A novel autonomous orbit determination system using earth sensors (scanner)

Abstract

With a pair of conical Earth scanners operating as attitude sensors, four horizon points are obtained during each scan. Using three of the four points, a unique method is derived to obtain the distance and direction of the Earth's centre with respect to the satellite position. The fourth point, used in a least squares or averaging manner, improves accuracy. Combining this data with on-board 3-axis attitude, it is shown that a position accuracy of better than 4 km could be achieved. A simple digital low pass filter incorporated in the measurement path improves the accuracy to better than 2 km, and also improves the transient response of the Kalman filter during sudden attitude disturbances due to panel rotations. A simple state prediction (position, velocity vectors) based on f, g series makes the state dynamics linear without deteriorating the performance. The average behaviour of the errors are based on 20 simulation runs.