Attitude determination using combined GPS and three-axis magnetometer data

Both GPS and three-axis magnetometer (TAM) measurements can be combined using an optimal filter to provide robust attitude determination of spacecraft. The main advantage of this approach is that the TAM attitude solution is always available throughout the orbit, and may be used to estimate the attitude when GPS outages and anomalies occur (such as cycle slips). A more fundamental utilization of the combined sensors will be explored in this paper. The crucial aspect in GPS attitude determination is the resolution of the integer ambiguities. Several approaches have been investigated to resolve these integers. A new motion-based algorithm for GPS integer ambiguity resolution has been recently derived which converts the reference GPS sightline vectors into body frame vectors. This is accomplished by an optimal vectorized transformation of the phase difference measurements. The result of this transformation leads to the conversion of the integer ambiguities to vectorized biases, having the form identical to a TAM bias problem. Since the GPS integer resolution problem has been converted to a magnetometer-bias problem, if a calibrated TAM is used in conjunction with the GPS measurements then the integers can be resolved very quickly and reliably. Simulation results will be provided to show the usefulness of this approach.