Time and Frequency from A to Z:  Ch to Cy

An extended test of the performance characteristics of a clock or oscillator.  A characterization is more involved than a calibration.  The device under test is usually measured for a long period of time (days or weeks), and sometimes a series of measurements is made under different environmental conditions.  A characterization is often used to determine the types of noise that limit the uncertainty of the measurement, and the sensitivity of the device to environmental changes.

A device that generates periodic, accurately spaced signals used for timing applications.  A clock consists of at least three parts:  an oscillator, a device that counts the oscillations and converts them to units of time interval (such as seconds, minutes, hours, and days), and a means of displaying or recording the results.

A measurement technique used to compare two clocks or oscillators at remote locations. The common-view method involves a single reference transmitter (R) and two receivers (A and B). The transmitter is in common view of both receivers. Both receivers compare the simultaneously received signal to their local clock and record the data. Receiver A receives the signal over the path d_ra and compares the reference to its local clock (R - Clock A). Receiver B receives the signal over the path d_rb and records (R - Clock B). The two receivers then exchange and difference the data as shown in the figure.

Common-view directly compares two clocks or oscillators to each other. Errors from the two paths d_ra and d_rb ) that are common to the reference cancel out, and the uncertainty caused by path delay is nearly eliminated. The result of the measurement is (Clock A - Clock B) - (d_ra - d_rb ). 

Common-view measurements were made for many years using land based transmitters as the reference.  Today, nearly all common-view measurements use a  GPS satellite as the reference transmitter, as illustrated below.  This enables clocks to be compared over transcontinental distances, with uncertainties of just a few nanoseconds.

The international atomic time scale that serves as the basis for timekeeping for most of the world.  UTC is a 24-hour timekeeping system.  The hours, minutes, and seconds expressed by UTC represent the time-of-day at the Earth's prime meridian (0° longitude) located near Greenwich, England.

UTC is calculated by the Bureau International des Poids et Measures (BIPM) in Sevres, France. The BIPM averages data collected from more than 200 atomic time and frequency standards located at about 50 laboratories, including the National Institute of Standards and Technology (NIST).  As a result of this averaging, the BIPM generates two time scales, International Atomic Time (TAI), and Coordinated Universal Time (UTC).  These time scales realize the SI second as closely as possible.

UTC runs at the same frequency as TAI.  However, it differs from TAI by an integral number of seconds. This difference increases when leap seconds occur. When necessary, leap seconds are added to UTC on either June 30 or December 31. The purpose of adding leap seconds is to keep atomic time (UTC) within ±0.9 s of an older time scale called UT1, which is based on the rotational rate of the Earth.  Leap seconds have been added to UTC at a rate averaging about 8 every 10 years, beginning in 1972.

Keep in mind that the BIPM maintains TAI and UTC as “paper” time scales.  The major metrology laboratories use the published data from the BIPM to steer their clocks and oscillators and generate real-time versions of UTC, such as UTC(NIST).  You can think of UTC as the ultimate standard for time-of-day, time interval, and frequency.  Clocks synchronized to UTC display the same hour, minute, and second all over the world (and remain within one second of UT1).  Oscillators syntonized to UTC generate signals that serve as reference standards for time interval and frequency.

A change in the signal tracking point of a carrier frequency that occurs during a measurement.  Cycle slips introduce phase shifts equal (in time units) to the period of the carrier frequency, or to a multiple of its period.  For example, if a WWVB receiver changes its signal tracking point during a measurement, a phase shift equal to a multiple of 16.67 microseconds (the period of 60 kHz) will result.  Most cycle slips are caused by a temporary loss of lock due to a weak or noisy signal.