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Acronym: TAI In French: Temps Atomique International
TAI is an international time standard. It is calculated by the BIPM from the readings of more than 200 atomic clocks located in metrology institutes and observatories in more than 30 countries around the world.
TAI's unit interval is exactly one SI second at mean sea level. The origin of TAI is such that UT1-TAI was approximately 0 on 1 January 1958.
BIPM estimates that TAI does not lose or gain with respect to an imaginary perfect clock by more than about one tenth of a microsecond (0.0000001 second) per year.
TAI is a continuous time scale which is not connected to Earth rotation. UTC is derived from TAI by introducing leap seconds.
according to IUGG 2007 Resolution 2, the ITRS is the specific GTRS for which the orientation is operationally maintained in continuity with past international agreements (BIH orientation). The co-rotation condition is defined as no residual rotation with regard to the Earth’s surface, and the geocenter is understood as the center of mass of the whole Earth system, including oceans and atmosphere (IUGG 1991 Resolution 2). For continuity with previous terrestrial reference systems, the first alignment was close to the mean equator of 1900 and the Greenwich meridian. The ITRS was adopted (IUGG 2007 Resolution 2) as the preferred GTRS for scientific and technical applications and is the recommended system to express positions on the Earth.
