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.