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CHAPTER IX
Radar Recognition Systems--IFF

A.Introduction: Definitions; recognition methods; development of IFF.

B.Operation of an IFF system: Definitions; principles of an IFF system; transpondor; interrogator-responsor; G-band operation; destructors; Mark IV IFF system; Mark V IFF system.

C.Limitations of the IFF system: Operational; propagation; resolution; matériel.

A. INTRODUCTION

1. General.

The friendly or enemy character of a radar contact cannot be determined from the appearance of the echo alone; some auxiliary means for recognition or identification must be provided. That this problem is most urgent is brought out by the following quotation from the War Instructions, United States Navy (FTP. 143):

"516. A most important factor in war is positive recognition and identification of friend or enemy. Through lack of thorough training, absence of coordination, and due to misunderstanding, friends lose their lives or the enemy is permitted to escape. It is essential that personnel receive proper indoctrination on this subject."

The terms "Recognition" and "Identification" are defined by the War Instructions as follows:

Recognition: The process of determining the friendly or enemy character of others.

Identification: The process of indicating your own friendly character.

Several methods of determining the identity of contacts are in common use. The basic means of recognizing ships and aircraft is by familiarity with silhouettes and markings. Recognition of radar contacts sometimes can be effected by coordination of radar data with reports received from observers at a distance who can see the target. A contributory aid to recognition is the maintenance of a continuous plot of both the ships in company and own planes, utilizing both radar and visual observations. Recognition is possible by means of a process of elimination if definite knowledge is available on the location and expected course of action of all friendly units, including aircraft, in the area of operations. But, to quote the War Instructions again, "When a friend approaches whose movements are unknown to the personnel charged with recognition and identification, command has failed in an important function. Therefore, it is requisite that responsible commanders disseminate within and without their commands adequate information of expected movements and the locations of friendly units who may possibly contact each other. However, meeting a friendly ship at an expected time and place is no assurance that an enemy ship is not also in the vicinity. This is particularly true of submarines. Relying too implicitly on advance information is unsound and not an acceptable substitute for effective recognition." In some cases, a vessel identifies itself by a simple, coded radiotelegraphic transmission to a direction finder system. A submarine frequently identifies itself by keying its radar transmitter to produce coded interference on the radar indicator of another submarine.

All of these methods involve considerable coordination, and consequent time delay. Hence, it has been found essential to provide means of direct recognition at the point where the target is detected by radar. Various systems have been developed whereby aircraft and surface vessels are provided with equipment which allows them to establish their friendly character, either directly to the primary radar equipment or to additional apparatus associated with the radar. Such systems are known as IFF (Identification, Friend or Foe), although the definitions stated in the War Instructions indicate that IFF should be called a recognition system.

2. Development of IFF.

The development of IFF began almost as soon as the first radar set was built. In one of the earliest experimental forms a simple dipole antenna was installed in an aircraft or surface vessel. The dipole was resonant to the radar frequency, and was switched so as to produce regular fluctuations in the size of the echo received at the radar. So simple a system was soon found to be inadequate due to such factors as its uncertainty, the introduction of new radar equipment using different frequencies, and the persistent demands for recognition at greater range.

These difficulties were in part overcome by the successive introduction of IFF Mark I and Mark II. Both of these systems employed a combined receiver-

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transmitter by means of which airplanes could identify themselves to radars. The set normally was in the receiving condition but when energized by the receipt of a radar signal, it broke into oscillation and became a transmitter. The signal emitted from the identification equipment was radiated to the radar station together with the normal echo from the target, and the echo was thereby distorted in such a manner as to make recognition possible. The tuning of the set was mechanically swept through the bands of radar frequencies then in use, so that the recognition of targets on any radar was aided by the reception of periodic identification signals as the receiver-transmitter set tuned through the radar frequency.

Radar equipments now operate on such a large number of widely separated frequencies that it has become impracticable to produce a single IFF set capable of tuning to all of them. To provide an adequate identification service operating in this manner it would, therefore, be necessary for aircraft and ships to carry simultaneously several different types of IFF sets. Further, it would be necessary to introduce additions and modifications to this equipment each time radar equipment on a new frequency was introduced.

Such increases in the amount of equipment carried, particularly in aircraft, could not be accepted. The difficulty has been overcome by the introduction of a universal frequency band for IFF, separate from that of the radar equipments on which the echoes must be recognized. In this manner, though the need for extra equipment still exists, it is possible to save installation of several IFF sets in each aircraft by the expedient of fitting auxiliary apparatus to the radar equipment in the ship, where considerations of weight and space are in general of less critical importance.

B. OPERATION OF AN IFF SYSTEM

1. Definitions.

The components which make up an IFF system are as follows:

  1. Interrogator: A radio transmitter which emits signals on some frequency in the IFF band, and which is associated with the radar equipment whose echoes must be recognized.

  2. Transpondor: A combined receiver-transmitter, fitted in all friendly aircraft and ships, which receives a challenge pulse from an Interrogator and automatically returns a signal on the same

Figure 152--Block diagram of IFF systems
Figure 152--Block diagram of IFF systems.

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Figure 153--Tuning of A-band transpondor
Figure 153--Tuning of A-band transpondor.

    frequency (or a different frequency, depending on the IFF system in use). The form and duration of the reply signal are controlled by a coding system.

  1. Responsor: A radio receiver, associated with the radar equipment, which receives the reply returned from the Transpondor and produces an output suitable for feeding to a display system. The Responsor is usually combined into a single unit with the Interrogator.

2. Principles of an IFF System.

A block diagram of an IFF system associated with a radar system is shown in figure 152. Both the radar indicator and the interrogator are synchronized from the radar transmitter. Thus the interrogator pulse and the radar pulse are transmitted nearly simultaneously. The IFF antenna and the radar antenna use a common reflector in the case illustrated, so that the two pulses are radiated from the same place. The radar echo from the ship is shown at X on the indicator and the echo from a second target, which is not illustrated, appears at Y. During the same time that the radar pulse is travelling out to the target, the interrogator pulse goes out and actuates the transpondor on the friendly ship. The transpondor response then returns to the antenna along with the radar echo. As a result, a negative pulse appears on the lower sweep almost directly under the echo pulse. Since the target at Y does not have a transpondor, it cannot identify itself as a friend. However, it must be realized that the lack of an IFF response is not a clear indication of the enemy character of a contact. The target at Y, then, is not necessarily an enemy, because the transpondor may simply not have been turned on, or the equipment may be inoperative.

3. Transpondor.

A transpondor consists of a receiver, a transmitter, and an antenna. The transmitter normally is quiescent and the receiver ready to receive. When an interrogating pulse is received, it is amplified many times. This large pulse is used to trigger the transmitter, causing it to reply to the interrogation on the frequency to which the transpondor transmitter is tuned at that instant. Circuits in the transmitter control the character of the pulse sent out, so that the interrogating pulse serves only to start the action. The action of the transpondor is entirely automatic; it needs no attention from operating personnel after it has been turned on. Whenever a pulse is received, the set automatically transmits a reply; the transpondor may be regarded as a pulse repeater or a pulse amplifier. A weak pulse is received and this device then automatically sends out a strong pulse with almost no delay. As a result, the IFF reply usually is much stronger than the radar echo, because the echo contains only a small fraction of the power of the transmitted pulse which is reflected back toward the radar.

The Mark III IFF system is in use at present throughout the world by all the United Nations. The transpondor that is used on board ship in the Navy is called the BK. An identical transpondor for use in aircraft is termed ABK, and for use by the U. S. Army, SCR 595. Other aircraft equipments that incorporate a Mark III transpondor are called ABF, SCR 695, or AN/APX-1, and AN/APX-2.

The Mark III IFF frequency band is called the A band, which is a 30-megacycle section of the P band of frequencies. Another frequency band, called the G band, is also used in the Mark III system for the recognition of fighter planes.

A Mark III transpondor is tuned mechanically over the A band in 2.5 seconds, and approximately one-third of a second is required to reset the tuning adjustment. Thus the time required for one cycle of operation of the transpondor is a little less than three seconds, but the transpondor is operative during only the 2.5-second interval of the forward sweep. This condition is indicated by the heavy lines in figure

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153. During the flyback time, which is indicated by broken lines, no response can be returned.

Because of the nature of the transpondor tuning, a reply is returned at intervals approximately 3 seconds long. For example, if the transpondor is challenged on some frequency within the A band, such as X in figure 153, a reply will be returned every time that the set is tuned to this frequency. As a result, replies will be transmitted at A, B, C, D, and E, which are at intervals of 2.9 seconds.

The length of time that the transpondor is tuned to the interrogating frequency determines the duration of the interval in which replies will be returned in each cycle. However, the characteristics of the receiver in the transpondor are such that it can be actuated for a longer interval by a strong interrogating pulse than by a weak one. As a result, replies are returned for a relatively long time at short ranges, while at long range the reply is visible for only a brief time.

In the Mark III system the transpondor reply is coded by means of varying the width of the pulses transmitted on four successive frequency sweeps. A complete cycle of the code, then, is completed in a little less than 12 seconds. The elements that are used to make up the code are a narrow pulse, a wide pulse, and a blank, which, is a sweep during which a response is not returned. The narrow pulse appears approximately 0.6 mile wide on the indicator, and the wide pulse approximately 1.5 miles in width. Thus, in figure 153, the responses may be:

  1. During the first sweep, a narrow pulse at A.

  2. During the second sweep, a narrow pulse at B.

  3. During the third sweep, a wide pulse at C.

  4. During the fourth sweep, no response at D, or a blank.

The fifth sweep duplicates the first, so that the code cycle is repeated.

The six codes that are available with the Mark III IFF are:

  1. Narrow, narrow, narrow, narrow (NNNN)

  2. Narrow, blank, narrow, blank (N-N-)

  3. Narrow, narrow, narrow, blank (NNN-)

  4. Narrow, narrow, wide, wide (NNWW)

  5. Narrow, blank, wide, blank (N-W-)

  6. Narrow, narrow, wide, blank (NNW-)

Note that the codes are so selected that each one is distinctive. No code can be mistaken for any other, no matter at what point in the cycle the response is first observed, provided the range scale is such that it is possible to distinguish between narrow and wide pulses. On a 200-mile sweep, for example, the distinction between a pulse that is 0.6 mile wide and a pulse 1.5 miles wide may not be apparent, making it difficult to read the code. Note also that there are no codes that use only wide pulses; the narrow pulse is fundamental in this system, and it is always included in the sequence for comparison.

The transpondor is capable of returning a very long pulse as a distress signal. This pulse, which appears approximately 9 miles wide on the indicator, is very distinctive and should be readily recognized by the operator. When the switch on the transpondor control box is thrown to "EMERGENCY", the coding mechanism is by-passed so that the very wide pulse is transmitted on every sweep of the transpondor, regardless of the code that has been selected previously.

4. Interrogator-Responsor.

In the Mark III IFF system the responsor is tuned to the same frequency as the interrogator. The frequency of the interrogator can be set by the technician to any point within the A band, but in general, once the frequency is set, there will be little need to change it. The interrogators of several ships operating together should be spread as wide apart over the band as possible to reduce interference and to avoid overinterrogation of the transpondors, since these sets are limited in the maximum number of responses that they can return. The responsor is a conventional radar receiver which should require little attention from operators aside from a periodic check to insure peak tuning.

The antennas in use with Mark III interrogators may be either omni-directional, and therefore fixed, or directional and mounted on the same reflector as the antenna array of the radar with which they operate. Because of the low frequency used with this IFF system, the antenna is not highly directional.

The interrogator-responsor may sometimes have its own indicator, but in almost all shipboard installations in the Navy, Mark III IFF information is displayed directly on the radar scope. In general, the responsor is so connected to a type A indicator that IFF responses produce downward pulses, while the radar echoes produce upward pulses on the screen. In older systems, both pulses were deflected from the same sweep, but in more recent designs a separate sweep is provided for the IFF display. The double-sweep type of display, which is illustrated in figure 152, retraces the echoes perhaps three times and the IFF responses once out of each four successive pulse repetition intervals. The advantage of the separate sweeps is that neither trace interferes

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with the other, and the picture therefore remains more clear.

Interrogators are generally fairly small equipment. Therefore, in order to get a reasonable amount of power out, they are usually operated at a low repetition rate. This allows the transmitting tubes to be very greatly overloaded during the pulse, but it allows them a long time between pulses to cool, so that no harm is done. When the radar operates at a high repetition rate, it is necessary to divide the pulse rate by a factor of two, three, or four to operate the interrogator properly. Even though the interrogator may be pulsed only once for every four pulses sent out by the radar, the interrogator pulse must be in synchronism with the radar pulse for satisfactory operation. Most interrogators employ some sort of counting-down circuit to meet this requirement.

In most existing radar equipment the counting-down function is performed in the interrogator itself, but in new radars an auxiliary device called an IFF Coordination Unit is provided for this purpose. The IFF Coordination Unit also makes possible the adjustment of the inherent delays in the system so that IFF responses will appear at the same range as the echo from the target carrying the transpondor, and it supplies the means for producing the second sweep on which IFF responses are displayed.

The shipboard interrogators for use on the A band

Figure 154--Time-sharing between A and G bands
Figure 154--Time-sharing between A and G bands.

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of the Mark III IFF system are designated BL, BM, and BN. The BL is for use on large ships, and the BM is an improved version incorporating higher power and other advancements for the same purpose. The BN is a small, medium power equipment designed primarily for use in small vessels, and as an auxiliary interrogator in large vessels. A fourth type of shipboard interrogator is the BO, which is designed to be used in conjunction with a radar used in fighter direction to interrogate on the G band only. Airborne interrogators are the Army SCR 729, which formerly was called ABL by the Navy, and the AN/APX-2.

5. G Band Operation.

The normal A-band IFF does not show up clearly on a PPI because there is no way of insuring that the transpondor will be tuned to the interrogator at the instant that the radar antenna is pointed at the target. If an omnidirectional interrogator antenna is used, IFF responses may be received on any bearing, but always at the correct range. Thus if several targets are present on the screen, it will be very difficult to recognize any of them by their A-band IFF responses.

In the control of airplanes from ships it is very necessary to be able to recognize the friendly planes so that a successful interception can be made. To aid in this respect, a special IFF channel on a higher frequency than the A band is provided for fighter planes in addition to the normal A band. This special channel is called the G band, and it is provided in the ABF or AN/APX-1, and the AN/APX-2 IFF equipments.

These transpondors normally operate in the conventional manner on the A band. But, whenever the pilot is directed by voice radio to identify himself, he simply pushes a button that causes the set to operate alternately on the A band and G band for as long as the button is held down, and for a holding period of 10 to 20 seconds after it is released. The transpondor may also be made to alternate continuously between the A and G bands if it is so desired. By this system, the G band is made operative for approximately one-twentieth of a second and the A band blanked. Then the A band is made operative for approximately three-twentieths of a second while the G band is blanked. The two bands share the operating time, then, in the manner illustrated in figure 154A. The holding period that follows opening of the G band switch is illustrated in figure 154B. At time 1 second the switch is thrown off but the mechanism is such that timesharing continues for several seconds afterward--in this case 12 seconds.

Because of the time-sharing operation, the G-band response is not absolutely continuous in spite of the fact that the tuning does not sweep. Instead, the reply is chopped, or broken, at the time-sharing rate. Thus if G-band responses can be obtained over a 30° arc and the antenna is rotating at 5 revolutions per minute, the responses will be visible for one second. In this time the G-band transpondor will be turned on and off five times, causing the IFF

Figure 155--G-band IFF response
Figure 155--G-band IFF response.

reply to be the dotted arc shown in figure 155 instead of a solid arc like the echo. The dotted arc is longer than that painted by the echo because the IFF antenna beam is broader than the radar beam. However, the center of this longer arc represents the bearing of the target just as the center of the echo arc does. No confusion should be caused by the fact that the arc is broad, because the G-band section of the transpondor is made operative only at the request of the intercept officer controlling the fighter plane. The fixed frequency on which the G band operates is chosen from an 8-megacycle band and all transpondors and interrogators are set to it before the planes take off. This allows the rapidly chopped IFF response to be obtained on the radar PPI whenever the intercept officer requires it. The chopped nature of the G-band reply, coupled with the fact that the pulses are approximately a mile wide in range should make the response sufficiently distinctive to prevent confusion when many responses and echoes are present. The instantaneous characteristic of this response is especially valuable in that the IFF shows up well on a PPI scope.

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6. Destructors.

The IFF system does not provide an absolute means of recognizing radar contacts. Therefore, to help insure that the responses are authentic, it is important to deny the enemy use of any transpondors that he may capture. As a means to this end, all transpondors are provided with small explosive charges which may be detonated either by an impact switch operated by the deceleration of a crash landing, or by a switch to be operated manually by the pilot whenever there is a possibility of the plane's falling in enemy territory.

The case of the transpondor is designed to withstand the force of the explosion without rupture, so that there is little danger to operating personnel from the explosion. The small charge used is intended primarily to make the set entirely useless, but not to conceal all details of the operation. Damage done by a destructor should be supplemented by any means available to insure complete destruction of the equipment when circumstances indicate such a course is warranted.

7. Mark IV IFF System.

A different system of IFF has been developed for use if the Mark III system becomes extensively compromised. Components of the Mark IV system have already been produced, but an extensive program of operational and maintenance training will be required to put this system into effective operation. The greater complexity of the Mark IV will probably cause more maintenance difficulty than is being encountered with the Mark III equipment.

The same general types of equipment are required with the Mark IV as with the Mark III. That is, challenges are sent out by interrogators, identifying responses returned by transpondors, and these responses received by responsors. The transpondor used with the Mark IV IFF system is called the ABA-1 or the BA-1 by the Navy and the SCR-515 by the Army. The AN/APX-1 and AN/APX-2 can be converted to Mark IV operation by the installation of a modification kit designated TN-35/APX. A kit called the BP has been developed to convert the BN to Mark IV band operation, and the interrogator-responsor designed as part of the Mark IV system is called the BG.

The frequency of operation is higher, being in the L band, and the type of coding different than in the Mark III system. In the Mark IV system a fixed frequency is used for the challenge and a slightly different fixed frequency is used for the reply. The use of fixed frequencies eliminates the wait for a response that is necessary in the Mark III system because the transpondor sweeps over a band of frequencies. As a result of the continuous response that is received in the Mark IV system, this type of IFF is much more satisfactory for use with radars that use a PPI. In fact, the Mark IV IFF response is very similar to the G-band response in the Mark III system, except that it is not chopped.

The response pulses returned by a Mark IV transpondor are all of the same duration. Coding is accomplished by turning the transpondor off and on, so that the IFF response flashes on the screen very much like a coded blinker light. The coding cycle is of approximately twelve seconds duration. Therefore, the antenna must be stopped for this length of time to allow the code to be read; the code will not show on a PPI.

An antenna array for the Mark IV system is already built on the reflector of the antenna of search radars such as the SA-2, SC-2, and the SK. Because of the frequency used, the beam width of the Mark IV array is of the order of 7°-10°, which should provide reasonable bearing resolution for this IFF system.

8. Mark V IFF System.

Development is nearly complete on still another IFF system which has been designed in an effort to overcome many of the objections to the present system. This new system also operates in the L band, and its design takes full advantage of all the advances in ultra-high-frequency techniques made during the war. The Mark V system provides much greater flexibility of operation and much better security (in the form of an improved coding method) than is available in either the Mark III or Mark IV systems. However, this new system is complex and an extensive training program will be required to put it in service. The enormous number of ships and aircraft that the United Nations now have requires that thousands upon thousands of Mark V units be produced before the system can be put in service, since whole combat areas must convert simultaneously. Because of the enormity of the production problem, it will be a relatively long time before Mark V is put to use.

C. LIMITATIONS OF THE IFF SYSTEM

1. Operational.

The recognition of a radar contact that results from the use of IFF is in a sense a negative process. Barring compromise of the system, a properly coded response from the target enables the radar operator to recognize it as friendly, but the lack of the proper IFF response is not a clear indication of the hostile nature of a target. However, any contact which does

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not respond correctly to an IFF challenge must be treated as enemy until some other means of recognition establishes its true character.

An IFF system is the only means so far developed whose capabilities of recognition match the radar's capabilities of detection. To be sure, there are several limitations inherent in the Mark III IFF system that prevent recognition of radar contacts under all conditions. Although a few of these limitations may be eliminated in future systems, it is most improbable that any practical system will be developed during this war that will be able to overcome all of the objections to the present IFF system.

In the early part of 1944, a commission was sent to the Southwest Pacific to determine the reason for the repeated reports of failure of the Mark III IFF system. Their findings may be summarized as follows:

  1. IFF is ineffective in nearly all theaters because the operation is poor.

  2. The equipment is reliable in a material sense, and it can be maintained so by relatively simple measures.

  3. The primary obstacle to improved effectiveness is lack of interest on the part of many responsible officers.

Operational surveys of IFF performance indicate that the percentage of ground station radar plots which are identified by IFF in the various theaters of operation are:

  Percent
Southwest Pacific 50
South Pacific 80
Mediterranean 70
Central Pacific 85
U. S. A. 75
Alaska 80

Although no such quantitative data are available on IFF performance on board Naval vessels, the fact that only a few references are made to IFF in action reports leads to the conclusion that average performance is perhaps somewhat better in the Navy than that indicated in the table above. In any case, the existing IFF system is capable of determining the identity of well over 90% of the contacts made by radar, if the system is properly employed.

Aside from the obvious danger that may result from unsuccessful operation of the IFF system, other undesirable effects may appear. When 20 or 30 unidentified flights approach daily there is no choice but to alert the ship and send fighters to intercept the contacts. If bogey after bogey turns out to be friendly, the pilots making the interceptions begin

to feel disgusted and suffer a Ioss in morale; the AA batteries may well become a bit slack after a series of such anticlimaxes; and all such personnel who were needlessly alerted become disgruntled. The worst feature of such failure is that the effect is cumulative. Because the gunners may have some hesitance to fire on what might be one of their own planes, the pilot whose IFF transpondor is not functioning may not be fired on. The gunners' inaction therefore tends to make the pilot even less likely to use his IFF the next time, and this failure leaves the gunners' quandary still unresolved. Such a progressive breakdown of the system can result only in mistrust of IFF as a means of recognition, with ineffective air warning and fighter direction systems as a consequence.

The recent survey of the operation of the Mark III IFF system showed that of 100 unidentified friendly radar contacts, 66 were caused by errors on the part of air personnel, 30 by obvious faults in the airborne equipment, and 4 by defects in the ground equipment or errors on the part of ground personnel. The kinds of personnel error that occur in aircraft may be given as follows:

  1. IFF transpondor not turned on.

  2. IFF turned on at wrong time (contrary to doctrine).

  3. IFF not in use at proper distances from own forces or territory.

  4. Use of improper code.

That the first of these is the most important is clearly demonstrated by the survey. Simply briefing the pilots and stationing a man at the end of the runway with a large sign bearing the legend "IFF ON?" reduced the bogey contacts from 25 to 10 per cent. Simple maintenance procedures were then instituted, and the inoperative IFF dropped to 3 per cent.

Errors in the use of IFF may arise from lack of knowledge concerning IFF, lack of indoctrination as to the use of IFF, carelessness, confusion as to the doctrine in different areas, or confusion due to different doctrine prescribed by the different branches of the services in the same area. It is apparent that these factors, which cause the largest part of the failure of the IFF system, are in no way connected with the matériel aspect of IFF. Therefore, if the Mark III system is replaced by the Mark IV or Mark V, the same problem of making the system operate effectively will be present. However, it has been demonstrated that interest and positive action by commanding officers can make the Mark III system work. Therefore, the Mark IV and any succeeding system also can be operated effectively if the officers in command maintain an interest in its operation.

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Aside from the operational difficulties that may arise in the functioning of IFF as a system, certain features of Mark III IFF may hinder recognition to some extent. The responses are coded by the use of combinations of wide and narrow pulses and blanks. When a slow sweep speed is used, as in long range searching, the distinction between the wide and narrow pulses may not be sufficiently apparent to allow rapid recognition of the code. An expanded A-scope sweep, called an R scope, will be very useful in reducing this difficulty.

Since the tuning cycle of the transpondor takes nearly three seconds, the probability is small that during several successive tuning cycles a constantly-rotating antenna will point at the target at the instant when the transpondor can reply. Therefore, it is not possible to read the code from a PPI, so that the antenna must be stopped on the target for at least 12 seconds to insure recognition of friendly contacts.

Although the Mark IV system replies immediately to every interrogating pulse that strikes it, it will still be necessary to stop the antenna to read the code, since the coding cycle in this equipment also takes 12 seconds. The only type of IFF response which leads to definite recognition without stopping the antenna is that returned on the G band. This type of reply is instantaneous, but it shows only when called for from the ship, and it is "coded" by selecting the frequency on which the G band operates.

If several planes in a group all have their transpondors turned on, the many responses received will make it impossible to read the code. The operator may be able to tell if any wide pulses are being returned, but the blanks and the order in which the narrow and wide pulses occur will be obscured. The many responses returned under this condition need not confuse the echo presentation on the screen, since the responses show only when the operator throws the IFF switch on the radar operating console to the "ON" position. Similar interference is often produced by the many responses returned from a few transpondors when they are triggered by a large number of interrogators in a task group. The responses triggered by other interrogators will show as pulses that move back and forth across the trace, producing considerable confusion on the indicator. The EMERGENCY pulse is sufficiently wide that it can be recognized through considerable interference or on a slow sweep, such as the 375-mile range scale on the SK radar. Little confusion should be caused by the reception of a large number of Emergency pulses since this signal is to be used only when in distress or when standing by a friendly craft in distress.

2. Propagation.

The energy radiated from the interrogator antenna strikes the water, and is therefore formed into lobes and nulls by interference. The nulls are not as deep nor as extensive as those in the radar pattern because the interrogator radiates vertically polarized waves. Since the frequency of the Mark III IFF system is lower than that of any radar in use on board ship, the coverage pattern of the IFF is not the same as that of the radar to which it is connected. In general, the IFF energy does not stay as close to the surface as the higher frequency radar energy does. As a result, low-flying planes and surface vessels are often detected by microwave radar well before an IFF response is visible. The low-frequency used in the Mark III system limits recognition between two destroyers to approximately 10 miles, while the radar range may be 15 miles or greater. Between two submarines IFF is practically useless because the combination of very low antenna heights with the low frequency requires the subs to approach to a dangerously short range to obtain a response.

The lobes and nulls of the radar and IFF patterns do not always overlap. Thus, an airplane can sometimes be in a maximum of the radar radiation while at the same time it is in a null of the IFF pattern. As a result, an IFF response may not be returned at some ranges even though the echo is visible, and of course the reverse can also be true. However, because the patterns of the two antenna arrays in general are different, there probably will not be many areas in the pattern where this phenomenon occurs. In any case, the plane will fly out of the null in a minute or two, so that if the radar operator is familiar with the fade pattern of the radar, there should be little confusion caused.

When the fade charts of the radar are calibrated by flight tests, an IFF fade chart can be derived by keeping the transpondor in the plane energized, and observing both the radar echo and the IFF responses during the test. Such a pattern for IFF would serve not only to indicate the areas in which IFF and radar nulls overlap, but also to provide additional data for estimating the height of friendly planes. The IFF pattern will be more reliable if the transpondor tuning is fixed at the interrogator frequency during the test run, since the continuous reply thus produced will make it easier to determine null areas.

Since the Mark IV and Mark V systems use frequencies in the L band rather than in the P band, they should be capable of greater range on low-flying aircraft and between surface vessels than the Mark III system. The greater range is possible because the high-frequency energy stays closer to the water than

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Figure 156--Transpondor coverage patterns of airplanes
Figure 156--Transpondor coverage patterns of airplanes.

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the lower-frequency radiation of the current system.

Japanese radar pulses can trigger both the A and G bands of the Mark III IFF transpondors. The responses show up on the Japanese indicators more or less like normal echoes that vary with the code set on the transpondor. The range from which the responses are received is always greater than that from which the radar echo is returned, because the transpondor serves as an amplifier for the Japanese radar pulse. For example, if the maximum range at which a plane can be detected is 70 miles, the transpondor conceivably could be triggered by the enemy radar at a range of 100 miles. Since the IFF response will in general be much stronger than the radar echo, the IFF response will show even though the echo is not visible. This is a very great advantage to the enemy since it extends the range of his radar for detecting our planes. Therefore, IFF transpondors should be turned off as airplanes approach enemy territory.

In areas covered by both enemy and friendly radars it is necessary to strike a balance between the advantage of keeping the transpondor energized to assist in recognition of radar contacts and the danger of aiding the enemy by extending the effective range of some of his radars. It is important to realize that the Mark III IFF can serve as a recognition system for the Japanese as well as for us, without their having to capture a single transpondor. From the Japanese point of view, targets that return an IFF response are enemy.

In general, it is more important for returning planes to have their transpondors energized than for planes that are flying away from their base. However, on long strikes the pilots must be briefed on the approximate positions of any friendly forces along their route, so that IFF can be on when the planes approach such forces. Unfortunately, several planes have neglected this simple precaution and have been shot down by friends.

The range at which an IFF response can be obtained from a target is dependent on the height of the interrogator antenna and the height of the transpondor antenna. The range is not related to the size of the target in any way except that antennas probably can be mounted higher on large craft than on small. Thus, the IFF range on a single fighter plane is just as great as that on a flight of large bombers. Of course, the maximum range at which an IFF response can be returned is dependent also on the power radiated by the interrogator and the transpondor, the sensitivity of the receivers in each of these pieces of equipment, and on the existing propagation conditions. If the radar itself were capable of triggering the transpondor, IFF would always show at ranges greater than those from which echoes can be obtained because the transpondor would serve as a pulse amplifier. However, the gain introduced by the transpondor is offset almost completely by the relatively low power radiated by the interrogator in the Mark III system. As a result, the range at which a challenge can just trigger the ABK transpondor is very nearly the same as the range at which an echo is just visible.

At very short ranges the interrogator-responsor can act as a radar. Since relatively low power is radiated by the interrogator, echoes from nearby large targets may clutter the first mile or two of the sweep. This clutter may limit the minimum range at which IFF responses can be read, and it may confuse the operator by the constant downward deflection of the trace on the indicator screen. The Mark IV IFF system will not be affected by echoes from nearby targets because the responsor receives on a different frequency than that on which the interrogator transmits.

Omnidirectional antennas are always used with transpondors in order that the ship or aircraft can be challenged and reply in any direction. However, these antennas rarely have perfectly uniform coverage because they cannot be mounted in a position free of obstruction on all bearings. Representative coverage patterns for large airplanes are shown in figure 156. Note particularly the poor forward coverage of the antenna mounted below and toward the rear in the case of the PB4Y-2 in B, and the sharp null to the starboard side forward caused by the off-center installation. Notice the generally good coverage of the B-17 installation in A. In C, the transpondor antenna is mounted on the top of the B-24 so that the coverage underneath is poor.

Since there are more obstructions on ships than on planes, it is reasonable to expect that the coverage patterns of shipboard omnidirectional antennas will be less uniform than those on planes. In figure 157B is shown the pattern of a BN antenna which is mounted on the port side, amidships. The pattern is far from uniform, having very poor coverage on relative bearings of 015° and 100°, with deep nulls at other angles. With a pattern such as this it would be very difficult to get a response from a target bearing 100° relative, for example. Figures 157 C and D show the coverage patterns for two BK's on the same ship. Where possible two transpondors are supplied so that the antenna of one may fill the gaps in the pattern of the other. However, note here that both patterns are weak on the port beam, although the coverage to starboard is excellent. One of the antennas should be moved to a point that would produce more uniform coverage. In A is

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shown the pattern of a BM, which uses a directional antenna mounted on the air search antenna reflector. Because the array is on top of a stub mast amidships and has negligible obstruction in its field of view, the coverage is good at all bearings.

3. Resolution.

Since an omnidirectional antenna sends out energy in all directions, no resolution in bearing is possible. Thus, if several targets are at nearly the same range, but at widely different bearings, a single friendly contact may identify all the targets as friendly. An omnidirectional antenna on an interrogator, then, is useful only when all targets are separated by a distance great enough to permit range resolution.

The directional antenna provided with some Mark III interrogators has a very broad pattern because space limitations will not permit a highly directional array to be built for the low frequency at which the IFF system operates. In general, these antennas have bad side lobes in addition to rather broad main lobes, but the side lobes in figure 158B are accentuated by reflections from obstructions in the antenna field. The pattern in A is more nearly like the free-space pattern because very little of the ship's structure interferes with the radiation. Note that the main lobes in both cases are asymmetrical. The asymmetry and the many side lobes are indicative of reflections from the ship's structure. Since the amount of

Figure 157--Coverage diagrams for shipborne IFF equipment
Figure 157--Coverage diagrams for shipborne IFF equipment.

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interference caused by reflection varies as the antenna is rotated, the shape of the antenna pattern is dependent to some extent on the relative bearing to which it is trained. The beam widths shown in figure 158 are approximately 20°, but in some cases the beams may be considerably wider. Consequently, the bearing resolution of a Mark III IFF interrogator is worse than that of any of the radars with which it may be associated. The bad side lobes that appear on some relative bearings, as in figure 158B for example, distort the antenna pattern so badly that IFF responses may be returned from a nearby friendly target over nearly the full 360°, irrespective of antenna bearing. In order to permit the operator to observe IFF responses from either of two targets at the same range without confusion from the other, the angular separation between the targets must be 10° to 20°.

The side lobes of the interrogator antenna may often result in IFF responses being returned from bearings at which no radar echo appears. Recognition of friendly ships in a melee is almost impossible because of the confusion produced by the poor bearing resolution of the interrogators, the false responses triggered by side and back lobes, the multiplicity of responses that make it impossible to read the code, and the short IFF range possible between surface vessels. The IFF responses from a single friendly target show over a wide arc on the indicator because of the wide antenna beam and the many side lobes. This may lead the operator to believe that echoes

Figure 158--Patterns of directional Mark III interrogator antennas
Figure 158--Patterns of directional Mark III interrogator antennas.

from enemy targets are friendly because part of the IFF response appears near the echo. Maintenance of an accurate surface plot is probably the only reliable means of insuring that all radar contacts can be recognized during a melee.

Because the Mark IV IFF system uses higher frequency than the Mark III, the interrogator beam width can be made narrow enough to provide reasonable bearing resolution. The Mark IV antennas which are built on the reflecting screens of current air search radars provide beam widths on the order of 7° to 10°, so that bearing resolution with this system should be better than 5°.

The nature of the IFF system is such that reducing responsor gain has less effect on side lobe IFF responses than reducing receiver gain has on radar echoes produced by side lobes. The power radiated in the side lobe is very much less than that in the main lobe. The signal developed at the radar receiver terminals as the result of a given echo power entering a side lobe is much less than if the same power entered the main lobe. These two factors act together to keep side-lobe echoes smaller than the desired echoes produced by the main lobe. Reducing receiver gain causes a greater decrease in the size of the side-lobe echoes than in the desired echoes, and in many cases, the side-lobe radar echoes can be entirely eliminated from the screen without serious loss of the desired signals. In the IFF system, however, if the interrogating power is sufficient to trigger the transpondor, a full-power response is

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returned, and the strength of the response is not increased if the interrogating signal is made stronger. For example, in figure 158B, the minor lobe at 235° Relative is only 5% of the major lobe. If a target is close enough that its transpondor will be triggered by this minor lobe, full-power responses will be returned in every lobe of the antenna. The strength of the signal at the responsor terminals produced by the side-lobe response is less than that produced by the main lobe response, but not nearly as much so as in the case of a radar echo. Consequently, the responsor is not as able as a radar receiver to discriminate between main-lobe and side-lobe signals. Reducing the responsor gain enough to eliminate most of the side-lobe IFF responses will also cause a serious decrease in the size of the desired main-lobe IFF replies.

The range resolution with IFF is dependent on the width of the response pulses. However, since every code in use includes narrow pulses, the range difference required between two targets to allow the operator to read the code from one target without interference from the responses of the other is approximately the range to which a narrow pulse is equivalent. Thus the range resolution in the Mark III IFF system is between 1,000 and 1,500 yards, which is poorer than that of almost all radars now in use in the Navy.

Since the interrogator antenna does not elevate, except for interrogators on the G band, no altitude resolution is possible with IFF. This limitation is largely academic in practice, however, because it is not likely that two airborne targets, or an airplane and a ship, will remain for long within the distance required for range resolution. The limited physical size of the G-band IFF antenna necessitates a broad antenna beam in both horizontal and vertical directions. As a result, the position angle resolution for G-band interrogators is poor.

4. Matériel.

There is inherent in the IFF system a small amount of delay that prevents the IFF response from coinciding exactly with the radar echo on the indicator screen. In some cases, as when the counting-down circuit of the interrogator is improperly adjusted, the delay may be as much as a mile. In general, however, the delay will be between 400 and 1,000 yards. In some cases it may be desirable to leave a small delay in the system to facilitate viewing the IFF responses. This delay causes the response and the echo with which it is associated to be slightly separated on the indicator. Radars of recent design incorporate an IFF coordinator which provides for a double trace on the A scope, and permits adjustment of the delays in the system so that the IFF response will appear directly under the proper echo.

Vibration that is encountered in normal use may cause the tuning range of a transpondor to change. If this change tunes the set out of the high end of the band, it may be triggered directly by some of the lower frequency P-band air search radars. Such responses would appear as upward deflections, rather than downward in the manner of normal IFF signals received by the responsor. An adequate program of inspection and testing should prevent such trouble.

The sensitivity of a transpondor decreases as the vacuum tubes age. An insensitive transpondor will not reply to a challenge until it is much closer to the interrogator than a set having full sensitivity. On the other hand, too great sensitivity is also harmful because the set may be triggered by ignition noises or other sparking, or the set may be triggered by itself, should certain components fail. Self-triggering or responding to noise, which has been termed "squitter", is objectionable because under this condition many unwanted pulses are radiated without any challenge being received. Thus, squitter causes excessive interference with the whole IFF band, and it may prevent recognition of any of the contacts detected by radar, as well as providing a continuous signal for the enemy's direction finders.

On an aircraft carrier with many planes warming up on deck, much IFF squittering may be encountered because of the strong ignition noise. The squittering may interfere with the air search radars to such an extent that detection of targets is impossible during the warm-up interval. In most cases this sort of interference should not appear on the indicator because the responsor output shows only when the IFF switch is thrown on. However, in aircraft carriers where the radar frequency is not far from the IFF frequency band, the transpondor pulses will cause interference because they are strong enough to go through the radar receiver in spite of the frequency difference.

When the Mark III IFF was first installed there were some air search radars that operated at frequencies within the Mark III band. This overlap caused confusion since it was impossible to refrain from interrogating every friendly target detected, and the responses caused upward deflection rather than downward. To avoid this difficulty all "red" SCs were set to another frequency band, and the "green" equipments were tuned to the high end of the band.

The Mark III transpondor transmits very low power pulses continuously at a 300 kilocycle rate, even though no interrogating pulses are received. These small pulses may cause considerable interference that has the appearance of strong grass each

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time the transpondor sweeps through the radar frequency. As a means of reducing this interference from a BK to a radar on the same ship, the transpondor is made inoperative, or suppressed, for a time equivalent to 100 miles range every time the radar pulses. The suppression is synchronized to the pulse repetition rate of the SA, SC, or SK radar. In some cases this same sort of interference is produced through the responsor, even though the radar receiver is tuned to a frequency considerably different from the IFF band. Thus, the BK usually will need suppression from the air search radars, irrespective of the frequency to which the radar is tuned. The policy of the Bureau of Ships relative to suppression of Mark III IFF transpondors is published in section 5 of the Radar Maintenance Bulletin.

In some cases the TBS and other VHF equipment causes interference with the IFF system. When the communication signals are picked up by the responsor, the whole trace on the radar indicator is deflected in accordance with the radio signal.

On the other hand, the transpondor may interfere with communications. The transpondor puts out noise signals at frequencies up to approximately 20 megacycles, but most of these frequencies are detectable over a very short range only. The principal interfering signals produced by the transpondor are on 300 kilocycles and 600 kilocycles, so that communications on these frequencies may be somewhat impeded. If serious interference is encountered on 300 kc or 600 kc, the frequency of the oscillator that generates this signal in the transpondor may be shifted without affecting the operation of the set, provided the AGS circuit is also retuned.

Interference with the IFF system and that produced in other equipment by IFF can be reduced by the proper selection of the operating frequency of the interrogator-responsor, by the use of filters in the equipment that suffers the interference, and by cleaning and, checking insulation in the radar. Much ignition noise can be avoided on aircraft carriers by proper grounding of the cowling and magneto and by thorough checking of the spark plugs.

When a plane is in the air, the pilot has no reliable means of determining if his transpondor is operating properly. There is a jack on the control box into which he can plug his headphones and hear a tone when the set in replying to interrogation, but if he hears no tone, he has no way of knowing whether he is not being challenged or whether the transpondor is defective. An adequate maintenance program can remedy this difficulty by insuring that all installed transpondors are in excellent condition. The frequency limits and sensitivity of the transpondors should be checked at frequent regular intervals, and the whole installation given a superficial daily inspection in order to find and replace any broken parts.

The Mark IV IFF system has certain advantages over the Mark III because of the higher operating frequency. But, since all the interrogators operate on the same fixed frequency and all the transpondors on a slightly different fixed frequency, keeping all the units tuned properly will pose a considerable maintenance problem if this system is put in use. However, if adequate test equipment is made available to the Fleet, there should be little difficulty in keeping the Mark IV system operative after the operational experience gained in the use of the Mark III IFF system.

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Transcribed and formatted by Larry Jewell & Patrick Clancey, HyperWar Foundation