The Space Hydrogen Maser, designed in Switzerland, is developed by Spectra Time.
Hydrogen maser performances do not depend on the micro-gravity environment. However, because of their simplicity and predicted reliability, H-maser are expected to be key instruments in future satellite positioning systems and other science missions such as ultra-high resolution space-space VLBI (Very Long Baseline Interferometry) experiments.
H-masers are based on the hyperfine transition of atomic hydrogen at 1.420405751 GHz. H2 molecules are dissociated by a plasma discharge and the resulting beam of H atoms is selected and sent to storage bulb. The bulb is made of a sapphire which loads a microwave cavity that, tuned on the resonance frequency, supports the maser action.
The ACES mission will be a test-bed for the space qualification of the active hydrogen maser. The on board frequency comparison between the SHM and PHARAO will be a key element for the evaluation of the accuracy and the short/medium-term stability of the caesium clock. Furthermore, it will allow to identify the optimal operating conditions for PHARAO and to select a good trade-off between frequency accuracy and stability.