Algorithms onboard the Oersted micro satellite stellar compass

The state of the art attitude determination instrument for spacecraft applications is a stellar compass (star tracker). It can determine the attitude of a spacecraft relatively to the stars with an accuracy better than 1 arcsecond (4.8 microradian). This is achieved utilizing a CCD camera and a powerful microcomputer.

The microcomputer analyses the CCD images using an onboard software star catalogue. The objective of the danish Oersted microsatellite is to measure the magnetic field of the Earth. The field is measured with a very accurate vector magnetometer. In order to utilize the accurate vector measurements it is necessary to relate the measurement to some fixed coordinate system.

The only instrument that is capable of doing so with the required accuracy is a stellar compass. Hence the Oersted microsatellite is equipped with a stellar compass, which will be discussed in this paper. The design of the stellar compass is novel compared to conventional star trackers, because it can make the initial attitude acquisition autonomously (lost in space).

This is achieved by pattern recognition of star constellations in the CCD image and a preflight compiled version of the star catalogue. The technique will be described and the performance analyzed. Also, the stellar compass is more accurate than conventional star trackers, because conventional star trackers typically tracks 3-10 star in a single frame, whereas the stellar compass tracks up to 200 stars, yielding more accurate attitude estimates due to statistical means.

The accuracy, the performance and the high sky coverage of this new approach is also discussed.