Abstract: 1. A system for passively measuring the range between a given point and a second point from which detectable radiations having distinguishable characteristics are emanating, comprising means for receiving at said given point direct radiations from said second point and indirect radiations from said second point which have been reflected from a reflecting surface, means for measuring the angle of elevation of said second point from said given point, means for measuring the altitude of said given point relative to said reflecting surface, a gate circuit for rejecting all direct signals which are not followed by a reflecting signal within a predetermined time span, and computing means for determining the range from said given point to said second point as a function of said direct and reflected signal, said angle of elevation, and said altitude of said given point.