> I expected this accuracy at km or greater. On the ground? Sure. Orbiting? I'm amazed they reach sub-mm. The gravitational field of a planet or star is not un

> I expected this accuracy at km or greater. On the ground? Sure. Orbiting? I'm amazed they reach sub-mm. The gravitational field of a planet or star is not uniform, subtle variations in their density are enough to impart tiny variations to an orbiting object, and they add up over time. Can't find a reference right now but I recall someone proposed even a positioning system relying only on accurate gravimetric sensors and a (very good) map of the strength at every relevant location on Earth. Good for submarines, GPS reception is not so good underwater.