October 24, 2023

Space Hydrogen Maser (SHM)

Space Hydrogen Maser
Space Hydrogen Maser

The Space Hydrogen Maser (SHM), designed in Switzerland, is developed by Spectra Time.

Hydrogen maser performance does not depend on the microgravity environment. However, because of their simplicity and predicted reliability, H-masers are expected to be key instruments in future satellite positioning systems and other science missions such as ultra-high-resolution space-space very long baseline interferometry (VLBI) 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 a storage bulb. The bulb is made of a sapphire, which loads a microwave cavity that is tuned to the resonance frequency and maintains the maser signal.

The ACES mission will be a testbed for the space qualification of the active hydrogen maser. Onboard frequency comparison between the SHM and PHARAO will be key to gauging the accuracy and the short/medium-term stability of the caesium clock. Furthermore, it will enable optimal operating conditions for PHARAO to be identified and a good trade-off between frequency accuracy and stability.