Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Aeronautical vehicle
Reexamination Certificate
2000-02-09
2002-01-22
Cuchlinski, Jr., William A (Department: 3644)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Aeronautical vehicle
C244S158700, C244S164000, C244S171900
Reexamination Certificate
active
06341249
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to simultaneous orbit and attitude control for acquisition and maintenance techniques for individual satellites as well as for multiple satellites in a constellation or formation, in which Modern Feedback Control is used for determining precise real-time autonomous on-board navigation, attitude estimation and both orbit and attitude control. The orbit control function of this control system can place any satellite in any orbit position in a formation or in a constellation, including the acquisition of the initial distribution for the constellation/formation after satellite separation from launch vehicles, and can also maintain the constellation/formation distribution. The attitude control function simultaneously estimates the attitude state and acquires and maintains desired attitude, including providing the required attitude maneuvers. In formation flying this unified control system establishes and maintains the satellite separation and phasing with respect to the ‘head of the fleet’ satellite, and synchronizes the satellite orientations with respect to this ‘head of the fleet’.
BACKGROUND OF THE INVENTION
The orbital control of satellites, in both geostationary orbits (GEO) and low-earth orbits (LEO), has primarily been ground-based. Orbit maintenance and station keeping have historically required involvement of Control Center personnel in all phases of operation. The computational burden for satellite control, including orbit analysis, maintenance and station keeping, has been on the ground computers. The ground computers provide both the off-line functions of orbit determination and maneuver planning as well as the on-line functions of commanding and telemetry processing.
With evolution of the concepts of operating large number of satellites in a constellation, attention has been focused on developing autonomous on-board orbit control system that removes the need for ground-based orbit control. One such design, shown in U.S. Pat. No. 6,089,507, provides for autonomous orbit control through modern feedback control, using GPS and a controller such as the Linear Quadratic Gaussian (LQG) with Loop Transfer Recovery (LTR) and/or H-infinity Controller.
Traditionally, all earth orbiting satellites have had some form of on-board attitude determination and control system. Many attitude determination systems used some form of attitude sensing devices, such as earth sensors, sun sensors, star trackers, gyroscopes, and the like to provide attitude information. Actuators, such as reaction wheels, momentum wheels, magnetic torquers and thrusters, used this information to provide the attitude control based on pre-programmed parameters.
Various methods have been studied for autonomous control of satellite navigation. U.S. Pat. No. 5,109,346 to Wertz discloses autonomous navigation control using Global Positioning Satellites (GPS) for orbit determination, and a method for providing orbital corrections. However, Wertz uses a non-feedback control system, which is subject to unstructured uncertainty. Additionally, Wertz is limited to real-time orbit and attitude determination, not real-time control and/or correction. Furthermore, position finding using GPS is known, as described for example, in U.S. Pat. No. 4,667,203 to Counselman, III.
Various methods have also been studied for developing the required relative dynamics and kinematics equations that are required for formation flying orbit and attitude control. For example, Bauer, F. H., Hartman, K., Forta, D., and Zuinn, D., in their paper “Autonomous Navigation and Control—Formation Flying in the 21
st
Century,” present the coordinates for control of multiple satellites moving in formation, without providing any detail on the method of obtaining this information for implementation.
Similarly, in Wang and Hadaegh, “Coordination and Control of Multiple Micro Spacecraft Moving in Formation,” each of the spacecraft moving in formation is modeled as a rigid body with fixed center of mass, and various schemes for generating the desired formation pattern are discussed. While they provide explicit laws for formation-keeping and relative attitude alignment based on nearest neighbor-tracking, they do not study or provide any method of obtaining this measurement information nor the actual implementation.
For attitude control, systems were designed using other controllers, such as proportional-integral-derivative (PID) compensators using a variety of frequency response techniques. However, the PID design requires trade-offs with conflicting design objectives, such as gain margin and closed-loop bandwidth, until an acceptable controller is determined. When the control dynamics are complex, or poorly modeled, or when the performance specifications are particularly stringent, PID system performance erodes. In cases where optimal control design have been used, no measurement feedback was incorporated.
SUMMARY OF THE INVENTION
It is an object of the present invention to utilize modern advanced multivariable feedback control techniques in the design of an unified real-time on-board orbit and attitude control system using GPS feedback. For orbit control, the control techniques to be used are the LQG/LTR for orbit maintenance and Feedback Linearization Control for orbit acquisition. For attitude control, the control techniques to be used are the Nonlinear Lyapunov Control and Sliding Robust Control.
These more powerful design tools result in a higher level of satisfaction only if a solution exists to the problem being solved. Achieving both satisfactory performance limits and ascertaining the existence of a satisfactory controller involves using an optimization theory. Use of an optimization theory helps avoid searching for solutions to problems for which there are no solution. A further benefit of optimization is that it provides an absolute scale of merit against which any design can be measured. These more powerful design tools utilize modern advanced multivariable feedback control techniques.
This invention provides a unified orbit and attitude control system on-board a satellite in orbit, the system having two closed loop multivariable controllers, a receiver that receives data for both satellite position and orientation, and a converter that converts the orbit and attitude control problem into a state-space form. For orbit control, the converter converts the control problem into a tracking problem and a regulator problem in order to minimize the position error and velocity error between the body in motion and a target. For attitude control, the converter converts the satellite orientation of the various satellites in formation into a relative attitude state, and the regulator minimizes this relative error to track and maintain the desired attitude orientation. The receiver receives data from the Global Positioning Satellite System or other external location information provider. This data typically comes in the form of Code pseudorange, which provides the positioning information, and Phase pseudorange, which provides attitude information. Both the attitude and orbit controllers are modern feedback closed loop controllers.
This invention provides a combined orbit and attitude control system that uses the orbit state vector to describe the orbit control system and relative attitude kinematics to describe the attitude control system. Control is provided by modern advanced multivariable feedback control techniques, for example, linear quadratic Gaussian/loop transfer recovery (LQG/LTR) controller for orbit maintenance control, a feedback linearization controller for orbit acquisition control, and a Lyapunov controller for attitude control. These controllers enhance the control system performance by minimizing the control error and control effort. Additionally, the real time feedback control results in optimum implementation of an on-board autonomous control system.
Attitude determination and control for formation flying requires the additional development of the relative attitude dynamics and kinemati
Parvez Shabbir Ahmed
Xing Guang Qian
Cuchlinski, Jr. William A
Green Robert S.
Marc-Coleman Marthe Y.
Rader, Fishman & Grauer P.L.L.C.
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