Coherent Jammer
Understanding Coherent Jammer
Non-coherent noise jammers raise the radar noise floor but are defeated by matched filtering and integration gain. A coherent jammer overcomes this by generating signals that pass through the victim radar's pulse compression and Doppler processing with minimal loss. The DRFM captures the radar's transmitted waveform, including its chirp slope, phase coding, and pulse width, then replays it with controlled modifications. Because the replayed signal matches the radar's matched filter response, it appears as a legitimate target return rather than broadband interference.
The core DRFM architecture consists of a wideband receiver front end (typically 2 to 18 GHz), a high-speed analog-to-digital converter sampling at 2 to 40 Gsamples/s with 8 to 14 bits, a circular buffer memory storing 1 to 100 μs of waveform data, a digital signal processor that applies time delays and frequency shifts, and a digital-to-analog converter feeding a power amplifier. The entire capture-modify-retransmit cycle completes in 50 to 200 ns, fast enough to inject the false return within the victim radar's range gate before the real target echo arrives at the receiver.
Jam-to-Signal Ratio
J/S = (Pj Gj) / (Pt Gt) × (4πR²) / σ
Self-Protection J/S (monostatic):
J/S = (Pj Gj × 4πR²) / (Pt Gt σ)
RGPO Capture Requirement:
J/S ≥ 3 to 6 dB for range-gate capture
Where Pj = jammer power, Gj = jammer antenna gain, PtGt = radar EIRP, R = range (m), σ = target RCS (m²). Self-protection J/S increases with R² (advantage to jammer at long range).
Jamming Technique Comparison
| Technique | Coherence Required | J/S Needed | Effect on Radar | ECCM Vulnerability |
|---|---|---|---|---|
| Noise (barrage) | None | 10 to 30 dB | Raises noise floor | Matched filter rejects |
| RGPO (coherent) | Full waveform | 3 to 6 dB | Breaks range track | Leading-edge tracking |
| VGPO (coherent) | Full waveform + Doppler | 3 to 6 dB | Breaks velocity track | Acceleration limits |
| False targets | Full waveform | 0 to 3 dB | Creates ghost tracks | Track correlation |
| Cross-eye | Phase matched pair | 0 dB (angle only) | Angular deception | Multiple apertures |
Frequently Asked Questions
How does a DRFM-based coherent jammer work?
A DRFM coherent jammer intercepts radar pulses through a wideband receiver, digitizes them at 2 to 40 Gsamples/s with 8 to 14 bits of resolution, stores the complex samples in high-speed memory, and retransmits modified copies through a power amplifier. Because the digitized samples preserve the original pulse's phase, frequency, and modulation, the retransmitted signal passes through the victim radar's matched filter. The jammer can add time delays for range deception, frequency offsets for velocity deception, or angular modulation for angle deception against monopulse trackers.
What jam-to-signal ratio does a coherent jammer need?
Effective deception jamming typically requires a J/S of 0 to 10 dB at the victim radar's receiver. Range-gate pull-off needs 3 to 6 dB to capture the tracking gate. Velocity-gate pull-off requires similar levels but must maintain coherence over longer intervals. Against modern radars with sidelobe blanking and pulse-to-pulse frequency agility, the required J/S can increase to 15 to 20 dB to overcome discrimination algorithms.
What radar features counter coherent jammers?
Modern radars use pulse-to-pulse frequency agility hopping across 500 MHz or more bandwidth faster than DRFM capture-retransmit latency, intrapulse coding with authentication bits, leading-edge tracking that detects the 50 to 200 ns DRFM retransmission delay, sidelobe blanking to reject off-axis jamming, and home-on-jam modes. Cognitive radar systems adaptively change waveforms to exceed the DRFM's instantaneous bandwidth or memory depth.