Operational Limitations on Radar
A. Proficiency of operator: Skill and experience; manipulation of controls; training.
B. False echoes: Side lobes; multiple reflection echoes; blind zones; second trip echoes; phantom contacts.
C. Radar operation near land: Blind zone near shore; radar shadows; aircraft over land; ranging on land; use of radar in piloting.
D. Spotting of shell splashes.
A. PROFICIENCY OF OPERATOR
1. Skill and Experience.
The amount of reliable data that can be obtained from any radar is dependent to a great extent on the proficiency of the operator. It is admittedly difficult to operate a radar with a high degree of effectiveness under all conditions. Therefore, the operators must combine high native intelligence, good visual acuity, manual dexterity, and great concentration with an intense interest in the work if radar is to be used to maximum benefit. To become proficient, the operator must practice continually.
The more the operator is able to understand about his radar equipment and about the tactical situation in which it is used, the more useful will be his performance. Many peculiar results have been obtained with radar that are variously attributed to phantoms, pixies, gremlins, and the like for want of a reasonable explanation for their occurrence. If the operator understands fully the limitations of his radar and knows the disposition of nearby friendly vessels, he will often be able to recognize the false character of many types of phantom contacts.
Long experience in observing a radar indicator enables the operator to notice many minute variations in the echo that are not apparent to a less skilled man. By noting these small variations, the operator is often able to estimate the size and type of the target. For example, the pip from an airplane is different from that of a ship, and the pip from several planes in a group is usually different enough from that returned from a single plane to enable the operator to estimate the number of planes in the group. In at least one case, it is reported that a very alert operator has often been able to tell not only the approximate number of planes in a group, but also the type of plane by observation of only the radar indicator. Such a performance is extraordinary, of course, but it illustrates the excellent results that can be obtained by very close observation coupled with wide experience. An unskilled operator can see only range and bearing, and that perhaps erratically; a skilled man sees in each pip much information beyond the normal range and bearing measurements.
In general, a skilled operator can detect a target at a greater range than an unskilled operator. This ability results from very close observation of the scope and a "feeling" for the appearance of echoes that are lost in the grass to the untrained eye.
2. Manipulation of Controls.
For detection of targets at maximum range, the grass must show strongly on the A scope and the "snow" on the PPI. However, the tendency of many operators is to cut receiver gain to a low level in order to obtain an indicator presentation which is sharp, and which shows strong contrast between signals and background noise. This is a dangerous practice. Large signals may continue to show up well on indicators, but the ability of the operator to detect small signals can be greatly reduced. It is necessary that small targets, such as life rafts, buoys, periscopes, and airplanes, be detected at the greatest possible range.
There is no way to eliminate receiver noise entirely. It is always present in sets which are operating properly, but it may not show if the receiver gain is too low. Signals smaller than the noise add to the noise to make the grass higher or the snow brighter. By noting the small change in the appearance of the trace caused by this addition, a good operator is enabled to discern very weak echoes. However, these echoes, which are equal to or less than the noise voltage, cannot be seen if the noise itself does not show.
The curves in figure 125 illustrate the decrease in performance of an SF radar on a small target as the gain is reduced. Compare the curves showing the range on the A scope and the PPI with the curve that shows the grass height on the A scope. When the gain is reduced sufficiently to make the grass only
Figure 125--Variation in maximum range as function of receiver gain.
1/8 inch high (receiver gain control set at 55), the radar is approximately 95% effective. If further reduction in gain is made, the performance falls off very sharply. Similar effects will be noted in all radar equipment in which receiver gain is a manual control.
A properly trained operator will measure ranges always in exactly the same way so that his personal error is small and constant. As a result, the range and bearing data obtained by a good operator will be more consistent and more reliable than that obtained by an unskilled operator. The skill and dexterity developed by constant practice enable a good operator to measure a range and bearing quickly. Not only is it desirable that each operator be trained to measure ranges in precisely the same manner on each target, but it is also very desirable that all radar operators be trained in exactly the same operating procedures. On a battleship, for example, there may be two identical surface search radars. Unless the operators of these two sets measure ranges in exactly the same manner, considerable confusion may be caused by an apparent sudden change in range when a target is shifted from the forward SG radar to the after SG.
Radar equipment of recent design usually incorporates a circuit that automatically keeps the receiver tuned to the transmitter. The circuit which is used to accomplish this tuning is called AFC (Automatic Frequency Control). The use of AFC enables the operator to concentrate on observing the indicator, without his having to check the receiver tuning to insure peak performance. Other features of forthcoming radars that will simplify and facilitate the operator's job are: improved range scales so that there will be little necessity of shifting scales at a crucial phase of an operation; a range ring and a cursor on the PPI to enable both range and bearing to be read directly from the scope without the necessity of stopping the antenna; and a ship's head marker (SHM) to indicate the course of the ship on the PPI scope.
The setting of the controls of a radar is a function of the tactical situation in which the equipment is being employed. For example, a surface search radar may be used either for station keeping or for searching for surface targets. In station keeping the operator must normally use low gain to prevent masking of close-in targets by sea return. However, this low gain setting prevents detection of targets at long range. This conflict in requirements in the past often has restricted one radar to a single function or it has forced the operator to vary his controls so often that operational effectiveness is reduced. However, the use of Sensitivity-Time Control (STC) causes the receiver gain to be low for the first few thousand yards of the sweep and at whatever level the operator wishes to set it for the remainder of the sweep. Thus, the radar is enabled to perform several functions simultaneously without serious conflict. On one repeater PPI close-in targets can be observed on a short range scale so that the ship can maintain station, while at the same time other repeaters can be set to other range scales to search for targets beyond the formation proper.
To establish a high state of efficiency in the use of radar, constant training is necessary. In conducting training exercises, it is important to remember that the operating habits developed in practices will carry over into combat. If it is expected to use full radar control of gunfire in night action, in practices none of the director crew should ever be permitted to rely on any optical equipment. Training conditions, then, must be devised with a view to combat, and care must be taken to insure that training exercises do not involve equipment that will reduce the fighting ability of the ship. For example, in a surface action the fire control radars normally will be engaged in ranging on the targets under fire. Therefore, in training exercises chief reliance must not be put on the fire control radars as aids to navigation or station keeping.
In developing search techniques in training exercises, the importance of a thorough all-around search should be emphasized, for it is all too easy to fall into the habit of concentrating on the sector in which a target appears. Investigation of a contact to the exclusion of general search encourages the use of feints and deceptive tactics on the part of the enemy. Ships have been lost because an adequate search was not maintained around the full 360°.
Extensive training is required to enable the fire control or height-finding radar to get on a target detected by the search radar. Coordination of this transfer is a function of CIC. With adequate training, fire control radar can get on a surface target and begin to track it within 30 seconds to a minute after the position of the target is indicated. Because of the additional complexity introduced by altitude and the difficulty of measuring position angle accurately with search radar, it usually requires a longer time for the director to get on an airborne target.
To develop the high degree of coordination necessary among the several operators in a director, the men should rotate positions during practice exercises. In this way, each operator will understand the problems confronting the others. As a result, the time required to produce effective teamwork will be reduced.
In. order to realize the full accuracy possible with fire control radar, the operators must practice making precise measurements of range and bearing. Since bearing accuracy often depends on matching pip heights, extensive practice in this operation is
Figure 126--Radiation pattern of a lobe-switching fire control radar.
necessary to develop the operator's judgment to the point where reliable bearings can be determined within the rated accuracy of the equipment. Accurate pip matching is difficult, and it cannot be done well by inexperienced personnel.
B. FALSE ECHOES
1. Side-Lobe Echoes.
Although a directional antenna radiates energy principally in one beam, a small fraction of the energy is radiated in other directions. This small radiation is confined generally to small beams, called minor lobes or side lobes, which are displaced from the axis of the main radiation. In addition, since the reflector is not perfect, some energy is radiated in a direction 180° away from the direction of the main lobe. The radiation pattern of an air search radar antenna is shown in figure 92, and in figure 126 is shown the pattern of a fire control radar that employs lobe switching. Note in figure 126 that large side lobes exist at 345° and 015°, and smaller side lobes are at 35° and 50° each side of the antenna axis.
In spite of the fact that these minor lobes are usually small, they contain enough energy to produce echoes from targets at short range when the receiver gain is high. Side lobe echoes, even from relatively large minor lobes like these shown in figure 126, are seldom received from targets at ranges beyond 10,000 yards. With most radars, however, side lobe echoes will not be troublesome at ranges greater than 5,000 yards.
If echoes are obtained from side lobes as well as the main lobe as the antenna rotates, an isolated target at close range will produce several spots on a PPI. All of these echoes will be at the same range, but they will cover an arc on the screen equal to the angle over which minor lobes appear. If the receiver gain is high and the target very close, the resultant poor bearing resolution may cause the several echoes to merge, forming a complete circle on the screen. Normally, side lobes are not strong enough to produce this extreme condition, but often ah arc of 50° to 120° length will be seen.
In the case of an isolated target, the effect of side lobes is easy to evaluate. The echoes are all at the same range, and they are symmetrically placed about a central bright echo. Side lobe echoes are weaker than main lobe echoes, but this distinction often may not be apparent. With high gain, echoes from both main and minor lobes may be limited in the indicator so that they all produce spots of equal intensity on the PPI. If the gain is reduced, the side lobe echoes in general will be less intense and of less angular width than the main echoes.
If the operator is thoroughly familiar with his equipment, he will know the range at which to expect confusion from side lobe echoes and he will know the angular separation from the main echo at which side lobe echoes normally appear. If the system is improperly tuned, however, the size of the side lobes may be increased and their position shifted. The technician should be able to correct this condition by returning the radar.
If the radar is close to a convoy or a formation of many airplanes, each target may produce echoes not only from the main lobe but also from several side lobes. In the resultant confusion of echoes on the PPI screen, it would be difficult to select the correct bearing of any individual target, and almost impossible to determine how many real targets are present. This same effect is often a handicap to reliable station keeping by radar.
The effect of side lobes can be very easily reduced by turning down the receiver gain, but this adjustment is undesirable from the search point of
Figure 127--False contact caused by reflection from ship's structure.
Figure 128--False contact caused by reflection from nearby ships.
view. However, Sensitivity Time Control will be of great help against side lobe echoes because it reduces receiver gain over only the first few thousand yards of the sweep, and then restores the sensitivity to normal for search over the remainder of the sweep.
2. Multiple Reflection Echoes.
The energy radiated from a radar antenna sometimes may be deflected from its normal straight path by reflection from some part of the ship's structure. This reflected energy can produce echoes from targets at short range, but the bearing on which the echo is indicated is false.
For example, in figure 127, R is a radar antenna and T is a nearby target. When the antenna is directed into the ship's structure, as along RS, the radiated energy will be reflected from the superstructure along ST to the target, and echo energy can return to the radar along TSR.
Since the antenna is trained to approximately 030° relative in the illustration, the echo will seem to be on that bearing at a position such as T'. The indicated range of the false contact will be almost the same as the true range because RST is almost the same length as RT. The echo is real, of course, but the contact is false since no real target exists at the range and bearing shown by the radar. The true position of the target will be indicated when the antenna beam is directed along RT, at a relative bearing of approximately 150°.
When the operator becomes thoroughly acquainted with his equipment, he will know the bearings on which to expect doubtful echoes. One very common cause of false contacts on destroyers results from mounting the SG antenna on a platform just forward of the mast. When the antenna is trained aft, echoes are often returned from the next ship ahead in column. If the operator is aware of this possibility he will not be confused by its occurrence. However, a very simple remedy has been found for eliminating phantom echoes caused by reflection from the mast. It consists of a metal screen reflector which is so mounted that the energy that strikes it is reflected straight up. This does not remove the blind spot caused by the mast, of course, but it does prevent echoes from being returned on a false bearing.
Figure 129--Multiple reflection used for calibrating radars.
Under certain conditions, false contacts can be caused by radiated energy being reflected from one ship to another, as in figure 128. The destroyer radar radiates energy toward the CV along DA. Energy is reradiated from the broad side of the carrier in all directions. Some of this energy returns to the radar as the normal echo from the CV, and some proceeds to the cruiser along AC. The energy that strikes the cruiser is again reflected, so that some energy returns to the radar along path CAD. Thus, on a relative bearing of about 040°, the destroyer radar will show a very strong echo at 1,000 yards and a weak echo at 2,000 yards. Because the image of the cruiser produces a weak echo, it may be interpreted as a submarine when first detected. However, plotting contacts of this sort usually reveals their false character.
Since the radiated energy must be reflected three times from the ships before it returns to the radar, the energy is so greatly attenuated that false echoes caused by multiple reflection can be encountered only when ships are in close formation. If the carrier in the illustration yaws or rolls, the false contact will fluctuate badly and perhaps disappear completely. In general, rather small changes in the relative positions of the ships involved will cause the false contact to disappear.
A slightly different case of multiple reflection has been found to be very useful in calibrating radars. When two ships are steaming abreast on parallel courses, multiple reflections can take place between them in the manner shown in figure 129 if the ships are separated by no more than 3,000 yards. The transmitted pulse is sent out along path , and it is reflected from ship B. Part of the reflected energy returns to the radar along path  and part of the energy may follow a path such as . This energy in turn will be reflected back toward ship B along path , where it is again reflected, following path  back to the radar. The radar indicator will show a strong echo at the range of ship B and a weak echo at twice the range. If the inherent error in measuring range is E and the true range is R, the radar indicates a range of R + E on the strong echo and a range of 2 R + E on the weak echo. The difference between these two values is the true range to ship B, since the error in measurement cancels. A detailed procedure for using the method to determine the Zero-Set correction for a radar is given in the Radar Maintenance Bulletin, page MK 3/4 5-12.
3. Blind Zones.
A radar antenna will have a clear field of view only when there are no obstructions on the ship in the path of the radiated energy on any relative bearing. Any metallic surface in or near the radar beam will reduce the intensity of the radiated field. Therefore, in every installation, a given target can be detected at a greater range on bearings where the field of view is unobstructed than on bearings where metallic masses aboard the ship intercept part of the radiated energy. Almost all radar equipments have areas of reduced coverage due to this cause, and many have sectors that are completely blind. A few illustrative horizontal coverage patterns are shown in figure 130. These curves may be interpreted as showing the relative ranges at which a particular target is first detected on various relative bearings. In B the coverage is practically perfect since the antenna is mounted up in the clear where no obstructions are present. However, the other patterns show, in varying degrees, how the signal returned from a target can vary as a function of relative bearing. These patterns are not necessarily typical of all classes of ships; they are extreme cases which are intended to show how poor the coverage can be in some installations.
Patterns of this sort can be taken by observing the height of the echo returned from a remote, isolated target as the ship turns. To avoid error, the ship should turn slowly in the smallest possible circle and the target should be stationary at a range of 30,000 yards or more. Although the pattern can be determined more accurately with special test equipment, a few simple tests involving only the radar and an isolated target will indicate satisfactorily blind zones and areas of reduced coverage.
In some installations it will be found that the extent of an area of reduced coverage will be dependent on the position of another radar antenna. When the SK antenna, for example, is trained so that
it intercepts a maximum amount of energy radiated by the SG, the blind zone for the SG will be more extensive than when the SK is turned 90° from this position. It is desirable, then, to note the effect of the large radar antennas on the coverage patterns of the various radars on board. When any of the sets that have large antennas are secured, they should be trained to the relative bearing on which they interfere least with thorough coverage by other equipment.
The only cure for a blind zone caused by interference from the ship's structure is to relocate the antenna. Since most naval vessels are now equipped with several radars, it is likely that only one antenna can have a clear field of view. For example, figure 131 shows two photographs of the antennas installed on the U.S.S. Yorktown. It is immediately apparent that not all of these antennas can have a field of view that is free of obstructions on all relative bearings. When the SM looks over the stern of the ship, the beam is almost wholly intercepted by the SK antenna. Thus the SM has a large blind zone aft. Similarly, both the SK and the after SG-a have blind zones forward, while the SC-2 has greatly reduced coverage over a wide angle to port, depending partly on the position of the SK antenna. Since it is not possible for all antennas to be in the clear because of the limited number of possible
Figure 130--Horizontal radar coverage patterns.
Figure 131--Superstructure of U.S.S. Yorktown.
positions in the superstructure, the radars should be installed so as to minimize the reduced coverage areas and the totally blind zones. Areas in which good coverage is obtained should overlap insofar as possible, so that a single ship may be able to detect both air and surface targets over the full 360°.
Another type of blind zone exists for all radars above the maximum position angle at which the vertical width of the antenna will permit detection of targets. The size of this zone is dependent principally on the characteristics of the antenna itself, and it is affected only a little by the site of the antenna. For example, the SG radar cannot detect any targets above a position angle of about 8°.
4. Second-Trip Echoes.
Radio waves travel through the atmosphere at a speed of 328 yards per microsecond. However, a radar pulse must travel out to the target and back again, so that the effective velocity of a radar pulse is 164 yards per microsecond. The maximum range that the radar can measure directly is then the interval between pulses, in microseconds, multiplied by 164 yards per microsecond.
A target that is 230,000 yards away normally will not be detected by either fire control or surface search radar. If the target is high land and the weather is such as to produce a duct, the radiated energy may be guided to the land and the reflected energy guided back to the radar. Under these conditions, the echo may show up on the SG or Mark 4 indicators, but since neither equipment uses a range scale long enough to measure the range directly, the indicated range will be incorrect. If the SG uses a repetition rate of 800 pulses per second, the interval between pulses will be 1,250 microseconds, which is equivalent to 205,000 yards range. In figure 132 the land target is 230,000 yards away so that the time of flight of the pulse is longer than the interval between pulses. As a result, nothing shows on the first sweep, and in A the land echo shows at 25,000 yards on the second trip of the electron beam across the face of the indicator. To get the correct value of the range, the 205,000 yards represented by the interval between pulses must be added to the indicated range.
It is apparent, then, that the range at which this target appears depends on the repetition rate of the radar. For example, in B the SG sweep is shown for a repetition rate of 750 pulses per second. In this case the pulse interval represents 218,700 yards, so that the indicated range is 11,300 yards. Thus, the simplest method of showing up a false contact caused by a second trip echo is to change the repetition rate. However, this is not possible with all radars, so that a second check would be to range on the target with other radar equipment. In figure 132C is
Fiqure 132--Second trip echoes.
illustrated the sweep of a Mk 4 radar which is ranging on the same land target. Because one pulse interval represents 100,000 yards with this equipment, the echo will appear on the third sweep. The indicated range is 30,000 yards. In D is shown part of the SK sweep. This equipment uses a 60 cycle pulse rate so that the pulse interval represents 2,733,000 yards, or 1,367 nautical miles. The 375-mile sweep on the SK, which is possible only on a radar having such a low repetition rate, is able to indicate directly the range to the land. Therefore, if all three ranges were compared, the false contact would be evaluated as a second or third trip echo. In some cases it will not be possible to check second-trip echoes on an SK or SC-2 because the atmospheric conditions that cause trapping of L and S-band radiation may have no effect on P band. When this occurs, the SK will not show the distant target at all. In a few rare cases unusually strong trapping conditions may cause second-trip echoes even on sets that have range scales 400 miles long. However, it usually is quite safe to assume that second-trip echoes are impossible with a radar whose pulse rate is less than 200 pulses per second.
It is interesting to note that a phantom enemy was fired on for over an hour as a result of a false contact caused by second-trip echoes. In this case, however, an unfortunate coincidence occurred to prevent thorough checking of the contact. The SG repetition rate was almost exactly half that of the Mark 3 and 4, so that both equipments indicated the same range--one on the second trip, the other on the third. Comparison of figure 132A and C will indicate how this mistake occurred. Another factor that prevented
Figure 133--Geographical plot of second trip echo contact.
a complete check of this contact was the fact that the duct formed by the weather apparently was too shallow to guide the P-band energy of the SK, while the L and S-band energy of the Mark 3 and SG was guided. As a result, the SK did not detect the land, which prevented a direct indication of the range of the contact.
A geographical plot can be used as an auxiliary means of evaluating a contact suspected of being a second trip echo. As an example, assume that a land mass is 230,000 yards away and that the pulse interval of the radar represents 200,000 yards. The indicated range of the initial contact will then be 30,000 yards. As the ship moves, the actual range to the land will change only a little, but the indicated range varies rapidly. Consequently, plotting the ranges and bearings indicated by the radar produces a track for the contact, as in figure 133A. If the same data are plotted to a smaller scale, and the 200,000 yards added to each indicated range, the land target will plot at a stationary position at T, as in figure 133B. It is evident from B that, for the radar of this example, the track of this second trip echo must always plot as an arc of a circle of 200,000 yards radius. Thus the apparent course of the contact changes in an orderly manner, regardless of the maneuvers of the ship carrying the radar. The apparent speed of the contact is always less than the speed of the ship, and since the triangle TGE is approximately similar to triangle TBA, the apparent speed of the second trip echo contact always must be proportional to the speed of the ship.
Thus, a second trip echo contact may be evaluated as such by changing the speed of the ship and noting the change in the apparent speed of the contact. Other features of the plot that may assist in the evaluation are the progressive change in course of the contact, and the fact that the plot can indicate a definite point of land if the range represented by the pulse interval is added to the indicated range. Since a considerable time is required to gain enough information from the plot for a conclusive evaluation, chief reliance should be placed in varying the repetition rate to identify this type of false contact.
5. Phantom Contacts.
Radar echoes can be returned from a large variety of objects that are of no consequence as targets. For example, clouds, birds, fish, weather discontinuities, and intersecting wakes all reflect radar pulses to some extent. In general, echoes from these sources can be recognized by an alert operator.
The wakes of two ships on parallel courses intersect at a point on the quarter of both ships. The intersection travels at roughly the same speed as the ships, and can produce a fluctuating echo that will remain at nearly the same range and bearing as long as the relative position of the ships is unchanged.
In some cases, heavy waves at close range produce echoes that have fairly high rates of speed. The size of the echo and the speed at which it approaches may temporarily lead the operator to believe that the wave is a motor torpedo boat, or some such small, fast vessel.
The echoes from flying fish or birds fluctuate rapidly as a rule. In addition, the behavior of birds is usually so different from that of any other type of airborne target that observation of the movement of the object should reveal it as a bird. Since birds and flying fish are relatively small, they can return echoes only at short ranges.
Clouds and weather discontinuities vary greatly in character, so that their ability to reflect radar pulses is far from constant. In general, echoes produced by atmospheric phenomena fluctuate rapidly and they produce a hazy indication on the cathode-ray tube screen. (See figs. 102 and 103.) In a few cases, however, clouds may produce sharp, bright indications that may seem to be reflected from real targets. Cloud contacts can be recognized by determining the approximate extent of the reflecting area, comparing the velocity of the contact with wind velocity, and finding the position angle, in addition to observing the character of the echo that appears on the screen.
It has been reported that star shells produce excellent echoes on radar. Since the parachute falls slowly at almost exactly the same range as the target being illuminated, radars that have poor resolution in position angle will be unable to range accurately on the target. Surface search radars are usually not seriously affected by star shells because the projectiles pass quickly through the narrow antenna beam. However, the air search radars and some fire control sets encounter difficulty from this cause.
C. RADAR OPERATION NEAR LAND
1. Blind Zone Near Shore.
The limitations imposed by the range and bearing resolution of most radar equipment prevent the detection of targets close to shore. If the radar beam looks at right angles to a straight shore line, the minimum offshore distance at which a target can be detected is the distance required for range resolution by the equipment. However, when the radar beam makes an angle other than 90° with the shore line, the minimum offshore distance at which a target can be detected is controlled by the bearing resolution. Since the separation of targets required for bearing resolution is generally greater than that required for
Figure 134--Radar blind zones near irregular shore line.
range resolution, a fairly extensive blind zone exists close in to shore.
An irregular shore line is shown in figure 134 along with the blind zones near the shore that exist with radars of different beam widths. The radar is assumed to be at point A. Since an echo will be received over an angle of train approximately equal to the beam angle, each point on the shore produces an arc on the PPI. As a result, the shore line seems to extend out to sea a distance dependent on the angle at which the radar beam strikes the shore. For example, when the axis of the SK beam lies along AB, any target that lies on the arc BC will be hidden by the echo from the land at B. However, if a radar with a narrower beam, and consequently with better bearing resolution, is used, the extent of the blind zone close to shore will be reduced. Therefore, an SG radar at point A could detect a target at point D instead of at C, and the Mark 8 could detect a target even closer to the land. Where the edge of the radar beam does not strike land at a particular range, the boundary of the blind zone is determined by the range resolution of the radar. For simplicity, it has been assumed that the range resolution is approximately 500 yards for each of the three radars illustrated in figure 134.
The extent of the blind zone for any shore line can be approximated if the position of the ship relative to the shore is known, and a chart of the area is available. For each bearing, a point is plotted at the range of the land, but offshore by an angle equal to the resolution angle. Thus, if the resolution angle of the SK is the 10° angle BAC, and the antenna is trained to a bearing of AB, it will be impossible to detect a target at the same range as point B until the antenna is turned away from the land by an angle great enough to eliminate the echo from the shore line. It is usually considered that this angle is the resolution angle of the radar, so that at the particular range and bearing shown, point C represents the closest point to land that an SK radar can detect a target. The outer boundary of the blind zone can be determined by plotting such points for all possible bearings. However, when the beam is directed perpendicular to the shore, bearing resolution does not affect the blind zone. Therefore, a contour representing the range resolution must be drawn to cover the areas where bearing resolution does not affect the blind zones.
This method is approximate only, because the echo is not necessarily returned from the junction of water and land. Often high land back from shore
Figure 135--Radar shadow behind high land.
produces the echo that is seen on the radar, while a low beach is not detected. In places where the shore line is a clear low beach, surface targets may often be detected much closer to shore than the method used in figure 134 would indicate. However, when the shore is steep, the great size of the land causes the bearing resolution to be poorer than normal because echoes are returned by the land even when the antenna axis is turned away by more than half the beam angle. As a result, in places where the shore line reflects radar pulses very well, the resolution angle used in determining the blind zone should be greater than simply half the beam angle.
In general, little decrease of the blind zone can be effected by the use of low radar sensitivity because the barges, sampans, or PT boats that are likely to operate close to shore cannot be detected with low gain.
2. Radar Shadows.
If the promontory shown at the right of figure 134 is high ground, the blind zone contours shown in the cove behind the headlands will be incorrect. This region will be in a radar shadow, as illustrated in figure 135, and a target behind the promontory will not be detected by any radar on the ship. High land blocks the radiated energy, preventing it from striking any low points beyond. Thus, high land produces a blind zone irrespective of the resolution characteristics of the radar.
In figure 135, notice that there is shown, directly over the land masses, a blind zone caused by the poor position angle resolution of the radar. In most search radars, the antenna cannot be tilted. Therefore, if a large target is detected at a given range, its echo will mask the echo from any other target at the same range even when the two targets are at different altitudes. Thus, in figure 135 a blind zone exists above the land, so that an airplane cannot be detected in this area.
3. Detection of Aircraft Over Land.
As a result of their poor position angle resolution, search radars generally are not able to detect airplanes that approach from over land. Since the Japs are apparently aware of this limitation of radar, operators must be especially diligent in observing the echoes returned from land for the slightest indication of a moving target. It is very difficult to observe the slight changes in the indications produced by airplanes flying over hilly land when the antenna is in constant rotation, since the echoes are retraced only intermittently, making it hard to distinguish the desired echo in the clutter produced by the reflections from the land. Any device which can reduce the amount of clutter without materially reducing the desired echoes will therefore aid considerably in allowing the operator to detect moving targets over a land mass. The Instantaneous Automatic Volume Control and Fast Time Constant coupling circuits that are being provided in radars of recent design will prove to be valuable not only as anti-jamming devices, which was their original purpose, but as anti-clutter devices as well. For example, in figure 136 three photographs of an experimental radar PPI are shown. In A, nearly the whole screen is covered by sea return, and the land at the bottom of the picture shows as a solid mass of echoes. When IAVC is used, in B, the sea return is made less severe, and the land mass is broken up to some extent. Finally, with the combination of IAVC and FTC, shown in C. the land mass is broken into many discrete pips and the sea return is greatly diminished. An operator would have a fair chance of detecting an aircraft that approaches over land if the echo can be broken up as in C because he could watch for targets moving in the cluttered area. To be sure, it is still a difficult task, but the improvement produced by these new circuits makes it a possible one. Even with these improvements, it may often be desirable to assign
Figure 136--Effect of IAVC and FTC on ground clutter on PPI scope.
Figure 137--Elevation of antenna to detect airplanes over land.
one radar to search carefully over the land, using manual control of the antenna, when the ship is anchored close to shore, since there is a better possibility of seeing a fast-moving target on the A scope than on the PPI under these conditions.
If the radar antenna can be elevated, the beam can be pointed so that it just misses the land. Thus in figure 137A, good protection is afforded by the radar, while if the land rises sharply, as in figure 137B, the operator must wait to see if any planes come over the peak of the land. The chance of missing the target under these conditions is great, so that it is very difficult to maintain an effective search.
If the radar gain is reduced to the point at which land echoes do not saturate the receiver, friendly planes may be tracked over land by following their IFF responses. This method is not highly reliable, and of course it gives no indication of the approach of enemy planes.
4. Ranging on Land.
The maximum range at which land can be detected by radar depends to a great extent on the character of the land. Sand spits and smooth clear beaches do not show up on radar at ranges beyond one or two miles because such targets have negligible area that can reflect energy back to the radar. Two radiated rays from the antenna are shown striking a smooth sand spit in figure 138. Since the target is so low in the water and because it is flat, these rays are reflected on beyond the target. If waves are breaking over the sand bar or on the beach, echoes may be returned from the surf. However, the waves may break well out from the actual shore, so that ranging on the surf may be misleading when a radar position is being determined relative to the beach. As the tide rises and falls, the apparent shore line, as determined by radar ranges on surf, will appear to shift.
Mud flats and marshes generally reflect radar pulses only a little better than a sand spit. The weak echoes that are received at low tide disappear at high tide. Mangroves and other thick growth may produce a strong echo. Therefore, areas that are indicated as swamps on a chart may return either strong or weak echoes, depending on the density and size of the vegetation growing in the area.
When there are sand dunes covered with vegetation well back from a low smooth beach, the apparent shore line determined by radar will be along the line of the dunes rather than the true shore. Under some conditions, sand dunes may return strong echo signals because the combination of the vertical surface of the vegetation and the horizontal beach may form a sort of corner reflector.
Lagoons and inland lakes generally show as blanks on a PPI scope because the smooth water surface
Figure 138--Radar reflection from sand spit.
returns no energy to the radar antenna. In some cases, the sand bar or reef surrounding the lagoon may not show on the radar because it lies so low in the water.
Coral atolls and long chains of islands may produce long lines of echoes when the beam is directed perpendicular to the line of the islands. This is especially true when the islands are closely spaced, since the spreading that results from the width of the radar beam causes the echoes to blend into continuous lines. However, when the chain of islands is viewed lengthwise, or obliquely, each island may produce a separate pip. Surf breaking on a reef around an atoll often may produce a ragged, variable line of echoes.
Submerged objects do not produce radar echoes. If one or two rocks project above the surface of the water, or if waves are breaking over the reef, it may show on the radar. When the object is entirely submerged and the sea is smooth over it, no indication will be seen on the radar. However, in one case a totally submerged reef was definitely detected by a P-band radar. This is theoretically possible for P band frequencies if the reef were submerged in fresh water, but an S-band radar was unable to detect the reef at all. Such an occurrence is very rare.
If the land rises in a gradual, regular manner from the shore line, no part of the terrain will produce an echo that is stronger than the echo from any other part. Consequently, a general haze of signals will show on the indicator and it will be very difficult to determine the range to any particular part of the land. As a matter of fact, while the antenna is held
Figure 139--Ship approaching land that rises gradually back of shore.
still and the ship is not rolling, the apparent range to a shore of this sort may vary as much as 1,000 yards. This variation may be caused by slight changes in propagation conditions that cause the beam to be moved up and down the slope.
When a ship approaches or goes away from a shore behind which the land rises gradually, a plot of the ranges and bearings to the land may show an apparent course and speed of the landfall. The reason that the land contact may show a course and speed may be explained by reference to figure 139. In A the ship is 50 miles from the shore line, but since the radar beam strikes at point , well up on the slope, the indicated range is 60 miles. At some later time, when the ship has moved 10 miles closer to land, the indicated range is 46 miles because the radar echo is now returned from point . In C, when the ship has moved another 10 miles, the radar beam strikes even lower on the slope, so that the indicated range may be 32 miles. If these ranges are plotted, the land will appear to be coming toward the ship at a speed of 12 knots, as shown in D.
In the illustration a smooth gradual slope was assumed, so that a consistent plot was obtained. However, in practice, the slope of the ground usually will be irregular and the plot will be erratic, making it difficult to assign any definite speed to the land contact. The steeper the slope of the land, the less will be its apparent speed. Since the slope of the land does not always fall off in the direction from which the ship approaches, the apparent course of the contact need not always be the reciprocal of the course of the ship, as assumed in this simple illustration.
Blotchy signals are returned from hilly ground because the crest of each hill returns a good echo while the valley beyond is in a shadow. If high gain is used, the pattern may become solid except for the very deep shadows.
Low islands generally produce small echoes. However, when thick palm trees or other foliage grow on the island, strong echoes are often produced because the horizontal surface of the water around the island forms a sort of corner reflector with the vertical surfaces of the trees. As a result, wooded islands can be detected at a much greater range than barren islands.
In general, the echo produced by land will be strong if the land is high and if it rises steeply. Therefore, high volcanic peaks can be detected at a much greater range than low, flat coral atolls.
5. Use of Radar in Piloting.
The PPI scope often can show the approximate shore line clearly. In some cases, it is possible to match the indicator with a chart of the proper scale as a means of determining the position of the ship. If no map is available, or if there is no reliable information about the configuration of the land, it is not possible to identify signals as certainly due to specific features of the shore. The operator cannot tell, from the indicator alone, that one signal comes from a ship, another from a rock, a group of signals from a group of small islands, or a solid block of signals from a mountain. The signal pattern from a group of small islands or rocks several hundred yards apart may simulate the appearance of a convoy of ships, and vice versa. An irregularly hilly shoreline with stretches of low beach between the hills may look on the indicator like a string of separated islands. A blank in the signal pattern may represent a body of water, but it may also represent simply a valley that is shadowed from the radar beam by higher land in the foreground.
The PPI pattern is a map presentation, but it is not a complete map. Even with the receiver gain turned up full, the shore line can show only if the radar beam reaches it. If it is so low as to be hidden by the earth's curvature, or if it is shadowed by higher headlands in the foreground, no energy can reach it and therefore none can be returned to produce signals on the indicator. Signals should not even be expected to appear from all over the land surface that is exposed to the radar beam. The configuration of that surface may be such that some parts of it will reflect practically all of the pulse off in a direction that will miss the antenna.
When a reliable map of the area is available, it will often be possible to match the pattern on the PPI with the shore line on the chart. However, this may be a rather slow and tedious process in many cases. Two devices have been developed which facilitate the comparison of the PPI picture of a shore line with a navigational chart. The first of these is the VPR (Virtual PPI Reflectoscope), which permits the radar operator to see both the PPI screen and a chart superimposed. The superimposition is accomplished by the use of a special transparent mirror through which the operator can see the PPI screen, while a virtual image of a chart is reflected into his field of vision by the mirror. The NMP (Navigational Microfilm Projector) operates on the same principle, but instead of using paper charts, a microfilm chart is projected onto the transparent mirror.
The limitations of range and bearing resolution tend to distort the shape of the coast line, making it difficult to match shore lines accurately without thorough training. For example, the coast line shown in figure 131 when seen on the SK is apparently shaped like the outer blind-zone boundary. Notice that the cove on the left will not show at all since
echoes from the shore extend over a great enough area to conceal it. The same illustration shows that as the bearing resolution of the radar is increased, the apparent shape of the shore on the indicator pattern is more nearly a true indication of the actual shape of the shore line.
Most navigational charts do not show the contours of the land back of the shore, so that special charts will have to be provided for use with both the VPR and the NMP. Even when the contours are shown, it is still not at once apparent just which parts of the shore line will return echoes and which parts will not. In many cases, for example, high land back from the beach will produce an apparent shore line that will not register with the charted shore. Other factors that may produce a false shore line on the PPI indicators are:
- Numerous off-shore rocks or very small islands may produce merging echoes.
- Very heavy surf breaking over a reef or sand bar.
- A row of pilings off shore.
- Many small boats off shore may produce merging echoes.
- A row of sand dunes back of a low shore line.
Figure 140--Shore line on PPI viewed from different angles.
The appearance of a shoreline on a PPI will vary when viewed from different directions. For example, the smooth line in figure 140 represents a charted shore line. The PPI pattern shown in A is unlike that in B because the radar shadows cast by high ground are different, and because the width of the antenna beam causes different distortion at the two positions of the radar.
The position of a ship can be determined within 100 to 200 yards by the use of either the VPR or the NMP when the vessel is within 10,000 yards of the shoreline. At greater ranges, the accuracy is less, but the necessary precision is also reduced. The accuracy of this aid to radar scope interpretation depends on several factors. Prominent among these factors are such things as the accuracy with which the chart is prepared, the range scale used on the radar indicator, the range and bearing resolution of the set, and the accuracy of ranging with the radar. This last factor includes not only the limitations on range measurement inherent in the equipment, but also the precision with which the set is adjusted and the skill of the operator in matching the pattern and in reading the ranges. The accuracy of VPR or NMP radar fixes also varies with the character of the shoreline. When the shoreline has a distinctive form that is capable of a precise match, as a steep, vertical cliff that falls down to the water's edge, the position of the ship may be rather quickly determined within the limits of ranging accuracy of the radar. However, when the shore is low and featureless and there is no reliable information available of the inland topography, an exact match is very difficult to make, and the position of the ship is correspondingly determined with less certainty.
The chief advantage of either of these auxiliary pieces of equipment is in the speed with which the fix can be obtained. They do not make the radar any more accurate, nor do they do anything that cannot be done by other, slower means. However, they should prove of great value in all classes of ships in expediting radar piloting, especially in small vessels that do not have the full CIC facilities available to the larger naval vessels.
In piloting a ship, tangent bearings to a piece of land are often taken as a means of finding the position
of the ship relative to the land. In cases where it is not possible to identify particular features of the land mass on the radar screen, it is not possible to get a fix using radar range and bearing. In such cases, tangent bearings can be measured by radar and a position obtained. However, since the width of the radar beam causes the echo from any point to appear as an arc, each tangent bearing determined by radar must be corrected by approximately half the beam angle. If this correction is not applied, the plotted position of the ship will be much closer to the land than is actually the case.
It is possible to predict the appearance of the PPI screen from charts, but this is a rather tedious process. As an aid in briefing units for missions in which radar is to be used, the RPD (Radar Planning Device) may be very useful. In this device a topographical model is made of the area to be attacked. A special light projector may be put at any desired point on the model and the light beam rotated to simulate antenna rotation. A photograph is taken from a point directly over the light source as the beam rotates, and only the surfaces that are illuminated--which are the surfaces that produce radar echoes--appear on the film. A series of these photographs, suitably printed, will show the operator what to expect on his screen as he approaches a target area.
The RPD, or some other device that allows the appearance of the radar screen to be predicted readily, will be of considerable value in conducting shore bombardments. By this means, it is possible to predict a good reference point for offset firing. The operators of the fire control radar are then able to study the predicted scope patterns before the action so that the bombardment can be carried out more effectively. For example, a cantilever pier on the island of Nauru was predicted as a good offset-fire reference point by study of charts and reconnaissance photographs, although no RPD photographs were made in this case. When the actual bombardment was carried out, it was found that the cantilever pier showed clearly, allowing the position of the ship relative to the target area to be established accurately.
RPD photographs are fairly good approximations to what the radar pattern will look like at a particular place. However, it must be remembered that they are approximations only. The light beam used in producing these pictures does not act in the same way that the radar beam does in that it is not attenuated by the same law. At present, there is no way of producing an RPD photograph that will simulate the appearance of the indicator screen at low gain, which limits the use of these photographs to some extent. For example, the VPR requires low gain for
best results, so that best matching cannot be done from an RPD photograph which represents high gain conditions. Also, a single photograph is good for only a single point in the approach to a particular shore. Thus, if a whole approach is to be covered by RPD predictions, a large file of prints will be needed, and if plans are changed, the file will have to be reshuffled quickly. Consequently, the RPD serves best as a briefing device, and either the VPR or the NMP as an aid to piloting in conjunction with a specially prepared map.
In general, it is very difficult, if not impossible, to select particular targets on land by means of radar. When the terrain is hilly, the echo is usually not well enough defined to permit recognizing detailed features of the land. However, if there are large steel tanks or towers on the land, they produce strong echoes. When the receiver gain is decreased, the operator may be able to see the strong echoes returned by the metal targets against the weak clutter of ground echoes. It is possible for the Shore Fire Control Party to carry ashore a radar beacon to use as a means of identifying their position to the radar. The operation of this equipment is such that the radar must be detuned to show the beacon response, so that the clutter from the land is not apparent on the scope, and the beacon response shows clearly.
D. SPOTTING SHELL SPLASHES
It has been found possible to follow projectiles in flight and to observe the splash thrown up when they hit the water. Since the splash lasts for only three to five seconds, considerable training is required before the operator can range accurately on the point of impact. Large shells throw more water up in the air than small shells. Therefore, the splashes produced by heavy guns can be detected at a greater range than those from light guns. In general, the maximum range at which splashes can be detected compares well with the maximum effective range of the guns producing the splashes.
Radar spotting is practicable up to the ranges indicated in the following table:
Size projectile Approximate maximum range Inches Yards 16 30,000 12 25,000 8 20,000 6 16,000 5 15,000
As radar equipment is improved and as the operators gain spotting experience, these values may well be
exceeded. This range is dependent primarily on the size rather than on the type of projectile fired. Armor piercing projectiles do not produce a larger splash at the point of impact than non-explosive practice rounds because, if the shell is detonated, the explosion normally occurs well beyond the point where the shell first hits the water. However, high capacity bombardment projectiles fitted with contact fuses may produce a splash that is larger than normal if the shell is detonated on impact with the water.
It usually is impossible to detect the "splash" of a salvo that hits land. But in one bombardment action where the target area was a cleared space in a wooded section, the flying debris thrown up by the exploding shells produced an echo that was visible because no ground echoes were returned from that range and bearing. However, such a fortunate situation exists rarely. Therefore, reliance for checking the accuracy of gun fire in a radar-controlled bombardment must be put in ranging on an occasional salvo purposely dropped in the water off shore.
When the range is purposely altered to cause a salvo to fall in the water, care must be taken to insure that the salvo will fall far enough off shore for the radar to detect it. If the splash is too close to the beach, the weak echo returned from it will not be visible because of the limits of range and bearing resolution.
Projectiles that hit very close to the target may appear to be on because of the limitations in resolution inherent in radar. Therefore, although the shells are followed toward the target on the radar, the absence of splash echoes is not an absolute indication of a hit. In general, projectiles that fall short are easier to detect than those that fall beyond the target, because the range resolution of the radar coupled with the extent of the target in range combine to produce a dead space beyond the target.
Rapid fire may produce a sort of enlarged grass in the area of the splashes because of the continuous fall of projectiles. With salvo fire, however, each projectile produces a single fuzzy echo of short duration. Therefore, salvo fire is much easier to spot by radar than rapid fire.
Because the separation in range between targets' required for range resolution is usually much less than that required for bearing resolution, spotting in range by radar is more effective than spotting in deflection. Unless the bearing resolution of the radar is very good, the operator will not be able to detect shell splashes that are at the correct range, but off in deflection. A radar with excellent bearing resolution, such as the Mark 8, is very effective for spotting in deflection.
Anti-aircraft bursts produce radar echoes much less effectively than splashes in the water. The echo is returned partly from the metal fragments scattered through the burst and partly from the hot ionized gases that are the product of the explosion. As the gases expand, they cool; when the gases cool, they lose their ionization. Since the explosion very quickly scatters the metal fragments over a wide area, and because the gases expand very quickly, an AA burst does not reflect radar pulses for more than a second or two.
Within the short time that the echo is visible it is very difficult to determine the range, bearing and altitude of an AA burst relative to the plane at which the shell was fired. A range spot can sometimes be determined satisfactorily on AA bursts. However, the time is so short that it is unlikely that reliable bearing and altitude spots can be obtained unless the operators are exceptionally alert and well trained.
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