Various radar systems have been proposed to avoid or mitigate the effects of automobile collisions. In these systems, one of the most critical problems is that of identifying potentially dangerous objects as opposed to those alongside the road or in an adjacent lane, especially on curves.

The authors have devised an improved pulse-Doppler radar with two antennas which are used alternately in scanning. In addition to range and range-rate information, the radar can obtain the lateral component of the relative motion of an obstacle by detecting the phase difference between two Doppler signals, each of which corresponds to the relative velocity between the obstacle and the respective antenna.

Test results show that the phase difference signal effectively discriminates between an impending collision and an object which will safely pass by.