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Abstract

An apparatus and a method for measuring the distance to an arbitrary target includes a radiation source producing a beam of coherent radiation the frequency of which is continuously varied. The beam is divided into a ranging beam and a reference beam. The ranging beam is coupled to a ranging interferometer, which directs a portion of the ranging beam at the target. The ranging interferometer produces a first signal indicative of the phase difference between a portion of the ranging beam directed at and scattered by the target and another portion of the ranging beam which has traveled over a path of fixed length. The reference beam is coupled to a reference interferometer. A portion of the reference beam is directed by the reference interferometer along a reference path of a predetermined length, and the phase difference between the portion of the reference beam directed along the reference path and another portion of the reference beam which has travelled over a path of a fixed length is measured. The number of fringes resulting from the wave interference produced in the ranging interferometer and the number of fringes in the wave interference pattern produced by the reference interferometer are counted and used, together with the known length of the reference path, to determine the distance of the target from the ranging interferometer.

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