NASA NTRS2004
A report describes the time-transfer system of the Gravity Recovery and Climate Experiment (GRACE), in which information on the distribution of Earth mass is extracted from position and time measurements for two spacecraft about 200 km apart in a circular, nearly polar orbit. Each spacecraft carriers a Global Positioning System (GPS) receiver, a K/Ka-band ranging (KBR) instrument, and an ultra-stable oscillator (USO) that serves as a clock for the GPS and KBR units. The long-term errors of the USOs are cancelled by use of a technique, called dual-one-way phase measurements, in which the phases of the KBR signals from spacecraft A as measured at spacecraft B are added to the phases of the KBR signals from spacecraft B as measured at spacecraft A. GPS data are used to synchronize time between the USOs to within approximately 150 ps as needed to enable the dual-one-way phase measurements: For each spacecraft, the GPS data are used to solve for orbital positions, and the difference between the onboard clocks and a ground clock every 5 minutes. The relative clock rate between the spacecraft is then determined from the difference between the two solutions.