Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
1999-12-03
2001-07-24
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S013000, C244S171000
Reexamination Certificate
active
06266616
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods for navigation and control of spacecraft, and in particular to a system and method for determining the attitude of a spacecraft having at least one star sensor.
2. Description of the Related Art
Satellites enjoy widespread use for a variety of applications, including communications, surveillance, and data gathering. To perform their design mission, most satellites require accurate information regarding the spacecraft and payload attitude.
For applications where high bandwidth satellite attitude data is required, such data is typically obtained by on-board inertial measurement instruments such as inertial reference units having a plurality of gyros and accelerometers. However, while such instruments can provide high bandwidth information regarding the spacecraft attitude, they can only do so with respect to a datum attitude reference. Errors in this attitude reference propagate throughout the satellite navigation system. At the same time, it occasionally occurs that satellites lose their attitude reference, and must reacquire the reference with on-board sensors such as sun and earth sensors. When the satellite's attitude datum has been entirely lost (a lost-in-space-scenario), the satellite must determine its attitude reference completely anew. When the accuracy of the attitude datum has been compromised, but not lost, (a confused-in-space scenario), the satellite must also determine its attitude reference anew.
Most attitude acquisition algorithms assume a “Lost-in-Space” problem, which use no a priori attitude-related knowledge, and use a single star tracker to estimate the satellite attitude. Such “Lost-in-Space” algorithms can take a substantial amount processing capacity and memory. Unfortunately, current attitude acquisition algorithms have no capability to relax memory or processing requirements when attitude-related information is supplied, or deal with multiple star trackers. For example, the system disclosed U.S. Pat. No. 5,745,869, issued to van Bezooijen on Apr. 28, 1998 is designed to work with data from a single star tracker, and assumes a lack of a priori satellite attitude information.
Further, many acquisition algorithms are designed to work with “smart” star trackers that are capable of using a built-in star catalog to determine which star is being tracked. Such an algorithm is disclosed in U.S. Pat. No. 5,412,574, issued to Bender, Parks, and Brozenec on May 2, 1995. Such systems cannot utilize minimum capability star trackers, such as the CT-602 available from the BALL CORPORATION, which can only identify star positions (in the star tracker reference frame) and corresponding star instrument magnitudes.
Hence, there is a need for a device and method for estimating the attitude of a spacecraft using supplied a priori information and information from multiple star trackers. The present invention satisfies that need.
SUMMARY OF THE INVENTION
To address the requirements described above, the present invention discloses a method, apparatus, article of manufacture for determining the attitude of a spacecraft having at least one star sensor.
The method comprises the steps of preparing, for each star sensor, an associated star catalog having an entry describing star parameters for each star potentially observable by the star sensor. The determination of which stars are potentially observable by the star tracker is performed using the supplied a priori attitude-related information. An identification is then determined between the stars observed by each star sensor with the associated star catalog entries. The set of identifications for each tracker is referred to as the “correlation” between the star observations from each star sensor and the associated star catalog entries. A reference star tracker is selected as the star tracker tracking the largest number of identified stars. (In the event of a tie, the reference star tracker is selected as the tracker tracking the stars with the greatest spread). Finally, the spacecraft attitude is determined from the stars identified by the reference star sensor. The apparatus comprises a means for performing the foregoing operations, and the article of manufacture comprises a data storage device tangibly embodying instructions to perform the method steps described above.
Thus, the present invention has several advantages over the prior art.
First, while the present invention may work with data from a single star tracker, it is designed to accept and use data from multiple star trackers, if observations from more than one tracker are available. If such is the case, the present invention can produce an estimate of the misalignment of each tracker, and, in certain cases, an estimate of the misalignment of the sun sensor, with respect to one “reference” tracker, in addition to the attitude estimate.
Second, the present invention does not assume a lack of a priori attitude knowledge. Instead, this a priori attitude knowledge is used to efficiently arrive at an accurate spacecraft attitude estimate.
Third, the present invention allows the use of lower capability star trackers that can only identify star position (in the tracker frame) and star instrument magnitude.
Fourth, the present invention is flexible in its requirements for a priori attitude knowledge. It can function with either a known point-of-reference (typically, the position of the sun), or a rough estimate of the attitude, supplied. It is an improvement over the technique of Bender et al, which assumes the position of the Earth as a point of reference.
And finally, the present invention has modest memory requirements; the size of the star catalog involved is about 75 kilobytes.
REFERENCES:
patent: 5745869 (1998-04-01), Van Dezooijen
Gates & Cooper LLP
Hughes Electronics Corporation
Nguyen Tan
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