GENERAL IFF PRINCIPLES
General IFF Principles
INTRODUCTION 2-2 Need for IFF and history relating to its development 2-2 PRESENT UNIVERSAL SYSTEM-MARK III IFF 2-3 Equipment and operational characteristics 2-3 Coded response 2-5 One cycle analysis of IFF operation 2-6 Reporting distress with IFF 2-8 SECURITY 2-12 Necessity of safeguarding IFF security 2-12 The destructor--the safeguard against enemy possession 2-12 ADDITIONAL USES OF IFF 2-13 EQUIPMENT FAILURES 2-14 LIMITATIONS OF MARK III IFF 2-14 Non-directional interrogation and range identification 2-14 Wide beam of BL and faulty identification 2-15 360 degree IFF due to minor lobes 2-16 Red and green band triggering 2-16 IFF interference and code readability 2-17 IFF signals too narrow on long range 2-17 SUMMARY 2-17
GENERAL IFF PRINCIPLES
In this section we are concerned with a major problem: that of identification. Having detected a target and interpreted it to be a plane or a ship, how can you he certain whether the target is friendly or enemy?
Consider for a moment how ineffective a sentry patrolling an outpost in a war zone would he if he were unable to distinguish between the men attached to his unit and the enemy trespassers. There would he two courses of action open to him: he could either sound a general alarm for each person who approached his post, or he could allow everyone to pass unchallenged into the camp, assuming that they could he identified later. Such practice would inevitably lead to disaster. However, we are confronted with much the same problem in radar operation. Radar is a long-range sentry that reports the presence of all trespassers, and is constantly on the alert for enemy ships, planes, and other objects.
Of course, the customary Navy methods of identification are still available. But in challenging by blinker light and by high frequency radio voice codes, the sender must disclose his position to the challenged contact. Both methods have the added disadvantage of range limitations.
In the accounts of the earliest tactical use of radar, all the evidence points to a lack of suitable means of target identification as the most serious limitation of radio detection and ranging.
Need for IFF and history relating to its development.
We are indebted to the British for development of the equipment that we so sorely needed. They had their own system of radar and its use helped in no small measure to save England in her gravest hour of peril. Still, at the outset, one serious limitation threatened to destroy radar's effectiveness, and that was this same problem of identification-the problem of IFF, Identification, Friend or Foe.
While the Luftwaffe was striking and the RAF was strictly on the defensive, the British radar operator had no identification problem, for his own fighter rarely ventured far from home. Thus, it could be taken for granted that all planes approaching from across the Channel or flying in from the North Sea were enemy. However, the Germans soon abandoned their daylight assaults because they could not stand the appalling losses of men and aircraft. This was the first real setback that the Luftwaffe had encountered in its wholesale bombings.
Driven from the sunlit skies, the Nazis resorted to all-out night bombings, relying on darkness to shroud the destruction-laden bombers from the deadly sting of the Spitfires and Hurricanes. But this freedom from effective fighter opposition did not last long. Radar, constructed so compactly that it could be taken aloft in night fighter planes, unerringly sought out the almost invisible black bombers. When the keen-eyed pilots had jockeyed their planes into gun range, the Nazis were met with a rain of hot steel pouring at them out of the darkness. As the night fighters were sent up in greater numbers, the toll of enemy planes increased, but so did the problem of target identification. Before he opened fire, the RAF pilot had to be certain that the pip was from an enemy plane and not from a comrade's plane (and the pips looked the same on his scope). The old problem of identification grew steadily more troublesome and the need for a solution became more critical.
It was the RAF's switch from the defensive to the offensive that really brought the need for an infallible system of identification to the forefront. Realizing that merely guarding their tight island fortress was not enough, the RAF had launched its own offensive. Soon British raiders were making the trip back and forth across the Channel.
The Nazis, foiled and confused by the magic of radar, started dispatching their bombers close on the tails of the returning British squadrons. Unaware that German planes were trailing them, or, in some cases even flying among their formations, the British pilots led the Nazis safely through the defensive radar network unrecognized. The radar operators were unable to distinguish between a pip from the British planes which they were expecting and one from a Nazi, for there was no observable difference in the appearance of either pip. The first indication of the electronic sentry's failure to detect the enemy was the
crash of bombs on factories, hangars, and other objectives. The British defenses were taken completely by surprise. The raiders withdrew before the antiaircraft guns could get into operation and before a fighter could get off the ground. The radar operators were powerless to stop the leak so long as the British carried on their own cross-channel operations.
Faced with realization that the radio locator was totally ineffective in coping with the new tactics, the English radio technicians working feverishly developed and put into production in one short week a type of identification radar, the first IFF unit. This special unit produced an identifying pulse that appeared on the screen along with the target pip. With this new equipment installed in the RAF planes and working in conjunction with land-based and shipborne radar, the operator was at last able to identify the target as friendly by observing whether the additional signal accompanied the target pip. Since the enemy aircraft were not equipped with the special unit, they could immediately be identified as hostile simply by the lack of the distinctive identification pulse. The addition of IFF made radar a relatively trustworthy long-range sentry that could distinguish between friend and foe.
PRESENT UNIVERSAL SYSTEM--MARK III IFF RADAR
The addition to the radar family, IFF, is designated as the "B" group, with different models indicated by a second letter; however, an "A" precedes the two letters on the name plates of airborne equipment. The modifications and improvements on the original system, the ABA (also referred to by the "Mark" designation) or Mark I, have been numerous, but the basic principles of operation are similar. In this discussion attention is centered upon the Mark III, the IFF system in use today.
Equipment and operational characteristics.
Mark III IFF consists of two distinct units. The BL or BN is the set we will consider first. It is really just a "baby radar" with some of the regular radar units missing. It has an antenna, a duplexer, a transmitter, and a receiver, but no modulation generator or indicating device, using instead the radar set's timing system and indicator. The BL is a shipborne or land-based recognition radar placed alongside the search radar. It is the function of this unit to "challenge" the detected target and identify it as either friendly or hostile. Only friendly craft answer the challenge (providing they are equipped with the necessary transponding unit). In the cabinet housing the BL are two separate sections; the interrogator and the responsor. The section that challenges the unknown craft is called the interrogator. As its name implies, it interrogates or questions the units carried by friendly aircraft and surface vessels. The responsor section of the BL intercepts the answers to the challenge.
The interrogator is a low-power transmitter that creates a short-duration pulse of radio energy which is beamed out as a pulse of radio waves. This pulse from the BL or BN is the questioning signal or challenge directed at the detected craft, and although it is slightly wider (i.e., it lasts a few millionths of a second longer) than the radar transmitter pulse, its strength or power is weak in comparison. From your study of basic radar you will recall that it was necessary to send out a very high-power pulse in order to get even a small reflection off the target in return. The "echo principle" is not used by the BL. A different principle is involved. All that is required from the interrogator is enough power to send the questioning radio waves to any craft within the range of the search radar. Of course, the signal must be strong enough when it reaches the target for it to be picked up by a special "receiver" carried solely by friendly air or surface craft.
The BL transmitter transmits on a narrow band of frequencies, the upper end of which overlaps the band employed by air-search radar transmitters. Different bands are necessary to prevent the air-search operation from interfering with the identification. Without this separation a maddening jumble would result, similar to that heard on any radio when more than one station is picked up at the same spot on the dial. That situation is undesirable in radar when one station is the interrogator and the other the search transmitter. The pulse rate of the BL is generally the same as the pulse rate of the radar transmitter, and the two units start to pulse synchronously, since the "keyer" of the search radar that triggers its own transmitter also triggers the BL transmitter.
Invariably the BL is connected to the available model of shipborne radar designed for air search. There are several reasons for the BL being used in conjunction with air-search gear: first, air-search radar can detect both air and surface targets; second, the range of search for this type of radar is longer; and third, the size and construction of the antenna pedestal and framework wake it possible to fasten the directional BL antenna system to the air-search antenna structure (SA, SC-1, SC-2, SC-3, and SK all incorporate the directional BL antenna). The BL
antenna sends out its spurt of radio waves in a wide but somewhat directional beam to the object on which the search beam is trained. Thus the questioning signals are sent out to the craft that has been detected at the bearing toward which the antenna is pointing. The BL radiating section serves to emit the challenging pulse as well as to intercept the answer from the unit aboard the challenged craft which indicates it is friendly. But it is not practical to mount the directional BL antenna array on the barrel stave or spinner antennas connected with surface-search radars. Either a "stove pipe" or a "steering wheel" antenna is used with these latter types, which are non-directional because they are merely single dipoles. The three types are shown in figure 2-1 below.
Figure 2-1. IFF antennas.
The BN is identical to the BL insofar as component parts and operational characteristics are concerned. The main differences between them are in size and power output. The BL is the larger, more powerful unit used with air-search radar on the larger ships; the BN answers the need for a compact, lower power unit that will operate on both large and small craft in conjunction with the surface-search radars, The BN is generally connected to the non-directional type of antenna.
The second section of the BL, the responsor, functions as the receiving or "listening" apparatus; it receives the answer to the challenge. The responsor is a powerful receiving set which is always tuned to the same frequency as the interrogator. By the action of the duplexer, the antenna is switched from the BL transmitter, at the end of the pulse, to the BL receiver, allowing each to work in its turn without interference from the other. The weak reply signal is amplified or strengthened by the receiver into a sizable signal. With the "remote" IFF (receiver) gain control (located on the control panel of the search-radar set) the operator can adjust the amount of amplification or gain to the required level. After the signal is amplified, it is changed or converted into a positive voltage (or negative output which is required by some search radars) available for application to an indicator unit. Since the BL has no indicator unit of its own, the indicator of the search radar must picture the IFF output. In order to make the identification signal visible, the receiver delivers the impulse to the A scope of the search indicator. The positive signal voltage is applied to the lower vertical deflection plate of the C.R.T. Attracted downward by the momentary positive signal, the electron beam is pulled down, tracing a pulse below the time base line. This is an important point to remember: the identifying signal appears below the trace line, and by appearing there, lessens the possibility of confusing IFF indications with target pips which invariably appear above the trace line.
The unit of the IFF communication system that receives the challenge and sends back the identifying pulse or "answer" is termed the transponder. The transponder is most commonly referred to by its abbreviated designations: the BK, or shipborne model, and the ABK, or airborne model. (The BK and the ABK are practically identical in all respects.)
Since the transponder was originally designed for aircraft use only, the ABK-BK is small, compact equipment weighing about 30 pounds. Housed in the cabinet are two sections comprising the receiver and the transmitter. In its operation, the transponder is entirely automatic once it has been turned on. Although it can tell a ship or ground station that the plane carrying it is friendly, it can not furnish the pilot of the plane carrying it the same information about the ships below him. Only the ship or land station finds out that it is a friendly airplane. The BK in your ship hookup is not associated in any way with your own radar equipment. The plane equipped with ABK may not have any other radar at all, but if it should have, it has no connection whatsoever with its IFF unit. The ABK, located in the plane, is connected to a small non-directional antenna which intercepts the questioning signals from any direction. An antenna with this characteristic of picking up signals sent out from any direction is very desirable, for the safety of the craft depends upon its ability to receive the challenges from all points of the compass and send back the answers which appear as the identifying pips on the scope of the search-radar indicator.
The steering wheel dipole antenna is generally used with BK in shipboard installations, and a single dipole is utilized for aircraft (ABK) installations.
The ABK receiver tunes the same wave band on which the BL transmits. If the ABK were tuned to the low end of the band, would it receive the challenge from the BL interrogator transmitting on the high end of the same band? Obviously it would not, and on account of its inability to receive the question, there would be no answer forthcoming. To make your identity known to all interrogators, you must he able to "hear" each of them separately. If only one fixed frequency were used by all BL and all ABK equipment, the receiving of challenges would be like listening to a crowd of people all talking at once. In such a case, many of the questioners would be either indistinguishable or blocked out entirely. To avoid this, BL equipment is tuned at different points on the frequency band. But how can the ABK receiver pick up those different signals if it is tuned to a fixed frequency? Only the challenges from interrogators transmitting at the same frequency would be intercepted.
In the past when you tuned in your radio, you twirled the dial of your receiver from one end of the broadcast band to the other, catching brief snatches of programs until you found the one you wanted. The tuning action of the transponder is quite similar. The tuning mechanism itself is automatically operated by a motor-driven cam, so that the band of frequencies is being continually swept. Each time the tuner reaches the high end of the frequency sweep, the device flies back to the low end and repeats the operation at a slow, constant rate. Approximately two and a half seconds are required to tune from the low frequency to the high frequency end of the band. Another half second elapses while the tuning device flies back to the starting point at the lower end of the hand. This means that one tuning sweep requires a total time of three seconds. Therefore, the receiver is tuned for a short time to each interrogator frequency along the band every three seconds. The ABK receives a challenge at the instant its frequency matches the interrogator frequency, and this match takes place at regular three second intervals (as long as the search-radar antenna remains trained at the target with the BL on).
Whenever the ABK tunes in a EL challenge, it automatically amplifies and converts the signal into a strong voltage, which is applied as a "kick" to the transmitter section of the transponder. The transmitter is normally at "standby," -not functioning- while the receiver is on constantly. As soon as the positive signal voltage is fed from the receiver, it immediately triggers or turns on the transmitter and the receiver is turned off or blocked, This ABK transmitter functions like any other transmitter. It creates a pulse; actually it generates the reply or response to the challenge. The generated pulse is radiated in all directions from the non-directional antenna. This pulse is a low-power transmission, but it is much more powerful than the echo signals of ranging. The pulse rate of the ABK will he the same as the rate at which the BL sends challenges to the ABK. At the completion of each reply pulse, the transmitter rests, and after a recovery period the receiver comes on again to receive another questioning signal; upon receiving the signal, the triggering is repeated. If the receiver picks up no challenges, the transmitter will remain at rest because of the lack of the triggering voltage.
A common tuning section serves to tune the receiver and the transmitter across the wave band. Whenever the receiver is tuned to the frequency of an interrogator, the transmitter (tuned to the same frequency) responds at the same frequency as the challenger's.
Finding that a single IFF response might be imitated by the enemy, the designers of the transponder found a way to make the answers more complicated. With three circuits available for connection to the ABK transmitter, and with the characteristics of each such that the pulse width can be changed, means are provided for coding the replies. When the ABK transmitter is honked up by the cam system to one circuit, the pulse width is seven microseconds, the narrow or short pulse; connection to another circuit produces a 21 microseconds, or wide pulse; the third blocks the transmitter from answering, resulting in the "blank" response or "silent" period. Each time the tuning sweep begins, the cam connects the transmitter to one of the circuits. If the narrow pulse circuit is first hooked in, any challenge received in the three-second period will be answered by a narrow pulse. At the completion of the sweep of the band, the narrow pulse circuit is disconnected by the cam action and the wide pulse circuit is connected to the transmitter; all responses in this three-second period are wide pulses. During the next three-second interval, if the blank circuit is cut in, there will be no responses, for this is the silent or "skip" period.
The three different responses are made into combinations or codes, a code consisting of any four signals, pulses, and/or blanks. Twelve seconds are required to send or complete the code of four characters which are made up into some of the following combinations: narrow, narrow, wide, blank; or narrow, narrow, wide, wide. When the blank is part of the
code, an additional three seconds exists between two pulses,-the skip period. The timing in the first example above would be first, a narrow pulse; three seconds later, a narrow pulse; three seconds later, a wide pulse; six seconds later, a narrow pulse, etc. The code is repeated every 12 seconds, and will continue so long as the antenna points at the target when the IFF switch is on. With six separate codes available, the particular code to be used can be selected by means of a six-position switch on the control box. (The six codes used are highly confidential and will not be described in this book.)
A special response feature is provided on the ABK, -a fourth circuit separate from the coding set of three, that gives an 80-microsecond, very wide pulse. This reply is controlled by a separate switch, the emergency switch which is protected by a guard that must be lifted before the switch can be snapped on. When the switch is thrown to the emergency position, the 80-microsecond pulse, regardless of the position of the code switch, is sent out to all challengers at three-second intervals, and is the signal of distress.
One cycle analysis of IFF operation.
Given an understanding of the makeup of the Mark III IFF system and the function of each unit, let us trace, the operation of the equipment through one complete cycle.
Assume that the operator has detected a target bearing 080 degrees, range 60 miles, and has identified it as a plane. His next step is to determine whether it is enemy or friendly. Keeping the antenna trained on the target, he turns on the IFF switch to put the BL interrogator into operation. He is now in the position of a sentry challenging a trespasser. (Normally the IFF switch is off and the BL is in standby, i.e.: the transmitter is resting and the receiver is effectively disconnected from the ship's radar indicator). The BL pulse and the pulse of the search gear travel out to the target at the same speed. Both beams strike the plane surfaces and reach the antenna of the transponder. Part of the waves from the search radar are reflected, and a small part of the challenge pulse is bounced back, but it so trivial that it dies out almost immediately.
Of the two wave fronts striking the ABK antenna, only those from the BL can he tuned in by the ABK receiver; the ship's radar transmission is above the frequency range of the ABK. (This is presuming that the ABK has tuned to the BL frequency at this instant). The echo from the search pulse has begun its return trip before the ABK receiver succeeds in triggering the transmitter. This delay may consume from three to five microseconds.
(For this example let us assume that the pilot of the detected plane has set his ABK to the code position that would give the response: narrow, narrow, wide, wide). The echo returns to the search antenna where it is intercepted and fed to the receiver tuned to pick it up, the search receiver. The echo signal finally appears as a pip showing above the trace line. At the instant the target pip is completed, the narrow response is intercepted by the BL antenna (the search receiver will reject this signal just as the BL receiver had previously rejected the echo signal due to differences in frequency). The signal, delivered to the responsor, is amplified and then converted to a positive voltage, whereupon it is supplied to the indicator unit of the ship's radar. The trace is pulled downward, forming below the trace line a narrow pulse that is just to the right of the target pip (owing to the longer time required for the two-way IFF communication). The IFF indication is much stronger than the pip from the plane because it is the result of a stronger signal. The downward indication is connected to the pip it identifies, making it possible to associate the proper target with its identifying signal.
The action just described is repeated each time the search transmitter pulses, so long as the ABK is tuned to the BL output. The narrow IFF signal is easily distinguished from the wide, as it appears to be just slightly wider than the target pip of one plane, and when measured along the time base its total width covers approximately one mile of range. After appearing on the screen for about a tenth of a second, the narrow pulse disappears (the ABK has tuned past the BL frequency so it is no longer triggered by your interrogator unit), and not until three seconds later does the IFF signal again flash on the screen. Since the narrow signal is next in the code series, a narrow pulse flashes briefly on the scope, three seconds after the first, and soon disappears, leaving only the target indication.
Three seconds later the wide pulse appears on the indicator. After another three-second wait a wide pulse again flashes downward; it appears for a fraction of a second and then it, too, disappears. Three seconds afterward, a narrow signal flashes, to begin anew the coding sequence. Thus the code is repeated time after time (so long as you continue to interrogate) with each code requiring a space of 12 seconds. As the wide IFF signal is the result of the 21-microsecond pulse, it is about four times as broad
as the target pip of one plane and is roughly two times the width of the narrow IFF pip. In other words, the wide indication measures almost two miles in width along the trace line. (All measurements are made from the leading edge of the signal to its outer edge, and it is a good idea to make an estimation to determine these values because the pulse is on the screen for such a short time). A person experienced in operating radar and observing the IFF codes can identify the code appearing on
Figure 2-2. Block diagram of a typical radar system with IFF.
the indicator in at least 24 seconds; the time of two coding periods.
Both the Germans and the Japanese have had some luck in building an airborne machine that triggers off from the challenges of our interrogators. The first models gave only the one-size signal in answer. With our ARK-equipped aircraft giving any one of a possible six codes, the operator may encounter little trouble in detecting which was the false response of the hostile planes. By establishing specific periods of operation for each of the six codes, the system of frequently changing the password minimizes even further the possibility of the foe hitting upon a signal that might he confused with the friendly combination. For instance, from 2400 to 0600 all surface craft and aircraft in a specified area will set the BK and ABK code switch to Code No. 3, and all operators of search radars are instructed to report all but the No. 3 response as unfriendly during the same period. Then, during the hours 0600 to 1200 Code No. 6 will be the selected password; thus, throughout the day, the acceptable answer is varied. (The radar officer keeps the men informed of the code to be accepted for any corresponding hour). The system of changing codes throughout the day or week is similar to changing passwords regularly to reduce the possibility that an enemy will discover the password and get by a sentry.
As soon as a target is identified as friendly, the IFF switch is turned off and the identifying signals cease, for the ABK is no longer being triggered. To continue to challenge a plane or ship is undesirable after it has been identified, especially near hostile areas, for the IFF signals travel over long distances and continuous questioning of one plane might enable the enemy to pick up the signals in the powerful receivers that are constantly combing the air for our transmissions. The B!. should he kept in a standby condition, ready to challenge when the IFF switch is turned on.
The following are, in brief, the main characteristics which aid in recognizing an IFF signal on the A scope:
It appears below the time base.
It is a stronger or longer indication than the target blip.
It is a broader signal than a one-plane target pip.
It is to the right of the target it identifies.
It appears at regular or periodic intervals and remains on the scope for only an instant.
IFF will also appear on the PPI indicator, but identification is more difficult if it is used instead of the "A" scope. As illustrated in figures 2-3, 2-4, and 2-5, it appears as a bright spot, or spots, depending on the antenna speed and ABK code used, and will always be outboard of the target indication.
Failure to get a response from any target that is challenged generally indicates that the target is hostile. This is not always reliable, however, due to ABK failures, or the chance that the unit may be turned off. From this fact, it can be seen how vital trouble-free IFF operation is to our ships and planes that rely on the BK and ABK to identify them to other ships and ground stations. While interrogating a target, it is possible to receive other IFF signals from the bearing at which the antenna trains without detecting a target pip. This is due to the longer effective range of IFF.
Reporting distress with IFF.
If the target being challenged sends back a very wide response that flashes below the trace every three seconds, it is the emergency or distress signal. However, before reporting the signal as an emergency, it is a good practice to cheek to be certain that it is the extremely wide distress signal and not the wide pulse of the coded combinations. One reliable method of checking is to measure the distance that the IFF indication occupies on the time base. Since the ABK sends out an 80-microsecond wide pulse on emergency, the signal will pull down about seven miles of the trace line. Comparing this seven-mile wide signal with the two-mile wide signal of the code series, you can see that there is no excuse for mistaking the true distress signal. Not only is the emergency pulse about four times as broad as the wide reply, but the distress signal also appears at the regular three-second interval,-a succession of very wide signals.
Therefore, if you observe a wide pulse as the first IFF response and are in doubt about its being the distress signal, continue to watch the replies closely for nine seconds. If three wide pulses appear in succession, it is the real thing, the emergency signal, for the six codes are made up in a manner that makes it impossible for the 21-microsecond reply to appear three times in succession (it can appear twice, and then some narrow pulses are thrown in). It is a good practice to make one or both checks before reporting the signal as "distress," It must meet these specifications: (1) at least a seven-mile wide pulse, and (2) pulses
flash on the screen three seconds apart with no variation in width.
Second only in importance to identification is this application of the Mark III IFF system to the transmission of distress signals. Whenever a pilot is in trouble and finds that he must make a forced landing at sea or in a trackless jungle, he cannot turn on his radio to send out an S.O.S. to his home base, if he is in a war zone. He does not want the enemy to intercept his distress call. Generally, he does not know what his exact geographical location is so his message would be of little help in any event. But by using the emergency position on his ABK he can show his location to the search-radar operator. Consequently, he switches to Emergency" on the ABK control box. Very likely a radar operator somewhere is searching the horizons. Detecting the plane, he turns on the IFF to identify it and on the screen sees the extremely wide signal flashing below the trace at three-second intervals. Quickly reporting the bearing and range of the emergency signal, the operator continues to track and report the plane until both it and the emergency signal disappear from the screen. The rescue plane or ship is promptly dispatched to the last reported hearing and range position near which the plane has probably crashed. The use of IFF to indicate distress is responsible for the rescue of innumerable pilots and crewmen of disabled Allied planes and ships.
The emergency signal is often the indication that a plane has lost its way. When the distress signal is picked up at a land station, the information is reported to the nearest air base. The apparently aimless course indicates that the pilot has lost his bearing and communication silence may be broken in an attempt to contact and give directions that will set him back on his course. In ease all attempts to establish contact by radio fail, a plane is dispatched to the lost plane's position to lead it to safety.
Using the emergency signal to signify anything unrelated to distress may have its disadvantages. In recent operations a carrier task force decided to use the signal as the code for "the enemy has broken through our outer battle line." Considerable confusion developed when the pilot of one of the patrolling aircraft threw on the emergency switch because engine trouble was forcing him down. Unaware that the signal was just the ordinary indication of distress the force was hastily prepared for an attack that never materialized.
When you are identifying one target, an emergency pulse may appear on the scope at some greater range without the target being revealed by a pip. But you can range on the signal by judging where the leading edge of the pulse appears on the time base and reporting that figure along with the bearing to the C.I.C. The target can be plotted from the reports on the movement of the signal as it flashes at different ranges along the time base, with the range always figured to the point at which the pulse starts to pull down. Pointing the antenna to maintain maximum strength of the emergency signals provides more accurate bearing information on the craft in trouble.
Always be on the alert for the emergency signal, for prompt and accurate reporting of this distress appeal may be the means of saving many lives.
Necessity of safeguarding IFF security.
Because we are entirely dependent on the ABK-BK equipment to determine which of the ships and planes detected by radar are friendly (at long range), it is of vital importance to keep the apparatus or any information about it from falling into enemy hands. A similar unit installed in enemy planes and ships could deceive us completely and would render our own radar entirely unreliable.
Security of information must he strictly maintained to guard against such a possibility. IFF equipment is even more confidential than the rest of radar materiel. The regulations applying to strict observance of the rules of security regarding radar must be emphatically followed in matters pertaining to IFF. Certain parts of the ABK related to the coding operation and the code itself, while classified "Confidential," are treated in much the same way as "Secret" material.
The ABK is safeguarded by measures similar to those taken to protect the bombsight. It is removed from the plane at the completion of the flight and placed in a guarded vault for safekeeping. If it should be necessary to leave the unit in the planes, an armed guard must be maintained over the craft carrying the equipment. Even then, certain confidential parts without which the unit cannot operate are removed from the ABK as an added safeguard.
The destructor: the safeguard against enemy possession.
But there is still the problem of maintaining the security of the equipment when planes fly over enemy
territory. There is always the possibility of aircraft being forced down or shot down with the ABK itself still in an undamaged or repairable condition. To guard against this contingency, a detonator is installed in every ABK unit. This detonator or "destructor" is an explosive charge inserted into the side of the cabinet. When exploded, it can destroy the insides of the unit so completely that it is impossible for the enemy to determine what the resulting wreckage originally was. Composed of thermite, the charge can be set off without rupturing the case, and though the terrific heat melts everything already blown up, it cannot injure the pilot who sets it off nor can it damage or set fire to the plane. It does a thorough job of guarding the security of this important IFF unit.
The designers made certain that the detonator would function regardless of the fate of the crew of the airplane. The pilot has two switches to push to explode the demolition charge if he must land on hostile ground, so that the foe is rewarded only with a picture of destruction when he attempts to investigate IFF. If all the crew are killed, an impact switch does the job for them. It sets off the destructor when the plane crashes. The greatest care must be taken when handling the detonator itself, for this small bomb can spell doom if carelessly handled outside the equipment. Numerous safety precautions are taken to prevent the ABK from being accidentally destroyed, as for example. in a rough landing. If the impact switch were not disconnected immediately before landing on the home field or on a carrier, such a landing would set off the charge.
ADDITIONAL USES OF IFF
By taking advantage of the adaptability of the ABK, the air forces have extended the range over which radar search information can be gleaned. In using only one of the six codes to indicate friend or foe, and assigning the remaining live to meanings of a special nature, the airmen scouring the seas for the enemy can relay invaluable information without informing the enemy. Suppose that Code No. 2 is to signify "sighted sub," Code No. 3 "sighted convoy." Code No. 4 "sighted enemy battle force," Code No, 1 "normal operating position-okeh-friendly, etc." The radarman on the carrier or land station tracks the patrol planes with the interrogator turned on. After he loses the target pips, he continues to receive the No. 1 coded signal, possibly from beyond 100 miles, which tells the operator that there is nothing to report so far. But as soon as one of the pilots sights, let us say, a convoy by visual means or with his own radar, he switches the ABK to Code No. 3. The search operator observes that he is no longer getting No. 1 response; instead, he recognizes the new code as No. 3. The bearing and range of the significant signal is reported at once. Long before the search radar could detect the convoy's approach, the ship has obtained useful information, and transmission of this advance word has been accomplished without breaking radio silence.
Reference was made previously to the possibility of following a plane's or ship's progress by observing the IFF signals changing bearing and range. In plane-tracking operations, the target pip often disappears, and in spite of all the operator's efforts, he is unable to bring the antenna to bear on the target with sufficient energy to produce a pip. Consequently, he must report that he has lost the target.
But the operator could have turned on the IFF and checked near the bearing at which the pip faded out. It is possible that the radarman could again have found the target pip when he trained the antenna to get the strongest IFF responses. In case the target pip failed to reappear, due to the extreme distance or adverse conditions, the operator could continue to track the aircraft by following the IFF signals.
By checking the range of the nearest edge of the IFF signal on the time base, an approximate range on the target can be obtained; bearing readings are taken when the pulse pips flash at maximum strength. Therefore, the distances over which a plane can be tracked are greatly extended, and the likelihood of losing the target is materially reduced when IFF is used as a tracking aid.
IFF has its fade zones in which the signals disappear, just as in the case of air-search radars. So continue interrogating, a "bogey" may become "friendly" when he gets out of an IFF fade zone. IFF fades, however, do not occur in the same places as the fades of the associated radar. Consequently, the target may be fading while the IFF response is very strong; next, the target signal may give a strong indication while the IFF has faded out completely.
The ABK transponder is the universal IFF unit used by the Army, Navy, and Marine Air Forces and by the Allied Navies and Air Forces as well. Needless to say, the chief use of IFF is for identification of ships and planes. In our all-out anti-submarine campaign in the Atlantic. it was extremely useful when ships were detected by radar and later sighted by lookouts, the appearance of the low-lying ship superstructures resembling surfaced submarine conning towers. Any indecision as to identity was dispelled as soon as
the target was challenged by the BL. The absence of the IFF response was the signal to press home the attack. This reliable system saved our forces afloat from the confusion that often brings about self-inflicted losses. IFF has been especially helpful to our own submarines and PT boats, enabling them to operate with some assurance that they will not be attacked by our destroyers.
The ABK transpondor is the most carefully and frequently checked and tested of all the radar equipment. It is inspected after each flight to make certain that it is in perfect condition. Parts of the set, such as the tubes, are replaced at the end of relatively short operating periods to insure against equipment failure due to old or worn-out parts. In spite of all the precautions taken to prevent ABK failures, the unit does develop troubles that render it inoperative. There is no visual indication that the equipment is not operating. However, there is an outlet jack into which earphones can be plugged. All the pilot (or operator aboard ship that is being challenged) hears is noise of the transmitter turning on and off, providing that the set is being triggered at the time. Yet there is always the chance that silence in the phones indicates that the unit is not being challenged, rather than not working due to some failure.
The BK or ABK must always be checked when there is a possibility of being challenged by one of our own planes or ships. Causes of trouble and failures in early models of the ABK have been studied, and modifications in the later sets have eliminated many of the shortcomings. Since the Mark III IFF system is composed of special vacuum tube circuits and mechanical gadgets that are all subject to failure, the chances are that only about 80% of all our ABKs function correctly all of the time. That means that what we sometimes think is an enemy contact is actually friendly. There is also the possibility that the transmissions of transpondors or interrogators may be blocked from some directions by either ship or aircraft structure.
In the Southwest Pacific, according to official reports, our naval gunners have mistakenly shot down Liberators, Mitchells, and Lightnings that failed to show IFF. In the Mediterranean theatre, our antiaircraft gunners have shot down troop laden airplanes through some IFF failure. We also have reports of U. S. Destroyers attacking our own PT's, and PT skippers have, on occasion, launched torpedoes at a friendly destroyer. Tragedies such as these have been variously attributed to equipment failure of the ABK units, failure of the interrogators, failure to turn on the ABKs, or no IFF installation.
Conditions of poor visibility due to weather conditions or time of day prevented the lookouts from correcting mistaken decisions on the status of sighted objects. Enumeration of these unfortunate incidents gives some idea of how important it is to have the IFF system working properly.
LIMITATIONS OF MARK III IFF
So far the IFF has seemed to be an easily understood, smoothly operating piece of equipment, and so it is, if all of the parts work as well as they appear to on paper. This unfortunately, does not always hold true. Because of the obvious value of identification radar, it is important to understand thoroughly its capabilities and its limitations. It must be constantly borne in mind that Mark III IFF is an aid in determining friendly or enemy character of a target and cannot be relied upon to give 100% identification.
Non-directional interrogation and range identification.
A few examples will serve to show just how much you can depend on all indications given by IFF of a friendly radar contact. First, assume that you have discovered two surface contacts at the same range (8,000 yards) but on different bearings on your radar. One is friendly, and the other is enemy, but unfortunately you do not know that. The fact that all surface-search radars use the non-directional IFF antennas leads to further trouble. If you stop your radar antenna on the first contact, and challenge it with your IFF, you will get a response which will tell you that it is friendly. Then you rotate the radar antenna to the second contact, challenge it, and get a second friendly response! Figure 2-6 will show you what happened.
When you stopped your radar antenna on the enemy ship 8,000 yards away, you got an echo on your scope at 8,000 yards. You also received an IFF response from the friendly ship 8,000 yards away, which led you to think that the enemy ship was friendly. Of course, when you pointed the radar antenna at the friendly ship you saw its echo at 8,000 yards on the "A" scope, with the same IFF response at 8,000 yards. Remember that this
condition can exist, and avoid being too quick to report two friendly contacts when it arises.
Should you have a friendly and an enemy ship at the same range when using the non-directional interrogator antenna, several checks on the position of the IFF signal in relation to the target pips will reveal which is friendly and which is enemy. The IFF response will always appear immediately to the right of the target pip. Since the possibility of a friendly and enemy ship maintaining the same range relationship to you over a period of time is quite unlikely, the pip which becomes separated from the IFF signal is the enemy ship. If there is no BN connected with your surface-search radar, you must depend upon your air-search radar and its associated IFF system to challenge the surface contacts.
Wide beam of BL and faulty identification.
Though designed as a directional antenna, the directivity of the BL array is limited, and it is to some extent subject to the limitations of non-directional interrogators. It follows, then, that some of the problems encountered with the non-directional system may also be a source of confusion when using air-search radar with the directional interrogator. Inability to identify positively two air contracts at identical ranges may result, for it is not unusual to observe IFF responses for a full 360 degrees, especially at short ranges. The limited directional characteristics are such that replies should be expected throughout a wide variation in bearing, especially below 12 miles range in the case of surface targets and below 50 miles range in the case of air targets.
Consider another confusing situation: assume that you have picked up a surface contact at 35,000 yards, and wish to challenge it with your IFF. If you have no 13L or BN connected with the surface-search radar, you must depend on the IFF equipment connected to the air-search set to do the challenging. The IFF band of frequencies lies just below that band used for our air-search sets, so you may expect the radiation characteristics of IFF radar to be very similar to those of the air-search sets. This means that radiation from IFF radar will not lie close to the surface of the water. Therefore, when you challenge the contact at 35,000 yards, it is very probable that no friendly indication will appear on your radar at that range even if the contact is friendly. The contact is unidentified until it closes to a range at which enough of your BL energy reaches the ABK to trigger it. That range may be as
short as 10,000 yards depending on the following factors: (1) height of the transpondor antenna above the surface of the sea, (2) materiel condition of both transpondor and interrogator, and (3) radiation pattern of the interrogator antenna in the vertical and horizontal planes.
As was pointed out earlier, however, you may get IFF responses from airplanes at greater ranges than you can see the actual echo from the plane for two reasons: first, the IFF radiation is directed upward from the water, just as air-search radar radiation is, and second, the power originates in an ABK, while power is only "reflected back" to our search radar after the search pulse hits an object. Inasmuch as the power transmitted by an ABK is greater than that reflected from the plane carrying the ABK, it is possible for the coded IFF response to appear below the trace even though the search radar has not yet indicated the presence of a target.
"A sense of false security must not be permitted because many friendly blips from a group of planes can be seen on the radar screen. Enemy planes may trait such a group and attempt to take advantage of their transpondors to press home an attack. It is also possible for blips from friendly aircraft to conceal enemy surface craft."
360 degree IFF resulting from back and minor lobes.
Because of the wide horizontal beam, back radiation, and prominent side lobes of the BL antennas, you will find that a contact at short range can be interrogated almost continuously as the antenna rotates through 3600. Under these conditions interrogation is non-directional and subject to the same limitations described under non-directional IFF. It is often possible to reduce the power of the interrogator until only, the major lobe of radiated energy has sufficient power to activate the transpondor. This can be done by turning down the plate voltage of the BL until you get an IFF signal only from the direction of the contact. As soon as you have finished the interrogation, turn up the voltage to its original setting.
Red and green band triggering.
For some reason, the Mark III IFF system was designed to operate in a band of frequencies that slightly overlaps the lower end of the air-search radar frequency band. This is illustrated in figure 2-7. The result is that a radar operator is placed in a difficult situation under certain conditions. With an air-search radar set tuned to any frequency within the red band, it is often possible for the radar itself to trigger the ABK every time the antenna points in the direction of the transpondor. The result is an IFF indication flashing above the trace on the "A" scope continuously, even though the BL is turned off. When the ABK is out of adjustment, it may overlap into the green band of frequencies as well (indicated by the dotted line on the graph), affecting green band radars as well as those in the red band. Moreover, if you challenge the target with your BL you can expect to receive the usual Mark III indication below the trace, but not necessarily at the same time as the upward deflection. That gives three separate indications on the scope: the regular echo, the upward IFF signal, and the downward IFF signal. The latter two may or may not appear at the same time, depending on the relative frequencies of the search radar and the BL radar, which determine, of course, the time that the ABK will respond to each set. The situation is serious when a number of friendly and enemy planes are flying fairly close together, since the "A" scope becomes a mass of bobbing echoes and downward IFF signals that have no apparent relationship to each other.
We have had an increasing number of reports from the Pacific Area stating that a response resembling the indications given by the obsolete Mark II IFF has been received. Authorities believe the Japanese are using the compromised ABE, since it puts the IFF response above the trace line. Hence the "positive" (above the time base) signals are to be regarded as no identification whatever-not as foe identification. IFF signals below the line identify "friendlies" whether or not accompanied by IFF above the line.
Figure 2-8. IFF interference.
IFF interference and code readability.
Since the ABK is able to receive challenges from any and all directions, there may be several dozen interrogators challenging at once. Yet the ABK is equal to this situation, for it can answer up to 30 different challengers at one time without interference. Special provisions have been made to prevent ABK's operating near each other from triggering one another should their frequencies coincide.
On the other hand, imagine how many IFF signals are received when challenging several planes in a group or a large formation of planes, all with their ABK's on. The whole scope will appear as a flashing, confusing muddle, making it impossible to distinguish the signal of any one of the planes or to make out the code. Actually, codes are almost meaningless because each answering ABK will respond at a slightly different time and all codes will overlap. Under these circumstances the radar operator can at least recognize that responses consist of all narrow poises, a combination of narrow and wide, or emergency signals.
IFF signals too narrow on long range.
Selection of proper range scales during interrogation is another problem. It is exceedingly difficult to distinguish the IFF signals when the long-range scale is being used. The narrow IFF indications especially, are so compressed on the long range scale that it is next to impossible to tell an IFF indication from BL Pulse interference. Due to the thinness of the IFF signals on this scale, considerable difficulty may be encountered in reading the codes, provided, of course, that it is possible to distinguish the IFF from the flashing interference. It is wise whenever possible to use the mid-range scale or the short range for identification of contacts. Either of these scales gives better definition to the IFF signals; hence, identification is more reliable and less difficult.
It is necessary to be especially alert when challenging a target, in order to recognize any or all of the confusing situations discussed in this section. In spite of the limitations of IFF radar, however, it is our only system for long-range identification of radar contacts. Combining a realization of its limitations with an up-to-the-minute understanding of the tactical situation will support the system so that satisfactory results may be obtained.
In the future, IFF will continue to serve our forces afloat and airborne, with increasing reliability, and in valuable capacities not hinted at in this manual. Thorough understanding of this phase of radar, and intelligent, alert interpretation of IFF may well be responsible for saving a ship or winning an important battle.
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