Calibration of the Milstar Attitude Determination System

The Milstar Attitude Determination System (ADS) uses a combination of gyros, an horizon sensor, and a digital sun sensor (DSS) to determine vehicle attitude. The calibration of this system once on orbit requires the determination of the gyro's drift and scale factor errors and the alignment of the gyros and DSS with respect to the horizon sensor. A Calibration Filter was developed which uses the on-board drift estimates as measurements. This allows calibration of the ADS without continuously down linking the gyro and horizon sensor data. In addition, the on-board drift estimates implicitly contain the horizon sensor data so that they are not needed by the filter. Observability studies show that it is necessary to collect data when the satellite is yawing in order to estimate all of the parameters of interest. Selecting the gyro or DSS z-axis misalignment as unobservable is equivalent to specifying the hardware which establishes the satellite yaw frame. The filter has been used to successfully calibrate the two Milstar satellites on orbit. Selected plots from a calibration of Milstar Flight 2 are presented.