Robust autonomous GPS time reference for space application

Details Robust autonomous GPS time reference for space application Robust autonomous GPS time reference for space application

1999-02-24

2001-07-24

Louis-Jacques, Jacques H. (Department: 3661)

Data processing: vehicles, navigation, and relative location

Vehicle control, guidance, operation, or indication

Aeronautical vehicle

C701S213000, C701S226000, C701S214000, C244S158700, C244S164000, C342S357490, C342S357490

Reexamination Certificate

active

06266584

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to space vehicle navigation and control and more particularly to a method and apparatus for providing an accurate Global Positioning System (GPS) time reference for spacecraft such as Low Earth Orbit (LEO) satellites.
2. Prior Art
Satellites and other spacecraft require a time reference for controlling their navigation and orientation and coordinating their operations with other devices with which they interact. A common source of a time reference is the GPS which provides signals from which the local satellite clock bias can be determined to generate an accurate time reference. Typically, receivers for detecting and using GPS signals require the pick up of signals from a number of GPS satellites, for example, the Three Axis Navigation System (TANS) Vector receiver for satellites, available from TRIMBLE NAVIGATION, of Sunnyvale, Calif., requires 4 GPS satellites to be in the view of the receiver in order to provide the receiver clock bias to generate the time reference. If there are less than 4 such satellites in view, then the clock bias solution is held fixed, which results in large errors in the time reference.
3. Problem to be Solved
It is therefore a problem in the art to obtain a reliable and accurate receiver clock bias signal for generating an accurate time reference for a spacecraft or satellite.
4. Objects
It is accordingly an object of the present invention to provide a method and apparatus that will obtain a reliable and accurate receiver clock bias signal for generating an accurate time reference for a spacecraft or satellite.
It is a further object of the invention to provide a system that uses GPS signals to obtain a reliable and accurate receiver clock bias signal, which system is capable of generating an accurate time reference for a spacecraft or satellite with less than 4 GPS satellites within the view of the receiver.
SUMMARY OF THE INVENTION
The present invention is directed to obtaining a receiver clock bias signal for a spacecraft in motion to generate an accurate time reference. The invention uses a real-time navigation Kalman filter solution of the receiver clock bias to provide a GPS time reference even when there are less than 4 GPS satellites within the view of the receiver in the spacecraft. The real-time navigation Kalman filter in the spacecraft is adapted to provide an accurate solution of the clock bias even when there are outages of less than 4 GPS satellites in view by the receiver, by means of a system which uses the knowledge of the dynamic motion of the satellite in conjunction with GPS signals for a robust estimation of time. The system provides an accurate GPS time reference for a receiver in dynamic motion, even when less than 4 GPS satellites are in view, by transferring timing information from an atomic reference standard (GPS) to a spacecraft in motion by directly measuring the GPS signal and without depending upon the tracking of multiple GPS satellites or a static receiver. The invention may be implemented in current aerospace qualified GPS receivers, e.g., the “GPS TENSOR™” produced by the assignee of the present application, and known orbital dynamics are used to predict receiver position which aids in the transfer of the GPS time.


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