Abstract: A method of preventing interference between `friendly` pulse doppler radars when in action against a common target or adjacents targets. Despite considerable separation of the radar r.f's the pulsed signals have repeated sideband pulses which may be taken by another radar as a doppler-shifted echo. This problem is to a large extent alleviated by frequency modulating the r.f. transmission at a very low cycle rate of the order of one cycle per second and at a modulation rate sufficient to indicate to a receiving radar that the source could not be a target accelerating at such a high rate.