About accuracy of an Earth remote sensing spacecraft attitude-keeping without using data from inertial sensors

The paper discusses attitude control of a spacecraft (SC) designed for imaging specified areas on the Earth surface at high spatial resolution (of 1,0 m when pictures are taken in nadir direction), operating in a circular orbit with altitude of 720 km and inclination 51,6°. During one observation session (lasting 20 minutes), the Guidance, Navigation and Control System (GN&CS) must provide support for imaging of up to 12 individual targets, or take 6 stereoscopic images. GN&CS includes: three star trackers, an angular rate sensor, satellite navigation equipment, a system of inertial end-effectors consisting of eight fly wheels, electromagnetic end-effectors for cancelling the angular momentum accumulated in the fly wheels. When the SC turns prior to the imaging session, the angular rate exceeds the measuring range of the angular rate sensor. The accuracy characteristics of the angular rate sensor immediately after entry into the measuring rate are not guaranteed by the developers of the instrument. That is why star trackers are used to determine the SC angular rate during the turn and the imaging session. A study was done on the feasibility of meeting the requirements for the SC stabilization accuracy and the number of observations in a session without using measurements from the angular rate sensors. Parameters were selected for the observer and regulator of the dynamic control loop for the SC attitude control. The paper provides SC dynamic mode simulation results.