Before DARPA was established, a President’s Science Advisory Committee panel and other experts had concluded that reliable ballistic missile defense (BMD) and space surveillance technologies would require the ability to detect, track, and identify a large number of objects moving at very high speeds. Responding to these needs, DARPA in 1959 initiated a competition for the design and construction of a large, experimental two-dimensional phased array with beam steering under computer control rather than requiring mechanical motion of the antenna.
Known as the Electronically Steered Array Radar (ESAR) Program, the focus of the effort was to develop low-cost, high-power tubes and phase shifters, extend component frequency ranges, increase bandwidth, apply digital techniques, and study antenna coupling. DARPA pioneered the construction of ground-based phased array radars such as the FPS-85. This radar system had a range of several thousand miles and could detect, track, identify, and catalog Earth-orbiting objects and ballistic missiles. The FPS-85 quickly became part of the Air Force SPACETRACK system and was in operation from 1962 until the SPACETRACK unit was deactivated in early 1967.