SPACE-FLIGHT TESTS OF ATTITUDE DETERMINATION USING GPS

Preliminary space flight results of attitude determination using GPS a re presented from a spacecraft in low Earth orbit. Relative position m easurements accurate to the sub-centimetre level are made among multip le GPS antennas mounted on the space vehicle. A Trimble Navigation TAN S Quadrex (a GPS receiver specially adapted for attitude determination by Stanford University) is used as a differential carrier phase senso r for the flight. Four GPS antennas are mounted on the zenith face of RADCAL, a polar orbiting, gravity-gradient-stabilized Air Force Space Test Program Satellite, built by Defense Systems, Inc. The four antenn as are equally spaced about the perimeter of the 30 inch diameter cyli ndrical spacecraft bus. The Quadrex receiver measures the phase of the L-band GPS carrier (1575 MHz) at each of up to four antennas for up t o six GPS satellites simultaneously. From these measurements, an initi al assessment of attitude determination in space is performed in post- processing. For RADCAL, the attitude solution is greatly overdetermine d. In a preliminary evaluation of system performance, the system accur acy is determined through measurement self-consistency. Analysis of th e attitude motion in the context of a gravity gradient dynamic model y ields further insight into the system performance.