Software for Navigation of a Spacecraft Flying in Formation

The Spacecraft Autonomous Navigation System using GPS for Earth Orbiters (SANS-GEO) computer program is designed to perform the navigation calculations that will enable the New Millennium Program's Earth Orbiter-1 (EO-1) spacecraft to fly in formation with the Landsat-7 (LS-7) satellite in orbit around the Earth. Scheduled to be launched in May 1999, the EO-1 will be required to follow 450±50 km behind the LS-7, and to keep its ground track within 3 km of the LS-7 ground track. Using data from an onboard Global Positioning System (GPS) receiver and empirical (purely kinematic) GPS navigation algorithms, SANS-GEO would compute parameters of the EO-1 orbit; these parameters would include rates of atmospheric-drag-induced decay. SANS-GEO would then utilize these parameters, along with the corresponding parameters and with maneuver plans generated remotely for the LS-7, to determine EO-1 velocity-change maneuvers needed to satisfy absolute orbital constraints (e.g., ground-track-repeat requirements) and/or relative orbital constraints (formation-flying requirements). The computed velocity-change maneuvers would be converted into commands for the EO-1 attitude-control system and thrusters.