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.