Description of the Milstar attitude determination system

The Milstar program uses a combination of gyros, an horizon sensor, and a digital sun sensor (DSS) to determine vehicle attitude. The horizon sensor provides a direct measure of roll and pitch and the DSS provides a measure of yaw when the sun is within the DSS field of view. Without DSS measurements, yaw is determined from orbital gyro compassing. The paper presents a tutorial development of orbital gyro compassing based upon observer theory to include the addition of a digital sun sensor and a yawing satellite. Although the system is linear it has time-varying coefficients which may result in an instability condition if the DSS window size and the ADS gains are not chosen wisely. A covariance analysis model is also developed and representative results are presented.