How Often Do the GPS Satellites occupy the exact same point in space during their orbits?
Answer by Bill Straka The simple answer to the question of when will the GPS vehicles will return to a given position is “Never.” The reason is to be found in the basics of celestial mechanics. If the satellites were point masses, and the Earth were the only gravitational source in the universe, and the Earth had a perfectly spherically symmetric gravitational field, and there were no magnetic fields, no atmosphere, no other forces, then the satellites would follow perfect Keplerian orbits. Oops, left out – there could be no relativistic effects. Since the universe contains a huge number of other objects, each exerting gravitational force on each satellite (the Sun and Moon being next in importance after the Earth for an Earth-orbiter), and the Earth has a decidedly non-spherically symmetric gravitational field, the positions are non-repeating (using an Earth-centered frame). There are other forces, like radiation pressure (due to solar radiation), atmospheric drag (yes, there is atmos
