Abstract: Methods and structures are provided that enhance the accuracy of the service attitude of an inclined-orbit spacecraft and, thereby, facilitate reduction of service error between a communication service area and the spacecraft's payload beam. The enhancement is realized by configuring a beacon-receiving antenna to have a beacon-receiving field-of-view that substantially matches a beacon-station window. Preferably, the beacon-receiving field-of-view is elongated and tilted to enhance its match with the beacon-station window in both size and orientation. The goals are also realized by configuring the beacon-receiving antenna to have a beacon-receiving field-of-view that is substantially smaller than the beacon-station window and successively steering a beacon-receiving boresight to successive beacon-receiving attitudes that maintain the beacon station within the beacon-receiving field-of-view over each solar day.

Abstract: Methods and structures are provided that enhance the accuracy of the service attitude of an inclined-orbit spacecraft and, thereby, facilitate reduction of service error between a communication service area and the spacecraft's payload beam. The enhancement is realized by configuring a beacon-receiving antenna to have a beacon-receiving field-of-view that substantially matches a beacon-station window. Preferably, the beacon-receiving field-of-view is elongated and tilted to enhance its match with the beacon-station window in both size and orientation. The goals are also realized by configuring the beacon-receiving antenna to have a beacon-receiving field-of-view that is substantially smaller than the beacon-station window and successively steering a beacon-receiving boresight to successive beacon-receiving attitudes that maintain the beacon station within the beacon-receiving field-of-view over each solar day.

Abstract: A solar wing control system for avoiding thermal shock to a solar wing includes an eclipse exit slew profile generator having an eclipse exit slew rate output for rotating the solar wing from an eclipse exit angle to a solar power generation angle to control solar wing heating rate.

Abstract: Methods and systems for stabilizing satellite spin about an intermediate inertia axis (Z) are disclosed. A set of gyros (22) sense the X component and the Y component of the angular velocity of the satellite body. A single degree of freedom momentum wheel (26) has a fixed transverse orientation with respect to the intermediate axis in order to store momentum. In one embodiment, the momentum wheel (26) is oriented to store momentum parallel to the Y axis. A tachometer (30) senses the rotation rate of the momentum wheel (26). A processor (24) forms a control signal representative of a control torque to be applied to the momentum wheel (26). The control torque is based upon the X component and the Y component of angular velocity of the satellite, and the angular velocity of the momentum wheel (26).

Abstract: A method, apparatus, article of manufacture, and a memory structure for controlling the eccentricity of an orbit. One embodiment of the present invention is described by a method comprising the step of performing a program of eccentricity control maneuvers wherein the control maneuvers are executed at times and in directions selected to apply substantially all eccentricity control along a line of antinodes. In one embodiment, the control maneuvers arc tangential maneuvers, and each control maneuver is applied near the line of antinodes. Provision is made for performing additional maneuvers, including node retargcting maneuvers during specified intervals. Another embodiment of the invention is described by a satellite in a substantially geosynchronous orbit having a satellite control system with a processor implementing instructions to perform control maneuvers as described above.

Abstract: A satellite (30) programmed for sun-nadir steering and having a solar wing (36) mounted to the satellite body (35) and being selectively moveable about two mutually orthogonal axes (A, B). The solar wing (36) is mounted to the satellite body (35) by a pair of gimbals (43, 45) thus allowing two degrees of freedom and thereby permitting the solar wing (36) to be rotated about the two mutually orthogonal axes (A, B). A first motor (42) in operative engagement with the gimbal (43) selectively rotates the solar wing about the axis (A), while a second motor (46) in operative engagement with the gimbal (45) selectively rotates the solar wing about the second axis (B). A control circuit (40) is in operative engagement with the first and second motors and selectively causes the first motor to rotate the solar wing about the first axis to a predetermined position, and selectively causes the second motor to rotate the solar wing about the second axis to a predetermined position.

Abstract: A laser crosslink attitude determination system using the payload laser crosslinks as the principal attitude sensor and thereby eliminating the need for additional high-cost, high performance dedicated celestial body or inertial sensors to provide attitude information. The method and system establishes optical crosslinks among orbiting spacecraft for attitude control by performing a series of deterministic scans. The need for separate dedicated attitude sensors is minimized or eliminated by increasing the acquisition field of uncertainty and/or reducing the crosslink acquisition field of view.

Abstract: Spacecraft cost, volume and weight are reduced with gyroscope calibration methods that can be effected with structures (e.g., a single-axis Sun sensor and a wheel system) which are typically carried by spacecraft for other purposes. In a method embodiment, these methods include an initial step 1) of calibrating a selected gyroscope and subsequent steps for each uncalibrated gyroscope of: 2) slewing the spacecraft by a slew angle and controlling attitude with the uncalibrated gyroscope for a selected time period to thereby couple its drift into the inertial axis of a calibrated gyroscope, 3) backslewing the spacecraft to return the calibrated gyroscope to its inertial axis, and 4) measuring an error angle generated by the uncalibrated gyroscope during the selected time period (e.g., with a Sun sensor or with the calibrated gyroscope).

Abstract: A method for steering the payload beam of a satellite in a non-geostationary orbit toward an intended service area having known geographical dimensions in order to obtain improved pointing performance with a corresponding reduction in the demand on onboard hardware and software systems. The method comprises the steps of determining a subterranean target point and a direction fixed in the payload beam, calculating the orientation that points the payload beam direction through the subterranean target point, and maintaining this payload beam orientation using an on-board attitude control system.

Abstract: Structures and methods are provided to effect satellite maneuvers in the presence of disturbance torques with enhanced fuel efficiency. A thruster control loop is combined with a wheel control loop in which the wheels respond to an estimate of the disturbance torques (e.g., an angular acceleration estimate, a thruster torque command or a filtered thruster torque command) to create a counteracting gyroscopic torque. Simulations of the invention's methods have shown they have the authority of all-thruster control systems and fuel efficiency that rivals that that of all-wheel control systems.

Abstract: A method for simultaneously controlling satellite nutation, sun angle and attitude walk using a dual-slit sun sensor and thrusters. The sun angle and nutation angles are computed from sun sensor data, and the thrusters are fired when their use improves both sun angle and nutation. Precession around the sun line is controlled by constraining firing, in the simplest case, firing only when the angular momentum is precessed directly toward or away from the sun. Precession maneuvers are implemented by commanding a new sun angle and precession plane spin phase.

Abstract: Methods for performing attitude maneuvers of a spinning satellite without the use of thrusters, or with a minimal number of thrusters, are disclosed. The attitude maneuvers are primarily achieved through the use of gimballed momentum wheels and solar wing drives. Various maneuvers can be performed depending on whether the satellite has near-zero net momentum or significant net momentum. The maneuvers include sun acquisition, sun hold, Earth acquisition and inversion.

Abstract: Solar tabs are provided which have thermal emissivity and absorptivity properties designed to increase the attitude-control torques that can be generated with rotatable wings in a spacecraft. These increased torques enhance the ability to offset attitude-disturbance torques which typically tend to alter the attitude of a spacecraft from a desired attitude. The tabs include highly absorptive and nonemissive front faces, highly emissive back faces and a high thermal conductivity between the front faces and the back faces.

Abstract: A method and system are disclosed for inverting a satellite spinning about a first desired spin axis to spin about a second desired spin axis substantially antiparallel to the first desired spin axis. A tumbling motion is induced in the satellite so that a spin axis of the satellite oscillates between the first desired spin axis and the second desired spin axis. The tumbling motion is induced by sensing at least one component of the angular rate vector and controlling a single degree of freedom momentum storage device based upon the at least one component of the angular rate vector. The single degree of freedom momentum storage device has an orientation of variation substantially perpendicular to the desired spin axis. The single degree of freedom momentum storage device is controlled so that the first desired spin axis is made an intermediate inertia axis of an effective inertia matrix.

Abstract: A method and system of orienting a payload of an orbiting spacecraft (14) to maintain a desired pointing profile in the presence of orbit inclination. A cone (12) is determined which is traced in inertial space by a pitch axis of the payload to maintain the desired pointing profile throughout an orbit. A bias momentum vector of the spacecraft (14) is oriented at an attitude which lies along the cone (12). The attitude has a nonzero angle with respect to a plane spanned by an orbit normal vector and an equatorial normal vector. The payload is rotated about a single body-fixed axis perpendicular to the pitch axis to align the pitch axis along the cone (12). As a result, the desired pointing profile is maintained throughout the inclined orbit.

Abstract: Methods and systems for stabilizing satellite spin about an intermediate inertia axis (Z) are disclosed. A set of gyros (22) sense the X component and the Y component of the angular velocity of the satellite body. A single degree of freedom momentum wheel (26) has a fixed transverse orientation with respect to the intermediate axis in order to store momentum. In one embodiment, the momentum wheel (26) is oriented to store momentum parallel to the Y axis. A tachometer (30) senses the rotation rate of the momentum wheel (26). A processor (24) forms a control signal representative of a control torque to be applied to the momentum wheel (26). The control torque is based upon the X component and the Y component of angular velocity of the satellite, and the angular velocity of the momentum wheel (26).

Abstract: A method for filtering actuator commands for a satellite control system to reduce resonant mode excitation despite uncertain knowledge of the resonance frequencies, due to parameter variations, nonlinearities, and the like. The present method filters the actuator commands with staggered proportional plus delay (posicast) filters having distinct delay values, chosen to provide a desired resonance attenuation over a resonance uncertainty range. The present method may be extended to filter multiple resonances by cascading the staggered filters for each resonance. The present invention has wide application, such as in spacecraft precession and translation maneuvers, as well as gimbaled payload stepping. A particular advantage of the present invention is its direct applicability to fixed-amplitude actuators (such as thrusters and stepper motors), and reduced excitation of resonances at frequencies above those of primary concern.

Abstract: The three axes thruster modulation (8) of the present invention accepts three axes of input torque commands or angular acceleration commands and generates thruster selection and thruster timing (40) information which is used to fire thrusters (48) for the purpose of spacecraft attitude control and velocity change maneuvers. The modulation logic (8) works in all three axes simultaneously and is suitable for use with an arbitrary thruster configuration, including a configuration in which individual thrusters or thruster groups do not produce torques about mutually orthogonal axes. After thruster selection and on-times have been determined, the modulation logic (8) uses this information to compute a best estimate of the actual rate change (42) which is then compared to the commanded rate change (44) to develop a residual unfired rate change.

Abstract: The attitude corrections required to remove attitude errors induced by orbit inclination deviations from the normal orbit plane, as well as the residual errors, are minimized by placing the satellite bias momentum at an inertial attitude lying substantially between the normals of the nominal and actual orbits, and using a payload reorientation means to adjust the payload attitude about three axes based on a combination of sensor data and offsets computed from the known orbit kinematics. In one embodiment, a momentum bias satellite is in an orbit slightly inclined from the geostationary orbit. The desired angular momentum vector attitude is chosen based on the orbit, the desired payload attitude, and the gimbal capabilities, and executed using thrusters. This reorientation limits the required gimbal travel.

Abstract: Agile (electronically steerable) beam sensing with associated on-board processing, previously used exclusively for positioning of antennas for beam formation and tracking in communications systems, is now also used for satellite active attitude determination and control. A spinning satellite (100) is nadir oriented and precessed at orbit rate using magnetic torquing determined through use of an on-board stored magnetic field model (520) and attitude and orbit estimates (212). A Kalman filter (211) predicts parameters (202, 203) associated with a received signal (204) impinging on the satellite's wide angle beam antenna (201). The antenna system measures the error between the parameter predictions and observed values and sends appropriate error signals (207) to the Kalman filter for updating its estimation procedures.