Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2000-07-29
2001-12-18
Eldred, J. Woodrow (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S171000, C701S004000, C701S222000
Reexamination Certificate
active
06330988
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to satellite attitude determination systems, and more particularly, to a system for star catalog equalization to enhance attitude determination performance.
BACKGROUND ART
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude of the spacecraft. In many applications, the satellite must be positioned to direct communication signals in particular directions or to receive signals from specifically located sources. Without accurate control over spacecraft 3-axis attitude, the transmission of such signals is hindered and at times impossible.
Many modem spacecraft use star trackers for 3-axis attitude control. The systems for 3-axis attitude determination generally include one or more star trackers and a separate 3-axis orthogonal gyroscope. During normal operation, star trackers provide continuous attitude information and the 3-axis orthogonal gyroscope is needed to provide rate information and is constantly corrected by the star trackers.
To improve spacecraft attitude control performance, stellar inertial attitude determination algorithms employ a carefully designed star catalog. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, a star catalog is “equalized” by eliminating stars in the dense areas of the star catalog. The resulting star catalog becomes approximately uniformly distributed and is known to improve stellar inertial attitude determination performance.
Typically, computer memory and algorithm complexities (computer throughput) have always limited the size of on-board star catalog and prevented star catalog designs from using real-time star selection from an on-board star catalog for attitude determination purposes. Unfortunately, real-time star selection algorithms are complicated and have difficulty accommodating other star selection criteria.
The disadvantages associated with these conventional star catalog techniques have made it apparent that a new technique for star catalog equalization to enhance satellite attitude determination is needed. Preferably, the new technique would allow real time star selection while allowing the inclusion of other star selection techniques. Additionally, the new technique should improve stellar inertial attitude determination performance. The present invention is directed to these ends.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide an improved and reliable system for star catalog equalization to enhance attitude determination. Another object of the invention is to provide a system that does not require large amounts of memory storage and throughput. Additionally, an object of the invention is to allow real time star selection while allowing the inclusion of other star selection techniques.
In accordance with the objects of this invention, a system for star catalog equalization to enhance attitude determination is provided. In one embodiment of the invention, an apparatus for star catalog equalization to enhance attitude determination includes a star tracker, a star catalog and a controller. The star tracker is used to sense the positions of stars and generate signals corresponding to the positions of the stars as seen in its field of view. The star catalog contains star location data that is stored using a primary and multiple secondary arrays sorted by both declination (DEC) and right ascension (RA), respectively. The star location data stored in the star catalog is predetermined by calculating a plurality of desired star locations, associating one of a plurality of stars with each of the plurality of desired star locations based upon a neighborhood association angle to generate an associated plurality of star locations. If an artificial star gap occurs during association, then the neighborhood association angle for reassociation is increased. The controller uses the star catalog to determine which stars to select to provide star measurement residuals for correcting gyroscope bias and spacecraft attitude.
The present invention achieves an improved system for star catalog equalization to enhance attitude determination. The present invention is advantageous in that it reduces required computer throughput and memory requirements while improving stellar inertial attitude determination performance.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3713740 (1973-01-01), Lillestrand et al.
patent: 5206499 (1993-04-01), Mantravadi et al.
patent: 5745869 (1998-04-01), van Bezooijen
patent: 5935195 (1999-08-01), Quine
patent: 6227496 (2001-05-01), Yoshikawa et al.
Li Rongsheng
Liu Yong
Wu Yeong-Wei Andy
Eldred J. Woodrow
Gudmestad Terje
Hughes Electronics Corporation
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