An LPI radar whispers across a wide stretch of the spectrum, so that only someone who knows exactly where to listen picks up the signal. The radar sees the target, but the target hears the radar late or not at all.
In practiceModern fighters with an AESA radar, such as the F-35, can search this way while the opponent's radar warning receiver notices nothing, or only late. The listener's classic head start (Inverse square law) shrinks as a result.
From Peak power and average power of the pulse radar follows an uncomfortable truth for the radar: an ESM receiver picks up the signal one way (1/R²) and so usually hears the radar before that radar sees the target (1/R⁴). An LPI radar (Low Probability of Intercept) is designed to win back that advantage: it keeps working, but makes itself as hard as possible to detect, identify and locate.
The basic principle is the opposite of a classic pulse: not a short, powerful peak, but spreading the energy over time, frequency and space, so that the power density at every moment and every frequency stays below the detection threshold of the ESM receiver. The radar's own receiver recovers the signal with processing gain (Pulse compression and pulse integration, Processing gain: the key to everything) — the opponent does not.
| LPI technique | Effect on the ESM receiver |
|---|---|
| Low peak power, high duty cycle (CW/FMCW, Variation on the CW radar) | no powerful pulse to trigger on |
| Wide bandwidth (spread spectrum, noise-like waveform) | the energy per frequency disappears into the noise floor |
| Frequency hopping / agility (PRI modulation) | the receiver does not know where to look |
| Low side lobes + beam steering (Antenna characteristics) | hardly any signal leaks outside the main beam |
| Power management (only as much power as needed) | no "shouting" when a whisper will do |
An AESA (AESA — Active Electronically Scanned Array) is an ideal LPI platform: it can finely control power, waveform and beam per direction and so put exactly enough energy on the target without flooding the surroundings with radiation.
LPI is essentially EPM (Jamming avoidance): it protects the radar against ESM, direction finding, anti-radiation missiles and home-on-jam. The counter-move (counter-LPI) requires wideband, very sensitive ESM receivers with long integration times that try to fish out exactly these spread-out, noise-like signals — an arms race in hiding.