SPOT-1 Earth Observation Satellite Deorbitation

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The SPOT Earth observation system received French government approval in 1978. Designed by the French Space Agency CNES, and produced in cooperation with Belgium and Sweden, it comprises a series of low orbit satellites and associated ground facilities for spacecraft control and programming, image reception and production. The SPOT IMAGE company, a CNES subsidiary, is responsible for the distribution of SPOT products. SPOT-1 was successfully launched on 22-nd February 1986 with a 3 year expected lifetime, and is still operational after 16 years in orbit ! The SPOT-2 satellite was launched on 22-nd January 1990, and SPOT-3 on 26-th September 1993. SPOT-4 was placed into orbit on 24-th March 1998, with the secondary mission VEGETATION and the passengers DORIS, POAM from the Naval Research Laboratory (US), and PASTEL from ESA. SPOT-5, the latest satellite in the family, was successfully launched on 4 May 2002. SPOT satellites orbit is circular (altitude 822 km at the equator), near-polar( i = 98°), sun-synchronous (descending node at 10h30 a.m.), and phased (26 days cycle), with an orbital period of 101.4 minutes. To respect CNES orbital debris mitigation advice, it has been decided to de-orbit SPOT-1 when SPOT-5 is declared operational. Without corrective action SPOT-1 would likely to remain in orbit for a great number of decades before falling back to Earth... Unfortunately due to a lack of propellant, a straight atmospheric re-entry is not possible today, but a compromise solution was found to command SPOT-1 to use its residual propellant to maneuver into a lower altitude disposal orbit, from which re-entry could be completed within 15 years. The paper reports on the technical choices performed during the deorbitation strategy development : Final orbit features (circular or elliptical), attitude control in transient phase, type and number of maneuvers, and low cost aspect. It presents the main steps of a joint study between ASTRIUM and CNES teams, the consequent modifications on the onboard computer software (LV), and on the ground space mechanics software (OMGS), and finally, the operational scenario. The main objective of this project is « to preserve space environment », in accordance with the IADC (Inter Agency Space Debris Coordination Committee) recommendations.