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Simulators prepare star trackers for space flight

SSTL’s Optical Payload Group is currently building two Dynamic Multi Star-field Simulators (DMSS) at its Sevenoaks facility that will be used to test star trackers that will be flown in space under simulated operating conditions here on Earth. Star trackers are an essential subsystem in any satellite, providing information that allows the spacecraft’s on board computer to determine its orientation or “pointing” at any point in time.

They work by taking an image of a region of the sky using a specially designed optical camera, and comparing successive images to determine how much the orientation of the satellite has drifted. This information is then sent to an attitude control system that corrects for the drift by using on-board thrusters or other motion-generating devices to maintain the correct satellite pointing. Any time a satellite is manoeuvred in orbit, the pointing information from star trackers is vital to the spacecraft’s control systems. For example, solar panels must be aligned to capture the maximum available light from the Sun, and communications antennae pointed towards the Earth. Star trackers are now available that operate in autonomous modes. These units are able to dynamically monitor the star fields in orbit and provide correction information back to the satellite to counteract adverse platform manoeuvres, such as slews and spins, which is particularly useful in the early stage of mission operations or following a major system failure. To test these units on the ground prior to launch, it is essential to present the star trackers with a dynamically varying star field. This is where the DMSS comes in.