Laser time-transfer and space-time reference in orbit
  A high performance Space-Time Reference in orbit could be realized using a stable atomic clock in a precisely defined orbit and linking that to high accuracy atomic clocks on the ground using a laser based time-transfer link. This would enhance performance of existing systems and provide unique capabilities in navigation, precise timing, earth sciences, geodesy and the same approach could provide a platform for testing fundamental physics in space. Precise laser time- and frequency-transfer from the ground to an orbiting satellite would make it possible to improve upon the current state of the art in timing (about 1 to 30 ns achieved with GPS) by roughly a factor of 1000 to the 1 ps level. 
Paul Berceau and Leo Hollberg
physics.optics astro-ph.IM