Star Tracker Rate Estimation with Kalman Filter Enhancement
AssigneeThe Boeing Company
InventorsTung-Ching Tsao, Richard Y. Chiang
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Abstract
An attitude estimator that uses star tracker measurements and enhanced Kalman filtering, with or without attitude data, to provide three-axis rate estimates. The enhanced Kalman filtering comprises taking an average of forward and rearward propagations of the Kalman filter states and the error covariances. The star tracker-based rate estimates can be used to control the attitude of a satellite or to calibrate a sensor, such as a gyroscope.
Claims20
A method of estimating a three-axis attitude and rate of a spacecraft in a spacecraft body frame of reference, comprising:
- (a) measuring star vectors at a plurality of instants of time using a star tracker attached to a spacecraft;
- (b) calculating a respective spacecraft body frame-to-ECI frame quaternion for each instant of time based on information derived from said measured star vectors;
- (c) calculating spacecraft body rates based the quaternions calculated in step (b);
- (d) forward Kalman filtering the calculated spacecraft body rates;
- (e) backward Kalman filtering the calculated spacecraft body rates;
- (f) averaging the results of steps (d) and (e) to produce average rate estimates;
- (g) selecting one of the average rate estimates, or calculating a rate estimate that is a function of two or more of the average rate estimates;
- and
- (h) calculating a three-axis attitude of the spacecraft based on the rate estimate selected or calculated in step (g).
A spacecraft comprising a body, a camera mounted to said body, and a computer system programmed to perform the following operations:
- (a) processing images of stars captured at successive instants of time by said camera to collect star vectors;
- (b) calculating a respective spacecraft body frame-to-ECI frame quaternion for each instant of time based on information derived from said star vectors;
- (c) calculating spacecraft body rates based the quaternions calculated in operation (b);
- (d) forward Kalman filtering the calculated spacecraft body rates;
- (e) backward Kalman filtering the calculated spacecraft body rates;
- (f) averaging the results of operations (d) and (e) to produce average rate estimates;
- (g) selecting one of the average rate estimates, or calculating a rate estimate that is a function of two or more of the average rate estimates;
- and
- (h) calculating a three-axis attitude of the spacecraft based on the rate estimate selected or calculated in operation (g).
A method of estimating a three-axis attitude and rate of a spacecraft in a spacecraft body frame of reference, comprising:
- (a) measuring star vectors on an imaging plane at a plurality of instants of time using a star tracker attached to a spacecraft;
- (b) calculating respective spacecraft body rates for said instants of time based on the star vector measurements without using attitude information;
- (c) forward Kalman filtering the calculated spacecraft body rates;
- (d) backward Kalman filtering the calculated spacecraft body rates;
- (e) averaging the results of steps (c) and (d) to produce average rate estimates;
- (f) selecting one of the average rate estimates, or calculating a rate estimate that is a function of two or more of the average rate estimates;
- and
- (g) calculating a three-axis attitude of the spacecraft based on the rate estimate selected or calculated in step (f).
A spacecraft comprising a body, a camera mounted to said body, and a computer system programmed to perform the following operations:
- (a) measuring star vectors on an imaging plane of the camera at a plurality of instants of time;
- (b) calculating respective spacecraft body rates for said instants of time based on the star vector measurements without using attitude information;
- (c) forward Kalman filtering the calculated spacecraft body rates;
- (d) backward Kalman filtering the calculated spacecraft body rates;
- (e) averaging the results of steps (c) and (d) to produce average rate estimates;
- (f) selecting one of the average rate estimates, or calculating a rate estimate that is a function of two or more of the average rate estimates;
- and
- (g) calculating a three-axis attitude of the spacecraft based on the rate estimate selected or calculated in step (f).
A method for calibrating a rate sensor comprising:
- acquiring measurements using a rate sensor and a star tracker running simultaneously;
- calculating rate sensor rates from measurements made by the rate sensor and star tracker rates from measurements made by the star tracker during the same time interval;
- filtering the calculated rate sensor rates forward and backward Kalman filtering and then averaging the filtered results;
- filtering the calculated star tracker rates forward and backward Kalman filtering and then averaging the filtered results;
- determining differences between the filtered rate sensor rates to the filtered star tracker rates;
- and
- calibrating the rate sensor to take the differences into account.
Dependencies are inferred from the claim text.