Strategy of visible telescope on-board data processing during GRB localization in SVOM mission

SVOM (Space-based multi-band astronomical Variable Objects Monitor) is a Chinese-French mission dedicated to the detection, localization and study of Gamma Ray Bursts (GRBs) and other high-energy transient phenomena. The instrumentation in space includes ECLAIRs, a low-energy gamma-ray imager and spectrometer, GRM, a gamma-ray spectrometer, MXT, a low-energy X-ray telescope and VT, an optical telescope. The joint observation of the instrumentation is vital to the scientific outcomes of the mission. During the GRB localization sequence, after the detection of GRB and slew request sent by ECLAIRs, the satellite will slew towards the GRB target if the on-board constraints are satisfied. At the end of slew when the stability of instruments is reached, MXT starts the localization of GRB while VT starts taking images. When a valid localization of MXT is available, its corresponding position and size of the window in the VT images are determined. Due to limitation of the speed of SVOM VHF data link, the VT window image cannot be directly downloaded to the ground. The data processing onboard for VT is mandatory, if the detection of VT would be downloaded to ground as early as possible when the optical counterparts of GRBs are still bright enough to be detected by ground large spectroscopic telescopes. The detailed strategy for data processing onboard for VT are described in this paper, mainly focusing on the different MXT localization errors, interruption of SAA and occultations of the earth.