Abstract: Apparatus and a processing method that determine accurate angle of arrival data indicative of the direction to an emitter. An accurate angle of to the emitter using four antennas and receiver channels (processor). The present invention provides for an ambiguity function that is an implementation of a four-channel two-dimensional interferometer array. The interferometer array and method provide for a reference antenna and second, third and fourth antennas disposed in a predetermined plane with the reference antenna and offset from the reference antenna at locations such that no two antennas lie along a straight line that includes the reference antenna. Receivers are coupled to the antennas for detecting RF energy derived therefrom. Phase comparators make three phase difference measurements (.phi..sub.1, .phi..sub.2 and .phi..sub.3).