An attitude sensing system utilizing simplified techniques and apparatus includes a Kalman filter which receives signals from an inertial measurement unit, a GPS receiver, and an integrated optical assembly. The output vector of the filter includes estimates of attitude misalignments and estimates of gyro drifts corresponding to the axes of the inertial measurement unit. The optical assembly includes a sensor array providing signals to the filter representing detection of the Earth's horizon or the center of the Sun. More particularly, a local vertical vector, computed from fore and aft detections of the Earth's horizon, is used in combination with GPS received signals to initially determine attitude by means of gyrocompassing. This attitude information is thereafter maintained by the inertial measurement unit and azimuth error resulting from drift of the inertial measurement unit and the initial gyrocompassing error is corrected by detections of the Earth's horizon and the Sun. The integrated optical assembly, used to detect the locations of both the Sun and Earth relative to the satellite, eliminates a need for separate apparatus dedicated to each sensing function. Moreover, the GPS receiver eliminates the need for ground tracking apparatus by providing the satellite ephemerides (including orbital pitch rate used in gyrocompassing) for use by the filter, rendering the attitude sensing system substantially autonomous. Additionally, the GPS receiver, in combination with the Sun and Earth detections, eliminates the need for a complex star tracker by providing an accurate inertial frame of reference.

Apparatus and method for autonomous satellite attitude sensing

A satellite communication system has at least one satellite (1) with an antenna that generates a moving beam pattern on the surface of the earth. The beam pattern (3) is comprised of a plurality of sub-beams (4). A method of this invention determines an attitude correction signal for the satellite by the steps of: (a) providing at least one reference transmitter (10) at a known location on the surface of the earth; (b) transmitting at least one signal from the at least reference transmitter into at least one of the sub-beams; (c) receiving the at least one signal with the satellite antenna and transponding the received at least one signal to a ground station (8). A next step of the method (d) receives the transponded at least one signal with the ground station; (e) determines a gain of the received at least one signal; (f) compares the determined gain to a gain expected to be received based on a predetermined knowledge of a spatial variation in gain of the satellite antenna; and (g) and determines a difference between the determined gain and the expected gain to derive a correction signal indicative of an attitude error of the satellite. The method further includes the steps of transmitting the correction signal to the satellite; and correcting the attitude of the satellite in accordance with the correction signal. For the case where the correction signal has a value that is a function of the yaw error angle of the satellite, the method further includes the steps of transmitting the correction signal to the satellite; and correcting the attitude of the satellite in accordance with the correction signal by rotating the satellite about the yaw axis so as to reduce a magnitude of the yaw error angle.

Satellite beam steering reference using terrestrial beam steering terminals

A method of Kalman filtering for estimating the position and velocity of aracked object is provided. A Kalman filter is initialized with at least position and velocity error states in an inertial computational frame. Sensor measurements are used to develop a measured line-of-sight vector to the object. Matrix transformations are used to analytically rotate the sensor measurements into a measurement frame. The measurement frame is defined as having one axis pointing towards the estimated relative position of the object. The use of the measurement frame allows the method to be adaptable to any line-of-sight/computational frame geometry. Since statistical correlation of the measurements is not present in the measurement frame, the number of computations at each filter update is reduced.

Method of Kalman filtering for estimating the position and velocity of a tracked object

A digital beam forming system includes an array of computing units (60-76) for weighting incoming signals and a plurality of summing processors (80-84) for generating output signals that represent weighted sums corresponding to rows within the array. The digital beam forming system can be incorporated in either a transmitter or receiver used in a radio frequency communications system.

Method and system for digital beam forming

The present invention is a method and apparatus to control a pointing trajectory of an aim point with respect to one or more targets for a momentum bias spacecraft in an orbit inclined relative to an equatorial geosynchronous orbit wherein the spacecraft is provided with an onboard computer capable of using orbit information available to the spacecraft and spacecraft hardware to move the aim point according to a predetermined tracking model which may be either time-varying or invariant. The tracking model allows the controller to remove short-term nutational and long-term orbit rate dynamical disturbances, beyond simple-order sinusoidal functions having a single frequency.

Method and apparatus for inclined orbit attitude control for momentum bias spacecraft

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. The Kalman filter additionally outputs the spacecraft attitude error signals (215) to an attitude control law (213), which determines commands to attitude-altering magnetic torque elements (220) to close the control loop via the spacecraft dynamics (230).

Joseph H Hayden

Papers

Satellite attitude determination and control system with agile beam sensing