A Conversation for Radar - ECM/ECCM
Monsignore Pizzafunghi Bosselese Started conversation Jul 18, 2001
An example of the ECM-ECCM cycle:
A tracking radar must be capable of precise angle measurements, even if the beamwidth cannot be made as small as desirable. An early idea to accomplish comparably good results is the Conical Scan (or ConScan) technique: The beam is made to rotate around the center axis of the radar dish, by using a feed which is slightly offset from the symmetry axis and rotates or nutates about this axis. If the antenna is somewhat misaligned with the target direction then the echo will show an *alternating* strength. This is because there are times when the beam is right on spot, and times when it isn't. If the dish is fully aligned with the target then the echo will be constant in amplitude. The presence of a varying amplitude indicates a misalignment, and the time of maximum echo, in relation to some 'index' beam position, indicates the direction into which the dish should be moved in order to get the target into the center. This technique is able to measure angles down to 0.5° even with a beamwidth of some 8° (for example, the German Würzburg radar of WWII).
A countermeasure (ECM) was soon developed:
An on-board receiver detects the illumination signal and also detects a varying signal strength (because the beam sometimes is on target and sometimes it isn't). The jamming technique is to fool the radar with a replica of the signal which is transmitted from the jammer. The rule is to use a *strong* replica when the illumination signal is weak, and a *weak* jamming signal when the illumination is strong. The technique works because the jamming signal is *added* to the real echo and either
* evens out the weak phases of echo (fooling the radar into believing it was right on spot), or
* over-compensates the weak phases, and has the radar dish move *away* from the target.
Because of the 'weak when strong - and strong when weak' rule this technique is called 'Inverse Amplitude' or 'Inverse Gain'. In a taxonomy of jamming techniques, Inverse Gain is sorted into the 'Angle Deception' kind because the radar still measures range correctly and it's the angle indicator which shows the wrong readings.
A counter-countermeasure (ECCM) was developed somewhat later:
Inverse Gain relies on measuring the parameters of the rotating beam. The point which lead to the development of an ECCM was that there is no need to let the radar beam exhibit a rotating movement when transmitting the signal. It is fully sufficient if the 'rotating' is done only on the part of the radar's receiver. One way to do this is to use two antenna dishes, one with a fixed beam for transmission, and the other with a rotating beam for reception. This type of radar is called COSRO (Conical Scan on Receive Only) or 'LORO', for Lobe-On-Receive-Only.
The counter-counter-countermeasure (ECCCM) was the following:
COSRO is effective in denying the jammer any information about the scanning frequency (how fast does the beam rotate) and scanning phase (when is the beam on target). But the jamming equipment can 'assume' that there's a COSRO radar around, and more or less blindly *sweep* its signal through a range of probable scanning frequencies in a repeting cycle. This technique is called SSW (for 'Swept Square Wave'). It doesn't protect the aircraft at 100% probability, but it's better than nothing.
The counter-counter-counter-countermeasure (ECCCCM) and the related ECCCCCM didn't need overly long to make their appearance, but you'll already have got the idea of the 'game' now: Electronic Warfare is a loop.
* takes a breath *
Monsignore Pizzafunghi Bosselese Posted Jul 18, 2001
Range Gate Pull-Off (RGPO)
As was already explained, a Range Gate is employed to blank out all echos which originate from distances other than the target's range. Actually, there are two of them, one for closer ranges and the other for more remote ranges. Engaging the Range Gates is one of the steps which are called 'Lock-On': the radar has found a target and no longer looks out for more of them. But Range Gates can be 'stolen'. RGPO works as follows:
Upon detection (or assumption) that a tracker has locked on, the on-board jammer is switched on and starts to work in a couple of phases:
* first, a sample of the illuminating pulse signal is taken. This sample is amplified and sent back immediately, which at first seems not a good idea as the aircraft highlights itself on a radar screen. Phase 1 continues until the replica is much stronger than the echo from the aircraft's body (the 'skin echo'). The sensitivity of the tracking radar's receiver must now be reduced to avoid overload. This, in turn, has the effect that the skin echo vanishes below the noise floor.
* Now it's time for Phase 2: *another* replica is transmitted after each of the 'dummy' skin echos, and increases in power while the dummy is made weaker.
* In Phase 3 the tracker has *locked on to the replica*, while the skin echo has sunk into the noise. With respect to each of the radar's pulses, the replica is now delayed by small, but increasing amounts of time. The range gates will, of course, follow the dummy target which appears to be receding. This continues until the range gates have been moved away from the target's real position. The result is that the radar is tracking a phantom target and the skin echo is being blanked out by the range gates.
* Phase 4 is simple. The jammer is switched off and leaves the radar with just *nothing* in its field of view. It has to switch back into a search or acquisition phase, and after that, the whole cycle can start again.
In a taxonomy of jamming techniques, RGPO is sorted into the 'Range Deception' kind because the radar still measures angles correctly and it's the range indicator which shows wrong readings.
Techniques like Inverted Gain, SSW or RGPO are some of the features which are commonly combined in a set which is either built into an aircraft or carried below a wing, as a pod. There is one property which they have in common: they are only suitable for protecting this one aircraft, and hence the equipment is referred to as a Self-Protection Jammer, or SPJ. There is no great deal in employing the above techniques from some point outside the ground radar's beamwidth.
'Blinking' is a jamming technique for mutual protection of pairs of aircraft. The SPJ of the jets are switched in an alternating fashiong, and either transmitting one of the jamming signals mentioned above, or just repeating replicas of the illumination signal. The technique is used as a defense against incoming missiles, and its effect is somewhat like 'mum' and 'dad' playing with the dog: dad throws a stick, and Barky runs away to fetch it. When Barky returns, dad starts crying 'bring it here'. Once the dog has turned into dad's direction, mum starts crying 'bring it here'. Then it's dad's turn again. Rather than heading for mum *or* dad, Barky will follow some wiggly course, and end up completely confused in the middle between the parents. That's exactly the idea behind the Blinking technique: with some luck, an anti-aircraft missile aimed at the pair of aircraft will pass *between* them without creating any damage.
Protecting a *group* of aircraft by using Self Protection Jammers is expensive and consumes considerable amounts of payload capacities. This task is performed by dedicated aircraft which are further divided into Escort Jammers and Standoff Jammers.
* takes another breath *
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