The calibration methods of star trackers are mainly divided into ground calibration and on-orbit calibration. On-orbit calibration usually only calibrates the installation matrix of star trackers and related parameters for the space mission. The ground calibration is the most and the first choice to carry out the internal azimuth element, which directly determines the accuracy level of the star tracker. Here to introduce the accuracy analysis and calibration technology is mainly based on the ground calibration method.
Ground calibration implemented via equipment or real sky for imaging points. Literature [61-66] researched and discussed the non-equipment calibration: established a parameter model with star tracker distortion, focal length error, principal point error and image plane tilt error, and used the principle of the star tracker star angular distance equal and estimated the parameter by nonlinear least squares. This method is relatively simple, and almost no special calibration equipment is required. The research group also used this method to carry out calibration experiments, but the experimental results were not satisfactory. The main reasons are: the established model parameters are coupled with each other, then making it difficult to decouple and analyze, the system of non-equipment calibration is not only insensitive for the parameters, but also not a optimized out put, then as consequence of the initial value, the estimated parameters obtained are difficult to control. Simple completed by a real sky test, the non-equipment calibration system is a convenient, economic, simple principle, but rough accuracy calibration system for various applications.
Equipment calibration generally includes two methods: the 1st is using a single star point simulator and a high-precision turntable; the 2nd is using a multi-star point simulator. These two methods can calibrate the accuracy of the star tracker. For 1st method, a especially high precision of the turntable is required, and the measurement time is relatively long due to “point-by-point”measurement during the calibration process. For 2nd method, the development of a multi-satellite simulator with higher precision than the star tracker’s is required, and it is difficult to develop multi-satellite simulator with a globe star map, while the measurement process is relatively simple and no need an turntable or other measuring equipment. At present, both methods are widely used, and reliable calibration methods.
In addition, the star tracker also can be calibrated by camera. Set a series of calibration parameters to the camera, then use the optimization algorithm to optimize the calculation of the parameters, for example, the Astro10 star tracker of Jena-Optronik Company determine 40 calibration parameters by this method.