Fault Reconstruction in a Leader/Follower Spacecraft System Using Higher Order Sliding Mode Observers

This paper considers the use of higher order sliding mode observers to address the problem of actuator fault estimation in a satellite leader/follower system. It is argued that if only relative position information between the leader and follower spacecraft is available, the problem of reconstructing an actuator fault signal, modelled as an additive unknown input to the system, represents one of estimating the unknown inputs in a relative degree two type system. This means many of the existing unknown input observer schemes are not applicable. Here both a broken super-twisting scheme and a 3rd order sliding mode observer are considered. The results of using these schemes on a 'real world' simulation are compared.