Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Aeronautical vehicle
Reexamination Certificate
2000-05-25
2001-11-13
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Aeronautical vehicle
C701S003000, C701S004000, C701S008000, C701S010000, C701S013000, C701S222000, C701S223000, C701S226000, C701S300000, C701S301000, C244S164000, C244S166000, C244S171000, C244S171900, C342S354000, C250S347000, C250S353000
Reexamination Certificate
active
06317660
ABSTRACT:
BACKGROUND
The present invention relates generally to satellites, and more particularly, to methods that use satellite state vector prediction to automatically inhibit satellite sensor scans and/or provide sensor switching.
The assignee of the present invention has developed a satellite that is to be deployed in a predetermined orbit around the Earth that requires yaw steering in certain circumstances. The orbit is highly inclined and elliptical. A plurality of Earth sensors, one or more sun sensors and a plurality of gyro sensors are employed to control the attitude of the satellite. Certain sensors, in this case the Earth sensor, are sensitive to intrusion by bright bodies such as the sun or the moon in their fields of view.
Such intrusions can lead to loss of attitude control and special precautions must be taken when such an event is predicted to occur. Specifically, for the Earth sensor, the scan experiencing the intrusion must be inhibited and the Earth chord it measures must be captured during the inhibit period in order for the remaining scan to be used to maintain attitude.
In addition, in the elliptical orbit where the Earth apparent radius varies, the captured Earth standard chord must be biased as the Earth radius changes for attitude control. During orbit normal operations of the satellite in this particular orbit, sun intrusions in the Earth sensor occur daily (for low sun angles). In addition to sun intrusions, moon intrusions may be encountered when in orbit normal operations.
When the satellite is operated in yaw steering mode, for low to moderate cases of sun angles, once daily, near satellite local midnight, the sun will follow a trajectory that is centered between north and south scan lines of the Earth sensor. A typical moon intrusion in yaw steering transverses through both Earth sensor scan lines in a very short time period (see FIG.
2
). Moon intrusion can be expected to occur every day for four or five days in a row repeating approximately every two weeks.
Thus, the orbit and yaw steering of this satellite produce problems because of the interference of the sun and moon with the Earth sensor scans. Lunar intrusion because of yaw steering can come from any direction several times a week.
Due to the frequency of these intrusion events and the commanding required to perform inhibits manually, it would be desirable to have a method that automatically inhibits Earth sensor scans to eliminate sensor intrusions by the sun, moon, or other celestial bodies. Therefore, it is an objective of the present invention to provide for improved methods that use satellite state vector prediction to automatically inhibit satellite Earth sensor scans and/or provide sensor switching.
SUMMARY OF THE INVENTION
To accomplish the above and other objectives, the present invention is a method (algorithm) that automatically inhibits Earth sensor scans to handle sensor intrusions by the sun, moon, or other celestial bodies. In a preferred embodiment, the present invention comprises firmware that implements two algorithms that predict intrusions of the sun and/or moon into the field of view of the Earth sensor. The algorithms make independent recommendations for inhibiting an affected Earth sensor scan. While the present invention is designed to solve problem associated with the inclined elliptical orbit, it is valid in the geo-synchronous orbit regime as well and may be used to simplify required ground operation during eclipse season of geosynchronous satellites.
The first algorithm comprises automatic scan inhibit logic. The first algorithm takes predictions of sun and moon locations from an orbit propagator and determines if a scan needs to be inhibited. The second algorithm comprises enhanced course alignment sun sensor (ECASS) logic that takes the sun pitch angle from the orbit propagator and a sun roll/yaw measurement from an sun sensor array, for example, to determine if the sun is intruding on one of the Earth sensor scans.
In particular, the method processes the output derived from the orbit propagator as a primary source of satellite state (predicted state vector). In particular, a predicted state vector for the satellite is generated. An attitude profile for the satellite is generated. Then, the satellite state (predicted state vector) and attitude profile are processed to determine Earth, sun, and moon vectors in a satellite body frame of reference at any instant. The Earth, sun, and moon vectors are compared to the Earth sensor field of view and sensor field of view limit boxes to determine if scan inhibiting or sensor switching should occur. The affected sensor is inhibited or switched if an intrusion of the sun and/or moon into the field of view of the sensor is predicted.
If the Earth sensor is not inhibited before sun or moon intrudes on a scan, the satellite will lose Earth lock and is likely to lose attitude control. The present invention predicts when scan inhibits must occur independent of the attitude of the satellite. The present invention thus processes attitude independent state vector products and attitude profile data to calculate attitude dependent state vector products from which the position of the sun and moon in the spacecraft body reference frame is determined, and from which a determination of when to inhibit the Earth sensor is made. When the output generated by the method is within limits (predetermined limit boxes) the Earth sensor is inhibited or switched to prevent loss of satellite lock.
The present invention uses the output of a high-precision orbit propagator on-board the satellite or on the ground as a primary source of satellite state. The present invention uses the satellite state and attitude profile to determine Earth, sun, and moon vectors in the body frame of reference at any instant and compares that to the Earth sensor field of view and sensor field of view limit boxes to determine if a scan inhibit or sensor switch should occur.
The present invention reduces requirements for manual commanding of scan inhibits. The present invention provides real time scan inhibit commanding or sensor switching in highly dynamic cases such as yaw steering where timing of inhibits is critical. The present invention provides the capability to handle varying Earth size in inhibit strategy for other than circular orbits.
The present method autonomously inhibits Earth sensor scans and/or switches sensors. The entry condition is that the ground software predicts an upcoming sun and/or moon intrusion. To avoid unpredictable results from automatic scan inhibits, scan inhibit predictions are performed on a regular basis. This ensures that interferences are not overlooked, and that any required scan inhibit commanding is performed as scheduled.
Practical uses of the present invention include, but are not limited to, prediction and automatic scan inhibiting of Earth sensor scans for impending intrusions by the sun or moon, strategy for handling the potential for simultaneous sun/moon intrusions, and switching logic for selecting between multiple sensors (i.e., star trackers) to avoid the sensor with the intrusion.
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Barker Lee A.
Price Xenophon
Cuchlinski Jr. William A.
Float Kenneth W.
Mancho Ronnie
Space Systems Loral, Inc.
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