"A little inaccuracy sometimes saves tons of explanation."
H. H. Munro ("Saki") (1870-1916)
The USNO Reference System (Master Clock) is based on a cavity-tuned hydrogen maser.
Masers are atomic clocks which have outstanding short-term stability. USNO's current hydrogen masers are designed to operate with a maximum frequency instablility of 3.0 parts in 10 to the 15th for periods of from 1,000 to 10,000 seconds and 2.0 parts in 10 to the 13th * t**-(3/5) for 1 sec < t < 1000 sec. The drift is specified at initially less than 1 part in 10 to the 15th per day. The USNO masers NAV-2, 3, 4, and 5 have shown drifts of less than a part in 10 to the 16th/day over 500 days. These devices are 46 x 76 x 109 cm in size, and with batteries weigh 245 kg.
Some of the main components of a hydrogen maser are the hydrogen gas supply , a controllable "leak" for the gas into the high vacuum system , a gas discharge to produce atomic hydrogen, and a state selector which rejects atoms in the lower energy states and focusses the higher states into the storage bulb.
A small storage bottle supplies molecular hydrogen to the gas discharge bulb. In the discharge bulb, molecules of hydrogen are dissociated into atomic hydrogen. They then pass through a source collimator and a magnetic state selector which filters atoms at the desired atomic state and passes them to the resonance cavity.
The storage bulb is a quartz bulb about 20 cm tall which is coated inside with Teflon to control the recombination rate of atoms into molecules. The quartz bulb is located in a precisely machined pure copper cylinder which acts as a microwave resonant cavity for the 1.420 GHz frequency of the 21 cm hydrogen line. Once the atoms enter the resonance cavity, they find other atoms radiating and they fall in step. They "start to talk to each other" and echo what they hear. This produces a highly coherent oscillation. This is the signal to which the crystal oscillator is phase-locked. All of that is packaged in cabinets with power supplies, temperature control, and magnetic shielding.
Great care is required to keep environmental disturbances small so that the full performance potential of these sophisticated clocks can be realized.