Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Aeronautical vehicle
Reexamination Certificate
1999-05-26
2001-09-04
Zanelli, Michael J. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Aeronautical vehicle
C244S171000, C244S164000
Reexamination Certificate
active
06285927
ABSTRACT:
TECHNICAL FIELD
The present invention relates to spacecraft attitude determination and, more particularly, to a method and system for minimizing time-mismatch between attitude sensors in spacecraft attitude determination systems.
BACKGROUND ART
Spacecraft attitude determination systems often include more than one attitude sensor. Some examples of typical attitude determination sensors are: 3-axes gyroscopes, star trackers, earth sensors, sun sensors, and beacon sensors. A multi-sensor based attitude determination system uses two or more attitude sensors to construct the spacecraft attitude. Typically, each individual attitude sensor generates attitude data at a moment in time different from the other attitude sensors. This is due to the asynchronous nature of the individual sensors, or to differences between sensor processing delays or data transmission delays. Accordingly, the sensor measurements available to the spacecraft control processor for controlling spacecraft attitude at a particular time, have different associated time moments. If the time mismatch is constant, the attitude error due to time mismatch can be accounted for, and calibrated out of the system. Thus, for example, in an orbit-normal steered spacecraft in a circular orbit, time-mismatched attitude sensor data will result in a constant attitude error which can be calibrated out of the attitude calculations.
The time discrepancy between the attitude measurement sensors can cause significant problems, however, when the spacecraft is experiencing dynamic motions such as agile slew, sun-nadir steering, yaw-flip, and spacecraft orbiting in a highly elliptical orbit (HEO). Time-mismatch of attitude sensor measurements is also a concern for traditional orbit-normal steered spacecraft in a circular orbit, if the time-mismatch is time varying. This will often be the case if the attitude sensors are running on their own clocks, independent from the system clock, and communicating with the spacecraft control processor through an asynchronous data bus.
The issue of time-mismatch between attitude measurement sensors is best illustrated by example. In a satellite having a stellar attitude sensor such as a star tracker and an inertial attitude sensor such a gyroscope, the stellar-inertial attitude determination system has two solutions—the inertial solution and the stellar solution. The inertial solution is obtained by integrating the gyroscope delta angle output, and the stellar solution is obtained by algebraic transformations of the star tracker measurements. Typically, these two solutions are not synchronized, and at any moment, the most recent solutions available to the spacecraft control processor do not match in time. Thus, a time-mismatch exists between the inertially derived attitude integrated from the gyroscope delta angles, and the attitude derived from the star tracker measurements. Thus, for example, if the spacecraft is stewing at 0.3 deg/sec and there is a 0.125 second mismatch between the stellar and inertial measurements, a discrepancy of 135 arcseconds exists. This can lead to inaccurate corrections of the integrated attitude solution. For spacecraft systems which require a few arcsecond or better attitude accuracy, this is not acceptable.
SUMMARY OF THE INVENTION
In the present invention, the aforementioned problems are solved by providing a method of controlling the attitude of a spacecraft by time tagging a primary attitude value from a primary attitude sensor and storing this primary attitude value and associated time stamp in a buffer. A secondary attitude value from a secondary attitude sensor is also generated. Because the secondary attitude value has a different associated time stamp than the primary attitude value, the secondary attitude value is time-matched to the primary attitude value. The corrected spacecraft attitude is then calculated as a function of the time-matched primary and secondary attitude values. The spacecraft attitude is then updated in accordance with the corrected spacecraft values.
In one aspect of the invention, the primary attitude sensor, such as the gyroscope, generates attitude data at a higher frequency than the secondary attitude sensor which may be, for example, a star tracker. The gyroscope data (or the attitude derived from the gyroscope data) is stored in a circular buffer such that when the star measurement becomes available, the star measurement time tag is used to retrieve the nearest-in-time gyroscope data from the buffer. Several gyroscope data points are then used to interpolate a higher time-matched resolution gyroscope data point for use along with the star tracker data point in updating the spacecraft attitude.
Accordingly, an object of the present invention is to provide an improved spacecraft attitude determination method for dynamic space missions or time-varying mismatched attitude sensor measurements.
An advantage of the present invention is stringent attitude determination performance in spacecraft designed for dynamic maneuvers. Another advantage is improved attitude performance for spacecrafts which require very high pointing accuracy.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
REFERENCES:
patent: 4658361 (1987-04-01), Kosaka et al.
patent: 4679753 (1987-07-01), Landecker
patent: 4754280 (1988-06-01), Brown et al.
patent: 5109346 (1992-04-01), Wertz
patent: 5412574 (1995-05-01), Bender et al.
patent: 5546309 (1996-08-01), Johnson et al.
patent: 5922033 (1999-07-01), Milford et al.
patent: 5948044 (1999-09-01), Varley et al.
patent: 5963166 (1999-10-01), Kamel
Li Rongsheng
Wu Yeong-Wei Andy
Gibson Eric M
Hughes Electronics Corporation
Zanelli Michael J.
LandOfFree
Spacecraft attitude determination system and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Spacecraft attitude determination system and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Spacecraft attitude determination system and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2500029