The SVOM satellite is made out of Payload Module and a Platform Module, which will be used to gather all the bus servitudes. The platform module will require high-precision attitude determination ability because the exposure time of the Visible Telescope will be 100 seconds, or even longer, thus the Relative Pointing Error should not exceed 0.8 arcseconds for every 100 second time window about the Visual Telescope’s cross-boresight axis. Star Trackers and Fine Guidance Sensor will be used to provide attitude information with such a high accuracy.
One of the two most important AOCS components in the SVOM satellite are Star Trackers. Such devices provide the absolute inertial attitude information. Thus, 3 Star Trackers from the Jena company were chosen as the absolute attitude measuring unit. In order to optimize the thermoelastic deformations between the Star Trackers’ axes and the instruments lines of sight, 2 STRs and VT were installed on 10cm thick ODP.
Like a Star Tracker with no star catalogue, Fine Guidance Sensor is used to provide relative attitude data with an accuracy of 0.3 arcseconds. 2 CCDs of Fine Guidance Sensor were designed on the above and below sides of the Visual Telescope’s main CCD because, without star catalogue pre-installed in its memory, Fine Guidance Sensor can only provide high quality star image diversion between two frames. All of the 3 CCDs were assembled on the same focus plane because scientists believe that would minimize the thermo elasticity error and increase the relative measuring accuracy. Special information fusion algorithm was designed to fuse the data from multiple CCDs to increase the attitude determination accuracy.
Scientists achieved sufficient attitude measuring accuracy by using a combination of Star Trackers and Fine Guidance Sensors. Hopefully, there will be no changes in the current SVOM’s AOC system architecture because the satellite itself is planned to be launched early next year.
Main Reference: AOCS General Architecture Design for SVOM Satellite
Additional References Gamma-Ray Bursts The Visible Telescope