Communications: directive radio wave systems and devices (e.g. – Directive – Including directive communication system
Reexamination Certificate
1998-06-19
2001-02-27
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including directive communication system
C342S355000, C359S199200, C359S199200
Reexamination Certificate
active
06195044
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to spacecraft attitude determination and control systems, and more particularly to attitude control of a spacecraft orbiting a celestial body using laser crosslinks between the spacecraft and other spacecraft.
BACKGROUND ART
Presently there are many different types of attitude sensors employed by spacecraft to determine attitude. Star trackers, sun sensors, earth sensors and gyroscopes are common. The required accuracy of such attitude sensors is usually determined by the required pointing accuracy of the payload. Laser crosslink payloads typically require highly accurate pointing accuracy. The cost of such attitude sensors generally increases with the sensor accuracy. Thus, in a constellation of satellites, use of high performance dedicated attitude sensors on each satellite represents a significant portion of the overall constellation expense.
Prior art techniques for satellites with optical crosslink payloads have typically assumed that the crosslinks are initially acquired and closed by equipping the satellite with attitude sensors of high enough accuracy such that the uncertainty in the pointing angles of the crosslink to the target satellite (the “field of uncertainty”, or FOU) is smaller than the acquisition field of view (FOV) of the crosslink receive telescope or the FOV of the crosslink acquisition transmit beam. Calculated acquisition times, or crosslink closure times, for such systems are typically less than three minutes, and frequently on the order of a few seconds.
These high performance features of prior art systems are not otherwise needed during the mission since, once the crosslinks are closed, the crosslinks themselves provide extremely accurate attitude data that can be used to maintain the crosslinks and perform crosslink handoffs as needed.
SUMMARY OF THE INVENTION
In the present invention, the aforementioned drawbacks of prior systems are solved by increasing the acquisition FOU and/or reducing the crosslink acquisition. In order to achieve rapid acquisition and closure times, such systems typically require an acquisition transmit/receive FOV that is much larger than the typical operational transmit/receive FOV of less than 100 arcseconds. To initially acquire and close crosslinks in a matter of minutes or seconds, such systems also require an attitude determination uncertainty that is much less than can be readily achieved by such low cost attitude sensors as magnetometers or GPS attitude determination systems. Both of these factors—the magnitude of the discrepancy between laser terminal acquisition FOV and service FOV, and the increased accuracy of the attitude determination sensors required to provide the laser crosslink acquisition FOU above that required for spacecraft health and safety—represent considerable cost. Additionally, these high performance elements that provide a low acquisition FOU and a high acquisition FOV are used primarily to initially acquire FOV. The disclosed method of using laser crosslinks for attitude control provides accurate, responsive angle sensing that enables full and continuous attitude determination without the need for a multiplicity of separate dedicated celestial body or inertial sensors. The laser crosslink attitude determination system of the present invention uses the payload laser crosslinks as the principal attitude sensor and eliminates the need for additional high-cost, high performance sensors such as star sensors to provide attitude information.
Crosslinks between satellites are acquired by defocusing their respective transmit beams to broaden the beams while maintaining the detectable illumination intensity, and scanning the beams over the acquisition FOU which is larger than either the transmit or receive FOV of the respective satellites. Upon successive deterministic scans, one satellite's transmit beam will illuminate the other satellite's receiver telescope FOV. The illuminated satellite then uses its receiver telescope measurements to guide its transmit beam to point toward the detected direction of illumination. Once both satellites are linked by the broad transmit beams, the direction information is used to start another iteration with narrowed beams, until the satellites are linked with an operational beam width.
Accordingly, an object of the present invention is to provide an improved attitude control system utilizing laser crosslinks between orbiting satellites.
Another object of the present invention is to provide attitude control for a satellite without the need for separate dedicated celestial body or inertial sensors.
A further object of the present invention is to use a single low performance attitude sensor and gyroscope for initial acquisition. Thereafter, once a crosslink has been established between two satellites, the attitude sensor and gyroscope can be powered off, thereby saving power, reducing thermal dissipation and improving reliability.
Another object of the current invention is to reduce system costs by increasing the crosslink acquisition FOU and/or reducing the crosslink acquisition FOV relative to prior-art systems, and subsequently using laser crosslink pointing information for attitude data.
An advantage of the present invention is that established crosslinks can be used to calibrate both fixed and periodic errors in the alignments between a satellite's base body attitude sensors and its crosslink terminals, and between multiple crosslink terminals on the satellite. This improves the accuracy of crosslink orientation predictions based on body attitude sensor data, thereby reducing later reacquisition time and improving attitude determination accuracy.
Another advantage of the present invention is that crosslinks can be acquired between satellites even when the satellites are not in the service orbit. For example, crosslinks can be established during ion propulsion ascent of the satellites.
A further advantage of the present invention is that it allows crosslinks to be acquired when a satellite is not in its service attitude.
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.
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Gudmestad T.
Hughes Electronics Corporation
Issing Gregory C.
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