Algorithms onboard the Oersted microsatellite stellar compass

One of the state-of-the-art attitude determination instruments for spacecraft applications is an advanced stellar compass (ASC or a star tracker). It is able to determine the attitude of a spacecraft relative 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 analyzes 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. The accurate vector measurements must be related to some celestial coordinate system. The only instrument capable of doing so with the required accuracy is an ASC. Therefore the Oersted microsatellite is equipped with an ASC, which is discussed in this paper. The design of the ASC is novel compared to conventional star trackers, because it is able to make the initial attitude acquisition autonomously (lost in space). This is achieved utilizing pattern recognition of star constellations in the CCD image and a preflight compiled version of the star catalogue. The technique is described and the performance analyzed. Also, the ASC is more accurate than conventional star trackers. A conventional star tracker typically tracks 2 - 10 stars in a single frame, whereas the ASC tracks up to 200 stars, yielding more accurate attitude estimates with similar lens configuration. The accuracy, the performance and the high sky coverage of this new approach are discussed.