NMC maintains five caesium atomic clocks and one hydrogen maser atomic clock. The onboard passive hydrogen maser and rubidium clocks are very stable over a few hours. The hyperfine splitting in hydrogen, measured using Ramsey's hydrogen maser, is known with great precision. The Triangulum galaxy is a source of HO maser emission. The hydrogen maser is used as an atomic frequency standard. It produced pulses of maser emission lasting for a few hundred microseconds. Emission can also be stimulated, as in a laser or a microwave maser. In the active hydrogen maser, the cavity oscillates by itself. In the passive hydrogen maser, the cavity is fed from an external frequency. It was later extended to explain behavior of the single photon maser. He also participated in developing an extremely stable clock based on a hydrogen maser. This line is also the theoretical basis of Hydrogen maser. The development of a passive hydrogen maser for QZSS was abandoned in 2006. The maser is a non-electronic microwave amplifier. The concept originally was called an "optical maser".