Tracking gate servoed by relative range

A tracker for an air borne or maritime collision avoidance system includes digital logic to servo a track gate pulse about the target pulse. The logic is arranged to utilize range change data during the interval from the track gate pulse to a moving target pulse. This arrangement eliminates the need for a range counter to measure the entire range from the interrogation pulse to the target pulse. Instead, the range counter measures the range from the leading edge of the track gate pulse to the target pulse and compares the range counter value with a predetermined fixed number representing the range interval from the leading edge of the track gate to the target pulse of a perfectly centered target pulse, or, simply, one half the desired resolution of the system.