Vehicle diplex doppler near-obstacle detection apparatus

A vehicle near-obstacle detector in the form of a diplex Doppler radar apparatus provides range information between a vehicle and an obstacle based on the phase shift between a pair of Doppler signals (D1,D2) derived from two transmitted radar signals at slightly different frequencies. A speed dependent error introduced by the time constant of a filter circuit (54) converting duty cycle range information based on the phase shift to an analogue signal is compensated by introducing a small time shift (35) in the signal path of one of the Doppler signals to effect a shift in the duty cycle range information.