Flight results of GPS based attitude control on the REX II spacecraft

The REX II spacecraft, funded by the US Air Force Space Test Program, was successfully deployed from a Pegasus XL rocket in March 1996. This gravity gradient stabilized spacecraft contains a Trimble TANS Vector GPS receiver that has been substantially modified for space use by Stanford University and the Goddard Space Flight Center. This mission marks the first successful extended real-time GPS based attitude determination application for spacecraft (the only other known real-time attitude determination experiment, Crista-SPAS, was of limited duration on a Space Shuttle mission). REX II is also the first known spacecraft attitude control application using GPS sensor inputs. This paper presents results and lessons learned from the REX II spacecraft and other recent GPS space experiences. The REX II GPS solutions are compared to magnetometer solutions and dynamic simulations to assess the GPS attitude accuracy. These results are then contrasted with those from the RADCAL spacecraft, which was similar in design but performed GPS based attitude determination in ground based postprocessing. Issues and lessons for future spacecraft that will employ GPS receivers on-orbit, such as solution availability and controller performance, are discussed.