1998 - Draper Laboratory

Table 1. Comparison of GPCS systems (Ref. [3]).ConcordiaBorealisBorealisFigure 1. Ellipso TM Mobile Satellite System Orbits (Ref. [6]).The two critically-inclined orbit planes are known as the EllipsoBorealis TM subconstellation. The orbits of Borealis TM are Sun-Synchronous with a Frozen Line of Apsides (SSFLA). Its orbitplanes are aligned 180 deg apart, with one ascending node atnoon, and the other at midnight. Together with the Concordiaplane, they provide 24-hour coverage of the NorthernHemisphere with five spacecraft in each orbit Borealis TM plane.The two subconstellations of Ellipso TM give it unique flexibility inhow it covers the globe. The design is based on the distributionof land and population by latitude (Ref. [7]). The two Borealis TMplanes cover the northern hemisphere, where much of thecustomer base for the mobile communications system isprojected, with the use of elliptic orbits. The Concordiacomplements the Borealis TM planes by providing coverage to thepopulation and land masses in the equatorial and southernhemispheric zones. Since most of the relevant land masses lieabove the Southern 55th parallel, no requirement for satellitecoverage is stated below this line. Figure 2 illustrates theminimum elevation angles achievable by the Concordia andBorealis TM planes. The current plan is to use five satellites in eachBorealis TM plane, and seven satellites in the Concordia plane. Thisallows Ellipso TM to meet its coverage requirements with a 10-degminimum elevation angle in the southern hemisphere and a25-deg minimum elevation angle in the northern hemisphere.In previous work, Sabol (Refs. [9], [10]) analyzed the Dawn/Dusk(with ascending nodes at 10 deg and 190 deg) configuration ofthe Borealis TM subconstellation. He produced a set of refined orbitelements for this configuration, with a complete perturbation andstation-keeping analysis. His refined orbit has an 8:1 repeatground track cycle, i.e., eight revolutions of the orbit per nodalperiod.Recently, Sabol, Draim, and Cefola (Ref. [11]) revisited theBorealis TM analysis. There, it was noted that both atmosphericdrag and tesseral resonance produce strong perturbations on theAutomated Station-Keeping for Satellite Constellations3