GPS receiver architecture for autonomous navigation in high earth orbits

This dissertation develops the systems level design of a Global Positioning System (GPS) receiver for high Earth orbit (HEO) satellite missions. The prospect of using GPS for autonomous navigation of satellites in highly eccentric and geosynchronous orbits has long been considered, with the goal of increasing spacecraft autonomy and reducing operations costs for these missions. While GPS has been used extensively for navigation of satellites in low Earth orbits (LEO), existing GPS receivers are not capable of functioning well at higher altitudes, where GPS signal availability is extremely limited.; The primary emphasis is the development of algorithms and methods to add HEO capabilities to existing GPS receiver hardware; in particular, optimization of the receiver algorithms for space, and for the weak signals present in HEO. Software simulation tools have been developed and used to model aspects of the GPS signal geometries, dynamics, and power levels. At low altitudes, geometries and signal levels are favorable; however, dynamics are extremely high. At high altitudes, power levels are weaker and geometries are poorer, but the dynamics are more manageable.; Improved algorithms governing satellite selection, signal acquisition, and the overall design of the tracking loops are presented. Adaptability to highly variable operating conditions is a key design feature of the algorithms in a HEO receiver. Preliminary steps have been taken to implement these concepts in the PiVoT GPS receiver being developed by NASA Goddard Space Flight Center (GSFC). These steps to optimize the performance of the receiver for space are expected to improve overall navigation performance by increasing the sensitivity of the receiver to track weaker GPS signals between 28 to 35 dB-Hz. Preliminary test results conducted with a hardware GPS simulator and the PiVoT GPS receiver are presented.; The limiting altitude for GPS tracking is highly dependant on the capabilities of the receiver, the antenna configurations, and the pointing constraints of the spacecraft. For a conventional GPS receiver with a tracking threshold of 33 to 35 dB-HZ, this limit is approximately 25 to 30 Earth radii. Some of the weak signal tracking techniques discussed in this dissertation could extend this limit to perhaps 40 to 50 Earth radii.