a pulse doppler radar system with variable pulse repetition frequency has a coherent integrator to which the reflected pulses are supplied in order to avoid deterioration of the indication of a moving target given over - the - horizon reception . the coherent integrator subjects the reflected pulses to a simplified vectorial addition by the use of a number of filters . a precondition for the modified coherent integration is a staggering of the pulse repetition periods such that their sums formed over successive pulse repetition periods are constant , and a phase reference for the signal amplitudes is obtained such that the phase difference is equal to zero after every second pulse repetition .