On Measuring Gravitomagnetism via Spaceborne Clocks: A Gravitomagnetic   Clock Effect
  The difference in the proper azimuthal periods of revolution of two standard clocks in direct and retrograde orbits about a central rotating mass is proportional to J/Mc^2, where J and M are, respectively, the proper angular momentum and mass of the source. In connection with this gravitomagnetic clock effect, we explore the possibility of using spaceborne standard clocks for detecting the gravitomagnetic field of the Earth. It is shown that this approach to the measurement of the gravitomagnetic field is, in a certain sense, theoretically equivalent to the Gravity Probe-B concept. 
B.Mashhoon, F.Gronwald, F.W.Hehl, and D.S.Theiss