AZUSA refers to a ground-based radar tracking system installed at Cape Canaveral, Florida and the NASA Kennedy Space Center. AZUSA was named after the southern California town Azusa, California where the system was devised in the early 1950s. Radar interferometry Radio interferometry yields very accurate tracking angles when a target emits a radio signal. This angular precision of interferometry led to the development of the Azusa tracking system as part of the Army Air Corps NUL-774 Project, forerunner of the Atlas ICBM program, at the Vultee Field Division of Consolidated Vultee Aircraft Corporation (Convair) in Downey, California. Two of the basic patents (2,972,047 and 3,025,520) in the field of interferometer tracking are shared by James W. Crooks Jr., Robert C. Weaver, and Robert V. Werner, all members of the Azusa design team. By the spring of 1948, the Azusa team had built an interferometer operating at 148.58 MHz. In a strange circle of history, the U.S. Naval Research Laboratory (NRL) was working on underwater sound interferometers at the time Convair was developing Azusa. Since the two groups were in close contact, there was considerable interchange of ideas.* The circle was completed in the early 1950s when the Navy picked up the Azusa interferometer work for its Viking Project at White Sands, New Mexico. The Navy wanted to explore the possibility of converting the Viking or some derivative of it into a guided missile and it needed an accurate guidance system. In an early report from this program, NRL's J. Carl Seddon explained how the Viking would determine its position: "The Missile will detect its position relative to the hyperbolic guidance path by phase comparison of modulation waveforms derived from signals received from two pairs of stations." In this scheme, the missile would guide itself using onboard electronics and navigational signals received from the ground. This seems a far cry from Minitrack and satellite tracking, but phase comparison, the essence of Minitrack, was there. Within a year, NRL reports from the Viking program were diagramming ground-based tracking interferometers, which relieved the Viking of the burden of signal-processing equipment by computing the missile's position from the ground. Two precursors of Minitrack were evident in the interferometer arrangement. First, only a tiny radio beacon needed to be carried on the Viking itself, an important feature of the Vanguard "Minitrack," in which the prefix 'Mini" applied to the minimum-weight satellite transmitter. The second precursor