Development of the Space active Hydrogen Maser for the ACES Mission

The Atomic Clock Ensemble in Space (ACES) is composed of two atomic clocks: one cold Caesium clock (PHARAO) and one active Space Hydrogen Maser (SHM). PHARAO is necessary to ensure outstanding long-term frequency stability (τ ≥ 3000 s) and accuracy, while SHM is mandatory for its ultimate frequency stability in the mid-term range (3 s ≤ τ ≤ 3000 s). The combination of the two clocks via a double servo loop (short-term and long-term), will ensure an ultimate frequency stability. This configuration takes advantage of the best frequency stability for each integration time (PHARAO frequency locked to SHM for the short and mid-term, and SHM frequency steered to PHARAO for the long-term).