Why is Control Systems important?
The set tasks of many spacecraft require them to maintain specific absolute and relative pointing within acceptable errors and this is achieved by the on-board AOCS. Communication satellite antennas need to respect half-cone pointing errors for ground stations and users, while space astronomy observatories or Earth observing satellites have in addition to ensure high stability of the line-of-sight of optical or radiofrequency instruments.
Similarly, Guidance, Navigation and Control (GNC) systems ensure that planetary probes and transportation vehicles shall maintain not only their pointing orientation, but also their absolute or relative position in space in order to achieve a successful rendezvous with a target, or an autonomous and safe landing on a planetary body or to maintain the relative six degrees of freedom (DOF) positioning when it comes to a a formation flying mission. Specialised sensors are required in this case, such as rendezvous sensors, Lidar or Optical Navigation cameras, radio frequency and optical metrology systems.
So AOCS/GNC is a crucial subsystem – a mission will come to a premature end if it fails. The level of performance required from the AOCS/GNC varies from mission to mission: A communications satellite pointing to a ground station might require an accuracy of 0.1 degrees while a space telescope seeking a certain astronomical target needs to be thousands of times more precise. Formation flying and planetary lander missions currently in the planning stages will need also high precision for relative navigation and positioning.
Last update: 18 November 2009