SSC13-VIII-1 On-Orbit Calibration of Photodiodes for Attitude Determination

This paper presents a method for 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 higher attitude determination accuracy than achieved with the pre-flight calibration parameters. The calibration is implemented with an extended Kalman filter to simultaneously estimate spacecraft attitude and the calibration parameters. This approach, as opposed to an attitude-independent 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 demonstrated by application to flight data from the RAX-2 satellite and results in an average angular improvement of 10◦ in sun vector measurements with the photodiodes. Attitude determination accuracies of below 1◦ in each axis are demonstrated using the calibrated photodiodes in combination with a low-cost three-axis magnetometer and rate gyroscope.