> their onboard clock is pretty good They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend

> their onboard clock is pretty good They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend of mine is working on the next generation, which will be even more accurate. In contrast, the force of gravity experienced by the satellite is known to much less accuracy. In fact, changes on a monthly basis due to rainfall/rivers and tides. The GRACE satellites measure the change, although I couldn't find any information on how accurate their measurements are, except that they can measure the distance between the two satellites to within a micrometer (10^-6), so substantially less precision than the clock!