Atomic clock ensemble in space: Scientific objectives and mission status

Atomic Clock Ensemble in Space (ACES) is a mission in fundamental physics that will operate a new generation of atomic clocks in the microgravity environment of the International Space Station. Fractional frequency instability and inaccuracy at the 10(-16) level will be achieved. The on-board time base, distributed on Earth via a microwave link; will be used for space-to-ground as well as ground-to-ground comparisons of atomic frequency standards. Based on these comparisons, ACES will perform fundamental physics tests and develop applications in time and frequency metrology, universal time scales, global positioning and navigation, geodesy, and gravimetry. After a. general overview of the mission concept and its scientific objectives, the present status of ACES instruments and sub-systems will be discussed.