Time and Frequency from A to Z, To to Tw

Total Deviation

A statistic used to estimate oscillator stability. Total deviation reduces the estimation errors of the Allan deviation at long averaging times, and thus is well suited for estimating long-term stability.

Traceability

The property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.

Tuning Fork

A metal two-pronged fork that, when struck, produces an almost pure tone of a predetermined frequency. Tuning forks are used for simple frequency calibrations, such as tuning musical instruments, and calibrating radar guns used by law enforcement agencies.

Two Way Time and Frequency Transfer

A measurement technique used to compare two clocks or oscillators at remote locations. The two-way method involves signals that travel both ways between the two clocks or oscillators that are being compared, as shown in the illustration.

A half-duplex channel is a one-way system that is “turned around” to retransmit a signal in the opposite direction. In this method, the one-way delay between the transmitter and receiver is estimated as one-half of the measured round trip delay. The delay estimate can be sent to the user and applied as a correction, or the transmitter can advance the signal so that it arrives at the user's site on time. The latter is how the NIST Automated Computer Time Service (ACTS) system works. Internet time transfers using the Network Time Protocol (NTP) also use a half-duplex technique.

A full-duplex system uses one-way signals transmitted simultaneously in both directions, often through a geostationary communications satellite. In this case data must be exchanged in both directions so that the two data sets can be differenced, as shown in the diagram.