1. The development of a passive hydrogen maser for QZSS was abandoned in 2006. 2. Hydrogen masers are very complex devices and sell for as much as } }.3. In the active hydrogen maser , the cavity oscillates by itself. 4. In the passive hydrogen maser , the cavity is fed from an external GHz frequency. 5. Products included hydrogen masers , telecom synchronization supply units ( SSUs ), and timing test sets. 6. Atomic hydrogen determines the frequency of hydrogen masers which are used as precise frequency standards. 7. GIOVE-B carries three atomic clocks : two rubidium standards and the first space-qualified passive hydrogen maser . 8. The long-term stability of hydrogen maser standards decreases because of changes in the cavity's properties over time. 9. Hydrogen masers ( often manufactured in Russia ) have superior short-term stability compared to other standards, but lower long-term accuracy.10. Hydrogen masers have linewidth around 1 Hz; their coherence length approximately corresponds to the distance from the Earth to the Moon.
