Carl Christian Liebe

Star trackers for attitude determination

One problem comes to all spacecrafts using vector information. That is the problem of determining the attitude. This paper describes how the area of attitude determination instruments has evolved from simple pointing devices into the latest technology, which determines the attitude by utilizing a CCD camera and a powerful microcomputer. The instruments are called star trackers and they are capable of determining the attitude with an accuracy better than 1 arcsecond. The concept of the star tracker is explained. The obtainable accuracy is calculated, the numbers of stars to be included in the star catalogue are discussed and the acquisition of the initial attitude is explained. Finally the commercial market for star trackers is discussed. >

Pattern recognition of star constellations for spacecraft applications

A software system for a star imager for online satellite attitude determination is described. The system works with a single standard commercial CCD camera with a high aperture lens and an onboard star catalog. It is capable of both an initial coarse attitude determination without any prior knowledge of the satellite orientation and a high-accuracy attitude determination based on prediction and averaging of several identified star constellations. In the high-accuracy mode the star image aims at an accuracy better than 2 arc sec with a processing time of less than a few seconds. The star imager has been developed for the Danish Oersted satellite.<<ETX>>

The advanced stellar compass, development and operations

Abstract The science objective of the Danish Geomagnetic Research Satellite “Orsted” is to map the magnetic field of the Earth, with a vector precision of a fraction of a nanotesla. This necessitates an attitude reference instrument with a precision of a few arcseconds onboard the satellite. To meet this demand the Advanced Stellar Compass (ASC), a fully autonomous miniature star tracker, was developed. This ASC is capable of both solving the “lost in space” problem and determine the attitude with arcseconds precision. The development, principles of operation and instrument autonomy of the ASC is described. This is followed by a description of test and performance verification methods, and finally key physical and performance data are given.

Operation and Performance of a Second Generation, Solid State, Star Tracker, the ASC

Abstract The Advanced Stellar Compass (ASC) is a second generation star tracker, consisting of a CCD camera and its associated microcomputer. The ASC operates by matching the star images acquired by the camera with its internal star catalogs. An initial attitude acquisition (solving the lost in space problem) is performed, and successively, the attitude of the camera is calculated in celestial coordinates by averaging the position of a large number of star observations for each image. Key parameters of the ASC for the Orsted satellite and Astrid II satellite versions are: mass as low as 900 g, power consumption as low as 5.5W, relative attitude angle errors less than 1.4 arcseconds in declination, and 13 arcseconds in roll, RMS, as measured at the Mauna Kea, HI observatories of the University of Hawaii in June 1996.

Real sky performance of the prototype Orsted advanced stellar compass

The Danish microsatellite, Orsted, is equipped with an autonomous, advanced stellar compass (ASC). The ASC consists of two separate units, a Charge Coupled Device (CCD) camera head (based on a commercial Sony interline CCD detector) and a data processing unit with a powerful microcomputer (Intel 80486 type processor). The microcomputer memory contains a star catalogue which enables the microcomputer to recognize the constellations of stars in the field of view and thus derive the attitude of the ASC camera head. The mission, and the design, operation, and performance of the ASC are described. Results of ASC prototype tests at the JPL Table Mountain Observatory (TMO) facility are given.

Astronomical Performance of the Engineering Model rsted Advanced Steller Compass

The Danish geomagnetic microsatellite, Orsted, is an autonomous sciencecraft which is scheduled for a May 1997 launch into polar orbit. It is produced by a consortium of universities, industry and government and is Denmarks first national spacecraft. NASA support includes JPL real sky evaluation of its star tracker, the advanced stellar compass (ASC). The ASC features low cost, low mass, low power, low magnetic disturbance, autonomous operation, a high level of functionality and the high precision. These features are enabled by the use of advanced optical and electronic design which permit the direct integration of the ASC and the science payload. The ASC provides the required attitude information for its associated vector magnetometer and the sciencecraft. It consists of two units, a CCD based camera head and a data processing unit with a powerful microcomputer. The microcomputer contains two large star data bases which enable the computer to recognize star patterns in the field-of-view, to quickly solve the lost-in- space acquisition problem and to derive the attitude of the ASC camera head. The flight model of the camera head has a mass and a power consumption of 127 grams (without baffle) and 0.5 W, respectively. Typical, beginning-of-life, relative measurement precision in pitch and yaw are in the order of two arcseconds (1 sigma) or better have been achieved in the tests and are substantiated.

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.

New strategy for tracking planetary terrains

In a typical planetary spacecraft mission it is of interest to autonomously detect features on a planetary surface and to be able to track them. Various algorithms exist to track a part of an image in a continuous sequence, for instance correlation algorithms. However, these algorithms tend to be computational intensive, and in addition, they lack autonomy. Furthermore they are not robust toward occlusion, changed lighting conditions, and changes in vantage point. Therefore a more reliable strategy to track planetary terrains is needed. This is one of the major research topics of the Autonomous Feature and Star Tracking (AFAST) project at Jet Propulsion Laboratory, Pasadena. This paper presents an alternative method of feature tracking, namely by extraction of such features in the images that are reoccurring in consecutive images and that are invariant to various parameters. With these candidate features, it is possible to navigate the spacecraft based on identified terrain data. A novel methodology is proposed to track planetary surfaces by recognizing feature constellations, a method similar to those employed in recognizing star constellations in a star tracker.