The Atomic Clock Ensemble in Space (ACES) is a European Space Agency (ESA) payload to be deployed externally to the Columbus Laboratory on the International Space Station (ISS). ACES is ESA's most challenging ISS experiment under development.It consists of two high precision atomic clocks, namely the Cs cold atom clock PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite) and the active Space Hydrogen Maser (SHM). PHARAO is developed by the French National Space Agency (CNES) and the SHM is developed in Switzerland by Spectratime. The excellent short-term performance of SHM is combined with the long-term stability and accuracy of the PHARAO clock to generate an on-board ACES time scale with fractional frequency instability and inaccuracy of a few parts in 10~(-16).The ACES Ground Segment (GS) will be integrated into the overall ISS ground architecture providing the communication links between ground and space through the Columbus Control Centre and NASA ground segment.The funding for the ACES flight model was secured at the recent ESA Council at Ministerial level in November 2008. This included funding for the SHM development. CNES also recently indicated its commitment to deliver a flight model of PHARAO. This paper will review the status of the ACES payload and ground segment development activities. It will also highlight recent progress and outline the future activities necessary to achieve flight readiness for the ACES payload.
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