Electronic countermeasures

Two EA-6 distance disruptors each with three AN / ALQ-99 containers for EloGM

Electronic countermeasures ( EloGM ; English electronic countermeasures - ECM) are part of the electronic combat (EK) in addition to the electronic protection (EloSM) and support measures (EloUM ). They aim to prevent or limit the effective enemy use of the electromagnetic spectrum by using electromagnetic energy.

overview

Before taking countermeasures, it is necessary to obtain as much information as possible about the systems that are to be affected. This is the task of electronic support measures ( English measure electronic support, ESM ). All electromagnetic energy emitted by the opposite side is captured, located and recorded here. The appropriate countermeasure is selected through the evaluation.

Radar systems are usually to be disturbed, and radio connections are less common. If the encryption is cracked in a radio connection, Gromolo can be sent in addition to garbage to block opposing communication. Usually this is not the case, however, as frequency spreading and encryption stand in the way. Radio body armor such as Hummel or aircraft for electronic warfare such as the EA-6B therefore usually send white noise so that the radio signal at the receiver is drowned out in the noise of the jammer.

There are a number of jamming techniques in radar systems, which are listed below (not in full). In principle, the jammers work here in two different ways: with transmission pulses (introducing false targets) and with white noise (masking existing targets). Active extinction is a special case because, although transmission pulses are used (if the radar to be disrupted is not a continuous wave radar ), false targets can also be generated.

The principle of interference from transmitted pulses is simple: a pulse radar pings into the room and listens for an echo. The jammer now pings the radar permanently and generates so many echoes which, depending on the size of the side lobes , can extend over a larger angular range, so that the radar no longer knows which input signal belongs to the transmission pulse. This is done either directly (impulse response disturbances) or indirectly (ground bounce), with phase manipulation (cross-eye, cross-polarization) in order to solve a connection (gate pull-off), or in a team (blinking).

EA-18G growler

In the event of interference by white noise, the jammer transmits simultaneously on all frequencies used by the opposing radar. As a result, the signal-to-noise ratio on the radar deteriorates (the receiver sensitivity decreases) and the effective range of the radar decreases. Depending on the size of the side lobes, the reduced detection range extends over a larger angular range. Since modern radars like the AN / APG-63 change the transmission frequency with each transmission pulse, the transmission of noise is the most effective type of radar interference. This makes jammers like the Boeing EA-18 Growler useful, because they can reduce the effective range of radars in a certain angular range (in extreme cases, the receiver overrides the radar, making it useless). However, there is usually the case that the echo of the radar protrudes from the transmitted white noise from a certain distance. This distance is referred to as burn-through distance ( English burn-through range hereinafter), is no longer concealed below the noise interfering with the radar target. The reason for this is the fact that halving the distance quadruples the energy density of the interference signal, but a 16-fold decrease in receiver sensitivity would be necessary to achieve the same effect.

In contrast to pulse radar , the transmitters of FMCW radars work continuously for the duration of the measurement process, so that the interference from transmitted pulses has no effect. Noise interference is mostly the only ECM solution here, whereas FMCW radars are more sensitive due to their low transmission power. Furthermore, FMCW radars can be controlled more easily in home-on-jam mode by missiles such as the AGM-88 HARM and Ch-31 .

techniques

Incorporation of false targets

Impulse response interference

In the case of impulse response disturbances, the greatest possible number of decoy targets is simulated in the receiver of the radar device by sending out short impulses. Their repetition frequency is either not synchronous to the pulse repetition frequency of the radar device, or the repetition frequency is synchronous to the pulse repetition frequency of the radar device, or is even derived from its own transmission pulse ( English repeater jammer ). The impulse sent out for the purpose of deception then has a different distance, a different side angle or a different speed than the real target sign. In the case of radar devices with automatic target recognition (plot extractor), the process computer can quickly reach the limit of its capacity. For example, the ST-68U can only automatically process up to 128 target characters, of which only 32 targets are then recognized as real target characters and automatically reported and accompanied. However, if this processing channel is overridden by a large number of decoy targets, some real target characters are lost or the process computer no longer manages to correlate all targets.

Digital Radio Frequency Memory (DRFM) is necessary so that the interference signal can be derived from the transmission pulse of the radar device. With this technology, the received signal is digitized and stored and can be processed and broadcast if necessary. As a result, the signal can be emitted with a delay, overlaid with a Doppler shift in order to fool the speed measurement of the radar, or sent specifically into the sidelobes of the radar in order to generate a decoy at a certain position.

Ground bounce

With ground bounce, a directed interference signal is sent at an angle to the ground, reflected from there and reaches the monopulse radar seeker of a guided missile. For semi-active and active missiles, the wave front comes from the direction of the ground in such a way that the missiles descend in home-on-jam mode (HOJ) until they collide with the ground. The jammer needs a certain amount of power for this, since the scattering losses on the ground have to be compensated for, and it must be directed at least in the elevation angle. The side lobes of the jammer must also be small in order to prevent a direct approach to the jammer in HOJ mode.

Cross-eye

Monopulse antennas can determine the angle to the target with just one pulse, as these usually have four feed lines. When the radar sends a pulse into space and the signal is reflected from a target to the right of the antenna, the wavefront of the echo arrives first on the right side of the antenna, then on the left. From the time difference between the received signals, the angle to the target can be determined, at which the antenna is then oriented. Cross-eye jamming manipulates this process by creating an oblique wavefront of the target so that the viewfinder chases a decoy.

For this purpose, two spatially separate transmitting and receiving antennas are required, which are connected to one another. A path shifts its receiving signal 180 degrees in phase to cancel the signal in the direction of the radar. Phase and amplitude controls are also included in a path to coordinate the repeater jammers with one another. This ensures that the signals from the two coherent jammers have the same amplitude and 180 ° phase shift, regardless of the angle to the radar. To be successful, the process must mask the target's true echo, which requires an interference signal-to-signal ratio of at least 20 dB.

Cross polarization

Some monopulse radar antennas output incorrect angle information if the received signal is polarized orthogonally with respect to the antenna polarization. This turns the monopulse antenna away from the signal instead of towards the signal as usual. If the normally polarized component outweighs the orthogonally polarized component, the jammer can be approached in HOJ mode. To prevent this, it must be possible to control the polarization angle with an accuracy of ± 5 °. Planar antennas and those with polarization filters cannot be disturbed with cross-polarization.

Gate pull-off

Gate pull-off is used when a radar system has already captured a target. To avoid interference and to improve the signal-to-noise ratio , the search system uses the measured Doppler effect to define a small distance (range gate) and velocity window (velocity gate) and masks out all incoming signals outside this window. Using Range Gate Pull-Off (RGPO) and Velocity Gate Pull-Off (VGPO), an attempt is now made to manipulate the signals in such a way that the target is apparently outside the window, which then breaks target acquisition and the radar returns to the Switch search mode to find the target again.

To achieve this, the pulse repetition frequency is determined and the incoming signal is initially emitted weakly and unchanged. Over time, the emitted signal is increased until it exceeds the target's radar echo. In order to prevent overloading, the radar now reduces its sensitivity, whereby the radar echo of the target is drowned out in the background noise. Now the radar is connected to the signals of the jamming system instead of the target. Next, another signal is generated which is delayed from the first and simulates a target at a different speed. The first signal is now continuously weakened, while the second becomes stronger and stronger. As a result, the radar switches to the second signal, the speed of which deviates further and further from that of the target. The speed window remains fixed on this apparent target, so that the radar echo of the real target is masked out. If the jammer stops its activity, the decoy disappears and due to the incorrectly defined speed window, the radar can no longer lock onto the real target and has to go back to search mode. Ideally, (eg. Tow or jammer is at the end of the process, the window in the region of other moving objects chaff ), so that the radar aufschaltet to this and thus is temporarily neutralized, at least.

Blinking

When blinking, several spatially distributed jammers emit a monopulse radar at different times. The radar therefore quickly changes target. If the blinking is fast enough, the radar servo can no longer keep up and the target is lost. If the blinking is even faster, the monopulse radar will average the sources of interference and head for a point between them.

Masking of existing goals

Noise jamming is a very simple form of interference, which is why it was used very early on. This technique uses white noise to attempt to worsen the signal-to-noise ratio of the receiver to such an extent that it can no longer receive the original signal. There are different types of noise jamming. The active cancellation of radar signals and the disruption of imaging processes, on the other hand, is relatively new, since fast computers with high computing power are required for this.

Broadband noise interference

For broadband noise interference ( English Barrage jamming ), the entire receiver is available bandwidth disturbed. However, since this usually only uses a fraction of this bandwidth, this technology is very ineffective, as large amounts of energy are required even to disrupt weak signals. This problem is exacerbated the larger the bandwidth of the receiver and the smaller the bandwidth of the signal. However, it is advantageous that barrage noise jamming is the only form of interference that cannot be neutralized by changing frequencies. The frequently used ECCM technology of frequency spreading is also ineffective here.

Targeted noise interference

In order to increase energy efficiency, with targeted noise interference ( English spot jamming ), only the frequency range that is used by the current signal is disturbed. However, this requires a quick measurement of the signal frequency and bandwidth as well as the possibility of quick frequency changes. Both frequency spreading and frequency switching are suitable countermeasures against this form of noise jamming.

Modulated noise interference

Modulated noise interference ( English Swept jamming ) are a further refinement of the spot-jamming technology. The emitted signal is significantly narrower and covers only a small part of the receiver bandwidth. The frequency of the interfering signal is changed at high speed so that it traverses the receiver bandwidth extremely quickly. In this case, the signal to be disturbed can usually not be completely covered, but it is highly likely that at least part of the transmission will be disturbed. Many radio or radar systems have problems using the partially disturbed signals effectively.

Pulsed noise interference

When cover pulse jamming a broad band, long noise pulse is generated, which covers the gate of the radar. To do this, the jammer needs to know when its own platform is being illuminated by the radar in order to start noise jamming shortly before that.

Active extinction

In the active cancellation ( English Active Cancellation ) sends the jammer signals which, frequency, pulse repetition frequency and polarization are identical to the radar signal in amplitude, but phase-shifted by 180 °. To do this, the jamming system must have a database with the radar cross-section (RCS) of its own platform from every angle, so that it can calculate the radar echo on its own object and accordingly send a signal in the direction of the radar, which cancels the echo. Since the calculation of the self-RCS is easier at low radar frequencies, this method is easier to apply here. According to simulations, this method can also be used to hide large warships such as helicopter and aircraft carriers from the radar. Only relatively low levels of power are emitted. Depending on whether the amplitude of the radar is hit perfectly, either a complete suppression of the radar echo or only a reduced RCS results.

Image disturbances

Picture disturbances ( English image jamming ) serve HRR, SAR - and ISAR imaging method to manipulate, so that a non-cooperative target identification for. B. spends a Eurofighter for a Su-30. It is also possible to manipulate the creation of SAR images of the soil in order to generate fake landscape images. This requires two spatially separate jammers that coherently irradiate the radar. The received signal is digitized (DRFM), the interference signal is applied and then re-emitted using the pulse compression method .

Application considerations

Warplanes

Older jammers for combat aircraft such as the AN / ALQ-131 emit non-directional radiation, i.e. only forwards and backwards. If the carrier aircraft is detected by a radar, the jammer pings through all frequencies of the band. If this z. B. is a Schuk-MSE that works in the X-band (8-12 GHz), the ALQ-131 will send out pulses on 10.3 GHz, 8.9 GHz, 11.7 GHz and so on. Since modern radars are frequency-agile and change their transmission frequency with each pulse, it rarely happens that the jammer sends out a signal on frequency X at the very moment when the radar is waiting for an echo with the same frequency. However, due to the broad radiation, it does not matter whether two or twenty X-band radars are in the transmission range of the AN / ALQ-131. Interfering with white noise would not be effective because the effective radiated power is very low due to the omnidirectional radiation.

Rafale at the 2007 Paris Air Show

Modern jamming systems like the SPECTRA the Dassault Rafale work with Active Electronically Scanned Array , so the noise energy can be directed specifically to a radar. It is also possible to transmit on different frequencies at the same time and to form several signal lobes . The potential for interference increases significantly compared to the above scenario. If a Rafale z. B. competes against two Su-30MKK with Schuk-MSE, the mechanical swiveling of the Schuk-MSE can be used: Since the system can calculate when the opposing antenna will pass, pulsed noise interference can be emitted in the direction of the radiating radar to prevent detection. At the same time, before and after the calculated antenna passage of the main lobe, impulse response interference can be emitted onto the radar sidelobes in order to generate false targets at a different position in space. With Digital Radio Frequency Memory (DRFM) , the transmission pulse can be manipulated and repeated in order to generate false targets despite frequency agility.

Due to the erratic scanning of the search volume, no targeted interference is possible against a Schuk-MFS with a passive phase-controlled antenna, ie permanent white noise or impulse response interference must be transmitted to the radar. It is no longer possible to schedule the work. The interference energy from the transmitter can no longer be fully focused on one radar when it is approaching the antenna passage, but must be divided between both radars. In this case, the effective radiated power can no longer be sufficient to protect one's own aircraft from being detected, so that both Schuk MFS antennas can only be affected with impulse response interference by the AESA jammer by forming two signal lobes on the radars.

If an active guided missile with a monopulse antenna such as the R-77 is fired on older aircraft such as an F-16 with AN / ALQ-131 , the jammer is almost powerless: Weak noise interference or impulse response interference would only allow the missile to operate in home-on-jam mode Lead the aircraft, since only the distance measurement but not the angle measurement would be disturbed. Since missiles usually strive for an interception course at which the angle to the target remains constant, the jammer guides the missile perfectly into the target in both cases. AESA jamming antennas with directional signal lobes can use ground bounce to direct the missile into the ground, deflect it with cross-polarization if possible or use blinking in a team with two or more machines . With two AESA jamming antennas per aircraft, which can irradiate a guided weapon at the same time, deflection by Cross-Eye is also possible.

In principle, electronic countermeasures are always applied en masse, as all modern combat aircraft have ECM antennas (exception F-22). Since modern combat aircraft also have AESA radars, which can themselves be used as AESA jammers with high transmission power, the result is an unmanageable jumble of different radar and jamming technologies. At the same time, distance jammers like the Boeing EA-18 are used to bring ECM antennas with high radiation power to the front, and external jammers like GEN-X are used to link monopulse radars. Examples of systems used by the military are:

Soldiers inspect an AN / ALQ-184 Electronic Attack Pod

Warships

AN / SLQ-32 (V) 3, the lower surfaces on the left and right are the EloGM antennas.

Systems for electronic countermeasures are also used on board warships . These are used to defend against marching and anti-ship missiles together with a short-range defense system (CIWS) and decoys . An early system was the AN / ULQ-6 decoy transmitter, which was first used in the US Navy. The system enabled a coarsely directed radiation in a certain sector through the central antenna group, which can be swiveled horizontally by a servo . The system was replaced by the passive phased SLQ-17 in the 1970s. This made it possible for the first time to focus the disruptive energy on a target, and it also made it possible to compensate for the ship's rolling movements. Due to the advanced technology, however, system failures occurred more frequently, so that the system was thoroughly revised by Hughes in 1985. The system was replaced by the AN / SLQ-32 currently used on numerous US ships (especially cruisers, destroyers and large amphibious vessels) , which works on the same principle. A similar development took place at the same time in the navies of other countries. In the German Navy , the frigates of the classes F123 and F124 z. B. the EADS EloKa system FL 1800 S II in use.

The main problem with these systems is that the need for jamming energy is proportional to the radar cross section (RCS) of the target. Since ships have a very large cross-section, very high effective radiation powers are required. The SLQ-32, for example, has an output of up to one megawatt. Since ship-to-ship combat no longer plays a major role today, and has been replaced by combat against aircraft and missiles, the possibilities for interference are limited. The jammers try to interfere with the radar systems of aircraft in order to prevent the dropping of bombs and the launching of anti-ship missiles. The same applies to the disruption as above in the section "Combat aircraft". Since the SLQ-32 works with passive beam swiveling, the X-band of fighter aircraft radars can only be "swept through" by modulated noise interference; With the APAR, however, broadband noise interference should be possible thanks to AESA technology. An approaching anti-ship missile cannot be repelled with on-board EloGM antennas alone, as it can approach the interferers in home-on-jam mode. That is why baits like Nulka are used, which are intended to deflect missiles with impulse response disturbances from the ship. Since external systems like Nulka cannot generate noise interference in the kilo- or megawatt range, impulse response interference is the only option, which, unlike noise interference, is unsafe.

Web links

YouTube: US Navy training film about the operation of the AN / ALQ-35, AN / ALQ-41/51 and AN / ALQ-55 (1962)

Individual evidence

↑ Radar tutorial: importing false targets , accessed on September 22, 2013.

^ ^A ^b ^c ^d ^e D. Curtis Schleher: Electronic Warfare in the Information Age , Artech House Radar Library, ISBN 0-89006-526-8 Chapter 4; PDF; 1.7 MB

↑ ^a ^b Kalata, Chmielewski: Range gate pull off (RGPO): detection, observability and α-β target tracking , Proceedings of the Twenty-Ninth Southeastern Symposium on System Theory, 1997

↑ ^a ^b Townsend et al .: Simulator for Velocity Gate Pull-Off electronic countermeasure techniques , IEEE Radar Conference, 2008.

↑ ^a ^b ^c Radar Tutorial: Masking Existing Real Targets , accessed September 24, 2013

↑ NATIONAL AIR INTELLIGENCE CENTER WRIGHT-PATTERSON AFB OH: Overall Early Warning Antiaircraft Jamming Technology in National Territorial Air Defense Systems (II). , 04 DEC 1995

↑ Qu et al .: Active cancellation stealth analysis of warship for LFM radar , 2010 IEEE 10th International Conference on Signal Processing (ICSP), 24-28 Oct. 2010

↑ Osman, Alzebaidi: Active Cancellation System for Radar Cross Section Reduction , International Journal of Education and Research, Vol. 1 No. 7 July 2013 (PDF; 376 kB)

^ Norwegian Defense Research Establishment: DRFM Modulator for HRR Jamming , "NATO RTO Target Identification and Recognition Using RF Systems", Oslo 11-13 October 2004 ( Memento of the original from May 17, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and not yet tested. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2
↑ Stefan Terzibaschitsch: Combat systems of the US Navy . Koehler, 2001, ISBN 3-7822-0806-4 , pp. 190-200 .

[1] Radar tutorial: importing false targets , accessed on September 22, 2013.

[artech-2] A ^b ^c ^d ^e D. Curtis Schleher: Electronic Warfare in the Information Age , Artech House Radar Library, ISBN 0-89006-526-8 Chapter 4; PDF; 1.7 MB

[kal-3] Kalata, Chmielewski: Range gate pull off (RGPO): detection, observability and α-β target tracking , Proceedings of the Twenty-Ninth Southeastern Symposium on System Theory, 1997

[town-4] Townsend et al .: Simulator for Velocity Gate Pull-Off electronic countermeasure techniques , IEEE Radar Conference, 2008.

[mask-5] Radar Tutorial: Masking Existing Real Targets , accessed September 24, 2013

[6] NATIONAL AIR INTELLIGENCE CENTER WRIGHT-PATTERSON AFB OH: Overall Early Warning Antiaircraft Jamming Technology in National Territorial Air Defense Systems (II). , 04 DEC 1995

[7] Qu et al .: Active cancellation stealth analysis of warship for LFM radar , 2010 IEEE 10th International Conference on Signal Processing (ICSP), 24-28 Oct. 2010

[8] Osman, Alzebaidi: Active Cancellation System for Radar Cross Section Reduction , International Journal of Education and Research, Vol. 1 No. 7 July 2013 (PDF; 376 kB)