Abstract: A system for detecting the distance L between two objects A and B, either or both of which may be in motion, comprising approach detector means and proximity detector means operational within overlapping distance ranges; each detector means includes an emitter mounted on a first one of the objects, which radiates a primary signal toward the second object, a responder on the second object, which radiates a similar secondary signal back toward the first object, and a signal comparator for measuring the time differential between the primary and secondary signals to determine the distance L between the objects. A range switch coupled to both comparators develops an output indicative of the distance L within a comprehensive distance range encompassing both the approach range and the proximity range and extending down to zero distance.