Accuracy assessment of the satellite remote sensing depends on the angular attitude estimation precision. The 1 arc second error in attitude estimation causes 2.5-meter error in the accuracy derived from remote sensing data for the 500 km orbit. Different kind of momentum wheels, propulsions and sensors help correct spacecraft torque moment to stabilize it in the orbit. Star tracker is the most precise optical sensor for spacecraft angular attitude estimation. An onboard guide star catalog containing data for star pattern identification is essential for star tracker operating. The total number of stars, the faintest stellar magnitude, completeness and uniformity are the key specifications of a star catalog influencing many characteristic of a star tracker. The steps of creating guide star catalog are: instrumental stellar magnitude estimation with respect to the star tracker spectral response, clusterization of nearby stars, removing of unreliable stars and final star selection. An iterative algorithm for thinning down the catalog allows reducing appreciably the number of stars in the catalog and improving its uniformity. The key point of the algorithm is the lower bound evaluation of star number in the FOV (field of view) for every boresight position within a triangle area. The algorithm uses recursive quaternary division of the icosahedron for the celestial sphere tessellation. The correction methods of stellar aberration and star proper motion are discussed as well.
The Wahba problem is the task of constrained optimization seeking the matrix from SO(3), which maximally converges (based on the least squares criterion) two sequences of unit vectors. The solution of this task is vital for satellite attitude determination using star trackers. An iterative method for solving the Wahba problem is proposed. Each iteration of the proposed method is reduced to sequential rotation of the vectors and solving the system of linear algebraic equations. Usage of the method implies that the corresponding vectors of both sequences are located sufficiently close to each other. Two variants of the method are proposed, having linear and quadratic convergence. The Wahba problem solution is interpreted in terms of finding the angular velocity of a system of material points, which have certain angular momentum. Taking into consideration the characteristics of state-of-the-art star trackers, one to two iterations are sufficient for finding the optimal solution using the small-angle rotation method. The primary advantage of the proposed method as compared with classical methods based on calculation of eigenvectors and singular decomposition is the simplicity of its implementation.
The paper is devoted to the results of the star tracker 329K flight tests on board of the satellites Luch-5A and Luch-5B
launched into geostationary orbit in December 2011 and November 2012 respectively. Emphasis is placed on accuracy
and photometric characteristics of the star tracker 329K.
The author presents a technique to calculate instrument stellar magnitudes for silicon photosensors used in modern star trackers as well as estimates the error of the said technique. The technique implies calculation of instrument stellar magnitudes as specifically selected functions of color indices B-V, V-J of the Tycho-2 and two micron all sky survey (2MASS) catalogs. This function is a sum of basis functions with coefficients determined based on star trackers' response. The coefficients are calculated individually for each star tracker response. Calculation of the coefficients of the function is done using the least squares method for color indices of artificial stars. Spectra of artificial stars are created on the basis of reddening of typical subclasses spectra without interstellar extinction, such spectra taken from the Pickles catalog. Thickness of interstellar medium for reddening is selected basing on random law. Validation of accuracy of the proposed methodology is performed by calculating star brightness for standard photometric bands R and I and subsequent comparison of the obtained results with actual data. Such check indicates that the root-mean-square deviation of error is not over 0.08 m. Due to usage of data related to star brightness in J band of 2MASS catalog, the number of stars in the guide star catalog is increased by 30% as compared with using data only from Tycho and HIPPARCOS catalogs.