Master Clock Reference System #2

"Time is nature's way of keeping everything from happening all at once."
-- Anonymous

UTC(USNO) is provided by the USNO Reference System #2 (Master Clock #2), which is a Datum Inc. hydrogen maser that generates a time signal. This time signal is steered in frequency by means of an Auxiliary Output Generator until its time is synchronized as closely as possible with the average of all weighted USNO clocks. The synchronization is generally kept within 20 nanoseconds. Furthermore, a small bias is added to the Master Clock's frequency to steer its time toward synchronization with the International Atomic Time (TAI) as well (actually UTC, which is TAI plus all the leap seconds which have accumulated since 1972; BIPM is the International Bureau of Weights and Measures in Paris, France). The frequency of the Master Clock, however, is changed slowly enough (less than 150 picoseconds/day/day with a time constant of 50 days) that its variation does not adversely affect the operations of our users. The frequency changes are made once a day at 1300 UTC. The USNO MC steering algorithm is described by L. A. Breakiron, 1996, Proceedings of the 1996 IEEE International Frequency Control Symposium, pp. 1113-1122.

Performance of MC #2:

The frequency stability "sigma" of MC #2 has been during the last year < 2 parts in 10 to the 15th for sampling times of 1 hour to 40 days.

Masers are atomic clocks which have outstanding short-term stability. They directly use the 1420 MHz atomic microwave radiation emitted by hydrogen atoms when the spin state of their single electron changes. The radiation (which is very weak, approximately 0.000 000 000 01 watts) is used to phase-lock a quartz crystal clock, which provides the actual oscillating signal. The cycles of this signal are counted, and since the signal's frequency (hence cycle wavelength) is well known, this count is a precise measure of time. Masers differ in this respect from cesium clocks , where one only detects a transition between electron orbital states.

Main Components

The main components of a 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 state so that only the higher state can enter the resonance cavity. 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.

The control computer also measures temperatures, humidity, and a host of diagnostic indicators to alert the operators to any possible trouble. If a problem is associated with the lead maser, then it is replaced with a backup maser by electronically switching the input to the clock designated by the electronic control system.

The control computer serves as a local data node. It reports the settings and measurements to the main Time Service Data Acquisition and Control System (DAS). It also receives instructions from the DAS on how to keep the system closely synchronized with the computed mean timescale.

Essentially, this system is a type of flywheel which implements and extends the computed time scale of the U. S. Naval Observatory. For this purpose, the driving oscillators, i.e., the masers, must have the very best short-time stability obtainable. They must also be very reliable. This latter point can only be achieved with redundancy. That is why two masers are combined in this system.

All of our reference clocks are real-time approximations of UTC (USNO), which is coordinated closely with UTC according to pertinent international agreements. The principle behind this arrangement is that measurements against the real-time reference MC #2 are obtained easily, while the small difference between UTC (USNO) and UTC are learned weeks later as more recent BIPM data become available. Also, improvements are made in our internal mean timescale A.1 as time goes on. This is unavoidable because systematic disturbances can only be identified with sufficient reliability after the fact. On the other hand, it is necessary to have a high precision real-time reference for operational systems that cannot tolerate ex post facto adjustments or changes. This procedure then provide the necessary decoupling between these two requirements.

A.1 is subject to small changes due to reprocessing and filtering after the fact to eliminate systematic changes. However, these changes are small and after about 50 days, the values become final. A.1 is based on all available cesium standards and hydrogen masers, with the masers phased out with time into the past in order to combine their short-term stability with the long-term accuracy of the cesium standards.

Additional Comments:

The requirement for the details mentioned above originates from the use of the UTC (USNO) as the real-time reference for many timed systems, particularly for GPS and timed communications systems such as the DSCS (Defense Satellite Communication System). Since most of these systems have global coverage and are widely used, there is need to approximate the international reference as closely as the state of the art allows - in real-time.