CROSSEYE JAMMING OF MONOPULSE RADARLars Falka, Claes Arvidssonb, Sven Berglundb and Anders Enerothb a FOI, Swedish Defence Research Agency, 172 90 Stockholm, Sweden,
[email protected] FOI, Swedish Defence Research Agency, 581 11Linköping, Sweden,
[email protected] ABSTRACT Radar seeking missiles represent a serious threat towards ships and aircraft. False targets can be produced electronically by crosseye jamming, a technique where small changes are introduced in the wave front to deflect a monopulse radar seeker. Experiments performed by FOI have confirmed the possibility of crosseye jamming using precise cancellation of phase between two transmitters. The principle of reciprocity has been used to analyse the conditions necessary to ensure equal path lengths and the limitations introduced by scattering from the terrain and non-reciprocal components in the crosseye system. 1. INTRODUCTION Radar seeking missiles represent a serious threat towards ships and aircraft. The superior acceleration of missiles makes them difficult to evade. They are also cheaper than their targets and can be shot in salvos. A radar seeker with monopulse antenna is particularly difficult to deflect. It will track all wave forms and noise jamming is consequently ineffective. The missile will keep tracking the signal by moving in a direction orthogonal to the phase front to reach the target. Missiles are traditionally deflected by chaff or noise jamming, but such methods are becoming ineffective. Modern techniques are based on towed decoys and range gate or velocity gate pull off. Imperfections in the target seeker can be used to introduce break-lock, but individual weaknesses are increasingly difficult to identify in a complex battle field. It is necessary to develop new methods of defence which are independent of the technical level of the target seeker. Crosseye jamming is a technique using two antennas onboard to modify the direction of the phase front in order to create a false target near the real one. The process is also called phase front distortion, since the principle is based on the general technique of tracking a target. If the false target is more powerful than the real echo it will attract the missile from the platform. 2. THE CROSSEYE PRINCIPLE The basic idea of crosseye jamming is to produce a false target sufficiently far away from a platform using antennas onboard a ship or aircraft (Figure 1). The false target is produced by repeating radar signals in order to make it as realistic as possible. It is also possible to add modulations and fluctuations to make the signal similar to a real target. This process is easily performed with digital components. Analogue components have been used in many previous crosseye experiments, but digital radio frequency memories (DRFM) are superior in this application π b Figure 1. A sea missile attacking a ship equipped with a crosseye system will detect a false target beside the ship. The idea of crosseye deflection is old, but electronic technique has only recently reached a level where jamming can be successfully performed in practice. Research programs have been going on in several countries, but most of them failed due to technical problems. Only recently Italy and Sweden demonstrated that crosseye may work in practice [1-9]. Crosseye targets are produced by interference between two signal sources of similar strength. The interference leads to angular glint of the same type as in complicated radar targets. If two signals nearly cancel each other the phase front will be distorted and the direction of the target is seen to fluctuate. This effect can be used to produce a false target some distance away from a ship or aircraft using two antennas onboard. The method avoids expensive off board decoys, which often require preparation. Crosseye is cheap, but there are many practical problems, like the need for calibration. The theoretical requirements for crosseye to function in complex environment were first clarified in a Swedish paper [4]. Representatives of many countries declared that crosseye is interesting but can not work in practice. This conclusion was based on the difficult problem of balancing amplitude and phase using analogue components, while wing tip vibrations and propagation were also mentioned as possible sources of error. However, Elettronica demonstrated a digital crosseye system produced for the Italian navy and the Italian version of the Eurofighter (EFA). Experiments have demonstrated a considerable deflection of missile radar seekers in air as well as close to sea and land [3,7]. Progress was based on the appearance of digital radio frequency memories (DRFM). Improved tactical analysis was also important to clarify under which conditions crosseye jamming may be used. By selecting phase and amplitudes correctly one can produce a false target with optimal parameters instead of requiring a system to work under all circumstances. Tactical considerations had previously been neglected in favour of fanciful constructions based on analogue components [11]. These experiments produced poor results, though the reasons for failure remained 1 A geometrical derivation A monopulse seeker affected by noise jamming can switch to Home On Jam mode and continue tracking the target.. The question was how to construct a system that would produce an angular deflection of required amount for any missile. Signals are shown in a polar diagram. The angular error may be computed from formulas describing radar glint [12]. The length of the arrows corresponds to signal strength and the direction indicates phase. A monopulse system can not resolve several targets. since the angular resolution of individual antennas is insufficient. This resolution is attainable with a monopulse radar system if the target is powerful enough and only a single target is located within the lobe. The antenna is limited by the size of the missile. Two out of five Exocet radar missiles launched during the conflict destroyed British ships. A crosseye system consists of two antennas placed at some distance from each other. This problem occurs in another form when monopulse systems are used against targets close to a water surface where reflections take place [12]. even if the radar waves were being scattered during propagation. Target T T ∆ρ Direction of propagation. which has to locate targets within a tenth of a degree. Both antennas repeat the incoming radar signals. 3. A target located directly in front of the radar will give the same phase in both receivers. The principle of monopulse measurements. If the signals are in phase the monopulse receivers will indicate a small phase difference equivalent to a forward position. The false target is produced by two antennas transmitting nearly out of phase in order to produce a displaced target. . If the two signals are out of phase a crosseye target will appear (Figure 5). This is the basis of crosseye jamming because the two antennas cannot be separated. Figure 3. Reflected signals will affect missiles seekers as well as a radar onboard a ship. If one monopulse antenna receives two components the other antenna will measure a small phase difference as shown in Figure 6. If the signals are in phase the target seeker will indicate a position somewhere between the antennas as shown in Figure 4.unclear in many cases. The sum is small but the direction of this arrow (the phase of the signal) is sensitive to the difference in direction between the two arrows (phase difference) and their length (amplitude difference). Radar is suitable for locating ships and aircraft at large range in view of all weather capability. Two antennas transmitting in phase will produce a target between the antennas according to a monopulse seeker. Phase errors will destroy the performance of a crosseye system and probably attract the missile towards the antennas. RADAR TARGET SEEKERS Most missiles use radar or IR to locate targets. The components nearly cancel and the sum is given by the difference between arrows pointing in the same direction. 1 2 Dashed: Dash-dotted: Antenna 1 Antenna 2 Figure 4. A radar seeker is equipped with two receiver antennas to locate the target by monopulse technique. while a slightly displaced target will give a phase difference proportional to angle. Phase difference ∆ϕ Phase difference ∆ϕ ∆ ϕ Phase front 1 2 Seeker antennas 1 2 Figure 2. The direction is obtained by comparing the phase difference of two signals as shown in Figure 2. The Falkland war clearly demonstrated the reality of the missile threat. but it is more instructive to give a geometrical derivation [9]. This ability is exploited in the crosseye system by introducing a false target that the monopulse system can not reject. 3. is ∆ρ Monopulse antenna 1 λ D ∆p The false target will appear several antenna base lengths D away from the antenna system if the signals have about the same amplitude. Both must be small enough to give a sufficient small miss distance. 4. the antenna diameter is less than a meter. ∆ρ ∆ϕ ∆θ = Figure 5. The radar wavelength is just a few centimetres. One jamming antenna Two sources of almost equal strength and nearly out of phase Figure 7. Phase and amplitude for waves produced by one transmitter and two transmitters out of phase (Hyberg [ 10. The sum arrow turns at a rate proportional to the distance between the receiver antennas and inversely to amplitude in Figure 6. The sum signal has small amplitude and its phase depends sensitively on amplitude (∆ρ) and phase (∆ϕ) differences. It appears as target in a direction different from the real one.3. The geometrical analysis indicates that this is not the case. d ≈ 0. The miss distance is independent of range. The sum arrow will obviously rotate faster than the individual components.5 Crosseye antennas are located 10-20 meter apart and can be resolved only if the missile is less than 200 meter away. A monopulse target seeker measuring the phase difference created by a crosseye system. The sum arrow turns approximately 1/∆p faster than two components and this means that the maximum miss distance. X.2 Miss distance A false target produced by phase front distortion will look like a target. which gives an antenna lobe several degrees wide. Direction of propagation. A detailed geometrical derivation gives the same result as the formulas for angular glint [12]. Direction of propagation. The two signals received by antenna 1 are exactly out of phase. because the target seeker can not resolve crosseye antennas until a few hundred meters from the platform. In general X/D = 5-10 would be enough to protect a ship.. The deflection angle is small and may be difficult to observe from a ship until the missile is very close. X= Crosseye phase Monopulse antenna 2 Figure 6. X = D ⋅ cosψ ⋅ ∆p ∆p + ∆ϕ 2 2 The expression D ⋅ cosψ is the projection of the base length in direction of the missile. CHOICE OF PARAMETERS 4. The phase difference (∆ϕ) just has to be smaller than the amplitude difference (∆p).02 ≈ 2o 0. which involve both phase difference (∆ϕ) and amplitude difference (∆p). X. A crosseye target is produced by two almost equally strong components nearly out of phase. The miss distance is computed by noticing that the angle between two components (long arrows) corresponds to a measurement of the distance (D) between crosseye antennas. which explains the term phase front distorsion: two crosseye antennas produce a signal which locally has an oblique phase front (Figure 7)..]). .1 Amplitude and phase The traditional view insists that phase and amplitude differences must be extremely small to create crosseye. there will be problems with circulators and other components which lack the required isolation at high power. 4π The false target should be at least 10 times stronger than the real target to deflect the missile. D B C Pout = (Flux )in ⋅ RCS ∆ϕ = π The power emitted by a crosseye system is determined by wavelength. This wasteful method was compensated by enormous amplification. a construction still mentioned in the literature [11]. since this would inevitably deflect the missile towards the platform. Components must also be reciprocal in order to ensure reliable function. 6. The reversibility of ray paths ensures equal phase change in both directions. D CONSTRUCTION OF A CROSSEYE SYSTEM Crosseye jamming is based on phase cancellation. The effect depends on the reversibility of ray paths and reciprocity shows .4. This means that many constructions suggested in the literature are invalid [11].2 Signal strength The main difficulty is to produce a false target that can compete in strength with a real target in spite of the necessary phase cancellation. Crosseye system based on analogue components. e g by scattering. Pout = (Flux )in ⋅ G 2 ⋅ Gamp ⋅ (∆p )2 ⋅ λ2 Figure 9. amplification Gamp. The power reflected from a target may be written. RECIPROCITY Crosseye systems can be effectively analysed by applying the principle of reciprocity. Interference is particularly sensitive to path length. This means that a crosseye system must have high antenna gain and amplification. No additional phase differences may be introduced. Crosseye systems based on analogue components have been constructed by using the geometrical fact that total propagation paths will be equally long from transmitter to receiver (Figure 9). A much better solution is to use digital components and avoid the apparent advantages of geometrical effects by storing the signal in DRFM. The phase difference is compensated on receive and ∆ϕ = π Figure 10. and the amplitude cancellation factor. phase differences will appear if receiver and transmitter antennas are separated as in Figure 9. Early systems attempted to cover large angles by using low gain antennas. r1 G r2 Power returned by a target and crosseye system. The two paths ABCA and ACBA differ by half a wavelength introduced by a phase shifter. A crosseye system based on analogue components requires linear amplifiers. It is important to determine the miss distance in advance to allow the amplitude factor ∆p to be as large as possible. but the argument does not apply directly to radio waves. However. A G π Gamp RCS Figure 8. A crosseye system based on TWT can handle several missiles transmitting on different radar frequencies. but this is only possible for limited field strengths. 5. which placed unrealistic demands on amplifiers and other components. Moreover. transmit in all directions (Figure 10). antenna gains. G. λ. The geometry guarantees that no phase differences are introduced. The main advantage of a construction using wide lobe antennas is that the system can decoy several missiles attacking from different directions if the band width is large. ∆p = p-1. The following conditions are necessary for the principle of reciprocity to apply [1]. Scattering of radar pulses from water waves and foam may affect a crosseye system. This may happen if a wave is propagating close to ground or over a water surface. Sequential lobing ensures that the propagation paths are equal in both directions. for instance by moving the antenna in a circle (Figure 13). even if they pass a highly asymmetrical water wave. Scattering Figure 11. Radar waves are one millon times faster than sound and the environment remains essentially frozen during a two-way pulse trip. since signals may be scattered from distant points in the terrain. where the same antenna is used for transmit and receive. This can easily be done by using an oldfashioned form of scanning. 2. The significance of reciprocity is illustrated in Figure 12. The argument will be presented in improved form compared with previous presentations [2. In practice the function of this system will not differ from an ”ideal monopulse system” if the targets do not fluctuate. ABSORBERS RAIN REMSOR CHAFF TRANSMITTER (RECEIVER) RECEIVER (TRANSMITTER) REFLECTORS WATER WAVES Figure 12. This principle ensures that signals transmitted and received by two antennas are equal if the feeds are swapped. If the cables feeding the two antennas are swapped the received signal will be the same in spite of the presence of scatterers and linear absorbers. Water waves and foam can affect radar propagation in a complicated manner (Figure 11) and thus influence crosseye. An ideal monopulse system used for analysing crosseye. Losses are linear functions of the field. The environment is stationary. This is far from obvious: radar waves propagating over sea will experience the same phase shift in both direction. Instead of using the reversibility of ray paths one can apply the principle of reciprocity for electromagnetic waves [1]. It is obviously important that the environment is stable so that no phase changes are introduced. Figure 14. because phase changes will be equal in . In order to apply the principle of reciprocity one can define an ideal radar seeker where the propagation paths are equal. Sea missiles travel close to the surface of the sea and the formation of a false target depends sensitively on cancellation between two sources. Even if a scatterer affects the propagation of radar waves it will not necessarily affect the function of the crosseye system. Presented in this way the principle of reciprocity appears unlikely. Wave propagation between two points will thus produce the same change in phase and amplitude in both directions. The medium is linear (superposition applies). sequential lobbing. The principle of reciprocity ensures that the phase differences will be the same both ways for an ideal monopulse system and an ideal reciprocal crosseye system even in the presence of scattering outside the direct propagation path.4]. The principle of reciprocity expresses ray reversibility in general form [1]. Wave propagation in three dimensions is different. 1. Seeker antennas Figure 13.that the false target can appear even under adverse conditions. The principle of reciprocity. π Waves and foam. If the feeds of transmitter and receiver antennas are swapped the signal remains the same in spite of the presence of linear absorbers and scatters. The speed of sound is slow and conditions will usually change during propagation. If the cancellation does not take place the missile will probably attack the antenna system. The final condition shows that crosseye is ineffective against torpedoes. 3. 4. Cross eye system The equations for electromagnetic waves must be linear and invariant under time reversal. Rays of light and signals on communication lines are reversible because they are restricted to one dimension. Falk. December 2000). L. Sweden.] [ 6. and our colleague Per Hyberg for valuable discussions. L. 126 – 130. RVK-99. “The reciprocity principle and cross-eye jamming of monopulse radar seekers. Falk. This argument requires that the crosseye system is completely reciprocal and that wave reciprocity ensures reversibility in the terrain. Karlskrona. May 2004.] L. C.” AOC Conference. december 1993. I Figure 15 this is illustrated by a displaced phase centre for transmit and receive.” RadioVetenskap och Kommunikation 99. 14 – 16 June 1999. L. Falk. 2000. A slightly non-reciprocal crosseye system will be affected by scattering. . Figure 15 illustrates how such an analysis may be performed.] [ 5. M. Stockholm. since this case would be particularly sensitive to nonreciprocity and scattering as shown in Figure 15. P.” AOC Conference. L.] [ 3. since opposite paths are no more equal. December 2000). Karlskrona. Applied ECM (EW Engineering 1995). ”Kompendium i Radarmotmedelsteknik. Arvidsson. B. Rome. e g when a missile is propagating over rough sea. L. All real systems contain some kind of non-reciprocity. “Experimental testing on cross-eye jamming. L. Eneroth. The reciprocity condition shows how to estimate under which conditions crosseye will work and when the system is affected by the choice of components. 178 – 181. “Cross-eye jamming. pp.” Invited paper. FOA-B--00-00617-170--SE. Many constructions found in the literature and based on analogue components fail to satisfy this condition. FOA-B--00-00616-170--SE. pp. e g water waves. The quality of components depends on scattering and can be analysed in general terms as shown in this report. "Vinkelvilseledning mot monopulsradar. We are indebted to Filippo Neri. Falk. C. Many practical considerations are important for the function of a crosseye system. 7. Lägesrapport”. As shown in Figure 15 scatter from areas outside the propagation path.1. This is particularly valuable in those applications where scattering takes place in a complicated environment. It is not difficult to estimate the amount from the scattering geometry.both directions (Figure 14). “Cross-eye jamming in Complicated Environments. In this figure the non-reciprocity is illustrated by an antenna where the phase centre is displaced between transmit and receive. L.] [ 11. RadioVetenskap och Kommunikation 99. in particular • distance between antennas • use of digital components • use of phased arrays ACKNOWLEDGEMENTS [ 10. Van Brunt. 8. 14 – 16 June 1999. The important question is when such a contribution will affect the false target. F.] [ 9. S. AOC Conference. Vittorio Rossi and Andrea Di Martino at Elettronica S.] [ 8.] [ 4. Monopulse Principles and Techniques (Artech House 1984). Zürich.” FOA Rapport A 10052-1. Neri. Las Vegas. FOI rapport FOI-RH—0148 (November 2002). “Physical basis for credibility of cross-eye for ship defense. REFERENCES [ 1. F. Neri. Falk.] [ 12. A crosseye system should in general be reciprocal. Introduction to Electronic Defense Systems (Artech House 1991). CONCLUSIONS Crosseye jamming can be based on general principles.] [ 2. They must be related to tactical requirements. Sherman. A. “Cross-eye jamming and the principle of reciprocity. May 2002.” 3rd International AOC Conference. (FOA Reprints. Sweden. Falk.] [ 7. Hyberg. p. This argument gives an exact formulation of the conditions. It is sensible to avoid signals of nearly equal strength. RVK-99. Las Vegas. Johansson. “Reciprocity in wave propagation.” AOC Conference. This condition depends on how close the signals are in amplitude.] Figure 15. Falk. May 2000 (FOA Reprints. will now give a non-reciprocal contribution to the signal. which should be used to test real monopulse seekers and crosseye system by comparing them with an ideal case. 2000. A.