A highly efficient attitude reference pointing law for Chinese “Einstein probe” satellite

The “Einstein Probe (EP)” is a small mission of the Chinese Academy of Sciences (CAS) dedicated to time-domain high-energy astrophysics. Its primary goals are to discover high-energy transients and monitor variable objects. The payloads of the EP include a wide field of view of X-ray telescope (WXT) and a following-up observation X-ray telescope (FXT). The goal of WXT is to implement a full-sky monitoring to achieve triggering and localization of X-ray transient sources and outbreak sources, while the FXT is used for deep observation of the trigger source, the opportunity target, and high sensitivity celestial survey. Considering the single observation time less than 20 minutes and illumination constraints of the payload, the attitude reference pointing law is designed based on the time of anti-subsolar, and the sun vector is constrained in the second quadrant of -XOZ of the body frame of the satellite in the attitude pointing law design. There are four observation fields in each orbit, the core part is the duration when the satellite is at the umbra of the Earth, which is named part II. In this part, the satellite is designed pointing to the right ascension of anti-subsolar point, and the declination is determined by the field of view of WXT in order to cover the declination range of -90 to 90 degree in the observation cycle of six orbits. The part I is the time interval from the point at which the satellite flies away the subsolar point to the start of part II. While the part III is the time interval from the end of the part II to the point when the satellite enters the light region. In part I and part III, the observation point of the declination is the same, while the right ascension is 120 degrees apart, twice of the field of view of WXT. Part IV is the duration when the satellite is around the subsolar point, the payload points to the Earth to avoid the sunlight. The attitude maneuvers between each part is obtained by spherical linear interpolation. The simulation results show that the attitude reference pointing law can ensure WXT covering more than 70.0% of the celestial sphere in one observation period of six orbits, and the total celestial sphere is covered in half a year, which can satisfy the requirement of the EP scientific observation.