Operational Collision Risk Assessment of CALIPSO and LANDSAT-5 Crossings

摘要

In late February 2010 the French Space Agency (Centre National d'Etudes Spatiales, CNES) and NASA (LaRC, Langley Research Center) operations teams in charge of the CALIPSO satellite were notified of an unfavorable spacecraft collision risk with the Landsat-5 satellite detected by the NASA Earth Science Mission Operations (ESMO) team. As a member of the Afternoon Constellation, CALIPSO is orbiting in a sun-synchronous frozen orbit following a repetitive ground track at a mean equatorial altitude of 705 km. Landsat-5, operated by the United States Geological Survey (USGS), is also orbiting in a sun-synchronous frozen orbit following almost the same ground track at the same mean equatorial altitude. Both orbits can be considered as nearly identical, the main difference between them being the mean local time of the ascending node. The assumed in orbit position difference between the two satellites was such that the relative phasing should not create any collision risk despite the orbit intersections. However, changes in mean local time of Landsat-5 and the Afternoon Constellation modified the orbital configuration and led to dangerous crossings during a significant period of time. This issue concerns not only CALIPSO and Landsat-5, but also all the current and future Afternoon Constellation missions. This paper will introduce the station keeping principles that led to the dangerous orbital configuration and the flight dynamics aspects taken into account to study the crossings. It will continue to present the CNES and LaRC tools developed to identify the crossings and to compute the maneuver trade space permitting to choose the maneuver parameters that mitigate the collision risk. Finally, it will describe the maneuver strategy agreed upon by all the concerned missions to manage the close approaches.