GPS Based Attitude Determination: The Rex II Flight Experience

A milestone in the annals of satellite guidance, navigation and control was achieved as REX II became the first spacecraft to employ GPS attitude knowledge for closed-loop control. Successfully launched and activated on March 8, 1996, this highly capable, low cost, 242 lb spacecraft, uses a GPS receiver to provide the attitude knowledge required by the control system to maintain a three-axis nadir pointed orientation. This paper describes the spacecraft, its attitude control subsystem and by evaluating its early flight history, captures our on-orbit experience using GPS. The resulting combination of GPS attitude knowledge coupled with our simple yet elegant controller has allowed REX II to exceed mission requirements via highly autonomous modes of operations.