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Atomic Clock Ensemble in Space (ACES) is an ESA mission in fundamental physics based on a new generation of clocks operated in the microgravity environment of the International Space Station. Installed at the external payload facility of the Columbus module, ACES will accommodate two atomic clocks: PHARAO, a primary frequency standard based on samples of laser cooled Cs atoms, and the active H-maser SHM. The two on-board clocks will generate a time scale with fractional frequency instability and inaccuracy of a few parts in 10¹⁶. The ACES frequency reference will be distributed to ground by a MicroWave Link (MWL) and used to compare distant clocks. These comparisons will allow precision tests of the Einstein's theory of general relativity, including a measurement of the gravitational red-shift, a search for time variations of fundamental constants, and tests of the standard model extension. ACES will …