Monitoring CloudSat reaction wheel friction via ground test and on-orbit data

Since joining the A-Train in May 2006, CloudSat actively maintained its orbit and flew in tight formation with CALIPSO. On April 17th of 2011 CloudSat experienced a severe battery anomaly which caused the vehicle to fault into a passive but power positive spin, and it could no longer maintain itself within its control box. Over the next several months, the team diagnosed the issue with the spacecraft battery, and developed a new method of operations which involved hibernating during eclipse, but allowed the vehicle to collect science data for 55 minutes during the sunlit portion of the orbit. By November 2011 the vehicle was returned to operational status. In order to ensure the health of the rest of the Afternoon Constellation (A-Train), the CloudSat team now consistently monitors any component, which if disabled, could cause CloudSat difficulty in exiting the A-Train. One of the components which is closely monitored is the four reaction wheels which CloudSat uses to maintain pointing and maneuvering capability to perform formation maintenance inclination burns. As a result of the new method of operating the vehicle, CloudSat carries additional momentum within the system and shuts all non-essential components off during eclipse. During this time, the reaction wheels spin to zero revolutions per minute (RPM) as the momentum in the wheels is transferred to the bus. At eclipse exit, the reaction wheels are powered back on and the vehicle regains control and continues science collection.