On-orbit thruster calibration

Significant errors in force and moment output as well as large control coupling forces can result if the attitude and translation control thrusters of a spacecraft are not calibrated property. Thruster calibration is also important for estimating fuel usage so that spacecraft life can be accurately predicted. A general procedure to accurately determine the true relationship between the commanded and the actual force output of a set of thrusters is presented. This relationship is determined from data generated by a spacecraft while it is in orbit. The calibration is based on the Kalman filter estimator and is verified by a digital computer simulation of the Gravity Probe B spacecraft dynamics. Gravity Probe-B is an Earth orbiting gravitational physics experiment, which will test several aspects of Einstein's general relativity theory. A set of 18 proportional thrusters generate attitude control moments around three axes and translation control forces along three axes for full six-degree-of-freedom control. A total of 108 parameters are needed to characterize both the force and moment outputs of all 18 thrusters. The ability to calibrate these parameters to an accuracy of better than 1% rms is demonstrated.