Abstract: A system for synchronizing a clock includes a phase-locked loop (PLL) circuit that generates or receives (304) timing errors that are based on timing information from multiple timing sources. Gain blocks (214) weight (306) the timing errors, which are then combined (308) into a loop time error. A loop integrator (226) integrates (310) the loop time error to produce an input used to adjust (312) an oscillator frequency. A corresponding oscillator clock signal is fed back (240) to one or more phase detectors (206), which receive (302) timing reference signals and generate timing errors. When a timing errors indicates that a problem exists with a timing source, the impact of the problematic timing source is reduced (430, 504), or oscillator frequency adjustments are suspended (608). When used on a satellite (700), at least one of the timing errors can be based on times of transmit and times of arrival of time messages exchanged between the satellite and its neighbors (716).

Abstract: A method of determining the position of a satellite in a near geosynchronous orbit includes receiving at least one main lobe signal from an antenna on a first GPS satellite. A GPS signal, including GPS time and Doppler, is received from a pseudolite positioned on the Earth and an approximate position of each of a plurality of second GPS satellites is determined from an onboard almanac. Side lobe signals and accompanying noise are received from antennas on the plurality of second GPS satellites. The GPS signal and known sequential data bits are used for sorting or integrating the side lobe signals from the accompanying noise and the position of the satellite in a near geosynchronous orbit is determined using the one or more main lobe signals and the sorted side lobe signals.

Abstract: A method and apparatus for maintaining the integrity of spacecraft based time and position using GPS (11) is disclosed. A GPS receiver is provided with a Receiver Autonomous Integrity Monitor (RAIM) processor (12) and a Kalman filter (14). GPS signals are received from at least four GPS satellites and processed in the GPS receiver to provide time and position information. Periodically, an extended fault vector is formed in the Kalman filter with propagated open-loop position and velocity measurements. The extended fault vector is used to isolate and remove failing GPS satellites.