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CHAPTER

A. Introduction--definition of terms.

B. Interception: Methods employed by the enemy; information gained; measures for limiting interception.

C. Jamming: Introduction; general effects of jamming on radar operation; electronic jamming; mechanical jamming; anti-jamming measures.

D. Deception: Methods of radar deception; uses of deception; detection of deception.

E. Evasion.

A. INTRODUCTION

The United States at present enjoys a considerable advantage over the Japanese in the superiority of its radar. To nullify this advantage, the Japanese are producing radars of their own, and at the same time, they are devising ways of decreasing the effectiveness of our radar. Consequently, more and more counter-measures activities should be expected as the Japanese continue to determine the characteristics of American radar, and develop improved countermeasures techniques. This should be especially true when the war is carried close to the islands of Japan.

The following definitions are stated to aid in drawing distinctions among the several allied fields of countermeasures:

Countermeasures are the means employed by the enemy to obtain information about our forces from our use of radar, and to prevent us from obtaining any accurate or useful information about enemy forces through the use of our radar. Countermeasures methods are of four types: interception, jamming, deception, and evasion. The techniques and equipment used to combat jamming are not considered to be a part of countermeasures, since they are, strictly, counter-countermeasures.

Interception is the detection of radar signals by the use of a special receiver. By this means, the enemy learns of our presence in his vicinity, obtains an approximate bearing on our position, and he may determine some of the characteristics of our radar.

Jamming is the deliberate production by the enemy of strong signals for the purpose of hiding his movements or position from our radar by obliterating or confusing the echoes on our indicators. The jamming signals may be produced by a modulated radio transmission, which is electronic jamming; or by echoes returned from many small metallic reflectors, termed Window.

Deception is the deliberate production by the enemy of false or misleading echoes on our radar by the radiation of spurious signals synchronized to the radar, or by the reradiation of radar pulses from extraneous reflectors. Small targets may be made to appear like large ones or echoes may be made to appear where no genuine target exists.

Evasion consists of tactics that are designed to take advantage of the limitations of radar to prevent or postpone radar detection, or to avoid revealing the true position of an attacking force. If attacking planes take evasive action, it may be impossible to determine the height at which they are flying, or the planes may be detected too late for an adequate defense to be made ready.

Interference is caused by the reception of confusing signals accidentally produced by the effects of either friendly or enemy electrical apparatus and machinery, or by atmospheric phenomena. Interference should not be confused with enemy countermeasures.

Anti-jamming, often abbreviated as AJ, is the art of minimizing the effect of enemy countermeasures in order to permit the echoes from targets detected by the radar to be visible on the indicator.

It is important to recognize that the counter-measures that can be used by the Japanese against our radars can also be used by us against their radars. In the same way, the means at our disposal for lessening the effect of Japanese countermeasures are approximately the same as those available to them. Therefore, practice in using and defeating counter-measures of our own devising against our own radar provides excellent training against the radar counter-measures that the enemy may employ. However, a very realistic approach must be made to training of this sort, and the enemy's probable line of thought must be approximated as closely as possible in devising training exercises.

B. INTERCEPTION

1. Methods Employed by the Enemy.

In the early stages of the war the Japanese had only a few types of radar and few installations in

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service. As a result, they were not faced with the difficult problem of eliminating interference from a large number of their own radars while they searched for our radar signals with intercept receivers. It is known that the Japs have intercept receivers in use for listening to our P-band radar. It is likely that they now have or soon will have equipment that will intercept our L and S-band radars. In fact, the receiver of the S-band radar with which some of their ships are equipped can be used for limited intercept work. As the war progresses, their intercept receivers are improving.

Several instances have shown that the Japanese sometimes depend on their intercept receivers for early warning of the approach of American vessels. Thus, they may not even turn on their own radars until alerted by the interception of American radar signals. Because of their early start in this field, the Japanese are probably more adept in interception than in any other countermeasures method.

The Germans have developed intercept receivers capable of detecting all our radar frequencies. German submarines therefore have often been able to escape detection by airborne radar through the use of the information obtained by interception.

2. Information Gained Through Interception.

The strength of a radar pulse decreases as the pulse travels away from the antenna. The small part of the pulse that is reflected from a target also falls off in strength as it returns to the radar. Since interception requires only one-way transmission of the pulse, the radar can always be heard at a range greater than that from which useful echoes are returned. For example, the SD radar can detect a large high-flying airplane at a maximum range of 30,000 to 35,000 yards under favorable conditions, but under the same conditions, the pulses can be detected beyond 45,000 yards by an intercept receiver carried in an airplane. A radar receiver, which is more sensitive than an intercept receiver, can detect an SD out to perhaps 75,000 yards if the radar frequency is accurately known and the receiver pretuned to this frequency. Normal interception range should be somewhat less than twice the normal maximum radar range when the intercept receiver is carried in a plane. If the intercept receiver is on a ship or on land, the range of interception can be only a little greater than radar range because of horizon limitations.

One type of information, then, that can be gained from the use of intercept receivers is early warning of the approach of radar-equipped vessels. It may be possible to make a rough estimate of the size of the approaching force my observing the number of signals intercepted.

If the intercept receiver is calibrated, the frequency of the enemy radar can be determined. Other characteristics of the radar can be found by the use of auxiliary equipment with the intercept receiver. Some intercept receivers for the S and X bands are capable of giving only early warning, since they produce an output for a wide band of frequencies without being able to be tuned to a single frequency.

The coverage of a radar installation can be determined by the use of an intercept receiver if the enemy operator is unwittingly cooperative. For example, a fairly complete coverage diagram of a Japanese radar installation was obtained on a radar reconnaissance flight because the radar operator stopped his antenna on the plane each time it appeared. By making "many approaches from different directions and at different altitudes, the operator of the intercept receiver was able to plot the whole coverage of the radar simply by noting the point at which the radar beam stopped sweeping to searchlight the approaching reconnaissance plane.

3. Measures for Limiting Interception.

The most obvious way of preventing interception of radar signals is to turn off the equipment. However, the use of radar silence is practical only when the tactical advantage of depriving the enemy of intercept information is greater than that to be gained by radar operation. Any condition of radar silence put in effect must apply also to IFF and VHF communications. The imposition of radar silence for long periods of time may result in a great decrease in the efficiency of the radar operators because they are denied practice.

It is possible for radar pulses to extend below the radar horizon to some extent as a result of diffraction. The diffracted energy usually is so weak that usable echoes cannot be returned from targets in this region. However, intercept receivers on the targets may be able to detect the radar pulses in the diffraction zone, even though the intercept antenna is well below the radar line of sight. If the weather conditions are such as to produce trapping of the radar signals, it will be possible for an intercept receiver within the duct to detect the radar far beyond the normal intercept range. The decision to use radar silence, then, should be based in part on the best available evaluation of the trapping conditions.

Training of operators should be aimed at developing the habit of keeping the antenna in constant rotation. If the antenna is stopped to investigate each target that is detected, an enemy with an intercept receiver may be able to determine the coverage of the radar.

There is a possibility that enemy interception may be delayed by intermittent operation of the radar. If both the radar antenna and the intercept receiver

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antenna are rotating, there is a good chance that they may not be pointed at each other at the same time. In addition, if the radar is turned off part of the time, the likelihood of the intercept receiver missing the radar will be increased. However, intermittent operation of radar cannot prevent interception; it can only defer it.

As a means of increasing the difficulty of interception, some of the new radars are being fitted with facilities for automatic intermittent pulsing. An extension of this same principle has developed into what is called jitter pulsing, in which the repetition rate of the radar is made random. When the enemy intercepts a radar that is jitter pulsed, he can hear only noise, very much like the noise that he would detect at high gain with no incoming radar signals. It has been shown in tests that jitter pulsing reduces considerably the maximum range of interception.

C. JAMMING

1. Introduction.

The effect of radar jamming is to produce a confusing pattern, on the screen of the radar indicator. The single trace that normally appears may become a network of interlaced lines, a solid pattern of light, very strong grass, or the trace may be wiped clean

Figure 141--Jamming on radar indicators
Figure 141--Jamming on radar indicators.

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of all signals. Variations of these patterns may be produced by changing the amount of and type of jamming, so that an almost infinite variety is possible. Figure 141 shows a few patterns that may be produced on radar indicators by some common forms of jamming. Additional pictures of jamming and operating instructions for combating jamming are presented in part 3 of RADTHREE.

There are two general types of jamming--electronic and mechanical. Electronic jamming is accomplished by the transmission of modulated radio signals, while mechanical jamming is performed by dispersing in the air many strips of metalized paper, called Window. The chief effect of either type of jamming is to prevent or to make difficult the determination of the range of a target in the jammed sector. In radars that use lobe switching or conical scanning to improve bearing accuracy, jamming can produce rather serious bearing errors on the targets that can be seen through the jamming, and the pattern on the scope is often made more complicated by the switching that must be employed to display the echoes from the separate lobes. Although jamming may deny range information to the radar, it usually is possible to determine the approximate bearing from which the jamming comes. Only the approximate position of the enemy can be determined in the presence of jamming, since the only available method may be to employ triangulation using several bearings determined with an accuracy of approximately ± 3°.

In general, the enemy will derive greatest advantage from jamming fire control radar, especially that used for AA fire control, because this type of jamming will permit attacks by enemy aircraft to be carried out with less likelihood of accurate AA fire. The ability of the operator to interpret the echoes as a means of establishing the size and composition of the targets is always impaired by jamming, and often interpretation will be impossible. The accuracy of the data obtained in the presence of jamming is less than that obtained in normal operation. Jamming usually can be detected on the radar at a greater range than is obtainable on the jamming vehicle as a target. Therefore, it is unlikely that the enemy will be so helpful as to begin jamming while he is beyond the detection range of an early warning radar.

An indicator on which the echo pips show as bright spots, for example a B or PPI scope, is much more vulnerable to jamming than one on which the echo pip shows as a vertical deflection of a bright trace. This limitation is due primarily to the nature of the cathode-ray tube.

If the signals applied to produce the bright spots are allowed to be of large amplitude, the spot will

be badly defocused and the pattern on the screen will therefore be fuzzy and indistinct. As a means of preventing this defocusing, the echo pulses applied to a PPI or a B scope are limited at a level just a little above the receiver noise level. This allows all the signals to be clearly in focus, but it also allows even a small amount of jamming to show with a strength equal to that of the desired signals. A second characteristic of the cathode-ray tubes used for PPI or B-scan presentations is the long persistence of the screen that is required because the echo pips are retraced relatively infrequently. As a result of the long persistence of the screen, both echo and jamming signals linger, and it is impossible to see a pattern in which a slight discontinuity may exist to indicate the presence of an echo.

On an A scope, on the other hand, the screen is of short persistence, and the echoes are retraced many times a second without their amplitude being seriously limited. As a result, the A-scope screen may appear as a jumbled mass of moving lines, but the echo will appear as a discontinuity in the pattern produced. Since the echo is synchronized, it will always appear at the same place on the sweep, while the jamming in general will move across the screen.

One serious result of the limitation of the utility of a PPI scope in the presence of jamming is that the system of repeater plan position indicators will be made ineffective. In order to combat jamming, the antenna of the radar must be stopped on the target, since even the simplest form of jamming produces a bright sector on the PPI and very little can be done to improve the picture if the jamming cannot be removed entirely. With the antenna stopped, very little useful information can be gained from any of the PPI's connected to that radar. The projection PPI, type VG or VG-1, is likely to be even more seriously affected by jamming than the normal equipments because of the very long persistence of the screen. However, the system of repeater PPIs provides the advantage of permitting a rapid check to be made of which radars are being jammed by simply turning the selector switch successively to all radars that can be connected to the indicator.

2. Electronic Jamming.

Electronic jamming is accomplished by transmitting modulated radio waves on a frequency as nearly the same as the radar frequency as possible. All possible types of modulation have been tried, but the most effective jamming is produced by modulating the radio wave with random noise. When such a signal is introduced into a receiver, the detected signal resembles the receiver noise in appearance on the indicator. Since it is impossible to eliminate receiver noise by any known methods, it is also

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impossible to eliminate noise jamming after the signal has entered the receiver because the electrical characteristics of this type of jamming signal are identical with those of receiver noise.

Types of modulation other than noise are less effective for jamming because they can be dealt with more easily. A few of the simpler forms of jamming can be eliminated almost completely by proper adjustment of the anti-jamming circuits, and some degree of readability can be effected against all but strong noise modulation, providing the receiver is not saturated by the jamming.

Off-target jamming can produce bearing inaccuracies up to half the beam width when it affects a lobe-switching radar. Since most fire control radars use lobe switching or conical scanning in an effort to obtain a high degree of training and pointing accuracy, they are very susceptible to bearing and position angle inaccuracies in the presence of jamming.

It is difficult to produce jamming signals that can obscure the echo signals completely if the operator of the jammed radar employs effective AJ measures. The appearance of the indicator can be improved in the face of almost all types of jamming when the operator is skilled. However, if adequate equipment is available to the enemy, and if it is used skillfully, he can jam a radar channel so completely that it will be useless. Fortunately, this seldom occurs in practice. In general, a single electronic jammer can jam effectively only a single radar set, because technical obstacles make it difficult to spread the jamming energy uniformly over a wide frequency band. Therefore, all the SG radars in a task group, for example, probably will not be completely jammed simultaneously unless the enemy employs a large number of jammers, or unless all the SG's in the group are on the same frequency.

Radar depends on two-way transmission. The power of a pulse sent out from the radar toward a high-flying airplane falls off as the square of the distance. The part of the pulse that strikes the target is reflected back toward the radar as an echo. During the reflection time the target is acting as a weak transmitter, and the signal it returns also is attenuated -as the square of the distance. The echo signal returned to the radar from the airplane therefore varies inversely as the fourth power of the range. Hence, the radar echo generally is a very weak signal because of the double attenuation encountered.

Jamming is a one-way operation, so that the jamming signal is attenuated proportional only to the square of the range. A jammer can have a considerably smaller peak power output than a radar, and yet because of the relatively low attenuation, the jammer can produce in the radar receiver a signal larger than the echo. However, the power output of the jammer is limited because, unlike a radar transmitter that produces a pulsed signal, most jammers must transmit continuously in order that the jamming signals may fill up the entire range of the radar indicator.

Figure 142--Variation of echo and jamming signals as functions of range
Figure 142--Variation of echo and jamming signals as functions of range.

The variation in relative echo strength with range is shown by curve 1 in figure 142, and the variation of the relative jamming signal strength in the receiver is shown by curve 2. It can be seen that as the target, which is assumed to carry the jammer, goes away from the radar, echo signal strength falls off more rapidly than the jamming. As a result, the effectiveness of the jamming is great at long range. However, at the range indicated as E the echo and jamming signals are of the same strength. Closer than this range, the echo rapidly becomes stronger than the jamming signal, and the jamming is ineffective. It is evident that in all cases there is a minimum range within which a target cannot conceal itself by transmitting jamming signals.

3. Mechanical Jamming.

Mechanical jamming is accomplished by dropping thousands of reflecting strips in the area where it is desired to prevent radar ranging. These metal strips, called Window, are designed to be approximately a half wavelength long for the radar frequency to be jammed. They usually are made of very thin aluminum foil backed by light, tough paper. Window most frequently is dropped in bundles from airplanes, but it can be dispersed from shells or

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rockets. The strips fall at about 300 feet per minute so that the signals returned from them persist on the jammed radar screen for perhaps 20 or 30 minutes, depending both on the height from which they are dropped and on the extent of the vertical coverage of the radar antenna. The signals returned from the cloud of Window become so scattered after the first 10 minutes that the Window no longer provides strong enough signals for jamming, since echoes from targets can be seen through it.

In order to be effective as a means of jamming, bundles of Window must be dropped within distances comparable to the range and bearing resolution of the radar. For example, a plane flying at 200 knots covers 112 yards in a second. If this plane were to drop Window as a means of concealing a large flight of planes from a radar using a one-microsecond pulse, it would have to release a bundle of thousands of strips every second. A tremendous amount of the material, then, is required to jam a radar thoroughly. The jamming operation must be planned carefully with full knowledge of the characteristics of the radar to be attacked. In many instances where the Japanese have used Window, they apparently did so without sufficient knowledge of our radars and the jamming was ineffective.

Unlike electronic jamming, Window has a relatively broad frequency response. It is true that it is cut for a particular frequency, but Window dropped to jam the Mark 4, for example, would also affect the Mark 12 and would produce some echoes on the SG and perhaps on the SK.

Mechanical devices other than Window usually are used only for deception.

4. Anti-Jamming Measures.

The most vigorous anti-jamming measure is the destruction of the jamming vehicle by direct action. To accomplish this, the jammer must be located by means of triangulation using radar bearings and then attacked by bombing, rockets, shellfire, or whatever means is most applicable.

The bearing of the jammer usually can be found within approximately 3° by maximizing the jamming on the radar indicator. If two or more bearings are taken simultaneously on a sufficiently long base line, the position of the jammer can be found by triangulation. Should the enemy use several jammers spread over a wide front, it will not be possible to obtain an accurate bearing on any single jammer unless the bearing resolution of the set in question is of very high order. Another factor that may complicate the measurement of the jammer's bearing is the reception in the side lobes of the antenna. For the purposes of obtaining bearing, the effect of the side lobes may be considerably reduced by a large reduction of the receiver gain, but for reading through the jamming, the optimum gain setting may be at some other point.

In the event that destruction of the jammer is not practicable, a more passive approach to the problem must be made. Thorough training of operating personnel is the most effective AJ measure that can be taken before enemy jamming is actually encountered.

Because of the difficulty of doing a complete job of jamming, the chief value of jamming is in the element of surprise. For example, on February 12, 1942, the Scharnhorst and Gneisenau passed through the English Channel under cover of the first large-scale use of radar jamming in this war. The ships were not seriously damaged by shore batteries because the jamming created a small panic among the radar operators so that the vessels were not tracked by radar, and they were invisible under the cover of the weather. It was found later that the type of jamming used in that action presented little difficulty to an operator trained in combating jamming. Therefore, the soundest AJ measure that can be taken is the thorough training of the radar operators. When they are completely familiar with appearance of jamming and when they know what to do if it appears, the surprise value of jamming will be negligible.

It is extremely important to recognize the cause of any unusual patterns that appear on the indicator. Jamming, interference, and internal faults in the radar all can cause the indicator presentation to vary from the normal. The operators can be made familiar with many of the common results of failure of the radar by having the technician remove certain of the tubes in the set so that the change in the indicator presentation may be observed. Interference that arises from some source on the same ship can often be recognized because it affects the radar uniformly at all bearings, or by noting the coincidence of the interference with the operation of some other equipment on board. Part of the training of the operator must equip him with knowledge of these effects so that he will be able to distinguish quickly and accurately between jamming and the effects of interference or failure of the radar.

One very common type of interference is that encountered between radars of the same type operating on several ships in company. This interference, which usually produces strong pulses that move across the A scope or spiral lines on the PPI, is seldom severe enough to reduce to any serious extent the effectiveness of the radars involved. Interference on both the SJ and SG radars has been encountered from Japanese shipboard radars, but no serious limitation on the use of the equipment was experienced.

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However, the Japanese radars also must have encountered interference, so that there is a possibility that the Japanese soon will attempt to jam some of our S-band radars, although no definite jamming has been reported, as of the end of 1944.

The confusion produced on the indicator by jamming usually is unintelligible to an inexperienced operator. However, after proper training, the operator will be able to read through most varieties of jamming with fair success. The use of special anti-jamming devices facilitates interpreting the indicator presentation; but, even though the AJ devices may be very elaborate, the echoes cannot be made as clear in the presence of jamming as when no jamming signals are received. Training devices have been developed for the purpose of simulating electronic jamming. By use of these trainers the operators can be given constant practice in working through jamming, so that they may become thoroughly experienced in this phase of radar operation. The trainers that are available for simulating radar jamming are: the OAV and OAV-1 for P band; the Mark 100, Mark 101, and Mark 5 for L band; and the AN/UPT-T3 (OBW) for S band. The Mark 6 trainer should replace the other L-band equipment when it becomes available, and a Mark 8 jamming trainer is being produced to enable jamming exercises to be carried out against S-band fire control radar. An attachment for some operational jammers, which is designated RF-9/UPT, is available for use in jamming exercises where it is desired to modulate the jammer with signals other than noise.

The enemy has no means of gaging the effectiveness of his jamming other than by observing the reaction of the radar. If the radar frequency is shifted markedly or if the radar is turned off, the enemy can well presume that his jamming is effective. Therefore, it is desirable to continue to operate the radar, even when the scope is unintelligible, in order to deny the enemy the satisfaction of knowing that his jamming is successful. Continued radar operation also has the effect of immobilizing the enemy jammer on one particular frequency, thereby forcing him to use more jamming equipment if he attempts to jam all our radars. The radar certainly is of no value when it is turned off; when it is on, there exists a possibility that the enemy may close to a range where the jamming is no longer able to obscure the echo from his ship. Since it is difficult to maintain the jamming at full effectiveness over long periods of time, it is well to keep the radar energized to allow the operator to maintain a constant watch on the indicator pattern. Small variations in the frequency transmitted by the radar may combine with variations in the jammer frequency to produce a condition that may permit the operator to obtain some useful information through a momentary gap in the jamming.

It is difficult to jam a centimeter-wave radar at all ranges because reflection of the jamming signals from the water produces many narrow lobes in the vertical coverage pattern of the antenna of the jammer. If the jamming is to be effective, the radar antenna must be kept within one of the lobes of the jammer antenna. This is relatively easy to accomplish at long range, but at short ranges the lowest lobe of the jammer may be below the radar antenna, so that no jamming energy is received by the radar. As the range closes, the radar antenna may again be receiving jamming signals from one of the higher lobes. This effect is not apparent in the radar echoes, since targets, especially ships, are much larger than the antenna alone, and they can therefore reflect energy from several lobes simultaneously. Since it is known that this effect does exist, it is well to keep microwave radars operating in the face of jamming because there may be null zones in the interference pattern of the jammer antenna where the radar operation will be nearly normal.

Since electronic jammers cannot cover a very wide band of frequencies, it is a good AJ measure to deploy the radar frequencies used over as wide a range as possible in order to force the enemy to use a great quantity of equipment if he wants to jam all the radars in a particular force. New radar equipment to be supplied to the Fleet will employ new frequencies as a means of making more difficult the task of jamming all of our radars. Additional flexibility in the form of sets capable of shifting frequency rapidly or of operating on a continuously varying frequency will be supplied in the future. The transmitters will be of higher power, and both the pulse repetition rate and the pulse duration will be variable. All receivers supplied with new types of radar will incorporate the most effective antijamming features known. No radar in current use is capable of operating on a constantly changing frequency. This will not be possible until the SR-4 or some radar of similar principle, becomes available to the Fleet, probably in 1946. However, jamming can be avoided, at least temporarily, by changing the radar frequency. Although it is possible to shift the frequency of some of our radars within rather small limits, not every set is capable of such operation. In any case a change of frequency affords only a few minutes' relief against an alert enemy, since equipment is available by the use of which the jammer can follow the radar frequency within a very short time. A slight advantage can be gained by tuning the receiver to the frequency to which it is intended to change before the transmitter is shifted. If the

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operators of the enemy jammer are slow, the advantage will be greater.

Since jamming produces on the radar screen many signals in addition to the desired one, the resultant confusion of signals makes it difficult to observe the desired echo. In many cases the jamming and echo signals interact to produce a distinctive break in the jamming pattern. Although this break may not have the appearance of a normal echo, the operator may still be able to get a range on the target. Antijamming techniques and devices, then, are aimed at eliminating as much of the jamming signal as possible, and at creating or accentuating the discontinuity in the remaining pattern on the scope.

Improvement of the indicator presentation in the presence of jamming usually can be effected by proper operating technique and the use of such AJ devices as are available. Varying the gain and the local oscillator tuning on the receiver are often helpful in reducing the amount of jamming signal on the screen, or in producing a clear discontinuity in the pattern. If the jammer is off target, a position of the antenna may be found in which the jamming signal can be greatly decreased without losing a large amount of the desired echo signal.

Anti-jamming circuits are now provided in many radar receivers. The most effective of these circuits are filters in either the intermediate-frequency amplifier or the video amplifier, Instantaneous Automatic Volume Control, and Fast Time Constant coupling. Both the use of the circuits and the operating techniques that help to minimize the effect of jamming are discussed in Part 3 of RADTHREE.

Noise jamming is difficult to counter by any of the AJ devices that are currently available because the character of the noise jamming signal is nearly identical with that of the receiver noise, which cannot be eliminated from the radar receiver output. The fact that the noise voltage is entirely random offers some hope of developing an effective anti-jamming device to counter it, for the echo signal is synchronous. However, manipulation of a few of the radar controls may result in improvement of the pattern on the scope. When the jamming signal is strong, the indicator will be covered with grass, and no echoes can be seen. If the radar has a variable transmitted pulse duration, use of the longest pulse may permit some improvement against noise jamming because the receiver is altered so as to pass only a narrow band of frequencies, which may eliminate a part of the jamming signal. On the other hand, if the enemy uses jamming modulated by noise voltage that is limited in amplitude, it may be helpful to use the shortest available transmitted pulse, because the i-f band width is increased. This may cause the jamming to produce an even pattern in which the noise appears like grass that is freshly mown instead of overgrown. It will be much easier to see the discontinuity produced by an echo in the even pattern than in the irregular pattern of jamming. Thus, improvement may be effected against noise jamming either by causing the receiver to pass a very narrow band of frequencies or a very wide band of frequencies. This type of adjustment is at present available in only a few of the newer radars.

The signals returned from Window are real echoes and it is therefore very difficult to produce an effective AJ device that can discriminate between the desired and undesired signals returned. It is very important to realize that Window does not act as a smoke screen; only the echoes from targets within the area infected with Window are concealed Targets outside the Window area remain visible, and the passage of the radar pulse through the infected zone does not reduce the power of the pulse. The better the range and bearing resolution of the radar the less effective Window will be in jamming the set. Therefore, when radars are available with variable transmitted pulse duration, the shortest pulse will be the most effective against Window. The use of AJ circuits such as Instantaneous Automatic Volume Control (IAVC) and Fast Time Constant video coupling (FTC), as are supplied in new models of radar or in the current change 50 to the SG, is of great help in reducing the effect of Window because these circuits can almost entirely remove echoes that extend over a relatively long range. Thus, IAVC and FTC are effective not only on sea return, clouds, and land echoes, but also on Window. The use of an expanded sweep, when available, is advantageous in combating Window jamming because the Window echoes are spread out on the fast trace, allowing full use to be made of the radar's resolution. If the Window is not sown closely, it may be possible to see targets between the Window return. A radar with a narrow antenna beam is better able to cope with Window jamming than one having a wide beam because the narrow beam provides better bearing resolution.

D. DECEPTION

1. Methods of Radar Deception.

False or misleading radar echoes can be produced in a number of ways, but the problem of making the characteristics of a false echo resemble those of a real echo is a difficult one to surmount. Many deceptive devices reveal themselves as false because they produce echoes that are nearly stationary in space.

Electronic devices have been developed to accomplish several deceptive tasks. For example, a fire

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control radar can be made to indicate an incorrect bearing by synchronizing a device with the lobe switching rate in such a way that the signal returned to one lobe is slightly stronger than the normal echo. As a result the radar operator will match pips on a bearing that may be several degrees away from the true target. Although such devices have been made to work in the laboratory, they are not yet practical for operational use. Very highly skilled operators and rather large equipment are required, and the advantage gained is the production of a bearing error equal at the largest to only half the radar beam angle. Another example is a device that transmits several pulses every time the radar pulse strikes the vehicle carrying it. The target then looks like several targets in a column along the line of bearing. The difficulty with such devices is that they are complicated and that identical sets must be used on every radar frequency to produce coherent deception. It is doubtful that electronic deception will be widely used against our radar by the Japs.

If Window is dropped properly it can produce an echo that looks for a time like a large ship, a flight of large planes, or a flight of small planes. Usually it is not practical to attempt to simulate a surface vessel with Window, since the cloud of reflecting strips would disperse too quickly to lead anyone to believe that the echo was returned from a real ship.

Japanese airplanes have dropped several varieties of deceptive devices other than window. For example, they have used some sort of apparatus that produces a fairly realistic echo, except that the speed of this artificial echo is much slower than that of the plane. These echoes are hard to distinguish without plotting, and since this takes time, the deception accomplishes its purpose of confusing AA fire. Little is known about this device because so far none have been recovered or captured. Another type of decoy reflector that the Japanese have used is apparently attached to a hot-air balloon. The reflecting member of this device is so designed that it can return radar echoes in almost any direction, but because of this property an alert operator usually can recognize it as a decoy, for the echoes are more steady than those from a real target. Other varieties of deceptive devices that may be encountered consist of long metallic streamers connected to balloons or to parachutes, or of floating reflectors like spar buoys, designed to appear like submarine periscopes. The variety of deceptive devices is limited only by the ingenuity of the enemy.

2. Uses of Deception.

The enemy has used deception for several purposes in the past, and as he learns more about our radar, it is to be expected that he will be able to devise new deception methods. In every case, the enemy's purpose is to introduce misinformation on our radar screens as a means of causing nuisance alerts, or in the hope of creating a local superiority for his forces.

By dropping Window, a few planes orbiting may simulate realistically many planes gathering for attack because the genuine targets in such an operation do not exhibit large range rates. Thus, Window can be used to conceal the number of planes in a strike, or as a means of creating a feint attack.

In some cases the Japanese have fitted small craft, such as sampans, with reflecting material in order that the small vessel may return an echo like that from a large ship. The additional reflecting material is much like the screening erected on target rafts used in radar-controlled firing practices. The sampans may then be deployed in a way calculated to reduce our local superiority in an impending engagement. A very effective counter to such a deceptive measure is thorough aerial reconnaissance.

Submarines as well as airplanes can release decoys. One device used by the Germans consists of a balloon tied to a sea anchor by a light wire or cord from which are streamed several long strips of aluminum foil. This device returns an echo that closely resembles the echo from a submarine periscope in both size and character. The deception provided by this device is in general excellent, although not equally good on radars of different frequencies. The echo returned from a decoy of this sort can usually be revealed as false by comparing the plotted track with the speed and direction of the surface wind. Because of the sea anchor the speed of the decoy will be less than that of the wind, but the course must agree with the wind direction.

In most cases the speed of such a decoy is a reasonable speed for a submarine, so that when the track of an object that appears to be a submarine is found to agree closely with the wind velocity, the target must be regarded with suspicion. The uses to which such a clever decoy can be put are numerous. For example, it has been used successfully as a lure to bring a destroyer into position for torpedo attack by the real submarine which lay in wait close by the decoy. In at least one case we lost a destroyer because of the German use of this type of decoy.

Another type of false reflector that is released from submarines is a sort of spar buoy which resembles a periscope. Deceptive devices of this sort are effective primarily against airborne search radar, although they may also be effective decoys against surface search radar. These devices apparently are fitted with a charge that will sink them several hours after their release.

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3. Detection of Deceptive Countermeasures.

Since it is difficult to make false targets return signals that are in all respects the same as the echoes returned from genuine targets, careful observation of all echoes will often reveal deceptive counter-measures. It is especially difficult to create adequate deception by electronic means.

Most mechanical reflectors used for deception move in a direction and at a speed that corresponds to the wind velocity. Therefore, plotting the suspected echo and comparing its track with the speed and direction of the wind will often reveal the false character of the contact.

Since it is difficult to make mechanical reflectors that respond equally to all frequencies, comparison of the echo received on several radars of widely different frequency should reveal some enemy attempts at deception. The repeater PPI system will be of great help in making such a comparison, since several radars can be connected to a single PPI by means of the selector switch.

Deceptive devices cleverly used with a reasonably full knowledge of our radar may often be highly effective. In general, however, careful scrutiny of echoes will enable an experienced operator to distinguish between the real and the false echoes. If jamming is used, even in small amounts that are insufficient to be effective alone, the clutter produced on the indicator will prevent the operator from observing the minor variations in the signal that reveal it as false. Therefore, deception used simultaneously with jamming is likely to prove very difficult to counter.

E. EVASION

Enemy planes often take advantage of the limitations of air search radar to avoid detection, and frequently they maneuver violently to avoid being tracked in altitude. However, with the installation of SP radars on many ships and the use of existing SM radars, along with the several members of the SR family which will soon be available, such evasive action will no longer be fully effective.

Because of destructive interference, the signal strength near the water is very small, which causes low-frequency, low-height radars to be unable to detect low targets at long range. Since air search radar antennas do not elevate, they have no position angle resolution. Therefore, planes that attack by flying from over land are often not detected until they are in position to begin attack. Another result of the lack of position angle resolution is the inability of the air search radars to follow changes of altitude. This limitation allows attacks to be made by approaching low and climbing sharply or by approaching in a long glide as ways of delaying interception. The wider use of low-angle search and height-finding radars such as the SM and SP should help to reduce the success of these tactics.

Captured documents indicate that the Japanese are aware of the limitations on the resolution possible with radar and that they have experimented with formations of airplanes arranged to exploit this weakness. If an attacking group of planes is formed so that the interval between planes is less than that required for the radar to resolve the echoes into separate pips, both bearing and position angle data determined by radar may be inexact. Tactics of this sort may be especially useful against anti-aircraft fire control radar because their use may reduce considerably the effectiveness of the fire. This is especially true of the Mark 4 and Mark 12 radars because the lobe switching used causes these sets to have rather poor resolution in both bearing and position angle.

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