In-flight Quality and Accuracy of Attitude Measurements from the Champ Advanced Stellar Compass

The German geo-observations satellite CHAMP carries highly accurate vector instruments. The orientation of these relative to the inertial reference frame is obtained using star trackers. This Advanced Stellar Compass (ASC) is a fully autonomous unit, which provides, in real time, the absolute attitude with accuracy in the arc second range. In order to investigate the in-flight accuracy of the ASC, the terminology to characterize noise and biases is introduced. Relative Instrument Accuracy (RIA) and Absolute instrument Accuracy (AIA) can in principle be determined in-flight. However problems with modeling external noise sources arise. The special CHAMP configuration with two star tracker cameras mounted fixed together provides an excellent opportunity to determine the AIA using the Inter Boresight Angle.