The 35 kg space active hydrogen maser (SHM-35) for ACES

A 35-kg active hydrogen maser is in an advanced stage of development at the Observatory of Neuchatel (ON) for the European mission ACES (atomic clock ensemble in space). The miniaturization of the instrument is based on a sapphire loaded microwave cavity. A frequency stability of 1.5/spl times/10/sup -15/ over averaging time of 10'000 s is made possible by the use of an automatic cavity tuning system. A prototype model of the sapphire loaded microwave cavity has been operated continuously for more than two years. The engineering (EM) and the flight (FM) models of the microwave cavity assembly have already been manufactured. In this paper, the design of the instrument is presented. The overall expected performances as well as measurement results are reported.