All the jamming in this chapter assumes that you first see and understand the threat. Behind every smart jammer — and every ESM sensor (EW: Electronic Support Measures) — there is therefore a receiver that has to find and characterise signals. This section opens that black box, and shows how a DRFM jammer creates a convincing false target.
The challenge for the ESM receiver
An EW receiver has to meet conflicting requirements at the same time: covering an enormous bandwidth, achieving a high probability of intercept (POI), measuring signals accurately (frequency, PRI/PRF, pulse width, angle of arrival), and all of this in an environment with thousands of pulses per second from dozens of emitters. No design does this perfectly; it is always a trade-off.
Receiver types and their trade-offs
- Superheterodyne: very sensitive and frequency-selective, but narrowband — it scans and can miss a short transmission while it is tuned elsewhere.
- Crystal video / IFM (Instantaneous Frequency Measurement): wideband and extremely fast, but less sensitive and weak in dense environments.
- Channelised: a bank of parallel filters — good coverage and sensitivity, but complex and expensive.
- Digital receiver (wideband ADC + FFT): the modern standard; flexible and programmable.
The common thread is the POI-versus-sensitivity trade-off: seeing everything at once (a wide instantaneous bandwidth) comes at the expense of how weak a signal can be and still be detected. This also explains why LPI signals (Radar) are so difficult: they are designed precisely to slip under that trade-off.
Measuring parameters and deinterleaving
From the pulses it picks up, the receiver derives characteristics — frequency, PRI/PRF, pulse width, scan pattern — and the angle of arrival (through interferometry or amplitude comparison between antennas). It then has to deinterleave the intermingled pulse trains of many emitters: unravel them into individual emitters, which are compared against the threat library.
DRFM: the engine of modern deception
A DRFM (Digital Radio Frequency Memory) is what distinguishes a simple noise jammer from a cunning deceiver (Deceptive jamming). The loop:
- Capture: the incoming radar pulse is picked up and digitised with a fast ADC.
- Store & manipulate: the digital copy is stored and processed — delayed (false range → RGPO), shifted in frequency (false Doppler → VGPO), or multiplied (several false targets).
- Regenerate: via a DAC, a coherent false echo is sent back that looks like a real one.
That very coherence is crucial: a coherent false target survives the radar's Doppler and pulse processing, where simple noise is filtered out. The limits lie in ADC resolution and bandwidth, processing delay (latency) and the instantaneous bandwidth.
This is the hardware reality behind the rest of EW: ES (EW: Electronic Support Measures) is only as good as the receiver underneath it, and the most advanced deception (Deceptive jamming) stands or falls with the digital speed and coherence of a DRFM. Anyone who understands the POI-versus-sensitivity trade-off understands why some signals are missed and why LPI (Radar) works.