1. A system for simulating wave reflections from an irregular-shaped movingbject which has been illuminated with energy of frequency F, said system comprising;a receiver (10) for said frequency F;a generator (14) of white noise voltage;means (13) for amplitude modulating frequency F with said white noise voltage to generate random bursts of frequency F;a radiating transducer (11);a bandpass filter (18) having a passband including said frequency F, the output of said filter being coupled to the input of said transducer, and the input of said filter being coupled to the output of said modulator means (13) to radiate said random bursts of frequency F;a second signal path between said modulator and filter including a time delay network (30) for randomly distorting the radiated bursts.