A Precise Algorithm for Computing Sun Position on a Satellite

ABSTRACT: To meet the high precision sun tracking needs of a space deployable membrane solar concentrator and other equipment, an existing algorithm for accurately computing the sun position is improved. Firstly, compared with other theories, the VSOP (variation seculaires des orbits planetaires) 87 theory is selected and adopted to obtain the sun position in the second equatorial coordinate system. Comparing the results with data of the astronomical almanac from 2015, it is found that the deviation of the apparent right ascension does not exceed 0.17 arc seconds, while that of the apparent declination does not exceed 1.2 arc seconds. Then, to eliminate the difference in the direction of the sun position with respect to the satellite caused by the size of the satellite’s orbit, a translation transform is introduced in the proposed algorithm. Finally, the proposed algorithm is applied to the orbit of the satellite designated by SJ-4 (shijian-4). Under the condition that both of the existing and improved algorithms adopt the VSOP87 theory to compute sun position in the second equatorial coordinate system, the maximum deviation of the azimuth angle on the SJ-4 is 35.19 arc seconds and the one of pitch angle is 19.93 arc seconds, when the deviation is computed by subtracting the results given by both algorithms. In summary, the proposed algorithm is more accurate than the existing one.