Cassini Thruster Calibration Algorithm Using Reaction Wheel Biasing Data

Abstract

Thrust force estimates for the reaction control thrusters onboard the Cassini spacecraft are presented in this paper. Cassini consists of two thruster branches, each with eight thrusters. The eight thrusters in each branch control the three body-fixed axes of the spacecraft. It is important to track the thrust force estimates in order to detect any thruster degradation and for supporting various activities in spacecraft operations (Titan flyby, spacecraft maneuvers, etc.). The Euler equation, which describes the rotational motion of the spacecraft during a reaction wheel bias event (wheel momentum change), is used to develop the algorithm. The thrust estimates are obtained from the pseudoinverse solution using flight telemetry during the bias. These results are similar to thrust estimates from the Kalman filtering algorithm. Cassini operations used the first thruster branch from launch until November 2008. Results show that two thrusters in this branch exhibited degraded thrust in November 2008. Because of the degraded thrust performance, the first branch usage was discontinued, and the prime branch was swapped to the second thruster branch in March 2009. The thrust estimates from this branch do not show any degradation to date. The algorithm is used to trend the thruster branch thrust force estimates as the mission continues.