Time & Time Transfer
Currently, microwave time transfer systems between ground and space reach a level of precision of the order of the nanosecond (about 1 to 30 ns achieved with the GPS). An integration on ~ 30 days would be necessary to reach the 10-15 level. That cannot support the performance requirements and goals of the future optical atomic clock in space missions (such as ESA-SOC) which aim 10-17 level or better.
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 by roughly a factor of 1000 to the 1 ps level.
This would also 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.