Global Positioning System - A Modification to the Baseline Satellite Constellation for Improved Geometric Performance

摘要

This investigation determined the effect of changes in eccentricity to the orbits of the proposed Global Positioning System (GPS) 18-satellite baseline constellation by analyzing the geometric performance obtained. The effect of satellite losses upon global coverage as also examined with an emphasis on determining which combination of remaining satellites provided the best and worst cases. The potential of GPS for navigation of the space-based user was explored by analyzing the geometric performance obtained for a variety of user trajectories and GPS antenna beamwidths. A computer program which analyzes the many aspects of the geometric performance of pseudoranging navigation satellite systems was used. The results of this analysis indicate that a simple modification to the baseline constellation could reduce outages on a global basis by nearly 50%. The modification consists of changing the shape of the GPS circular satellite orbits to slightly elliptical ones, resulting in more favorable satellite geometry and fewer outages to the user on a global average. Degradation of coverage due to satellite losses was found to be largely dependent on the combination of the remaining satellites which suggests that rephasing of the remaining satellites could significantly improve the degraded performance. The potential for conventional use of GPS for navigation in space was shown to exist for the low altitude user but will be very limited for the higher altitude user due to present GPS antenna design.