Electronic Protective Measures (EPM, Electronic Protective Measures) are all the measures that keep your own systems working when the opponent uses the techniques from Electronic Counter Measures. Much EPM has already been covered "along the way", because good radar design choices are EPM: PRI jitter against RGPI and cover pulses (PRI modulation), pulse compression with matched filtering against non-coherent jammers (Pulse compression and pulse integration), CFAR against noise saturation (Threshold, Constant False Alarm Rate and Automatic Gain Control), monopulse against inverse gain (Techniques of the target tracking radar), side lobe blanking and null steering against side lobe jamming (Main lobes, side lobes and back lobes), Doppler discrimination against chaff (Challenges for chaff) and sensor fusion against everything at once (Target acquisition and tracking by IR).
This chapter sets out the thinking behind them. Against jamming, a radar (or weapon system) has three fundamental answers:
- Avoid — make sure the jamming power never reaches you: jamming avoidance (Jamming avoidance).
- Exploit — use the jammer itself as a beacon: home-on-jamming (Home-on-jamming).
- Overpower — simply be stronger than the jammer: burn-through (Overpowering the jamming signal).
On top of that, the procedural layer remains just as important: emission discipline (EMCON), frequency planning, switching between sensors, and operators trained to recognise jamming rather than ignore it. The best EPM is often an operator who notices that "that target" is not behaving like a target.
Jamming avoidance
A jammer can only jam a frequency it knows. If the radar keeps jumping to a different frequency, the jammer is always one step behind.
In practiceModern radars hop per burst or even per pulse. A spot jammer cannot keep up and has to switch to wideband noise, which makes its power per channel a factor of 10–100 weaker. Tactical radios use the same principle (FHSS and DSSS).
Jamming avoidance is the most elegant defence: the jamming power never (effectively) gets into your receiver. After all, a jammer can only jam what it knows and can follow — so you make your signal unpredictable in every dimension:
- Frequency agility: the transmit frequency hops — per dwell, per burst or even per pulse — pseudo-randomly across a wide band. A spot jammer (Spot jamming) chases it hopelessly; the jammer's set-on receiver always has a reaction time, and with pulse-to-pulse hopping there is nothing left to follow. The price for the jammer: it has to go barrage (Barrage jamming) and dilute its power density by a factor of 10–100.
- PRI jitter and stagger (PRI modulation): unpredictable pulse timing breaks everything that relies on predicting the timing — RGPI (Range gate Pull-in), cover pulses (Cover pulse jamming), and placing false targets ahead of the real echo.
- Waveform agility: chirps and phase codes that change from pulse to pulse make DRFM copies (Digital Radio Frequency Memory) obsolete the moment they are played back: the matched filter of the current pulse rejects the copy of the previous one.
- LPI waveforms (Frequency Modulated Continuous Wave radar): low power, wide bandwidth, continuous waveforms — the opponent's ESM receiver detects your radar later or not at all, and what is not detected is not jammed.
- Spatial avoidance: narrow beams and low side lobes (Main lobes, side lobes and back lobes) limit the ways in; side lobe blanking throws away whatever still leaks through; adaptive arrays steer nulls towards the jammer's direction — the radar is made selectively deaf to the jammer.
- Time and place: transmit only when you have to (EMCON), use short bursts, and in networks: the radar that sees the target does not have to be the radar that transmits (bistatic/multistatic, Passive radar).
The common denominator: every jamming technique needs knowledge — frequency, timing, waveform, direction or scan pattern. Avoidance means systematically making that knowledge worthless. That is why modern radars are "agile in everything": frequency, PRF, waveform and beam.
Home-on-jamming
Home-on-jam (HOJ) reverses the roles: the jammer that wants to blind your radar is itself a powerful, continuously transmitting RF source — a beacon. A radar-guided missile that loses its echo in the noise can switch to passive homing on the jamming signal itself (compare passive homing, Passive homing): the seeker measures the direction from which the jamming arrives and steers towards it.
Why this is such a powerful lever:
- The jammer makes things easier for the missile, not harder. Noise jamming (Noise jamming) provides a strong, continuous signal — more comfortable for a direction-finding seeker than a weak echo. No range is needed: proportional navigation (Guidance methods) works on direction alone.
- It forces a devilish dilemma: jammer on = a beacon for the missile; jammer off = the radar can see again. Whatever the target chooses helps the defender. Many systems therefore switch automatically between semi-active/active guidance and HOJ, depending on what the target does.
- It explains how the ECM side evolved: towed and expendable decoys (Decoys) were developed precisely against this — if something is going to home on the jammer, let it be a throwaway transmitter hundreds of metres from the cockpit. It also explains why modern SPJs jam sparingly (deception and cover pulses instead of continuous noise, Cover pulse jamming): the less you shout, the less of a beacon you are.
Related but distinct: the anti-radiation missile (Passive homing) homes on the opponent's radar; HOJ is a mode of an anti-air or anti-ship weapon that homes on its target's jammer. The principle is identical — whoever transmits can be found — and it is the reason why emitting in an EW environment is always a trade-off and never free.
Overpowering the jamming signal
At long range the jammer wins, because its signal only travels one way. As the target comes closer, the real echo grows faster than the jamming and the radar sees through it again.
In practiceFor an aircraft relying only on its own jammer, the last kilometres towards an air defence site are therefore the most dangerous. The defender knows this too: keep tracking the target, because the closer it gets, the better the radar can see.
The third way is brute force: make sure the real echo is stronger than the jamming in your receiver — burn-through. Physics is on the radar's side once the range becomes short enough (Inverse square law, Radar Jamming): the echo grows with 1/R⁴, the jamming signal only with 1/R². As the radar closes on the target, the echo therefore grows faster than the jamming; the crossover point is called the burn-through range:
The radar can actively push that burn-through range outwards — anything that raises S or lowers the effective J counts:
- More energy on the target: higher (peak) power, longer pulses with pulse compression (Pulse compression and pulse integration), longer dwell time on the suspect sector, and with phased arrays (Antenna shapes): extra power and time only in the direction of the jamming — "burn-through mode".
- Processing gain: coherent integration of more pulses (Pulse compression and pulse integration) lifts the echo out of the noise, while non-coherent jamming noise partly averages out; the matched filter gives the real echo its compression gain of a factor of 100–10,000, which the jammer does not get without a perfect copy.
- Weakening the jamming: null steering and side lobe blanking (Jamming avoidance) take tens of dBs off J — every dB less jamming pushes the burn-through range further out.
- Geometry and networking: a second radar looking from a different angle looks past the jammer's beam; multistatic set-ups force the jammer to spread its power over several directions.
Operationally, burn-through means: jamming buys time and distance, not invulnerability. The EOB planner calculates at what range each radar will see through the friendly jamming, and the attacker plans his route and weapon employment within the window that the jamming buys. Conversely, the defender knows: the closer the target gets, the more physics works for him — hold on and keep building the track until the echo is back.