PURPOSE:To erase the secondary echo from a fixed object, by using an N-stage delay line eraser and 2(N+1) or more kinds of stagger repeat period and using a pulse width discriminator and a defruiter. CONSTITUTION:A coherent video CV which has the phase detected in a phase detector 5 is erased in a delay line eraser 6 in case of the receiving signal from a fixed target, but is outputted through the eraser 6 in case of the receiving signal from a moving target. When transmission pulses are radiated at plural repeat periods T1, T2... and a primary echo EF and a secondary echo ES are described on a basis of a radiation time point TP, the primary echo EF is always later than transmission pulses by a time TF, and the delay time of the echo ES is different in every receiving. When the number of stages of the delay line is set to 2 and the number of stagger repeat periods is set to 6, the echo ES passes through a discriminator 14 because its pulse width taus is shorter than a prescribed time taut, and it is inputted to a defruiter the output is turned off by detection of disaccord. Thus, the secondary echo of the fixed object is erased.
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