Method for joint estimation of stellar atmospheric refraction and star tracker attitude

A method for joint estimation of stellar atmospheric refraction and star tracker attitude, executed by a star tracker processor in a spacecraft navigation system to enhance attitude determination accuracy, comprising:

• capturing, by a star tracker, an image of stars, acquiring coordinates of the stars on an imaging plane to obtain all observed stellar vectors in the star tracker coordinate system, and recognizing the image to obtain a matching relationship between an observed star and a reference star;
• transforming the reference star to a terrestrial reference frame based on time and position of observation by converting the reference star from a celestial reference frame adopted in a navigational star catalog to the terrestrial reference frame at the star tracker's observation position to obtain a true zenith distance of the reference star;
• obtaining a stellar atmospheric refraction compensated reference stellar vector in the terrestrial reference frame by calculating an estimated stellar atmospheric refraction using a simplified atmospheric refraction model with the true zenith distance and an initial atmospheric refractive coefficient, then subtracting said estimated refraction from the true zenith distance;
• obtaining an attitude cosine matrix from a star tracker coordinate system to the terrestrial reference frame by inputting the observed stellar vectors and compensated reference stellar vectors into a QUEST algorithm to generate an attitude quaternion, which is converted to the attitude cosine matrix;
• obtaining a stellar atmospheric refraction error based on the attitude cosine matrix by transforming observed stellar vectors to the terrestrial reference frame via the matrix, comparing with reference vectors to calculate observed refraction, then finding the difference between observed and estimated refraction;
• subjecting the stellar atmospheric refraction error to linear least squares fitting to obtain a fit slope and a fit intercept, wherein if the absolute value of the fit slope is greater than a cutoff condition, an estimated atmospheric refractive coefficient is superimposed with a feedback gain that is positively correlated to the fit slope, and the method returns to obtaining a stellar atmospheric refraction compensated reference stellar vector in the terrestrial reference frame;
• if the absolute value of the fit slope is smaller than the cutoff condition, iteration ends, whereby an optimal atmospheric refractive coefficient and an atmospheric refraction error compensated star tracker attitude are obtained for real-time spacecraft orientation adjustment.