact. When two atoms collide with sufficient force some electrons are shaken loose.
Figure 1-26. Structure of on atom showing electrons in their orbits.
Scientists discovered that by heating material such as metal it is possible to make the atoms within the metal collide with sufficient force and such rapidity that the metal will emit or give off electrons in large numbers. They also discovered that some metals give off more electrons than others, indicating that some materials release their electrons more easily than others.
How well these electrons flow in materials is quite important when selecting materials for conductors and insulators. Conductors are materials used for electrical wires because the electrons are held together loosely and can move through the material with relative freedom. Insulators, on the other hand, are materials which hold their electrons very tightly around the center of the atom. Electrons have great difficulty in moving through insulators.
Electrons have a small negative charge. Usually, this negative charge is cancelled by the positive charge of the center part of our atom. This is true, of course, only if the atom has exactly the right number of electrons. If an atom has too many electrons, a negative charge exists, if it has lost some electrons, it is said to have a positive charge.
In conducting materials a large number of extraelectrons may he stored up, resulting in many negative charges. Such matter is said to be at a negative potential. Of course, electrons do not do this of their own accord. If free to do so, they will scatter so as to get as far apart as possible (until there is a uniform distribution of the charge. To make them congregate, you must do something to overcome their natural tendency. You can collect them by stroking cat fur, or by rubbing a glass rod with silk, or by connecting up a battery or generator.
If it is possible for them to do so, the electrons will return to atoms which do not have enough electrons. As soon as the electrons are paired off with atoms lacking electrons, everything is back to normal, or a state known as zero potential.
Whenever you collect electrons in one place to produce a negative charge, you must get these electrons front somewhere. When electrons are taken from atoms, those atoms do not have enough electrons to be neutral. Since there is a lack of electrons the atoms in question have a positive charge.
There are certain facts about negative charges and positive charges that may be stated in the following general law: electrical charges of like kind repel each other, and charges of unlike kind attract each other. In terms of negative charge and positive charge the law is: a negative charge will repel a negative charge; a positive charge will repel a positive charge; a positive charge will attract a negative charge. This is exactly like the principle of magnetism: like poles repel and unlike poles attract each other.
The following is a summary of the main ideas we have discussed on the fundamentals of electron theory.
Electrons are small particles of negative electricity.
Each atom has a certain typical number of electrons. So long as it has this number of electrons, it has no net charge, and thus has zero potential.
Conducting materials have some electrons attached very loosely. The atoms can gain or lose electrons easily.
Non-conducting materials have the electrons firmly bound to the atoms. Any free electron finds great resistance to its movement.
When electrons are caused to assemble on, or in, something, that object is said to have a negative charge.
When electrons are taken away (so that there is an insufficient number to satisfy all the atoms), the object has a positive charge.
Electrons will go from a negative charge to a
positive charge unless special measures are taken to prevent their doing so. This movement is called a current.
Like charges repel and unlike charges attract each other.
In order to measure the time it takes for the energy to go out, and echo hack, we must have some instrument to indicate its return. Mechanical means, such as an ordinary pencil with gear and lever arrangements Could not operate rapidly enough for this: their weight prevents them from being moved quickly enough to give an indication of the presence of the reflecting object. Since electrons are so extremely light, a beam of electrons may be made to move when the echo returns, and move in just a fraction of a microsecond. This electron beam is used as a convenient pencil to draw the picture of what is happening.
Structure of the electrostatic cathode-ray tube. In a cathode-ray tube the hot metal from which the electrons are boiled is called a cathode. This is usually only a small piece of fine wire (something like the filament of an ordinary light bulb), which is heated by an electric current. In order to make the electrons boil off more easily, this wire is usually given a chalky covering of a special material, which is merely a substance from which electrons can easily be boiled. The cathode, then, just furnishes the electrons.
You could control the number of electrons by varying the cathode temperature, but this is a very slow process. Some other method of controlling the number of electrons must be used. A man by the name of De Forest found that a negatively charged piece of metal in the path of these electrons could stop them altogether, while a wire with a smaller negative charge would permit some of them to go by, the number depending on how negative this wire was. He also found that by weaving wire into a grid he could do this more easily than by using just a single wire. We still call this controlling part of the C.R.T. the grid, but its actual physical shape is more like a miniature tomato can fitted around (but not touching the cathode). There is a small hole in one end to let the electrons out in a stream, or beam. The potential ofthis grid, being usually negative, repels most of the electrons, but a definite number get through for any particular grid voltage. The less the negative potential (or voltage), the more the electrons that slip by. Figure 1-27 shows the grid (with cathode inside).
The electrons coming out of the grid are moving relatively slowly, and more or less at random. If they are to he used, they must be speeded up (accelerated) greatly so that they will get from the cathode to the screen in a short time. To do this two more parts are placed in the C.R.T.: the first anode and the second anode. Remembering the properties of electrons, you know that a positive voltage attracts the electrons and causes them to rush toward it. This force of attraction depends on the magnitude of the voltage, so to get a strong force, the anodes are put at a high positive voltage. This causes the electrons to move very rapidly toward the anodes, and to shoot through them toward the center of the screen.
The anodes are cylindrical (see fig. 1-28) and are mounted so that the electrons may shoot through the hole in the grid, the hole in the first anode, and the hole in the second anode. Since the electrons emerge from the second anode at extremely high speed, the complete arrangement of the cathode, grid, first anode, and second anode is sometimes called the electron gun.
These electrons continue on and in a very short time strike the glass front of the tube. They are much too small to be seen even with the most powerful microscope, but they can produce effects which are visible. Certain materials will glow and give off light when these electron bullets strike them. If the inside of the tube could be painted with some of this material, you could tell just where they hit by looking for this glow. That is precisely what is done. Since the glass front, painted on the inside with fluorescent paint (a substance that glows), is what the picture appears on, we call it the screen. Its purpose corresponds exactly to the screen in a motion-picture theatre. Without it, you could not see the pip.
You know that you can attract the electron with a positive charge and repel it with a negative charge, and that the attraction or repulsion is proportional to the charge (or potential). You can move the beam upward by placing a positive charge near the top, or a negative charge near the bottom of the tube, or by both. Likewise you can move the beam to the side by putting positive or negative charges to the side. It is convenient to put these charges in place by using two pairs of metal plates, one pair horizontal and the other pair vertical. These are known as the deflection plates, since they deflect the electron beam.
One of the vertical plates is above and the other below the beam. If there is a positive charge on the upper one and a negative charge on the lower, the beam will move upward, since the positive charge attracts the electrons and the negative charge repels them. These charges can be placed on the plates this way by connecting the + terminal of a battery to the upper plate and the - terminal of the same battery to the lower plate. Since these plates move the beam up or down, they are called the vertical deflecting plates (V.D.P.). Remember that they lie horizontally in the tube, but the direction in which they move the beam is important. They are called the vertical deflecting plates because they can cause the beam to move only up or down, never horizontally. The vertical position (up or down) depends, of course, on the amount of the charge on these plates, or the potential difference between them. A large difference will cause the beam to be either far above or far below the center position, while a smaller voltage difference will cause the beam to appear nearer the center. The beam (or the dot on the screen) may be moved to the right or the left while it is going up or down, but its distance above or below center is always determined by the voltage on V.D.P. Its vertical height, in radar, is independent of the sideways position. For a given voltage on these plates, the dot will appear at a certain height above or below center. For every voltage on these plates, there is a certain definite vertical position of the dot, regardless of its horizontal position.
There is another pair of plates which are exactly like the vertical plates, except that they are turned half-way around so that they are at right angles to the vertical plates. They are able to move the beam to the right or the left, depending on the charges on these plates. Since they control the back-and-forth position of the dot, they are called the horizontal deflection plates.
It is obvious that by merely varying the vertical position and the horizontal position of the dot, you can make the dot take any position you want, anywhere on the screen. Consequently, by adjusting the voltages on the two sets of plates properly, you can make the dot appear at any place on the screen. You can make it move in any definite manner by changing the voltage on one or both sets of the plates properly.
Concept of sweep and time base. If the dot moves from left to right at a certain speed, you can use this spot movement as a yardstick with which to measure time. Suppose the distance the dot moves is three inches, and that it covers this distance in exactly 1,200
Figure 1-29. Movement of electron beam with change in voltage on vertical deflection plates.
Figure 1-30. Movement of electron beam with change in voltage on horizontal deflection plates.
microseconds. If its speed is constant, it must have moved one inch from the starting point in one-third of the time, or in 400 microseconds. If you drive a car 3 miles in 12 minutes, you could go one mile in one-third of 12 minutes or in 4 minutes. If you set your mileage at zero when you started, and kept the same speed continuously, you could measure the time by reading the distance you had traveled. You know that it would take one minute to travel one quarter-mile, and therefore for every quarter-mile you had traveled, you would have been traveling for one minute. If you had gone two and one-quarter miles, you would have traveled nine quarters of a mile, and since it took one minute to travel one-quarter mile, it would he nine minutes from the time you started. Since you move the same distance every minute, the time that has passed since you started is exactly in proportion to the distance you have gone.
The spot on the screen of the radar cathode-ray tube moves across the face of the tube in the same way. It starts at one place and moves toward some other place on the tube face. You can control the voltage variations, causing the dot to move in such a manner that it travels from its starting point to the finish point in a certain length of time. It moves across at an unchanging speed (approximately) and in a definite length of time. This is important the whole ranging procedure used in radar depends on it. Movement of this dot is termed the sweep.
It takes about 12 microseconds for radar energy to travel to and return from a target a nautical mile away. You can, therefore, determine the range (in nautical miles) corresponding to any point on the sweep by dividing the time represented by that point by 12 microseconds. Suppose the dot traveled a distance of three inches in 1,200 microseconds. Each inch represents 400 microseconds as before. Any distance along this sweep represents. a certain definite time, and therefore a certain definite range. Since each inch represents a time of 400 microseconds, each inch represents 400/12 = 331/3 nautical miles (approximately). If an echo returns when the dot
Figure 1-31. Electrostatic cathode-ray tube.
is one and one-half inches from the start, it would indicate a round-trip time of 600 microseconds, or a range of 50 miles. These actual numbers, of course, can be used only for a sweep that moves three inches in 1,200 microseconds. A sweep of any other speed will have different numbers to tell you what range an inch distance on the scope means.
Since the time and the range to a target always have a fixed relationship, let distance on the scope be marked in range instead of time. Do not forget, though, that you are actually measuring time. In all radar sets, the path of the sweep is marked in units of range such as miles or yards (and not time) for convenience in reading. This sweep-line is sometimes called the time basis.
So far, you have studied the electrostatic cathode-ray tube. You have found that you can deflect the beam by changing the charges (or voltages) on the deflection plates. You have learned that the sweep is produced by varying the voltage on these plates in a very definite way.
Electromagnetic cathode-ray tube. However, getting the sweep on a PPI scope is more difficult because the sweep must change direction but not speed. It is troublesome to get the voltages to cooperate and vary properly. Fortunately, another type of cathode-ray tube is available; using this tube you can get the sweep to move in the desired way with little difficulty. This type is known as the magnetic cathode-ray tube.
In order to understand how this tube works. you need to know something about the principles involved. You may have wondered how an electric motor could pull as strongly as it does. The explanation is as follows. The motor is an arrangement in which large wires carrying heavy currents pass through a strong magnetic field. When these currents flow through the magnetism, a strong force tends to push the current out of the magnetism. This force is always al right angles to both the magnetism and the direction of the current. Consequently, the force is in such a direction as to turn the motor. No wire is needed to carry the current because the streaming of these electrons from the cathode to the screen in the cathode-ray tube makes up a current. What happens, then, when you hold a magnet across the neck of the tube? You have placed some magnetism across the beam, and the beam is a current, so the beam tends to move out of the magnetism. It is neither repelled nor attracted by the magnets, but is bent sideways in tending to get out of the magnetism!
The stronger the magnetism, the harder it tends to get out, and consequently the farther from the center of the screen it will appear. If you vary the magnetism between a pair of poles mounted vertically across the neck of the tube, the spot on the tube face will move sideways, farther from the center if you increase the magnetism, and closer to the center if
Figure 1-32. Magnetic cathode-ray tube.
you decrease it. Then you can produce a sweep, by varying the magnetism.
If you have ever experimented with electromagnets you know that it is possible to vary the amount of magnetism simply by changing the current through the coils around them. That is just what is done in radar. To get the spot to move and form the sweep, you increase the current through the magnetic coils. The magnet (or held) current determines the position of the spot on the screen. The current in the control coils above and below the neck, controls the horizontal position of the spot, and that in the coils to the right and the left controls the vertical position.
By varying these currents (and thus the magnetism) you can change the spot position in exactly the same way as was done by changing the charges on the deflection plates. By varying the magnetism properly, you can make the spot draw any desired picture, duplicating any picture that could he drawn on the screen of an electrostatic tube. However, it is easier to produce certain voltage changes than it is to change the currents correspondingly. Likewise, current may sometimes be changed more easily than voltage. Making the spot on the screen move outward first in one direction and then in another (to form the PPI sweep) without changing the sweep speed is relatively easy in the magnetic tube. This action is difficult to achieve in an electrostatic tube. You know that the spot must be made to move in this manner, if you are to have a true picture of the PPI (Plan Position Indicator) scope.
The deflection coils are mounted around the neck of the tube within easy reach. Since the sweep is perpendicular to the direction of the magnetism, you can turn the coils around the neck of the tube to change the direction of the sweep. To produce a PPI scope, then, you merely need to rotate the coils as the antenna turns, change their magnetism properly, and intensify the beam when the echo comes back. It would be troublesome trying to use rotating deflection plates on the outside of the neck of the tube, otherwise we could use an electrostatic C.R.T. as well as we can use the magnetic C.R.T. Even if the coils are not rotated, it is easy to make the sweep behave properly for PPI purposes by employing simple electrical devices which keep the currents varying correctly.
Echo indication by deflection and by intensity method. You were told that the reason for using an electron beam is to have a "pencil" that can be moved very rapidly. Why is that necessary? Before you can measure the time required for the radio waves to strike an object and be reflected you must have some indication of the precise moment that the echo comes back. You can connect the receiver (which makes echo voltage larger), to either the upper or lower vertical deflecting plates of the C.R.T. When an echo is received, the receiver puts a voltage on one or the other of these deflecting plates, but only for a short time. What would happen if it took six microseconds for the pencil to be moved? The range would always measure about a thousand yards too large. Six microseconds is a very short time in which to move anything mechanical.
By connecting the receiver so that either a negative voltage is applied to the lower vertical plate or a positive voltage to the upper vertical plate of the C.R.T., it is possible to move the electron beam up and then down again in a very short time. It may jump up and back down again within a fraction of a microsecond producing the pip or blip. What happens vertically does not affect appreciably the horizontal movement, so time can be measured in a horizontal direction regardless of how much the beam jumps up and down tracing pips. It jumps up almost instantly when the echo comes bade, giving an accurate indication of the time that the echo comes back. Therefore, you can measure how long it has been gone with very little error. The electron beam, our "pencil," can be moved with amazing speed.
By making an echo move the electron beam, you get a pip as an indication of the echo's return. This is called the deflection method of showing the echo's presence, since the beam is deflected. There is another method by which the echo can be detected, and that is by the beam causing a bright spot to show along the sweep at the instant the echo returns. This method is called the intensity method, since the echo causes the screen to be momentarily illuminated by the greater intensity of the beam.
How would you make the sweep brighten up at a particular spot to tell you when the echo returned? Since the grid voltage controls the intensity (or brightness) of any C.R.T., disconnecting the receiver from the vertical plates and reconnecting it to the grid causes the positive output voltage of the receiver (when an echo returns) to intensify the beam and make a bright spot.
To make it easier for a radar operator to see this bright flash, an average voltage is maintained on the grid just sufficiently negative to keep the sweep from showing up except when the echo returns. Then it
is bright enough to be easily seen. Thus the target indication is a bright spot or bright smear.
Standard C.R.T. controls. The cathode-ray tube is one of the most important parts of a radar set. It may be thought of as the information center of the equipment.
All cathode-ray tubes have certain characteristics in common. For one thing, all use an electron beam which produces a spot on the screen. There must be some way to control the brilliance and position of this spot. There must also be some adjustment to bring the spot into sharp, clear focus. Controls to make these adjustments are found on the cathode-ray tubes. Let us see just what is the function of each of these controls.
The intensity or brilliance control enables you to adjust the grid voltage (with respect to the cathode) and thus control the number of electrons striking the screen. It should always be adjusted for minimum intensity allowable.
The horizontal centering control of an electrostatic (I.R.T. controls the direct current voltage on the horizontal deflection plates, permitting you to move the complete trace to the right or left.
The vertical centering control of an electrostatic C.R.T. permits adjustment of the voltage on the vertical deflection plates so that the complete picture may be moved up and down.
On a PP! scope, horizontal and vertical positioning is effected by actually moving the focus coil up and down or sideways. These are semi-permanent adjustments made by the technician.
The centering control of a PPI scope permits making the sweep start in the center of the screen. This is done by controlling the direct current flowing in the deflection coils of the magnetic C.R.T.
A focus control is used on every cathode-ray tube to permit bringing the dot to a sharp focus. This is done by varying the voltage between the first and second anodes of an electrostatic C.R.T., and by varying the current in the focussing coil of a magnetic C.R.T.
The astigmatism control is a secondary focus control. By varying the direct current voltage on both vertical (or both horizontal) deflection plates, any part of the sweep may be brought into sharper focus than is possible by using the regular focus control.
Development of trace by the electron beam. In order to see more clearly the action of the C.R.T., let us follow a pulse from the transmitter and see what happens on the scope screen (see fig. 1-33). At the same instant that a pulse is sent out the dot starts at the left-hand side of the tube screen and forms a peak. This peak is traced, because as the transmitter sends out a pulse, the radio receiver detects some of the energy and applies it as a voltage to the vertical deflection plates in such a way that the dot is pulled upward following the shape of the transmitted pulse. As the radio wave continues toward the target, the dot also moves at a constant rate of speed from left to right, leaving a trace behind it. In other words, we can look at the dot and see what the wave is doing.
Now, as the radio wave strikes the target and returns as an echo, the dot still keeps moving. Then at the same instant that the echo reaches our position, another peak is formed by the dot because the radio receiver detects and amplifies the echo energy and again applies a voltage to the vertical deflecting plates. This peak, caused by the echo, is called a pip.
After the pip is traced, the dot still continues its travel to the right on the scope until it completes the time base. Notice that before the next pulse is sent out there is a definite lapse of time after the echo is received. The most distant echo should have time to return before the next pulse goes out. This interval allows us to identify each echo and the exact pulse that caused it.
For the purpose of explanation, we have slowed down the action of a single pulse, its returning echo, and corresponding pulse trace and pip on the scope. The actual speed and flow of radio energy is that of
the speed of light, so that the time base with the pulse trace and echo pip on the scope will appear to he stationary or fixed. Keep in mind that there is a definite relation between the rapid dot travel on the scope and the radio wave travel between our position and the target.
The distance between the pulse trace and the pip on the time base is actually the "yardstick" for measuring the distance between own ship and the target. The C.R.T. translates the smallest fraction of time into exact physical distance of yards or miles. It can be seen that the scale most be calibrated so that its indications actually divide by two the total distance traveled by the radio wave.
Continuous and discontinuous sweeps. You know that the electron beam travels across the screen in a definite manner and in a definite time. You also realize that there is one sweep for every pulse sent out by the transmitter; consequently, the time between pulses puts a limit on the time length of the sweep Sweeps that use up all of this time differ slightly from those which do not, so there are different names for them.
A continuous sweep is a sweep in which the dot needs the complete time (practically) between pulses to get across the scope. It travels continuously, because it jumps back and starts over immediately after it completes one trip. A discontinuous sweep is one in which the dot completes its crosswise trip considerably ahead of the time it is to start across again, so it "rests" awhile. It shows no echoes during the rest period, as it does not sweep across continuously.
Suppose that you have a radar set pulsing 833 times every second. This means that the time between starts of sweeps is 1/833 of a second, or about 1,200 microseconds. If you are interested only in targets within about 10 miles, you need consider only echoes coming back within about 10 × 12, or 120 microseconds. Since the dot can be set to travel at any speed you choose, you can make it go across the screen in 120 microseconds. When it reaches the end, it has to wait 1,080 microseconds before it can start over. You would refer to this as a discontinuous sweep because of the rest period, during which the scope is blanked out (turned off).
If you decided to look for targets as far as 25 miles, you would have to slow the dot down until it took 25 × 12, or 300 microseconds to cross the screen. Its rest period now will be 900 microseconds (with 833 pulses per second). This would he called a nominal range of 25 miles, because that is the biggest range that can he read directly on the scope with this sweep speed.
You can increase nominal range to 50 miles by further slowing the dot until it takes 600 microseconds to complete the trace. Then an echo from a 50-mile target would get back just in time to show up on the scope. If an echo returned from a target beyond 50 miles, it would arrive during the 600-microsecond rest period, the time when the scope is blanked out.
What would happen if the dot were slowed until it took the full 1,200 microseconds to go across? For one thing, the nominal range would be 100 miles. Furthermore, there would be no (appreciable) rest period. Up to this point slowing the dot increased the nominal range. It is not possible to continue to increase the nominal range by slowing the dot still more since you are allowing only 1,200 microseconds between starts of sweeps. Hence, any further reduction in speed using this pulse repetition rate will only shorten the length of the trace. Further, slowing the dot cannot increase the pulse interval -the maximum time of travel. It will only shorten the distance the dot travels, which usually is not desirable.
You have found that so long as you have a discontinuous sweep, the nominal range can be changed by changing the speed of the sweep. The only way you can change the nominal range of a continuous sweep is by changing the pulse repetition rate. The nominal range is determined by the time the dot takes to cross the screen, and that, for a continuous sweep,
Figure 1-34. Continuous sweep.-Discontinuous sweep.
is the same as the time between starts of pulses. Increasing the number of pulses decreases the time between them, and so reduces the nominal range.
The following is the equation for calculating the nominal range of a continuous sweep:
N.R. = 1,000,000/12 × P.R.R. (approx.).
N.R. is the nominal range in nautical miles.
P.R.R. is the number of pulses the transmitter sends out every second.
There are advantages associated with both the continuous and the discontinuous sweep. As long as a discontinuous sweep is used you can switch scales simply by turning a knob. Since the only thing that needs to be done to change the nominal range of a discontinuous sweep is to change the sweep speed, you can put in a rather simple control to make a quick, easy change possible. Changing the sweep speed is easy. The pulsing rate, on the other hand, cannot, in some cases, be changed much, so sets with continuous sweeps usually have only one scale.
The sweep speed of a continuous sweep can be changed on some radars so that the sweep starts slowly, then speeds up, and finally slows down again without altering the time of the start and ending of each sweep. The total range indicated by the sweep is unchanged since the total time of the sweep remains the same. However, both ends of the sweep register a greater proportion of the total range, for they represent a greater part of the total time due to the reduced speed of the spot. The center portion that has been speeded up now represents less of the total range since its time is reduced, but it represents an increased part of the physical length of the sweep. This results in expanding the picture of any objects appearing in this center section of the sweep because it now covers a greater part of the sweep length. This has certain technical advantages which makes accurate ranging relatively easy.
Calibration of continuous and discontinuous sweeps is discussed in the following section on "Calibration." There you will find that the calibration of a continuous sweep is different from calibration of a discontinuous sweep.
Radar sets, like every other precision instrument, must be calibrated before they can give correct information. Calibration is the process of making the radar read the correct range, bearing, and position angle. It is a common error to think that the term calibration includes tuning the receiver, adjusting the dial lights, throwing switches, and everything else necessary to get echoes. Calibration simply makes the set indicate proper range, bearing, and position angle.
Range calibration is necessary in the ease of every radar set. Every operator must check his range calibration every time he takes over a watch. There are two things that must be done to make the scope read correct range, internal calibration and external calibration.
Internal calibration. The purpose of internal calibration is to make the divisions on the scales the correct length. It is the same problem as making the inch-marks on a ruler exactly one inch apart. You could not measure distances accurately with a ruler if the regular inch-marks were really three-quarters of an inch apart, for you would be reading each distance too long. An actual three inches would only occupy four of these three-quarter inch spaces, with the result that you would think the object had a length of four inches.
Radar internal calibration deals with the same sort of proposition. If the spot moves across the screen in, say, 900 microseconds when the scale is marked off for a 50-mile nominal range, you would read a range of 50 miles for a target that actually was at 75 miles.
Internal calibration is the process of making the time of the sweep match the scale: making the dot move from the start to the end in the correct amount of time. For a continuous sweep, this time is the time interval between pulses, so you must get the correct pulse repetition rate for a continuous sweep. If you have a discontinuous sweep, this time depends only on the speed of the sweep. Consequently, internal calibration requires determining either the correct pulse repetition rate or correct sweep speed.
External calibration. External calibration is the process of making the zero setting correct, to avoid any constant range error. Unless you make this adjustment, you may read a range too large or too small by a definite amount.
Imagine a yardstick with the first five inches cut off (so that the five-inch mark is right at the start). Unless you made proper allowances you would read a length five inches too great for anything measured with this yard stick. The 25-inch mark would be at the edge of a 20-inch box, and the length of the box would appear to be 25 inches. If you moved the yardstick over five inches, placing the five-inch mark five inches from the left edge, you could read the correct length of the box. Its edge would line up with the 20-inch mark.
You can do exactly the same thing with your radar set. You cannot actually read a zero range (because of the transmitter pulses), but you can detect nearby
targets on your radar and make their pips appear at the correct places on the scope. Double range (or multiple range) echoes are useful in doing this (see the section on Multiple Range Echoes in Part 3).
All radars should have the zero setting checked s often as possible, either by comparison with a set known to be accurate or by the double range echo method, which is the most accurate and convenient method to use, especially when at sea.
Figure 1-35. Measuring a box with two different scales.
Double range echoes occur at close ranges and result from the reflected energy striking your ship, returning to the target, and being reflected a second time. Therefore, you should see a blip created by the reflected energy returning on the first trip; then a second blip, which will be smaller, should appear at exactly twice the actual range if the zero setting is correct. With this information, the exact range to the target can be determined, for it is the difference between the second trip echo and the first trip echo (6,500-3,500). Next, you will learn how to find the zero setting so that the actual range will be correct.If you subtract the range you actually read for the double range echo from double the range read for the target, you will get the correction you should make in the zero setting, and reduce the zero reading by this amount. In the example, double the range read for the target was 7,000 yards. The range reading for the double range echo was 6,500 yards. Subtracting 6,500 yards from 7,000 yards leaves 500 yards, the number of yards too many your set is indicating.
If you move the zero setting back 500 yards, the normal echo will appear at 3,000 yards and the double range echo at 6,000 yards. Twice 3,000 is 6,000, so you know your zero set is correct.
This method is useful for calibrating fire-control radars. It can be used to a lesser degree with search type radars, if there is another ship parallel to yours at a short range.
Range calibration consists of two steps. If you have a discontinuous sweep, you must get the sweep speed (internal calibration) and zero setting (external calibration) correct. If you have a continuous sweep, you must get the pulse repetition rate (internal calibration) and zero setting (external calibration) correct.
Bearing and position angle calibration should be checked against the optical methods. Some of the search equipment in use does not require frequent calibration for bearing or position angle. This is not true of the fire-control radar, where extremely accurate hearing or position-angle readings are needed, and such a radar should be checked as often as possible.
Types of scopes.
Through the use of radar it is possible to get information when all other methods fail, but this information is absolutely useless unless it can be put in understandable form. For certain uses, this information is more understandable when presented in a particular way, while for another job, some other manner of presenting the data may be more desirable. There are several ways in which the same information
Figure 1-36. Zero set is off 500 yards.
can be shown, and the particular method used will depend on the specific job.
The information you are interested in primarily is detecting the presence and finding the location (range and bearing) of objects. Just how does the radar set show this? If there were a few million vacuum tubes, coils, condensers, and resistors in the set, you could make the beam perform somewhat as shown in figure 1-37.
Unfortunately there is no room for the many complicated circuits required for this outfit, so instead, simpler, easier methods may be used-many of them. Six of these methods and their uses will he discussed here: the "A" scope, "B" scope, "PPI" scope, "H" scope, "R" scope, and "J" scope.
The "A" scope. The fundamental type is known as the "A" scope. It is the type most frequently referred to during previous explanations. You remember that the spot moves from the left to the right side of the screen at some approximately constant speed, enabling you to measure time from the start to any point along the time base. The spot jumps up whenever a reflected echo returns simply because the receiver output is connected to one of the vertical deflection plates. The echo comes out of the receiver and regardless of where the dot happens to be on the time base, it gives the dot a kick upward, forming a pip or blip which indicates a target.
We may determine the range, then, by measuring the time from the start of the sweep to the spot at which the pip appears, translating the time into range. Since the speed of radio waves does not change, the indicator is usually marked off directly in yards or miles. Hence, the "A" scope indicates range horizontally and presence vertically. It tells nothing about the bearing of the target.
The "A" scope has advantages which often outweigh its failure to tell the bearing of a target. One important characteristic is its ability to tell you what you are "looking" at--just what the blip indicates (see section on Composition in Part 3).
The "A" scope is useful when trying to detect the presence of objects at long ranges. Fairly weak echoes may sometimes be detected on the "A" scope before they can be seen on another type of scope (especially when the pulsing frequency is high).
Ranges can readily be determined with accuracy on an "A" scope. Some scopes provide a range step in the sweep, and control its position electrically to facilitate the task of range measurement, The step appears because a voltage is suddenly applied to a vertical deflection plate, which causes the remainder of the sweep to he shoved down.
Dials or scales which will read the range of our step directly and accurately are provided. When you move the step under the blip of the target, the range of the target appears on these dials. This greatly simplifies the task of accurately determining range (this step is usually not so sharp as that shown in the
Figure 1-38. Two groups.
sketch, and it may be altogether different in shape, but the basic idea is the same).
Before leaving the "A" scope, let us review briefly the facts known about it. First of all, when an object is reflecting the transmitted energy the spot jumps up, tracing a triangular-shaped pip (this might well be the reason it is called an "A" scope since the blip resembles the letter "A" without the cross bar). The presence of the target, then, is shown vertically. The farther an object is from you, the more time the energy will require to go out and return, so the spot tracing the time base will get farther across the screen before the echo returns. Range can be found by seeing how far the blip appears from the start of the sweep. Range may he determined directly with ease and accuracy; presence may he detected on the "A" scope although the echo is weak. Variations in appearance of the blip can tell you much about what is on the reflecting end of the radar beam.
The "A" scope itself tells nothing at all about the bearing of the target. However, you can tell when the antenna is pointed directly at the object, thus the antenna bearing will be the same as that of the target. Thus by reading the bearing of the antenna you find the bearing of the target relatively easily, but there must be a bearing indicator to show the antenna bearing. This is usually made up of two circles marked off in°; one reads the relative hearing, the other the true hearing. A pointer (called the bug) which rotates in synchronism with the antenna points to the proper reading on the dials. This type bearing indicator is found on many radar sets in use at the present time.
The "J" scope. A new model radar recently introduced to the Fleet has a circular sweep. The dot, instead of traveling from one side of the screen to the other as on the "A" scope, goes around and around (somewhat like the sweep on some sound ranging gear). When an echo returns, the dot suddenly jumps outward to form a blip. This blip is straight out ("radially," we say), and time is measured by its distance from the start of the sweep around the screen. On this scope, range is shown circumferentially (around the circle) and presence radially.
For any particular size of cathode-ray tube, the time base is about three times as long as it would be on a corresponding "A" scope. So, by wrapping the "A" scope around in this way, it has been possible to increase the range accuracy about three times. The "J" scope presentation is the name given to this novel indication.
The "R" scope. A modification of the "A" scope, of great value in determining the composition of targets (see section on Composition), is now found on some sets. It is called the "R" scope, and is a magnified (expanded) portion of the "A" scope with a control which enables you to choose the part to be magnified.
Figure 1-39. Magnifying a portion of the "A" scope with an expanded sweep.
To magnify the pip, you cause the spot to move across the scope in a short time. It may cover the five inches (approximately) of the time base in just 25 or 30 microseconds. This, of course, will make the pip appear very wide. For instance, a pip five microseconds wide might cover a whole inch on the time base. Two targets separated by only a half mile will appear almost an inch apart. Consequently, the operator is not likely to read the indication as being a pip from a single target.
By increasing the width of the pip in this way, it is possible for the operator to recognize definite features thereof. You can count the separate peaks running up or down the sides of the pip, and separate targets close together which might otherwise be mistaken for a single target. The job of estimating size, number, etc., is greatly simplified by use of the "R" scope.
The "A" scope, the "J" scope, and the "R" scope all operate by deflecting the electron beam when an echo returns. Hence we say that they indicate presence by deflection. However, it would be possible to have a bright spot appear when the echo returns instead of deflecting the spot. This brightening of the spot is called the intensity method of showing presence, and it is used in both the PPI scope and "B" scope presentations.
The PPI scope. The PPI scope gives us a top view of the vicinity, with our own position in the center. Draftsmen call a top view a plan view; since the scope indicates a top (or plan) view of the position of everything around you it is called a plan position indicator, or as abbreviated, a PPI scope.
The pictures of the surroundings appear by this simple process: the sweep begins in the center and goes outward (at its constant speed), toward the edge in the direction the antenna is pointing, and a bright spot appears at a distance proportional to the range of the target. Those who have done any plotting will recognize this method: placing a mark in the correct direction and at the correct range on a polar chart. That is all that is needed to draw a true map of everything in the vicinity. This scope shows everything that can reflect the radar energy, and shows it in the proper place on the PPI map. You will see islands, your own ships and planes, as well as the enemy's craft, and anything else that happens to reflect the energy; all objects appearing in their actual positions. You can readily see how helpful this PPI scope is in task-force operations, in convoy duty, in working navigation problems, or in any one of various technical uses.
Remember that the sweep moves outward in a direction representing the antenna beating. Since the antenna can pick up an echo from a target only while it is pointing at that target, and since the rotating antenna points in any target's direction for but a short time, you can expect the bright spot to appear for only a moment. As the radar beam swings away from a target, you fail to receive any echo from it, and consequently, the beam does not make a bright spot any longer. (You recall that the beam can intensify only contacts on one bearing at a time.) This means that in order to continue to see that particular target, the screen must continue to glow after the sweep leaves it. In making the cathode-ray tube, the screen is painted with a chemical coating that glows longer than the "A" scope screen coating, and a tube with a longer persistence screen results. A screen is termed persistent because it persists in glowing after the sweep has left it and has moved on to another spot. Tubes which employ the intensity method are used in PPI and "B" scopes.
The "B" scope. The "B" scope resembles both the "A" scope and the PPI scope, but has some characteristics of its own. If you turn an "A" scope on its side, the range will be indicated upward, or vertically, and the blip horizontally. With the receiver disconnected from the horizontal deflection plates, it will not be possible to get a blip sideways. By connecting the receiver to the grid instead, a bright spot will result which will indicate a target just as a blip does. Seeing a bright spot is the indication of the presence of an object, and the spot's position tells you the range of the object causing it.
The horizontal deflection plates have been disconnected, and are not in use. What use could be made of them? Range is determined vertically and presence by intensity, so the one item of major information lacking is the bearing. The horizontal plates, then, can be used to determine the bearing of the object in the following manner.
If the whole time base could be moved, bright spot and all, to one side for a distance proportional to the movement of the antenna, it would provide a means of indicating the bearing of anything giving an echo, because the scope would show how far the sweep was from its usual position. For example, suppose the time base moved one inch to the right when the antenna was trained five degrees to starboard. If a bright spot showed on the time base one and one-half inches to the right of the center, the antenna must have been pointing seven° and thirty minutes to starboard when the echo was received. Consequently, the target was at an angle of seven degrees
and thirty minutes to starboard. In some sets (such as the Mk. 8), the scope tells the angle of a target to the right or the left of the line along which the director points.
Usually, the "B" scope is used with sets which do not show targets for the full 360° about the ship. Most of them show targets in only a limited area, such as a sector from 285° forward, to 075° relative, Other sets search a sector only 30° wide, 15° to each side. How you set the "blinders" depends on what you are looking for.
What, then, does the "B" scope show? Since it is similar to an "A" scope that does not stay right side up, it shows range vertically. The electron beam brightens up the screen when an echo returns, and consequently shows the presence by intensity. Finally, the horizontal plates are connected so that it shows the bearing horizontally. It gives the same information as the PPI scope, but in a different manner.
Since the sweep always starts at the bottom and goes upward-always in the same direction-you can measure range accurately. A movable pointer is sometimes used, or moving the sweep past a stationary pointer can serve to measure the range with reasonable accuracy. There is no problem here of measuring different directions, as in the case of the PPI scope. When no critical range accuracy is required, horizontal lines are placed on the screen to be used in estimating ranges, and vertical lines are used in estimating the bearing of a target.
The "H" scope. An "H" scope is a modified "B" scope. Azimuth is given horizontally, and range vertically. The signal appears as two bright spots, displaced laterally with reference to each other. The slope of the line that can be imagined as joining the dots gives an indication of target elevation. The "H" scope is often designated by the term double dot scope.
Summary. The following summary gives in brief form the function of each of the scopes, and provides a means of comparing features of the various types.
"A" SCOPE Range: Horizontally Presence: Vertically Bearing: None indicated Advantages: Ease in detection, ranging, and determination of target composition. "R" SCOPE Range: Horizontally Presence: Vertically Bearing: No bearing indication Advantage: Great ease in determining composition. "H" SCOPE Range: Vertically Presence: Double dot Bearing: Indication horizontally Advantages: Provides bearing and range plus data for altitude determination, "J" SCOPE Range: Around circumference Presence: Radially Bearing: None Advantage: Increased range accuracy. PPI SCOPE Range: Distance out from center of scope (radially) Presence: Intensity Bearing: Direction of sweep Advantages: Complete picture in a few seconds. Shows all objects in true relative positions. "B" SCOPE Range: Vertically Presence: Intensity Bearing: Horizontally Advantages: Good bearings at any range; good bearing resolution or target separation at short ranges as contrasted with PPI; shows all targets at the same time. Similarity to cross-hairs makes it especially good for gunnery.
The modulation generator.
So far, you have discovered that the radar set has these parts: the transmitter, the antenna, the receiver, and the indicator. There is more to the set than this, however. You know that an operator is needed to key the communication code set, for someone must turn it on and off to form the dots and dashes. Similarly, radar energy must he sent out in pulses (or extremely short dots) if you are to get the accurate range of an object. These pulses must all he of the same length, and must he spaced evenly over a period of time. This means sending a pulse only four or five microseconds long, and 1,640 of these must he transmitted in one second. Some radar sets require a device to do just that, so an electrical means of
Figure 1-40. Scope presentations.
keying the transmitter, called, naturally enough, the keyer, is used. The keyer is also known as the modulation generator.
The modulation generator keys the transmitter, forming pulses of definite duration and at regular intervals. The definite duration is called the pulse width or pulse duration, and the number of pulses every second is called the pulse repetition frequency or the pulse repetition rate.
The modulation generator also controls the start of the sweep. Why is this necessary?
You remember that range is determined by measuring the time taken by the radio waves to leave you, travel out, reflect from an object, and return. In other words, you measure the total time between those occurrences.
You recall that the keyer turns the transmitter on and off to form the pulses. If it can also he used to start the sweep every time at the same instant the transmitter pulses or at the same time after the transmitted pulse, each blip will appear at an identical spot on the time base every time. Fortunately, it is not difficult to utilize the keyer to do this. It does this second job by sending a synchronizing pulse to activate the indicator. Synchronizing means that the pulse makes the sweep start always at the same time. This is also called the synch pulse. Thus, in addition to its other functions, the keyer furnishes the synchronizing pulse, which starts the sweep at the same definite time with respect to the transmitted pulse.
The sweep may not start at the same time as the transmitted pulse. By knowing how long the start of the sweep is delayed you can figure in that time in calculating the range. In some sets, range is determined by changing this time delay until the blip is moved to the center of the sweep. You can then measure the amount the sweep was delayed and know the range.
You will find the same basic units in practically every radar set. Often several, and maybe all of them, will he in the same box or cabinet, but they are always represented in some form.
Most radar sets have large, bulky antennas. This is necessary in order to obtain good bearing accuracy. The transmitter, you remember, must he "coupled" to the atmosphere if it is to transmit its energy. The receiver, too, must he "coupled" to the atmosphere if it is to receive any of the reflected energy.
You recall that you transmit in pulses, and receive the echo while the transmitter is not sending. Consequently, while the receiver is receiving the transmitter is not transmitting, and vice versa. The transmitter uses the antenna only while it is transmitting (during this time no echo can be received), so it is possible to let the receiver use the same antenna while the transmitter is off. In that way, the weight of antennas needed, and the difficulties in tuning them are reduced. In addition, we have assurance that while receiving the antenna is pointing in the same direction that it was pointed when transmitting. Certain advantages, therefore, are gained by using the same antenna for both purposes.
However, this arrangement presents one difficulty. If the receiver and transmitter are both connected to the same antenna at the same time, the receiver will be unable to carry the load; hence means have been provided to disconnect the receiver from the antenna while the transmitter is sending out the pulse, and reconnect it when the transmitter shuts off. This switch is called the reprod (receiver protective device), or the TR box (for transmit-receive), or the duplexer.
If you are interested in nearby targets, this switch must operate very rapidly. A delay of only one forty-thousandth of a second in re-connecting the receiver would cause you to miss any targets within about two nautical miles. There are no mechanical switches which will work fast enough for this, so electrical switches are used, switches which have no moving parts except the tiny electrons in a tube. To work properly the duplexers must he tuned carefully. The technician should be called upon to do this.
Let us review the action again, and follow the course of a single pulse. As the wave travels out from your ship (see fig. 41), the action will be stopped from time to time so that you can see how far the dot has traveled on the scope.
As the pulse leaves the transmitter, (1), the dot starts at the left side of the tube and traces out the shape of the pulse. When the pulse has traveled half the distance to the target, (2), the dot has completed one-fourth of its travel. As the pulse strikes the target, (3), the dot has traveled but half the distance to the place on the scope where the pip will appear.
As the echo, (4), moves toward your position you can see that the dot must travel the remaining distance an the time base before the pip is formed. When the echo, (5), reaches the antenna the radio receiver detects and amplifies the energy, which, when applied to the vertical deflection plates, causes the pip
to be traced. Therefore, the time base actually measures the total time for the pulse wave to go out and return as an echo to your position. Because the speed of wave travel in each direction is the same, the scale on the scope can be calibrated to give the true range directly in yards or miles.
Broadly speaking, there are five parts involved in radar apparatus: the transmitter; the antenna, duplexer, and transmission lines; the keyer; the receiver; and the indicator or indicating devices. Of course, you must also have the required power supplies and controls in addition to the five basic units.
The transmitter is used to generate very short pulses of electrical energy which are radiated out into space. From the transmitter the energy flows through the transmission lines to the antenna which may he highly directional and concentrates the radio energy into a narrow- beam. As the wave travels out into space, reaches the target and returns as an echo, the scope traces a line along the screen forming the time-base. The echo strikes the antenna and is detected and amplified by the radio receiver and applied to the indicator scope in such a way that it causes the pip to appear on the time base.
Since the same antenna is used for both sending and receiving, you must have a receiver protective device to guard the receiver from the powerful outgoing pulses. By using the so called duplexing equipment
Figure 1-42. Block diagram of a typical radar system.
(which might he compared to a simple valve), you can keep the heavy flow of outgoing power away from the receiver. When the echo is received, the duplexing equipment works in reverse and forces the greater portion of the received signal into the receiver-indicator channel. The block diagram should assist you in fixing these component units of the equipment in your mind.
GENERAL RADAR CHARACTERISTICS
There are some common characteristics of air-search, surface-search, and fire-control radars than can be summed up as follows:
Long-wave radar "P" frequency band Antenna Bedspring (curtain or flat) Maximum range Average 100 miles Minimum range Relatively long Bearing resolution Poor, due to wide beam Range resolution Poor, due to wide pulse Pulse duration or width Long Fade zones Observable Accuracy Expected range measurement error on 75-mile scale of PPI and "A" scope using scotch tape scale is ± 1 mile.
Note: SM radar is an exception to the above.
Micro-wave radar "S" or "X" frequency band Antenna Dishpan or barrel stave Maximum range Approximately the line of sight. Minimum range Relatively short Bearing resolution Relatively good Range resolution Relatively good Pulse duration or width Short Fade zones None Accuracy In general better than that of air-search radar. For specific sets see Part 4.
Fire-control radars vary so much that it is difficult to generalize about them. For characteristics of individual sets the reader is referred to Part 4.However, bearing and range accuracies are comparable for all fire-control radars. The average expected error in range measurement is from ± 15 yards plus 0.1% range to ± 40 yards. The average expected error in bearing measurement is from ± 2 to ± 4 mils (± 1/10 degree to ± 1/5 degree).
FACTORS AFFECTING RADAR RANGE
Maximum range factors. In order to give you some reason for the variation in range performance of radar sets, we shall list the factors affecting the maximum range of any radar:
- Wave length.
- Long wave length radar is best suited for air search.
- Micro wave length radar is best suited for surface search.
- Size of target.
- Height of target.
- Height of mast for surface target.
- Height of plane for air target.
- Target presentation (target angle).
- Material of target.
- Height of antenna.
- Output power radar.
- Sensitivity of receiver.
- Atmospheric condition.
- Type of indicator ("A" scope most sensitive).
- Pulse repetition rate (determines maximum range scale that can be used).
- Beam concentration.
- Condition of radar equipment.
- Operator's technique and skill.
Minimum range factors.
There are also factors affecting the minimum range. They are:
- Pulse width.
- Receiver recovery time.
- Height of antenna.
- Receiver gain setting.
HOW DOES RADAR DETERMINE ALTITUDE?
When enemy planes appear it is necessary to know their elevation before putting the antiaircraft guns into action against them. How can this be done?
Position angle and range method.
At short ranges (up to several miles), the most accurate way to find the altitude is by calculation
using the angle of elevation (or position angle) and the range to the target. The antiaircraft fire-control set is equipped to do this.
How does this information-the range and position angle-give you the elevation? For every size position angle, there is a definite ratio of the vertical side opposite this angle to the slant range. This ratio is called the sine of that angle. Tables which list values for all angles are available. As soon as you find the position angle and the range, look up the sine of the position angle and multiply this ratio (the sine) by the range (the slant distance to the plane). In some of the later radars you can even make the set do this calculating for you. The elevation equals range times sine of the position angle.
The foregoing method works well for short ranges, but is not satisfactory for longer ranges. At a few miles range, you can get the elevation accurate within a very few feet, perhaps within 25 or 30 feet, or even less. However, several factors cause the accuracy to drop off at greater ranges. One of these factors is that you cannot measure small position angles (angles of elevation) accurately. When you use vertical lobe switching, for increased position-angle accuracy), the lower lobe is distorted by the ocean when you try to measure small angles. Naturally, this reduces the accuracy. With some fire-control sets, you cannot measure position angles of less than ten°. Of course, the higher the frequency of the set the smaller the angles you can measure, but you must train the lobe a few° above the surface. A very distant plane will be only a few° above the horizon; hence, the difficulty of measuring a small angle arises.
Another factor reducing accuracy is the curvature of the earth. Close targets can be considered to be above the same flat surface you are on, even though this is not actually the case. The resulting errors are small for targets within range of antiaircraft guns.
The fighter director officer is also interested in elevation. He must get the elevation of planes at long ranges. Once an enemy plane is within antiaircraft range the main effectiveness of the fighter director is gone. His business is to direct his fighter planes on a course which will bring them within visual range of the enemy planes and effect an interception before the enemy gets within antiaircraft range.
It was found that phenomena known as fades could be used in doing the job. The use of charts indicating areas of these fades (called fade charts) is the long-distance method of finding elevation. It is not very accurate; the closest you can expect to come to the proper elevation is about 500 feet, but you stand a chance of making greater errors unless you are exceedingly careful. Errors of 15,000-20,000 feet may he made, and errors of 2,000-3,000 feet are common enough. Still, it is the best method available for getting elevations at great distances.
Air-search radar for altitude determination.
Air-search radar operators in times past often believed their sets were not operating properly, for the echoes from a plane coming in seemed to fade out more or less regularly, until the pip was no longer visible. Of course the pip became visible again before very long, but the fading out was disturbing.
This situation continued to puzzle the radar experts for some time. The strangest part of it was that not all planes faded at the same range. Then someone made the discovery that all planes flying at the same elevation (or altitude) faded regularly at the same ranges. Planes flying at a different altitude faded at different ranges, but any plane flying at a specific altitude was consistent: at any specific altitude all faded at the same range. A plane at 5,000-feet elevation, for instance, might fade at 72 miles, again at 50 miles, at 35 miles, and at 21 miles. All planes at this elevation faded at these same ranges. Planes at other elevations faded at other ranges. This was an important discovery. The fighter directors had long been seeking a reliable way to find the elevation of planes coming in. Here at last was a means by which that information could be obtained.
The fade points were different for different radar sets, depending upon the frequency of the set and
height of the antenna, but once these fade points were found for a given set, they were constant. By sending our planes out at definite altitudes, the location of the fades zones for each elevation could be found and plotted on a chart.
Addition and cancellation of radio waves.
Before learning any more about fades, you should know why an echo from any type target should fluctuate in size, going from a maximum echo to a minimum or no echo, thence to maximum, and repeating this cycle as the target closes or opens. (A target is closing when coming toward you and is opening when going away.)
Since the radar waves leave the antenna at such a number of different angles from the horizontal, some of the energy (or radar waves) will hit the earth or water, and bounce off (like rubber balls) at the same angle as they hit (fig. 1-44). Certain of these waves will strike the water fairly close to the ship and may reflect right back into the antenna as sea-return, while others, due to their decreased angle from the horizontal, will strike the water farther out, be reflected, and continue on. Others with a smaller angle, will strike the water still farther out, until finally they will begin to miss the water on the horizon altogether, and so must be traveling nearly parallel to the waters surface.
When this occurs you must consider what is known as the phase relationship between the two waves shown going out from the antenna in figure 1-44. One cycle contains 360°. When a wave travels a distance of one wave length, it goes through one cycle, or 360°. If the two paths of the waves are of different lengths, the two waves will go through different numbers of° in traveling from the antenna to the target.
If the two waves arrive as illustrated in figure 1-45, both starting a cycle at the same time, they are said to be in phase.
Figure 1-45. Two waves in phase.
When the two waves are in phase, they are always acting in the same direction. They may be compared to two forces pushing on an object in the same direction; together they produce an effect as great as that of one force equal to their sum. The two waves in phase produce the same effect as one wave whose strength is the sum of the two, as shown by the dotted line in figure 1-45.
Figure 1-44. Determining elevation.
If the two waves do not arrive in phase, they are said to be out of phase. When this occurs, you must specify the amount they are out of phase. Thus, if one wave starts its cycle 60° after the other wave, the two are 60° out of phase. One wave may start a cycle anywhere from 0° to 360° after the other. Of course, if the two waves are 0° or 360° out of phase they are in reality in phase. Figure 1-46 shows two waves 60° out of phase. It is seen that during some parts of the cycle the two waves are opposing, and during the rest of the cycle are supplementing each other. The resultant will therefore he smaller than it would be if the two waves were in phase.
Figure 1-46. Two waves 60° out of phase
If the waves arrive as shown in figure 1-47 they are 180° out of phase. In this case the two waves are always opposing, and if equal in strength they will cancel out and the result will be zero. Figure 1-48 shows two waves 270° out of phase.
Figure 1-47. Two waves 180° out of phase.
Now, let us redraw figure 1-44, representing the radio waves traveling along both the direct and reflected paths by wave forms similar to those used in the discussion on phase, for radar or radio waves will combine and behave in the same manner.
Figure 1-48. Two waves 270° out of phase.
As these waves continue to leave the antenna, traveling paths of unequal length, there are places in space where the direct and reflected waves will be in phase. Thus, their forces add, as in the case of two forces pushing on an object in the same direction. In figure 1-49, plane B is in space where the direct and reflected waves reinforce each other. Therefore, the two waves striking it will add and return to its a maximum reliable echo. In radar these areas are called maxima areas.
At other places in space (see fig. 1-50, plane A) these radio waves traveling paths of unequal length will arrive at the target when they are 180° out of phase. Thus, their forces will cancel, as in the case of two forces pushing on an object in opposite directions. Therefore, if a plane is flying through such an area, the two waves striking it will cancel each other and a weak echo, or probably none at all, will be returned. In radar, these areas are called fade areas.
The result of ground reflection is to break the single free-space lobe into a number of smaller lobes with gaps between them. Figure 1-51 illustrates this effect.
The above theory is applied to the construction of a fade chart showing the places in which you get cancellation (fade area), or reenforcement (maximum area) for a specific antenna. By use of this chart, it is possible to determine the altitude of planes quickly and without the use of mathematics. Fade charts vary in appearance, because each one has to be made to fit its particular antenna installation.
Figure 1-52 shows a typical fade chart. The line on the left side of the chart is marked off in feet of
altitude. The bottom of the chart is marked off in nautical miles of distance, or range. The cross hatched lines on the chart represent the fade areas, or areas where the waves cancel, while the clear spaces represent the maxima areas, or areas where the waves add. The precise boundaries between the fade areas and lobe areas must be determined during calibration exercises.
The chart as reproduced here shows the theoretical maxima and minima areas, and is for practice in determining the altitude of a plane. It is not intended that this chart be used for actual altitude determination. Whenever theoretical charts of this nature are employed they should be checked when tracking planes of known altitude. Thus, the accuracy of the fade chart can be tested and any necessary changes made.
Let us take a hypothetical case in which the planes are assumed to be in horizontal flight and see just how their altitude is determined.
At 1000, radar reports large bogey (unidentified planes), zero-nine-zero true, range 90 miles. After several reports, the operator reports that the echo is fading, and finally at 1011 it disappears at zero-eight-six, range 56 miles. This indicates that the planes have entered a fade area, but you do not know which one, since it could be any one of three fade areas shown on the chart at that range. Therefore, it is necessary to mark each line on the lower side of each fade area at that range, since the planes are closing (points A, B, and C). (If planes are opening, the mark will be placed on top of fade area.) At 1014, radar reports
echo reappears, zero-eight-one, range 48 miles. This indicates that the planes have emerged from the fade area, so we place a mark on the top of each fade area at a range of 48 miles (points D, E and F). This is enough information for an estimate of the planes altitude. To find the estimated altitude, select the pair of points on the fade area which line up horizontally. The estimated altitude of the planes, indicated on the altitude scale, is 10,000 feet.
Later, radar reports that the planes have again entered a fade area at a range of 31 miles. Operator picked up target echo again at a range of 28 miles. This indicates that the planes have gone through a second fade area. By plotting the points as before on the fade area, you again find that the altitude is 10,000 feet. This verifies the original altitude estimated.
The use of a calibrated fade chart has just been demonstrated. These charts are calibrated to show not only the fade and lobe centers, but the exact size and shape of the area in spaces where planes will fade. The fade and lobe centers can be calculated mathematically, given the antenna height and wave length (see the engineering manual for air-search radar). However, the fade areas and lobe areas, i.e., the exact areas of radar visibility and radar invisibility, must be determined by observation using planes flying at various known altitudes. This is why fade chart calibration exercises are held from time to time. It is essential that your set be in excellent condition during the calibration exercises, because any change in the power radiated from the antenna will result in a change in fade zone size and re-calibration will he necessary. Conversely, the fade chart, when calibrated at a time when the radar is in good operating condition, serves as a first-rate device for checking performance. If the fade areas increase in size, the materiel condition of the radar has slipped below par.
One other thing which affects the size of the fade areas is the size of the target. Fade areas drawn for one small plane will be larger than those drawn for a large plane or many planes. In other words, the bigger the target the smaller the fade areas. Some fade charts show fade zones not for just one size target but for several.
Completely un-calibrated fade charts (showing only the positions of fade centers and lobe centers-points of minimum echo and maximum echo respectively) can be used fairly well even though the limits of the fade areas are not indicated, provided they have been drawn for the correct antenna height and wave length. If you have been unable to calibrate your fade chart, or if the materiel condition has changed considerably since the last calibration, you can still get a fairly good solution by estimating the range at
which a plane passes through a lobe center (and gives its strongest echo) and the range at which it is in a fade center. This use of the chart does not provide as rapid a solution of altitude in some cases as would be obtained with a calibrated chart, since it is necessary to wait until the plane has flown through a maximum and a minimum point. However, the condition of the radar and the size of the target need not be taken into account when using the chart in this way.
You have seen that in using the fade chart, the solution is reached in two steps. As the plane crosses a boundary between zones of visibility and invisibility (in other words, when it enters a lobe or fade) you get a number of possible solutions (one for each lobe ) because you do not know which lobe the target is entering or leaving. When it crosses a second boundary, however, all solutions but one are ruled out on the assumption that the plane is in level flight. Under certain conditions it is possible to tell which lobe a plane is in at the instant contact is made. You can do this when the plane is picked up at very long range. Anything beyond 120 miles would be in the lower (first) lobe or else its altitude would be over 40,000 feet, which is unlikely. Look at your fade chart and see what the maximum expected range is for contacts in the second lobe.
Instantaneous estimates are also possible when the plane is detected at fairly short range: For example, suppose you have been on the air-search radar for twenty minutes and the screen has been "clara."
Figure 1-52. Fade chart.
Then if a contact arrears at a range of 20 miles you can reason that it is most likely at an altitude of 300 feet and entering the first lobe. The plane is not apt to be entering the second lobe at an altitude of 3,500 feet, because if it had made a horizontal approach at that altitude you should have detected it entering the first lobe at a range of about 65 miles. In fact, if the plane had been at 3,500 feet, you should have detected it at all times between the ranges of 65 miles and 24 miles. Similar reasoning rules out the possibility that the plane could be entering the third or fourth lobes, so the altitude must be about 300 feet and the plane must be entering the first lobe. This example illustrates not only one technique of altitude determination, but also the necessity for alertness on the part of an operator. If a plane is closing at three miles per minute and it flies in (low) under the first lobe, it may easily come to a range of 20 miles before an alert operator detects it. You can give the ship a maximum of seven minutes warning if you are on your toes, or less if you are not.
So far, only planes in horizontal flight have been considered. It is possible for a plane to fly down a lobe so that it does not fade at all. Likewise it is possible for one to fly down a fade zone and escape detection for long periods of time. This accounts for an occasional plane getting in to less than the usual range of detection. It does not happen often and it is not done deliberately.
Before leaving the subject of fade charts, let us summarize briefly: To use the charts you must assume horizontal flight. Uncalibrated fade charts show the positions in space of lobe centers and fade centers. They can be used as they are if drawn for your antenna height and wave length, but it is best to calibrate them (by working with planes at known altitude) to show areas in which a plane will be seen and areas in which the plane will be in a fade. The size and shape of these fade areas will depend upon two things: the materiel condition of the radar and the size of the target. The better the materiel condition the smaller the fade zones will be for all types of air targets. The smaller the air target, the larger will be the corresponding fade zones. The fade chart can show fade zones for several sizes of air target. Calibration exercises should be conducted with the radar in the best possible materiel condition. The fade chart affords one of the most effective checks on the condition of the radar once it has been calibrated. If a plane is detected at extremely long range, or if it is detected at short range for the first time, it must be in the first lobe, so immediate altitude estimate is possible. Constant use of fade charts will familiarize you with the certain capabilities and limitations of your radar.
SPECIAL USES OF RADAR
Actual operation in the Fleet has proved that radar may frequently be called upon for special jobs that are ordinarily not in the daily routine of the radar operator. It is well, therefore, for the operator to have a general knowledge of these special applications of radar so that he may carry out intelligently any special assignment when called upon. The operational technique recommended in each case is discussed in some detail in Part 3.
Because fairly accurate bearings and ranges can be obtained on landscapes with radar, it is frequently used as an aid to navigation. The PPI is particularly useful in this respect, because it gives a fairly accurate picture of the surroundings. It is obvious that the PPI can be of great value in navigation when visibility is poor. Buoys can be seen-on the PPI, as well as islands, jetties and shore lines.
With sufficient experience the radar operator may aid navigation by taking tangents on landscapes. This calls for considerable practice, but the method is approximately as follows: The operator swings the beam toward the island or land until pips just begin appearing on the scope. He then assumes that the effective edge of the beam is just striking the land, and swings the antenna on it about half the effective beam width. The center of the beam should then be on the edge of the land and the bearing can be taken. A range is then taken either on the edge of the land or the nearest point of land on that bearing, and a fix may be obtained. Tangents on low-lying landscapes are to be avoided as a rule, because the operator can never be sure that the tangent point is actually being detected. Practice in taking tangents will reveal other equally effective methods of obtaining fixes.
By training the antenna in the direction of the target, it is possible to watch the shell pip move along the time base. A shell in flight will give the appearance of a mouse running under a sheer, as seen on the "A" scope. When the shell strikes the sea, the resulting geyser of water gives an even better echo for several seconds. Do not try to range on the splash, try instead
to estimate the range difference between the splash and the target.
In estimating this range difference, it is helpful to know the range width of an expanded "A" scope, the notch width, the width of a typical echo (expressed in yards) as seen on the "A" scope, and the range dimensions of anything else that can be used for comparison. If there is a scotch tape range scale on the "A" scope, it too can be used to estimate the range difference.
The radar receiver and antenna may be used as a direction finder to obtain the bearing of another radar or a jamming transmitter. It may be impossible to obtain the range of a radar or jamming transmitter, but bearing fixes from two receivers at separated positions may provide a fix. The receiver must be tuned to the same frequency as the radar or jamming transmitter in order to get an indication on the screen. The operator will see moving pulses, humps, or other indications on the screen, and the antenna should be trained until this indication is at a maximum. There are several reports of ships making a rendezvous with other ships by this method when visibility was poor, or radio silence maintained.
The fire-control radar may he put out of order during an engagement, whereupon the search radars will be called upon to give bearings and ranges for fire control. The operator should be prepared to handle this problem by knowing the range and bearing errors of the equipment. Constant check with fire-control radars will give the operator this information.
When shifting targets during an engagement some fire-control radars must be coached on the new target by the search-radar operator. The latter, having a complete picture of the situation, can easily and quickly coach on the fire-control radar with bearing and range information.Fighter direction.
The search operator never knows when he will be called upon to aid in fighter direction. Although he may be on a destroyer or small craft, there is always the possibility that someone aboard his ship may be acting as a fighter director officer in an emergency. The operator should practice giving bearings and ranges rapidly and accurately on a large number of targets so that he will be able to handle this strenuous job if called upon.
FUTURE OF RADAR
Radar will play an increasingly important part in our lives during the period following World War II. It will be on the job then as now, protecting our lives, and making this a safer world in which to live. Read what David Sarnoff, president of RCA, has to say about the postwar prospects of radar:
"Television and radar add new dimensions to radio. Wireless telegraphy was its first dimension, and broadcasting its second. Application of these new developments of radio to peace creates new fields of activity on land, at sea, and in the air.
"Radio instruments will emerge from the war almost human in their capabilities. They will possess not only a sense of direction, but a sense of detection that will open new avenues of service. The radio direction-finder, which heretofore had only an ear, now also has an eye. The safety of aviation will be greatly enhanced, for the aviator will be able to see the ground through clouds or darkness. By the scientific application of the radio echo, the radio "eye" will avert collisions, while the radio altimeter will measure the altitude and warn of mountains ahead or structures below."
There is no doubt about it--radar is a coming field. Learn all you can about your equipment, its maintenance, and care. What you learn will be of use to you in the future.
Table of Contents
Next Part (2)