Time and Frequency from A to Z: I
International Atomic Time (TAI)
A time scale maintained internally by the the BIPM, but seldom used by the general public. TAI realizes the SI second as closely as possible, and runs at the same frequency as Coordinated Universal Time (UTC). However, TAI differs from UTC by an integral number of seconds. This difference increases when leap seconds occur.
International Date Line
The line on the Earth, generally located at 180° longitude, that separates two consecutive calendar days. The date in the Eastern hemisphere, to the left of the line, is always one day ahead of the date in the Western hemisphere. The International Date Line passes through an area covered mainly by empty ocean, and most of the line is located exactly halfway around the world from the prime meridian (0° longitude) that passes near Greenwich, England. However, there are a few zigs and zags in the date line to allow for local circumstances.
Internet Time Service (ITS)
A popular NIST service that allows client computers to synchronize their clock via the Internet to UTC(NIST). The service responds to time requests from any Internet client by sending time codes in several formats defined by the Daytime, Time, and NTP protocols. The ITS handles hundreds of millions of timing requests every day. For more information, visit the ITS home page.
A standard (such as a frequency standard) based on an inherent physical constant or an inherent or sufficiently stable physical property. Technically, all atomic oscillators are intrinsic standards. In practice, however, only cesium oscillators are considered as intrinsic time and frequency standards, because the SI definition of the second is based on a physical property of cesium.
A device that allows ions to be trapped for long periods of time, during which the ions can be interrogated and their state changes observed. Since the ions are nearly motionless during the observation period, an ion trap can provide the basis for highly stable and accurate atomic oscillators that should eventually replace today's frequency standards. For more information, visit the Ion Storage Group web site.
IRIG Time Codes
The time codes originally developed by the Inter-Range Instrumentation Group (IRIG), now used in government, military and commercial fields. There are many formats and several modulation schemes, but they are typically amplitude modulated on an audio sine wave carrier. The most common version is probably IRIG-B, which sends day of year, hour, minute, and second data on a 1 kHz carrier frequency, with an update rate of once per second.