Radar lives at the front line of electronic warfare. There is a long, ongoing arms race between electronic countermeasures (ECM), which aim to deny or deceive radars, and electronic counter-countermeasures (ECCM), which aim to defeat ECM.
13.1 Jamming
Barrage jamming: blast a wide range of frequencies with high power. The radar's receiver saturates across the jammed band. Crude but effective with enough power. Defeats: frequency agility, spread-spectrum waveforms, and high antenna gain (a jammer outside the main beam contributes much less than a target inside it).
Spot jamming: focus all the jammer's power on the radar's operating frequency. More efficient than barrage, but the jammer must know the radar's frequency, which the radar can change.
Deceptive jamming (DRFM): a digital RF memory captures the radar pulse, modifies it (delay, Doppler, multiple replicas), and re-transmits. The radar sees a fake target that looks genuine. Defeats: leading-edge tracking, monopulse, multi-PRF cross-correlation, and home-on-jam (since a bright jammer is itself a bright beacon).
13.2 Chaff
Drop a cloud of small metal strips, each cut to roughly half the radar wavelength, behind your aircraft. The chaff cloud has an enormous radar cross section over the band the strips were tuned for, and it follows the wind. The radar's tracker, especially old non-monopulse trackers, will be drawn to the chaff cloud and ignore the actual aircraft.
Chaff was first used operationally by the British in 1943 (codenamed "Window"), sometimes credited as the most cost-effective counter-radar measure ever invented. It still works in modern conflicts, alongside flares (against IR-guided missiles) and DRFM jammers (against radar-guided ones).
13.3 ECCM techniques
- Frequency agility: hop carrier frequency pulse-to-pulse, randomly. A jammer that does not know the next frequency cannot jam it.
- PRF stagger: vary PRF too. Thwarts deceptive jammers that try to inject false echoes at the expected pulse intervals.
- Sidelobe blanking: a separate auxiliary antenna with low gain detects emissions arriving from outside the main beam. If the auxiliary signal is stronger than the main, the radar knows the emission is from a sidelobe (a jammer not in the main lobe) and ignores it.
- Sidelobe canceller: actively subtracts the auxiliary-channel signal from the main channel, weighted to null out the jammer.
- Polarization agility: switch polarization between pulses. A jammer matched to one polarization is mismatched in the other.
13.4 Low probability of intercept (LPI)
A radar that wants to operate without being detected by enemy receivers uses an LPI waveform: spread the energy over a wide bandwidth and a long time, with low instantaneous power. The matched filter at the legitimate receiver still pulls the signal out, but the enemy's wide-band intercept receiver sees only weak noise. Modern fighter radars like the AN/APG-81 have LPI modes that radiate at least 30 dB lower peak power than the equivalent conventional radar while preserving detection range.
LPI is the radar equivalent of side-channel-resistant cryptographic operations: the goal is to leak as little information as possible to a passive observer.
13.5 Stealth: lowering RCS
Already covered in Section 4. The F-117, F-22, F-35, B-2, B-21, J-20, and Su-57 are operational or near-operational stealth aircraft. Naval stealth uses the same principles (Zumwalt-class destroyer, Visby-class corvette). Even some ground vehicles get RAM coatings.