Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
1999-03-03
2001-05-22
Lobo, Ian J. (Department: 3662)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S013000, C244S164000, C244S171000
Reexamination Certificate
active
06236939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the determination and control of spacecraft attitude, and more specifically to a method and system for positioning star trackers to produce low error attitude control signals.
2. Description of the Related Art
Spacecraft such as satellites are used for various purposes including scientific research and communications. In many applications the spacecraft must be positioned in particular attitudes, or orientations, in space. Spacecraft attitude includes roll, pitch, and yaw as described in “Spacecraft Attitude Determination and Control”, edited by James R. Wertz, D. Reidel Publishing Company, Boston, U.S.A. 1986, pages 1-21. A precision spacecraft attitude determination using gyros and star trackers was described in Wu and Hein, “Stellar Inerial Attitude Determination for LEO Spacecraft,”
Proc. of the
35
th Conference on Decision and Control
, Kobe, Japan, December 1996, pages 3236-3244. In this system, a real-time knowledge of spacecraft attitude is computed by numerically integrating gyro data measuring spacecraft dynamic motion, and star tracker data is processed to generate corrections to attitude and gyro rate bias estimates which are provided by a six-state extended Kalman filter. Star trackers used for this purpose are also described in B. Kunkel et al.,“METOSAT Second Generation Enhanced/Visible/IR Imager SERVER I and its Environmental Monitoring Potential”,
Proc. of the Central Symposium of the International Space Year Conf
., Munich, Mar. 30-Apr. 4, 1992.
As used herein, the term “star tracker” includes all star tracking devices which image or project a star onto a pixel array, including but not limited to gimbaled star trackers, fixed head star trackers and star scanners. Star trackers are currently mounted on spacecraft without considering the direction in which an imaged star traverses their CCD (charge coupled device) pixel arrays. Because of star tracker spatially dependent errors stemming from the spatial position and movement of star images across pixel arrays, the attitude determination performance is sensitive not only to the star tracker temporal noise, but also to the tracked star motion across the detectors. Star tracker pointing error varies positively with this star sensor spatially dependent error.
SUMMARY OF THE INVENTION
The present invention seeks to provide a high precision attitude control for spacecraft. This is accomplished by positioning star trackers on the spacecraft so that the images of the stars or other celestial objects being tracked traverse the star tracker's pixel array diagonally. The result is a shifting of noise in the image position. signal towards higher frequencies at which it can be easily removed by a low pass filter. While a 45° traverse angle is optimum for square pixels, significantly improved results can be obtained with traverse angles in the range of 25°-65°. The traverse angle can be generalized for the case of rectangular pixels to be within 20° of tan
−1
(v/h), where v and h are respectively the vertical and horizontal pixel dimensions.
The high precision filtered signal is used to calculate and correct the spacecraft's attitude. Simulated results have shown a performance improvement of about 50% with a 45° traverse angle.
These and other features, aspects and advantages of the invention will be better understood from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 4159419 (1979-06-01), Wittke
patent: 5525793 (1996-06-01), Holmes et al.
patent: 6047226 (2000-04-01), Wu et al.
Liebe, C., “Star Trackers for Attitude Determination”, IEEE AES Systems Magazine, pp 10-16, Jun. 1995.*
Liebe et al, “The Advancing State-of-the-art in Second Generation Star Trackers”, Aerospace Conference, vol. 5, pp. 243-253, 1998.*
Andy Wu, “Stellar Inertial Attitude Determination For LEO Spacecraft”, Proceedings of the 35th Conference on Decision and Control, Kobe, Japan,IEEE, Feb. 1996, pp. 3236-3244.
Kunkel et al., “METEOSAT Second Generation Enhanced Viable-/IR Imager SEVIRI and its Environmental Monitoring Potential”, Proceedings of the Central Symposium of the International Space Year Conference, held in Munich, Germany, Mar. 30-Apr. 4, 1992, (ESA SP-341, Jul. 1992).
James R. Wertz, Editor, “Recursive Least-Squares Estimators and Kalman Filters”,Spacecraft Attitude Determination and Control, D. Reidel Publishing Company, pp. 459-468.
Augenstein David L.
Didinsky Garry
Hein Douglas H.
Wu Yeong-Wei A.
Gudmestad T.
Hughes Electronics Corporation
Lobo Ian J.
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