The purpose of the GPS Satellite Geometry Analysis Tool (GPSGEM) is to evaluate GPS satellite geometry for a given Earth-fixed location or for a provided trajectory. The tool will generate a listing of all satellites in view, the best satellite combination defined by the most optimum Geometric Dilution of Precision (GDOP), the GDOP profile expected if all satellites are available, and the worst-case GDOP profile when one or two satellites are removed from the available constellation. The tool will provide the navigation controller with insight into the expected performance of the GPS constellation, and allow an assessment of the implications to onboard navigation performance for the ascent, orbit, abort, and entry phases of flight.
GPSGEM reads an ephemeris of projected vehicle positions through to landing. Satellite positions are calculated from a GPS almanac for every satellite for every time step of the ephemeris, and the GDOP (a measure of robustness of GPS constellation geometry) calculated. At the direction of the software user, the program can also calculate the resulting GDOP assuming single or double satellite failures that would cause the greatest degradation of GDOP. Results over any desired time interval are aggregated, and any continuous periods with poor geometry are reported.
Using the output of GPSGEM, an analyst can assess whether GPS constellation geometry can support vehicle GPS receiver state solutions accurate enough for a safe landing, even if one or two satellites fail. The GPS constellation has enough satellites to provide a good geometry in nearly all circumstances, so geometry analysis is much more useful when possible satellite failures are taken into account.
The basic algorithms implemented in this software are applicable for supporting any use of GPS navigation critical enough that it would be worthwhile to project consequences of GPS satellite failures.
This work was done by Timothy Wegner and Guadalupe Cardona of United Space Alliance for Johnson Space Center. This software is available for use. To request a copy, please visit here.