Flying in a spacecraft constellation: a coordination puzzle.

CNES has been operating the PARASOL spacecraft as a member of the Afternoon (A-Train) Satellite constellation for nearly 5 years and the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) for more than 3 years. Whereas PARASOL is a French spacecraft operated by CNES, CALIPSO is a French platform with a National Aeronautics and Space Administration (NASA) payload (Lidar) and jointly operated by CNES and NASA. In this frame, a number of challenges and puzzling situations have been experienced in the A-train and in the CNESNASA cooperation.