Tracking Loop Optimization for On-Board GPS Navigation in High Earth Orbit (HEO) Missions

The tracking of GPS satellites in orbits which have an apogee above the GPS constellation is evaluated as a function of the code and frequency tracking loop bandwidths and predetection integration time. The distribution of the satellite carrier to noise ratio ( C=N0), acceleration, and jerk from all GPS satellites with a line of sight to the receiver antenna is computed at several different positions in the receiver orbit. Linearized models of the tracking loops are used to predict the thermal noise variance and steady state error. Requirements are also placed on the mean time to lose lock, commensurate with the duration of expected satellite passes at each orbital position. The closed loop bandwidths and integration time were determined to maximize the number of satellites tracked at different positions in the receiver orbit. Two orbits were studied: a geostationary transfer orbit and the second phase of the proposed Magnetospheric Multiscale (MMS) mission. It was found that, as a result of the slower dynamics in these orbits, the tracking threshold could be reduced significantly by decreasing bandwidth and increasing predetection integration time when satellite visibility is poor. The need to demodulate the 50 bps data message further reduces the number satellites available. However, it is proposed to record and buffer the data message during portions of a satellite pass which are above the threshold for low bit error rates in decoding that message.