On-Orbit Calibration of Photodiodes for Attitude Determination

A method is presented for the on-orbit calibration of photodiodes for sun sensing in an attitude determination system. The calibration estimates the scale factors and alignment angles of the photodiodes, resulting in a higher attitude determination accuracy than achieved with the preflight calibration parameters. The calibration is accomplished through the simultaneous estimation of the spacecraft attitude and calibration parameters. This approach, as opposed to an attitude-dependent method, enables the calibration of an arbitrary number of photodiodes mounted in any orientation on the spacecraft and facilitates the use of an attitude- dependent Earth albedo model. The method is formulated within both an extended Kalman filter and an unscented filter. The filters are demonstrated by application to flight data from the Radio Aurora Explorer satellites and result in an average angular improvement of 10 deg in sun vector measurements with the photodiodes. Attitude determination accuracies of below 1 deg in each axis are demonstrated using the calibrated photodiodes in combination with a low-cost three-axis magnetometer and rate gyroscope.