§13.1 Introduction

Electronic Support Measures (ESM, Electronic Support Measures) are the sensory pillar of EW: intercepting, direction-finding and identifying the opponent's transmissions, in support of immediate operational decisions — warning, evading, choosing countermeasures and cueing your own weapons.

The fundamental advantage was already established in Electronic Support Measures and cannot be repeated often enough: the listener hears the transmitter before the transmitter sees the listener (1/R² against 1/R⁴, Inverse square law). On top of that, the listener itself is completely silent — listening costs no signature.

What an ESM system does, in four steps:

1. Intercept: picking up the signal — wide frequency coverage, high sensitivity, high probability of intercept (POI: are you looking at the right frequency at the right moment?).
2. Measure: determining the parameters — frequency, PRF/PRI (modulation), pulse width, scan pattern, waveform, direction of arrival (Electronic Support Measures).
3. Identify: comparing the measured fingerprint with a threat library: what type of radar is this, which weapon system does it belong to, and which mode is it in (search / acquisition / track / guidance, Target tracking by radar: the Target Tracking Radar)?
4. Respond: warning the operator, prioritising (what is the most dangerous threat?) and — automatically in modern systems — triggering countermeasures: jamming programmes, chaff/flares (Chaff to Flares), manoeuvre advice.

The two most important ESM applications on board aircraft, ships and, increasingly, vehicles are the Radar Warning Receiver (Radar Warning Receiver (RWR)) and the Missile Approach Warning System (Missile Approach Warning System): the first hears the radars, the second sees the missile.