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Description of Driving Event:

Hydrazine fuel in pressurized tanks is used for satellite stationkeeping. Fuel consumption log indicated sufficient fuel to extend mission beyond date originally planned. During a north-south stationkeeping in July 1998, thruster firing was aborted due to satellite attitude instability caused by uneven thruster firing. This was determined to be caused by two-phase flow of liquid hydrazine and Helium pressurant gas to thrusters in the odd half-system. This only occurs when hydrazine in a tank is nearly depleted. The fuel log indicated that the tanks in both the odd and even half system should have had sufficient fuel for several more stationkeeping maneuvers, contrary to the observed performance.