At the centre is the threat simulator: equipment that generates realistic enemy signals — radar pulse trains, data links, seeker behaviour — to test ESM receivers and jammers without needing a real threat. It is how you validate the threat library (New concepts in EW): does the system recognise the threat, does it choose the right countermeasure, and is the mission data file up to date?
This is no formality: an outdated or incomplete threat library is a main cause of EW failure. A radar with a new waveform that is not in the library is not recognised — and therefore not countered. That is why simulators are constantly updated with new threat profiles collected through ELINT (EW: Electronic Support Measures).