SumbandilaSat and Interactive Attitude Control

Abstract


The lecture will describe the attitude determination and control system to support the multispectral Earth observation main payload of the SumbandilaSAT microsatellite. The satellite has only a single main Y-body mounted solar panel and the attitude control system must ensure a nominal sun-pointed attitude under all non-imaging conditions during the sunlit part of the orbit. The control actuators employed are 3-axis magnetic torquer rods and reaction wheels. During initial detumbling and safe mode operations a simple magnetic control law is used to bring the satellite to a sun pointed Y-Thomson spinning attitude for maximum solar power collection.

From this sun-pointed, spinning attitude an intermediate control mode is entered when the Y-reaction wheel is utilised as a momentum wheel, to absorb the body spin rate and to inertially stabilise the angular momentum vector towards the sun direction. During the intermediate mode the magnetic rods are used to maintain the momentum vector size and direction and to do nutation damping. The pitch angle is also controlled using the Y-wheel, to keep the main imager payload as close as possible to an earth pointed attitude and to thermally stabilise the imager telescope.

The final and nominal attitude control mode is entered when a zero biased 3-axis reaction wheel controller is enabled, for: 1) sun tracking for optimal solar power collection, 2) target tracking during view finder use or during imaging download communication with a ground station and 3) pushbroom imager scanning with a forward motion compensation capability. During the nominal mode the magnetic rods are used to dump the angular momentum from the reaction wheels during sun tracking periods.

A short introduction to the Sumbandila satellite will be given. All the control modes, the attitude sensors and estimators utilised, will be presented. Specifically, a unique agile view finder control mode to manually select targets for subsequent high resolution image scanning, when a control ground station is available within the communication footprint of the satellite, will be explained more thoroughly.