Midcourse space experiment precision ephemeris

The Midcourse Space Experiment satellite, hosting a suite of state-of-the-a rt sensors, was launched into an 898-km altitude, nearly sun-synchronous or bit in April 1996, One of the primary tasks of the space surveillance princ ipal investigator team was to evaluate the utility of the Midcourse Space E xperiment sensors in performing space surveillance tasks. A critical issue in assessing the sensors' performance is the metric accuracy of their obser vations, which depends on the ephemeris accuracy of the satellite. In parti cular, to support the accuracy requirements of the Space-Based Visible, the primary space surveillance sensor onboard the satellite, the ephemeris acc uracy goal was set at 15 m (1 sigma). There are two issues that had to be a ddressed in meeting this ephemeris accuracy goal: 1) the quantity and quali ty of the tracking data and 2) the dynamic modeling of the satellite's moti on. The tracking of the satellite was performed by the U.S. Air Force Space Ground Link System of S-band radars, while Lincoln Laboratory's Millstone Hill radar in Westford, Massachusetts provided tracking data that were used to independently assess orbit accuracy. The most difficult aspect of the d ynamic modeling of the satellite's motion was due to the effect of anomalou s accelerations from cryogen gas venting, Results are presented that show t he 15-m ephemeris accuracy goal has been met and exceeded.