Atomic clock ensemble in space

International audienceAtomic Clock Ensemble in Space (ACES) is an ESA mission designed to test theEinstein's Equivalence Principle with high-performance atomic clocks in spaceand on the ground. Installed onboard the International Space Station, the ACESpayload will generate a clock signal with fractional frequency instability andinaccuracy of 1 − 3 · 10 −16. Two links operating on microwave (MWL) and optical(ELT) frequencies will allow comparing the ACES clocks with the best atomicclocks on the ground in a global network. Space-to-ground and ground-to-groundcomparisons will provide tests of Einstein's theory of general relativity, atthe same time developing applications in geodesy and time & frequency metrology.The ACES flight model is close to completion. The cold atom clock PHARAO hasbeen tested and delivered for integration in the ACES payload. The ELT link hasbeen completed. MWL and the active hydrogen maser SHM are presently under test.System level tests have already started and will continue all along 2017. Thispaper will present the recent progress of the ACES mission and discuss futureperspectives for testing fundamental physics with clocks in space. 1 ACESMission Elements Proposed to the European Space Agency in 1997, the Atomic ClockEnsemble in Space (ACES) mission relies on PHARAO, a clock based on laser-cooledcaesium atoms, to generate a high stability and accuracy time reference in space1,2,3. The free fall conditions are crucial for PHARAO to reach or even surpassthe performance of the best atomic fountain clocks on ground, while keeping avery compact volume, small mass and power consumption. Installed onboard theInternational Space Station (ISS), at the external payload facility of theColumbus module, the ACES payload distributes its time scale to ground clocks byusing two independent time & frequency transfer links, a link operating in themicrowave domain (MWL) and the ELT (European Laser Timing) optical link. On theground, a network of MWL ground terminal and satellite laser ranging stationsprovides the physical interface between the ACES clock ensemble and atomicclocks on ground. The ACES payload is shown in Fig. 1. It has a volume of about1 m 3 , for a mass of 230 kg and a power consumption of 450 W. The main onboardinstruments are the cesium clock PHARAO and the active hydrogen maser SHM. ThePHARAO clock reaches a fractional frequency stability of 1.1 · 10 13 / √ τ ,where τ is the integration time expressed in seconds, and an accuracy of a fewparts in 10 16. SHM is the ACES flywheel oscillator, also providing thefrequency reference needed for the onboard characterization of the PHARAO clockstability and accuracy. PHARAO and SHM 100 MH