Spacecraft Attitude Estimations Using Phase Information of GPS Signals

This paper presents an implementation of an EKF (extended Kalman filtering) estimator for spacecraft attitude determination using GPS (Global Positioning System) signals ' The spacecraft dynamics under small rotation angles is modelled using Euler angles (roll ' pitch and yaw) parameterisations which are applicable for real-time operation onboard spacecraft. The torque from the Earth's gravitation field and magnetorquers i modelled in order to cope with perturbations ' The filtering estimator is tested with simulated GPS data, and flight GPS data collected from the real spacecraft. The results shows that the estimated attitude derived from both simulated and flight data were in agreement with the reference attitude. A contribution from this paper is that the analytical formulation identifies two state vectors that keep the pitch state independent of roll and yaw. This results in significantly reducing the dimensions of computation' Keyvords: Kalman filtering, spacecraft attitude, GPS' new attitude sensor and typical attitude sensors in three aspects. Measurement Availability: The tracking data from PoSat's GPS receiver showed that'four or more GPS satellites can be seen for 80 % of the time in low Earth orbit [4]. Improvement over this has been demonstrated with the next generation of GPS receivers. This suggests that a GPS attitude sensor can provide almost continuous measurements (whereas the Sun sensor cannot provide data in eclipse period)' Orbit/Attitude Determination Unit: A GPS receiver with multiple antennas for spacecraft navigation generally provides position ' software needs to include carrier phase measurement and attitude estimation sub-routines ' Then, the GPS receiver can be used for both orbit and attitude determination