Abstract: A pulse coherent Doppler radar allows both transmission frequency agility and pulse repetition frequency agility in association with phase encoding which accomplishes the transmission frequency agility by the use of a transmission oscillator, the nominal frequency which may be modified by a microcomputer, a local oscillator, a reference oscillator, and a phase-locked loop having a phase comparator with the oscillator and a mixer for receiving the signals issued by the oscillator and the signal issued by the local oscillator.

Abstract: A secondary radar system in which the transponder and central station possess for enciphering and deciphering complementary dispersive filters having expansion/compression factors of .theta..DELTA.F.sub.1, where .DELTA.F.sub.1 = 1/.DELTA..mu. is a measure of the spectral width of each reply pulse having a temporal width of .mu., .theta. being at least s .mu./.DELTA..mu., such that at least two adjacent pulses spaced s apart in the initial reply train are completely overlapped in the expanded reply.The application to secondary radar systems, especially of the IFF type, in order to improve considerably the secrecy of the link between an airborne transponder and the ground station.