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1-29 Figure 1-22.—Pulse detection. PULSE-DOPPLER METHOD Pulse radar systems may be modified to use the Doppler effect to detect a moving object. A requirement for any Doppler radar is COHERENCE; that is, some definite phase relationship must exist between the transmitted frequency and the reference frequency, which is used to detect the Doppler shift of the receiver signal. Moving objects are detected by the phase difference between the target signal and background noise components. Phase detection of this type relies on coherence between the transmitter frequency and the receiver reference frequency. In coherent detection, a stable cw reference oscillator signal, which is locked in phase with the transmitter during each transmitted pulse, is mixed with the echo signal to produce a beat or difference signal. Since the reference oscillator and the transmitter are locked in phase, the echoes are effectively compared with the transmitter in frequency and phase. The phase relationships of the echoes from fixed objects to the transmitter is constant and the amplitude of the beat signal remains constant. A beat signal of varying amplitude indicates a moving object. This is because the phase difference between the reference oscillator signal and the echo signal changes as the range to the reflecting object changes. The constant amplitude beat signal is filtered out in the receiver. The beat signal of varying amplitude is sent to the radar indicator scope for display. Q28. What factor determines the difference between the transmitted frequency and the received frequency in an fm transmitter? Q29. What type of objects are most easily detected by an fm system? Q30. What transmission method does NOT depend on relative frequency or target motion?