Time delay compensation for spacecraft motion compensation

Abstract: An instrument command logic circuit (11) generates slew commands that control pointing of an instrument. A pointing control system (14) points the instrument in response to the slew commands. A spacecraft motion compensation logic circuit (12) processes the slew commands from the instrument command logic circuit (11) and is responsive to a signal that is indicative of the time at which a disturbance is to occur that will cause motion that requires compensation. The spacecraft motion compensation logic circuit (12) outputs a compensation signal to the pointing control system (14) substantially at the time at which the disturbance occurs to compensate for the disturbance.

Description: Field of the Invention The present invention relates generally to spacecraft motion compensation systems and methods, and more particularly, to spacecraft motion compensation systems and methods that eliminate systematic error introduced by the time delay required to calculate and apply the actual compensation to instruments disposed on the spacecraft. Background to the Invention The assignee of the present invention manufactures and deploys spacecraft that house electro-optical instruments that observe the Earth or other stellar bodies. The instruments are pointed by adjusting pointing mirrors using closed-loop servo systems. Unfortunately, motion of the pointing mirrors induces motion in the spacecraft. Furthermore, in conventional spacecraft motion compensation systems and methods, there is a time delay between the time the induced motion is sensed and the time that the compensating commands are applied to the instruments. US Patent No. 4,687,161 entitled "Pointing Compensation System for Spacecraft Instruments" assigned to the assignee of the present invention discloses a system for reducing errors in pointing spacecraft instruments that are caused by spacecraft motion induced by motion of one or more of the instruments. This patent discloses a closed loop system wherein each instrument disposed on the spacecraft has command electronics for commanding motion in that instrument and a pointing control system for imparting motion in that instrument in response to commands provided by the command electronics. Spacecraft motion compensation logic compensates for instrument pointing errors caused by instrument-motion-induced spacecraft motion. Any finite number of instruments can be compensated, by providing each pointing control system and each command electronics with a link to the spacecraft motion compensation logic for the instruments that are to be compensated. The spacecraft motion compensation logic is an electronic manifestation of the algebraic negative of a model of the dynamics of motion of the spacecraft. The system of US Patent No. 4,687,161 has the ability to reduce dynamic interaction among spacecraft instruments and simplify ground operations, thus enhancing overall system performance. However, this compensation scheme suffers from a time delay that is required to calculate and apply the actual compensation. It would be desirable to eliminate systematic error introduced by this time delay. Accordingly, it is an objective of the present invention to provide for improved systems and methods that implement spacecraft motion compensation that are a function of the motion of instruments disposed on the spacecraft. It is a further objective of the present invention to eliminate the compensation errors in a spacecraft motion compensation system due to known time delays in the compensation for known disturbances. US 5,517,418 discloses a feedforward control system for compensating for predictable disturbances exerted on a spacecraft, for example thermal shock disturbances. The system provides control signals for actuators for controlling the spacecraft dynamics. Summary of the Invention According to a first aspect of the present invention, a system for correcting the pointing error of an instrument carried by a spacecraft comprises: an instrument command logic circuit (11) that generates slew commands that control pointing of the instrument; a pointing control system (14) coupled to the instrument command logic circuit (11) for pointing the instrument in response to the slew commands; and a spacecraft motion compensation logic circuit (12) for processing the slew commands from the instrument command logic circuit (11), wherein the compensation logic circuit (12) is responsive to a signal that is indicative of the time at which a disturbance is to occur that will cause motion that requires compensation, and outputs a compensation signal to the pointing control system (14) substantially at the time at which the disturbance occurs to compensate for the disturbance. According to a second aspect of the present invention, a spacecraft motion compensation method for correcting the pointing error of an instrument carried by a spacecraft, comprises the steps of: determining the time at which a disturbance is to occur in the future that will cause motion that requires compensation; computing compensation signals comprising slew commands that control pointing of the instrument and that will compensate for the disturbance; and applying the compensation signals to the instrument substantially at the time of the occurrence of the disturbance causing the motion that requires compensation. The present invention eliminates the systematic error introduced by the time delay that is required to calculate and apply the actual compensation in conventional systems. In accordance with the present invention, the compensation signal is applied at the same time as the occurrence of the disturbance causing the motion that requires compensation. An exemplary method comprises the following steps. The time at which a disturbance is to occur in the future that will cause motion that requires compensation is determined. Compensation signals that will compensate for the disturbance are then computed. The compensation signals are applied to the instrument substantially at the time of the occurrence of the disturbance causing the motion that requires compensation. Undesirable motion of any spacecraft sub-system caused by deterministic disturbances are compensated by calculating and removing the disturbance from the sub-system response. If the time of the impending disturbance is known, or if the disturbing event itself can be delayed, compensation electronics of the present invention calculates the compensation signal prior to the disturbing event. Therefore, the compensation is applied at the same time and/or during the same computer clock cycle as the disturbance. This eliminates introduction of artificial time delays into the compensation due to the time required to compute the compensation, known physical time delays in the actuators, and the digital nature of the control logic. Brief Description of the Drawings An example of the present invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 illustrates an exemplary spacecraft motion compensation system in accordance with the principles of the present invention; and, Fig. 2 is a flow chart that illustrates an exemplary spacecraft motion compensation method in accordance with the principles of the present invention. Detailed Description The present invention has utility on any type of spacecraft and for any finite number of instruments disposed thereon, such as cameras, antennas, or solar panels, that need to be precisely pointed, and will be illustrated with respect to a typical spacecraft manufactured by the assignee of the present invention. The spacecraft may be a GOES I/J/K meteorological satellite, for example. This satellite includes a solar array, an x-ray sensor, a magnetometer, an S-band transmit antenna, a search and rescue antenna, a UHF antenna, a telemetry and command antenna, earth sensors, an S-band receive antenna, a solar sail, an imager, and a sounder. The imager comprises a cooler, an aperture, and a pointing mirror. The sounder comprises a cooler, an aperture, and a pointing mirror. The pointing mirrors are each mounted on a two-axis gimbal which selectively positions the mirror with respect to orthogonal x and y axes at a scan rate of many successive positions per second. The x axis is referred to as the roll, north/south, or elevation axis. The y axis is referred to as the pitch, east/west, or azimuth axis. Referring now to the drawing figures, Fig. 1 illustrates an exemplary spacecraft motion compensation system 10 in accordance with the principles of the present invention. The system 10 may be advantageously used with the above-described or similar satellite, for example. Undesirable motion of any spacecraft sub-system (including the antenna, solar sail, imager, or sounder) caused by deterministic disturbances are compensated by calculating and removing the disturbance from the sub-system response. As is shown in Fig. 1, each of N instruments for which pointing compensation is desired comprises a command logic circuit 11, which is typically an electronics package that controls pointing of the respective instrument using slew commands. Each instrument comprises a pointing control system 14 for imparting pointing motion to the instrument. A spacecraft motion compensation logic circuit 12 normally operates continuously, and receives the slew commands as inputs from the instrument command logic circuit 11 of each instrument, and outputs a compensation signal to each of the pointing control systems 14 by way of a plurality of summing devices 13. The compensation signal can be interrupted at will by means of opening an override switch 16 upon command from the ground received by way of the telemetry system on-board the spacecraft. The spacecraft motion compensation logic circuit 12 may be implemented in analog or digital form. For the example illustrated, the compensation logic circuit 12 is implemented as part of an onboard attitude and orbital control electronics digital microprocessor, such as a miniaturized version of a Perkin Elmer/Interdata 5/16 minicomputer. The spacecraft motion compensation logic circuit 12 comprises the algebraic negative of a model of the dynamics of motion of the spacecraft. Thus, the compensation signal commands the instruments to do the opposite of what the spacecraft is expected to do in response to the slew commands. The compensation signal is input into the instruments by way of their respective pointing control systems 14. Since the system is closed loop, closed loop spacecraft dynamics 15 cause the spacecraft to move as a result of slewing the instruments, while pointing of each of the instruments is compensated for. A major source of pointing disturbances to the imager, for example, is the slewing motion of the sounder pointing mirror during operation of the imager. The slewing motion of the sounder pointing mirror has been found to introduce significant errors. The motion compensation system 10, using estimated spacecraft dynamics and controls, compensates for these motions so that their effects are minimized or substantially eliminated. The marked improvement provide by the present system 10 is that it substantially eliminates systematic errors introduced by the time delay normally required to calculate and apply the actual compensation in conventional systems. In the present system 10, the compensation signal is applied at substantially the same time as the occurrence of the disturbance causing the motion that requires compensation (or during the same clock cycle). The key to this is that the time of the occurrence of the disturbance causing the motion that requires compensation is known, and the desired compensation signal is precomputed and applied at the appropriate time. In particular, the compensation signal is applied at the same time as the occurrence of the disturbance causing the motion that requires compensation. Consider, for example, the response of the imager, the sounder, and the spacecraft to slewing of the sounder pointing mirror. Slewing of the sounder pointing mirror starts with a command generated in the sounder slew command logic circuit 11 that is sent to sounder mirror servo dynamics. The command conveys the desired mirror angles about its x and y axes, with respect to arbitrary reference angles. A mirror control loop tracks the command accurately, the momentum of the pointing mirror is exchanged with the spacecraft and can cause as much as a 50 microradian transient spacecraft disturbance which results in a like pointing error induced in the imager. To compensate for this disturbance to the imager pointing mirror, the compensation logic circuit 12 processes the slew command from the sounder, anticipates the resulting platform motion, and sends a compensation signal to the imager mirror servo dynamics by way of the summing device 13. Since the compensation signal is equal in magnitude and reversed in polarity to that representing the anticipated disturbance, the pointing mirror of the imager moves to cancel the spacecraft disturbance. The above example is intended to illustrate one typical application of the principles of the present invention. Reference is again made to US Patent No. 4,687,161, the contents of which are incorporated herein by reference in its entirety, which discusses other applications and examples in which the present invention may be used. Additional examples of types of disturbances that may be compensated and details of the spacecraft and its operation as it relates to the compensation provided by the present invention may be derived from reading this patent. A typical example involves compensation implemented using a GOES I type mirror motion compensation system for providing rigid-body motion compensation of instrument slewing motions. This system is disclosed in US Patent No. 4,687,161. In accordance with the present invention, commands to slew an instrument are delayed until the compensation for other instruments has been calculated, and then the respective slew and compensation commands are applied to all instruments at the same time as the disturbance. The model used for spacecraft motion compensation logic circuit 12 is given by the following equations. The compensation signal has x and y axis components, C x (s) and C y (s), respectively. This same compensation signal is sent to each of the pointing control systems 14. The equations are the algebraic negatives of the linear transfer functions of the spacecraft to disturbance torques produced by x and y axis motions of the instrument servos. The transfer functions are linear functions of inertias and angles, and their accuracy depends only on the tolerances in the knowledge of the rigid-body mass properties of the spacecraft and instrument servos. Propellant slosh has relatively little effect on satellite motion at the very low satellite accelerations produced by the servos. Similarly, structural flexibility effects at very small amplitudes of motion do not produce measurable disturbances to spacecraft motion. Because the mirrors have little mass compared with the spacecraft, and their servos are fast and track the input slew commands with little error, the dynamics of the servos do not have to be taken into account in these compensation equations. The spacecraft motion compensation logic circuit 12 is implemented as a time domain realization of the following transfer functions: where: s is the Laplace operator (differentiator), C x (s) is the compensation angle for the x axis component of compensation signal sent to each of the servos, I x is the moment of inertia of the satellite about its roll axis, I lx is the moment of inertia of imager mirror about its x axis, I Sx is the moment of inertia of sounder mirror about its x axis, A lx (s) is the angular position of imager mirror about its x axis, with respect to an arbitrary reference angle, A Sx (s) is the angular position of sounder mirror about its x axis, with respect to an arbitrary reference angle, H is the total angular momentum of the satellite, G, J, K, and L are constants that realize the closed-loop response of the satellite's roll and yaw attitude control loop, C y (s) is the compensation angle for the y axis component of compensation signal 6 sent to each of the servos, I ly is the moment of inertia of the satellite about its pitch axis, I Sy is the moment of inertia of imager mirror about its y axis, I ly is the moment of inertia of sounder mirror about its y axis, A ly (s) is the angular position of imager mirror about its y axis, with respect to an arbitrary reference angle, A Sy (s) is the angular position of sounder mirror about its y axis, with respect to an arbitrary reference angle, and P, Q, and R are constants that realize the closed-loop response of the satellite's pitch attitude control loop. In the general case, where more than two instruments are compensated, numerators of the equations for C x (s) and C y (s) are supplemented with terms representing disturbance torques emanating from the additional instruments. Fig. 2 is a flow chart that illustrates an exemplary spacecraft motion compensation method 20 in accordance with the principles of the present invention. The exemplary method 20 comprises the following steps. The time at which a disturbance is to occur in the future that will cause motion that requires compensation is determined 21. Compensation signals that will compensate for the disturbance are then computed 22. The compensation signals are then applied 23 to the instrument substantially at the time of the occurrence of the disturbance causing the motion that requires compensation. Thus, undesirable motion of any spacecraft sub-system caused by deterministic disturbances are compensated using the principles of the present invention by calculating and removing the disturbance from the sub-system response. In using the present system 10 and method 20, the time of the impending disturbance is known, or the disturbing event is delayed to a known time. The present compensation system 10 and method 20 calculate the compensation signal prior to the disturbing event. Thus, the compensation signal can be applied at the same time and/or during the same computer clock cycle as the disturbance. This eliminates the introduction of time delays to the compensation due to the time required to compute the compensation signal, the known physical time delay in the actuators, and the digital nature of the control logic.

Claims: A system for correcting the pointing error of an instrument carried by a spacecraft comprising: an instrument command logic circuit (11) that generates slew commands that control pointing of the instrument; a pointing control system (14) coupled to the instrument command logic circuit (11) for pointing the instrument in response to the slew commands; and a spacecraft motion compensation logic circuit (12) for processing the slew commands from the instrument command logic circuit (11), characterised in that the compensation logic circuit (12) is responsive to a signal that is indicative of the time at which a disturbance is to occur that will cause motion that requires compensation, and outputs a compensation signal to the pointing control system (14) substantially at the time at which the disturbance occurs to compensate for the disturbance. A system according to claim 1, wherein the spacecraft comprises a plurality of instruments, and wherein the system comprises: a plurality of instrument command logic circuits (11) respectively coupled to the plurality of instruments that respectively control pointing of each instrument using slew commands; and a plurality of pointing control systems (14) respectively coupled to the plurality of instruments for pointing the instrument, wherein the spacecraft motion compensation logic circuit (12) processes the slew commands from the instrument command logic circuits (11), and outputs a plurality of compensation signals to the plurality of pointing control systems (14) substantially at the time at which the disturbance occurs to compensate for the disturbance. A system according to claim 2, wherein the compensation signals are summed with the slew commands from the plurality of instrument command logic circuits (11) to produce the compensation signals that are applied to the plurality of pointing control systems (14). A system according to any preceding claim, wherein the or each compensation signal is summed with the slew commands from the instrument command logic circuit (11) to produce the compensation signal that is applied to the pointing control system (14). A system according to any preceding claim, wherein the or each compensation signal is applied during the same clock cycle as the disturbance. A spacecraft motion compensation method for correcting the pointing error of an instrument carried by a spacecraft, characterised in that the method comprises the steps of: determining the time at which a disturbance is to occur in the future that will cause motion that requires compensation; computing compensation signals comprising slew commands that control pointing of the instrument and that will compensate for the disturbance; and applying the compensation signals to the instrument substantially at the time of the occurrence of the disturbance causing the motion that requires compensation. A method according to claim 6 wherein the compensation signals are applied during the same clock cycle as the disturbance.

Inventors: CHU PETER Y, MCLAREN MARK, CHU, PETER Y., MCLAREN, MARK
Applicants: LORAL SPACE SYSTEMS INC, SPACE SYSTEMS / LORAL, INC.
Application year: 2000
Main classification: G05D1
IPC codes: B64G1, G05D1