Results of the ACES engineering model system test

Atomic Clock Ensemble in Space (ACES) is a European Space Agency (ESA) project to be deployed externally to the Columbus Laboratory on the International Space Station (ISS). Two high performance atomic clocks on-board ACES generate a frequency reference with stability and accuracy at the 1×10
^-16
level. The performance of the ACES clock signal results from the combination of the good short-term stability of an active hydrogen maser (SHM) and the long-term stability and accuracy of a primary standard based on samples of laser cooled Cs atoms (PHARAO). The two clocks are controlled by two servo loops, the first stabilizing the PHARAO local oscillator on SHM clock signal, the second correcting the long-term instabilities of SHM using the error signal generated by the PHARAO Cs resonator. This frequency reference, distributed to the ground by a link in the microwave domain is used to perform comparisons of distant clocks and to test Einstein's theory of general relativity. ACES main instrument and subsystems have now reached a high technology readiness level, demonstrated by the completion and the successful test of their engineering models. In particular, a dedicated test campaign has recently verified the performance of the ACES system, where PHARAO and SHM, locked together via the ACES servo loops, are operated as a unique oscillator to generate the ACES frequency reference. The test campaign conducted at CNES premises in Toulouse between July and November 2009 has concluded the engineering models phase, releasing the manufacturing of the ACES flight models. The setup of the ACES system test campaign, the specific tests performed, and the achieved results will be presented and discussed.