Time and Frequency from A to Z, I

International Atomic Time (TAI)

A time scale maintained internally by 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 is related to leap seconds, and increases whenever a leap second occurs.

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 oceans, and therefore 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 the Daytime, Time, and NTP protocols. The ITS handles billions of timing requests every day.

Intrinsic Standard

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 currently based on a physical property of cesium.

Ion Trap

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 optical frequency standards that should eventually replace today's frequency standards. 

Ionosphere

A region of the Earth’s upper atmosphere that ranges from about 60 km to 1000 km in altitude, and that is divided into a number of defined layers (including the D, E, and F layers).  

Ionospheric corrections are commonly applied when using radio signals for time transfer.  For example, when using terrestrial signals such as WWV, it is necessary to know the height of the ionosphere and the number of times that a signal was reflected off the ionosphere, to estimate the propagation delay.  These corrections can be large, many microseconds or even milliseconds in extreme cases.  When using satellite signals, such as GPS, the signals are transmitted from above the ionosphere.  Therefore, the ionospheric corrections are small (tens of nanoseconds or less) and are applied to compensate for the delay added to a signal as it passes through the ionosphere on its way to the Earth’s surface.

IRIG Time Codes

The time codes originally developed by the Inter-Range Instrumentation Group (IRIG) for military use that are now utilized by both the public and private sectors, in addition to the military. There are many IRIG formats and several modulation schemes, but they are typically amplitude modulated on an audio sine wave carrier. The most common format 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.