Abstract: In a receiving circuit a control circuit and a direction finding circuit generate a digitally encoded "descriptor" which characterizes pulses of r.f. radiation received in a particular azimuthal direction and having a particular carrier frequency. The "descriptor" may be compared with data stored in a processing circuit representing r.f. pulses emitted by sources which have already been detected or which are known to exist. To reduce the workload on processing circuit a number of emulator circuits generate pulse patterns which simulate pulses produced by known sources of little inherent interest. If the received pulses match the simulated pulses a gate inhibits control circuit.