Attitude independent Spacecraft Angular Rate Estimation and Rate Damping using Magnetometer data

India has been launching low-Earth Orbit (LEO) satellites for Resource management, Radar imaging Cartographic, Oceanographic and other scientific applications. With increase in number of missions, there is need for autonomy operations on-board especially during attitude loss to ensure power safety and attitude re-orientation. The autonomy and mission requirements make it essential to have a gyro-less algorithm for spacecraft rate estimation whenever spacecraft enters safe mode and gyros are off and any other times during gyro malfunction/gyro saturation. Post safe mode, the first step is to ensure power safe and thermal safe conditions for the spacecraft. In case of high rates it has to be damped by thrusters and brought to a threshold so that sun acquisition is possible with sun-sensors. A Magnetometer based rate estimation (MRE) algorithm is developed for estimating spacecraft angular rates. Further a MRE based rate damping (MRD) logic is used for damping the spacecraft rates using thrusters. MRE is a Kalman filter based algorithm to estimate spacecraft angular rates from sequential readings of magnetometer data alone. The assumption made here is that geomagnetic field vector does not change (relative to an inertial frame of reference) during the short sampling interval. Accuracy of MRE rate estimation is about 0.5°/s. Based on these estimated rates thruster selection is done autonomously and thruster ON time is computed for firing the thrusters in MRD. Maximum angular rate axis is always selected for rate damping. There are in built protection logics to take care of wrong firing, firing duration more than intended etc. MRE and MRD are validated during various levels of ground testing and performance is found to be satisfactory.