In-orbit AODCS performance of Sumbandilasat an earth observation satellite

Steyn W.H. (2010)

Conference Paper

The paper describes the commissioning and in-orbit performance results of the attitude and orbit determination and control system to support the multi-spectral earth observation imager payload of the SumbandilaSAT microsatellite. The satellite has been designed as a single string mission with little redundancy due to mass and volume restrictions and early in the mission the Z-axis reaction wheel was permanently lost due to a power switch failure. The paper will demonstrate how full 3-axis stability is achieved without the use this wheel. The pushbroom imager is still able to scan target areas accurately using a forward motion compensation method (FMC4), simultaneously doing a roll offset manoeuvre for cross track scanning. Target tracking by pointing the high data rate S-band antenna is also successfully demonstrated, while allowing the satellite to freely rotate around the antenna boresight during these periods. The satellite has a Y-body mounted main solar panel and the attitude control system must ensure a sufficient level of solar energy collected during the sunlit part of each orbit. The control actuators employed are 3-axis magnetic torquer rods and X/Y reaction wheels. During initial detumbling and safe mode operations, a magnetic control law is used to bring the satellite to a sun pointed Y-Thompson spinning attitude for maximum power collection. From this sun-pointed, spinning attitude an intermediate control mode is entered where the Y-reaction wheel is utilised as a momentum wheel, to absorb the body spin rate and to inertially stabilise the momentum vector initially towards the sun direction and finally aligned with the orbit normal. During this mode the magnetic rods are used to maintain the momentum vector in size and steer the pressesion direction while doing nutation damping. The pitch angle is controlled using the Y-wheel, to keep the main imager payload as close as possible to an earth pointed attitude and thermally stabilise the telescope. Finally, the nominal attitude control mode is enabled when a low Y-momentum biased X/Y reaction wheel and magnetic controller is enabled, to: 1) ensure a nominal nadir pointing attitude to collect a sufficient level of solar energy, 2) do target tracking during view finder use or during imaging download communication with ground stations and 3) implement pushbroom imager FMC4 scanning. During the nominal mode magnetic rods are used to maintain the Y-wheel angular momentum, dump the X-wheel momentum and zero any yaw angle offsets. The in-orbit performance will be presented of newly developed 2-axis sun and earth sensors, an autonomous star tracker and SGPS receiver. A Butane resistojet propulsion system's performance will be discussed, showing the results of circularising and raising an eccentric 490 by 505 km initial orbit. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

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