§11.4 Deceptive jamming

Where noise jamming blinds the radar, deceptive jamming lies to it. The jammer receives the radar pulse, processes it and sends it back amplified and manipulated — as an echo that looks completely genuine but reports the wrong range, velocity or direction. The target of deception is almost always a tracking radar: the three gates (range, velocity, angle) with which a TTR holds on to its target (Target tracking by radar: the Target Tracking Radar) can each be played separately.

Deception has two big advantages over noise: it costs little power (the false echo only has to be slightly stronger than the real one) and it is hard to detect — a radar following a ghost thinks everything is fine. The price is intelligence: the jammer has to be able to reproduce the radar pulse exactly and predict its timing — hence the final part of this section, DRFM (Digital Radio Frequency Memory).

Range gate pull-off

Range Gate Pull-Off (RGPO) attacks the range gate. A tracking radar follows the range of its target with a range gate: a narrow time window around the expected arrival of the echo (Target tracking by radar: the Target Tracking Radar). RGPO pulls that window off the target, in three steps:

1. Capture: with every radar pulse, the jammer sends back a copy echo exactly coinciding with the real echo but stronger. The radar's AGC (Threshold, Constant False Alarm Rate and Automatic Gain Control) adjusts to the strong signal; the gate now "hangs" on the false echo.
2. Walk-off: the jammer delays its copy a fraction more with every pulse. The false echo — and the gate following it — slowly walks away from the real target range. To the radar it looks as if the target's range is gradually changing; nothing suspicious.
3. Turn-off: hundreds of metres or kilometres "away", the jammer stops abruptly. The gate stares at nothing; the track breaks. The radar has to reacquire (Target acquisition and tracking) — precious seconds, and the game can start again.

The counters (EPM!): leading-edge tracking (the gate follows the front edge of the echo — a delayed copy by definition arrives after the real echo and does not take hold), PRI jitter (PRI modulation) so that the jammer cannot predict the pulse timing, and logic that recognises physically impossible changes in range (the apparent velocity must agree with the measured Doppler — see also Velocity gate Pull-off).

Range gate Pull-in

Range Gate Pull-In (RGPI) is the mirror image of RGPO: the false echo is sent back not ever later but ever earlier — the target seems to be closer than it is.

But wait: how can an echo arrive earlier than physics allows? By not reacting to the current pulse but anticipating the next one. The jammer measures the radar's PRI (Pulse Repetition Interval and Pulse Width), predicts when the next pulse will leave, and transmits its false echo in advance — timed so that it reaches the radar before the real echo of that next pulse. This only works against radars with a predictable, stable PRI; PRI jitter (PRI modulation) makes RGPI impossible at a stroke — that is why modern radars jitter.

Why would you want a target to appear closer? A few reasons:

• Spoiling the fire control: a weapon that calculates with the false (too short) range misses — proximity fuzes and intercept calculations get the wrong input.
• Making the defender fire too early: a target that seems to be within weapon range while it is still outside tempts the defender to waste his missiles.
• The same track break as RGPO, but with a walk direction that leading-edge tracking (the standard counter to RGPO) does not catch.

In practice pull-off and pull-in are combined and alternated, so that the radar designer is left without any "safe side" of the echo.

Velocity gate Pull-off

Velocity Gate Pull-Off (VGPO) is RGPO, but in the velocity domain. Doppler and pulse-Doppler radars (Pulse-Doppler radar and Moving Target Indicator) and CW illuminators (Continuous wave radar) follow their target with a velocity gate: a narrow filter around the target's Doppler frequency. VGPO pulls that filter away:

1. Capture: the jammer sends back a copy of the radar signal with exactly the Doppler shift of its own platform, but stronger than the real echo — the velocity gate now hangs on the jammer.
2. Walk-off: the jammer gradually shifts the frequency of its copy (a few hundred Hz per second). To the radar the target seems to accelerate or decelerate; the gate walks along with it, away from the target's real velocity.
3. Turn-off: the jammer stops; the gate sits at a velocity where nothing is flying; the track breaks and the radar has to find its target again in the Doppler spectrum — among all the clutter.

VGPO is the weapon against systems that rely primarily on Doppler: pulse-Doppler fire-control radars and semi-active missiles with CW illumination (Semi-Active Radar Homing). VGPO is often combined with a push towards the clutter: the false Doppler walks towards zero (the notch), after which the turn-off "leaves" the target where the clutter filter cannot see the real target anyway — deception and the beaming manoeuvre (Counter-IR tactics) in one.

Counters: a coherent plausibility check — the change in range (the range rate from the range gate) and the measured Doppler must agree with each other; if they diverge, the radar is following a lie. That is why modern deception carries out range and velocity pulls in a coordinated way (RGPO + VGPO together, kinematically consistent) — which is hardly feasible without DRFM (Digital Radio Frequency Memory).

Angle deceptive jamming

The third gate is angle: the direction in which the antenna holds the target (Techniques of the target tracking radar). Angle deception is the supreme discipline — whoever breaks the angle track breaks everything, because without direction there is no weapon solution. The techniques target the weaknesses of the angle-tracking method:

Inverse gain jamming (against conical scan)

A conical-scan radar (Techniques of the target tracking radar) derives its pointing error from the amplitude modulation of the echo as the beam goes round. The jammer measures that scan rhythm (or guesses it, in the case of silent lobing, from known system data) and sends back pulses modulated in antiphase with that modulation: strong where the echo should be weak and vice versa. The radar calculates a pointing error with the wrong sign, corrects in the wrong direction and pushes its own beam off the target.

Techniques against monopulse

Monopulse (Techniques of the target tracking radar) obtains its angle error from a single pulse and has no scan rhythm — inverse gain does not take hold. That is why there are specific (and much more difficult) techniques:

• Blinking jamming with several platforms (Blinking jamming);
• Cross-polarization jamming (Cross-polarization);
• Cross-eye jamming: two transmitters on the platform itself (e.g. nose and tail) send the same signal in antiphase. At the radar antenna this creates a distorted wavefront, so that the monopulse system apparently places the source outside the platform. Technically very demanding (phase stability to a fraction of a wavelength), but one of the few single-platform options against monopulse.
• Terrain bounce: reflecting the jamming signal off the ground beneath the platform, so that (semi-active) seekers dive at the mirror image below the target.

The common thread: angle deception has to attack the physics of the measurement (wavefront, polarisation, multiple sources) because the information flow of monopulse itself has no exploitable time pattern. That makes it expensive — and explains why monopulse became the standard (Techniques of the target tracking radar).

Blinking jamming

Blinking jamming uses two or more platforms (or two jammers placed far apart on one large platform) that jam alternately — blinking, typically a few times per second.

The mechanism: a tracker (monopulse included!) that sees two unresolved sources within its beam points at the energy centroid of the two. If the sources blink in turn, that centroid jumps back and forth. The consequences, increasing with the blink rate:

• the antenna's servo system (or the missile's steering command) starts oscillating between the two sources and never reaches a stable solution;
• if the blink rate is chosen around the natural frequency of the track loop, the oscillation can resonate and break the track completely;
• a missile steering at the swinging aim point wastes energy on useless corrections and, at best, flies exactly between the two platforms (the centroid is, after all, empty).

Blinking is thus a classic formation tactic: two aircraft that stay within one radar beam(width) of each other and let their SPJs blink in sync are much safer together than each on its own. Conditions: their separation must be smaller than the beamwidth at that range (Angular resolution) — otherwise the radar resolves the two sources and the trick is over — and the timing requires a data link or a tight procedure.

Counters: narrower beams (higher frequency, larger antenna), better resolution in range/Doppler to separate the platforms after all, and track logic trained to recognise centroid oscillation.

Cross-polarization

Cross-polarization jamming ("cross-pol") attacks the angle measurement through a back door: polarisation (Polarisation of the electromagnetic wave). Every real antenna is designed for one polarisation, but for the orthogonal polarisation it has a different, distorted reception pattern — with peaks where the normal pattern has nulls (the so-called Condon lobes). The monopulse network, which derives its angle error from comparing patterns, therefore produces wrong, often reversed angle errors for a cross-polarised signal.

The jammer exploits this by sending back the radar signal with a polarisation rotated by 90° relative to the original, and strong enough to overpower the real (co-polarised) echo. The radar calculates pointing errors based on its distorted cross-polar pattern and steers its beam — sometimes in large jumps — off the target.

Limitations and counters:

• High demands on the jammer: the polarisation has to be very purely orthogonal (contamination by a few per cent of co-polarised signal spoils the effect), and the power must comfortably overpower the real echo — because the antenna receives the cross-polar signal heavily attenuated.
• Antenna quality is the defence: the lower the cross-polar response of the radar antenna (good reflectors, polarisation filters/grids), the more power the jammer needs — up to the point of being unachievable.
• It works best against antennas with mediocre polarisation purity (simple reflectors, some seekers); less well against high-quality arrays.

Cross-pol neatly illustrates the pattern of this whole chapter: an obscure, unintended property of the system (the cross-polar pattern) becomes the point of attack — and the countermeasure is not a black box but simply better design.

Digital Radio Frequency Memory

All the deception techniques above stand or fall with one capability: being able to reproduce the radar pulse exactly. Against modern radars with pulse compression (Pulse compression and pulse integration) this is crucial — the matched filter rejects anything that does not carry the internal modulation (chirp, phase code), and an inaccurate copy loses the compression gain of a factor of 100–10,000. Analogue memories (delay lines) could not do this. Digital Radio Frequency Memory (DRFM) can.

A DRFM works like this:

1. The received radar pulse is digitised over a wide bandwidth (high-speed ADC) — including all phase and modulation information, coherently.
2. The samples are held in digital memory and are perfectly and endlessly reproducible — no degradation, however often and however late the copy is played back.
3. Before sending it back, the processor manipulates the copy at will: delaying it (RGPO Range gate pull-off), advancing it relative to the predicted next pulse (RGPI Range gate Pull-in), shifting it in frequency (VGPO Velocity gate Pull-off), modulating it (inverse gain Angle deceptive jamming), and multiplying it into dozens of false targets, each with its own kinematically consistent range and Doppler.

That last point is the core of the modern threat: coherent, consistent lies. A DRFM echo passes the matched filter (full compression gain — to the radar the false echo is just as "real" as the target), and range and Doppler data can be made to agree, so that plausibility checks see nothing. DRFM is therefore at the heart of practically every modern self-protection jammer and of towed/expendable active decoys (Decoys).

The race thus shifts to the radar side (EPM, Electronic Protective Measures): waveforms that change from pulse to pulse (so that a recorded pulse is already out of date by the next one), PRI jitter, non-repeating (random) phase codes, and searching for the small imperfections that every DRFM chain still has (quantisation noise, processing delay — a DRFM copy always arrives at least tens of nanoseconds late, which leading-edge analysis can reveal).