Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Aeronautical vehicle
Reexamination Certificate
2000-03-01
2001-06-26
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Aeronautical vehicle
C701S222000, C701S300000, C342S355000, C244S158700
Reexamination Certificate
active
06253125
ABSTRACT:
BACKGROUND OF THE INVENTION
An orbiting satellite is in continuous need of data relating to its position in order to maintain or change its orientation with respect to earth and its orbital path. In addition the adjustment of antenna arrays, solar arrays, and other common functions is also dependent on this information. Much of these data is currently uploaded from ground stations either automatically or by manual intervention. This cannot be accomplished on a continuous basis because of gaps in communication as the satellite orbits the earth.
Attitude control is a primary example of a function which requires such information. Modern satellites now have an extensive means to accumulate information by means of onboard sensing of the positions of the earth, sun, stars, and other satellites. In addition an orbital calendar (ephemeris) may be stored in onboard computers which contain the expected orbital path and schedule of attitudes. A further database of expected repetitive perturbations can be stored and updated by actual experience. From this data an orbital model can be predicted and used to correct errors in attitude, orbital position and the focus of the various arrays.
Where a geosyncronous orbit is involved, the data is relatively stable, since the relative position of earth remains the same. The data becomes more dynamic as the orbit becomes more complex. With the advent of commercial satellites, there is a greater demand for complex orbits which provide greater “hang time” over specified areas of high demand. An example of such an orbit is a so called Tundra orbit which is an inclined elliptical sidereal day orbit. The relative position of the earth in such orbits is no longer constant and there is a continuous need to update data with respect to earth and other referenced bodies in the galaxy. It is anticipated that, when such orbits are used, the orbital and attitude data will need to be updated as frequently as every ten seconds, depending on the application served by the satellite.
There is a need, therefore, for a method of propagating the required information which is resident onboard. The information needs to be translated into a reference frame suitable for use by the various functions. It is a purpose of this invention to provide an onboard system for generating near real time knowledge of the spacecraft position and velocity to more efficiently perform station keeping and more accurately control attitude, solar arrays, and antenna pointing based on data sensed onboard by earth, sun and other sensors.
SUMMARY OF THE INVENTION
Satellite control data in the form of state vectors and state vector products are generated representing the position and geometric relationships of the space craft to the Earth, sun, moon, and specific locations on the Earth. These data is used to control attitude, SADA steering, antenna pointing, momentum management, and Earth sensor scan inhibit scheduling. An onboard orbit propagator predicts the data and integrates the data into the various control systems for the respective function. The state vector, consisting of position, velocity, and epoch, is generated in a geocentric equatorial coordinate frame at fixed ten second intervals. From this data, a series of vector products are generated to serve particular functions. The vector products are organized into two groups, attitude independent vector products and attitude dependent vector products. A subset of data representing the attitude profile is based on the attitude independent vector products and is integrated with the attitude dependent group. These outputs are used by the various function controls to provide the ideal solution for the closed loop function controls. Sensor processors provide the real time position attitude based on sensed references for comparison to the ideal solution generated by the orbit propagator of this system.
A validity check is performed each time the state vector is generated to insure continuing performance. The predicted state vector is tested by two methods. One involves the calculation of an angular momentum vector based on the state vector position and velocity predictions. This is compared to prior momentum vectors to detect abnormal variations. Since in high energy orbits, of the type to be used, the momentum vector will remain quite stable, relatively small variations will be suspect and generate an invalid determination. In addition, the current argument of latitude is calculated and compared to the prior argument of latitude to obtain a rate of change. The rate of change is then compared to the fastest angular rate in the orbit. A higher angular rate will also generate an invalid decision. Invalidity will result in the insertion of the last valid state vector into the orbit propagator to stabilize the propagation process. A valid decision will result in the state vector being buffered for use in generating the next level of vector products.
REFERENCES:
patent: 5259577 (1993-11-01), Achkar et al.
patent: 5984238 (1999-11-01), Surauer et al.
patent: 0123456-A2 (2000-01-01), None
Nguyen Tan
Perman & Green LLP
Space Systems Loral, Inc.
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