GPS based attitude determination algorithm for the spin-stabilized microsatellite UNISAT

GPS receivers for attitude determination have been used in several satellite missions. The typical configuration exploits the carrier phase differences among four GPS antennas and estimates the attitude achieving accuracy of 1 deg or better. For spinning satellites it has been shown that GPS based attitude determination is possible using only two antennas. This simplifies the attitude estimation algorithm, avoiding to solve for the so-called integer ambiguity, and to take into account the line-bias. Several methods have been proposed in the literature to estimate the attitude of a spinning spacecraft: most of them make use of a two-stage procedure where, first the spin and nutation frequencies are determined, then they are used in the second step, to estimate satellite's attitude. In this paper a different approach is presented. A Recursive Least Square algorithm is implemented to fit the so-called displacement vector, computed making use of the double-differenced phase measurements, to an analytically predicted function. It allows to estimate at once six independent parameters which define the spinning motion properties. This paper deals with an application of this GPS based attitude determination algorithm to the design of the attitude determination system of the University of Rome microsatellite UNISAT. The angular momentum direction is determined to within 10-2 deg, spin and nutation rates to within 10-5 rad/s, and phase to within 10-1 deg.