SMART-1 STAR TRACKER IRRADIATION EXPERIENCE AND BENEFITS FOR FUTURE GEOSTATIONARY TRANSFER ORBIT MYRIADE MISSIONS

摘要

The ESA's SMART-1 mission to the Moon uses a shielded version of the highly advanced and autonomous Star Tracker (STR) "Advanced Stellar Compass (ASC)" developed by the Technical University of Denmark (DTU). SMART-1 was launched by an ARIANE 5 the September 23rd 2003. Its unique propulsion means to transit from the initial Geostationary Transfer Orbit (GTO) up to a low Lunar orbit is an ion-motor. Its perigee altitude raises from 656 km to 13,600 km within 3 months. Therefore, during that period, the equipment of SMART-1 was subjected twice per orbit to Van Allen belts radiation. Incidentally, several huge solar storms occurred within the same period. Consequently, bright spots appeared on the STR CCD and resulted in transient loss of attitude determination. Dedicated morphological filters were designed by DTU and loaded in the STR software permitting to recover to normal performance. This experience provided DTU with high heritage on the use of STR within radiation belts. The ASC also provides attitude measurement on the Low Earth Orbit MYRIADE platform developed by the French space agency: CNES. Spacecrafts based on this micro-sat platform can be launched together with 2 Geocom satellites on the ARIANE 5 launcher. So the DTU ability to work within radiation belts gives the opportunity to design and study a low cost scientific mission on GTO. To estimate the ASC capability to operate on a 2 years GTO mission, DTU has developed a model of temporal evolution of the number of hotspots on the STR CCD. Both for "permanent" and "transient" hotspots, pixel pattern and luminosity have been determined according to the CCD temperature. Using this model, it has been possible to conduct a high fidelity GTO representative test on the ALCATEL STR optical stimulator test bench. The test results demonstrate a very good robustness capability for CCD temperature up to 5℃, even at end-of-life conditions across the proton belts.