"A little inaccuracy sometimes saves tons of explanation."
H. H. Munro ("Saki") (1870-1916)
The short term stability of the USNO master clock is determined by an ensemble of hydrogen masers. The large number of these clocks that are maintained at USNO allow for the robust determination of a very stable time scale with excellent short term performance. Hydrogen masers also provide the references for the realizations of UTC(USNO) that are delivered to our customers.
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 1015 for periods of from 1,000 to 10,000 seconds and 2.0 parts in 1013 * t**-(3/5) for 1 sec < t < 1000 sec. The drift is specified at initially less than 1 part in 1015 per day. The USNO masers have shown drifts of less than a part in 1016 per day over 500 days. These devices are 46 x 76 x 109 cm in size with batteries weighing 227 kg.