ACES: A time and frequency mission for the international space station

ACES (Atomic Clock Ensemble in Space) will test a new generation of micro-gravity or cold atom Caesium clock with frequency stability in the range 10^-16 to 10^-17. This clock will be compared to a Space Hydrogen Maser in orbit. Both clocks will also be compared to primary ground standards world-wide with unprecedented accuracy via a specially designed Micro-Wave Link (MWL). The ACES payload has recently undergone its Preliminary Design Review (PDR) and is preparing for the Mission PDR. A description of the ACES mission is given and status of the ACES payload presented.