Satellite data management in DoD Navstar GPS receivers

The authors describe the software implementation of the satellite management function in the DoD Navstar GPS (Global Positioning System) receivers. The software implementation addresses three functional requirements: database management of satellite almanac, ephemeris, and deterministic corrections data; computation of precise satellite position and velocity for use by navigation software; and using satellite and receiver position data to periodically calculate the constellation of four satellites with optimum geometry for navigation. The authors present an implementation description, detailing how these functions are accomplished, with particular emphasis on the interfaces with the receiver satellite tracking functions necessary to acquire the 50 Hz downlink data. They also illustrate the solutions that were designed into the Navstar user equipment software to deal with the obstacles presented in creating the efficient, effective, and timely data management capability required by a real-time navigation system