IMPROVED SATELLITE CONSTELLATIONS FOR CONUS ATC COVERAGEImproved Satellite Constellations for CONUS ATC Coverage

The report examines the problem of designing a constellation of orbiting satellites capable of supporting an aircraft navigation/surveillance service over CONUS. It is assumed that the aircraft positions are determined by hyperbolic multilateration using all satellites visible at elevation angles exceeding a minimum angle. Comprehensive analyses are presented of three 'baseline' constellations. The constellations are representative of previous large, medium, and small constellations. The analyses include calculation of The Geometric Dilution of Precision (GDOP) during level flight, calculation of GDOP after a key satellite is deleted, and calculation of GDOP during aircraft banking. Comparison of the resulting GDOP's with the theoretical minimum values indicates that there is considerable room for improvement. A new method of calculating GDOP is described. The method suggests that improved GDOP's can be obtained by placing satellites in retrograde orbits rather than the previous posigrade orbits. Accordingly, nine new constellations are designed that employ retrograde orbits. When subjected to the same analyses as the baseline constellations, the new constellations exhibit significantly improved GDOP's.