Abstract: A matched transmission line coupling an RF generator to an antenna and receiving Doppler altered reflected signals from the antenna. A pair of diodes coupled to the transmission line 90 electrical degrees apart at the RF frequency and oppositely poled so that the RF signal from the generator and the reflected signal from the antenna are heterodyned in each diode to produce a heterodyne signal which is in phase. The heterodyne signals are band limited, filtered and passed through threshold detectors separately to provide separate in phase pulses which are applied to a coincidence gate to provide an output. Amplitude modulation is rejected because it produces out of phase signals and frequency modulation is rejected because the transmission line is substantially flat. The RF generator may be frequency modulated and the output of the coincidence gate is applied to a shift register to insure the triggering thereof by only a true Doppler signal.