Control tasks in automatic rendezvous and docking of spacecraft

Automatic rendezvous and docking/berthing operations will be one of the key techniques necessary for the operation of the International Space Station (ISS). Russian, European and Japanese transport vehicles will approach to, mate with, and depart from the ISS automatically, whereas the US Shuttle will perform these tasks in a semiautomatic mode. The rendezvous and docking process consists of a series of orbital manoeuvres and trajectories, the last one of which has to put the approaching vehicle into the narrow limits of position and attitude and their rates required for the mating process. All functions involved in the control of approach and mating are considered safety critical. Their failure tolerance requirements for safety and mission success are discussed. System designs for fault tolerant onboard control include different layers for Guidance, Navigation and Control (GNC), Mission and Vehicle Management (MVM) and Failure Detection, Isolation and Recovery (FDIR). Major tasks of these layers include: the execution of the various GNC modes, the switching between modes, the assignment of resources to each mode, the detection of failures and reconfiguration after failures, the high level monitoring of GNC functions and of the vehicle state. The GNC, MVM and FDIR control functions of the European ATV vehicle are discussed at the hand of block diagrams. In addition to onboard monitoring and control by onboard systems, the vehicle's state in orbit and the vehicle functions will be remotely monitored and the complete mission be supervised by operators on ground and (partially) in the target station. The philosophies for recovery or safe abort of the mission and, finally, the philosophy of interaction by human operators with the automatic onboard system are described.