  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.
