Relative time and frequency alignment between two low earth orbiters, grace

The two GRACE (gravity recovery and climate experiment) spacecraft were launched into a near polar circular orbit around the earth in March of 2002. The two spacecraft serve as test masses to measure the earth´s gravitational field. Both spacecraft carry ultra-stable oscillators (USO) with an Allan deviation of a few parts in 10^-13 for Tau=1 to 1000 s. The USO´s drive both the microwave links and GPS receivers. To cancel out long term errors on the USO´s a linear combination of the 1-way microwave links is used (dual-one-way). In order to form the dual-one-way measurement and cancel our long term USO error, time must be synchronized between the two spacecraft to about 150 picoseconds. This synchronization is accomplished using the GPS data. For each spacecraft, the GPS data are used to solve for the orbital positions and the difference between the on-board clocks and a ground reference clock every 5 minutes. The relative clock is determined by the difference of these two solutions. Validation of the relative clock accuracy includes the solutions from overlapping data arcs which are typically less than the 150 picosecond goal and unique combination of the one-way microwave links that allows independent comparison of the GPS determine relative frequency of the USO´s to a measurement made by the microwave link.