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| Terrestrial Time | |
|---|---|
| Name | Terrestrial Time |
| Abbreviation | TT |
| Introduced | 1984 |
| Epoch | 1977-01-01 TAI offset |
| Unit | second (SI) |
| Basis | International System of Units |
| Maintained by | International Astronomical Union, International Bureau of Weights and Measures |
| Related | Ephemeris Time, International Atomic Time, Coordinated Universal Time, Barycentric Dynamical Time |
Terrestrial Time Terrestrial Time is a modern astronomical time scale used as the independent variable for apparent geocentric ephemerides and for calculations of positions and motions in the vicinity of Earth. It provides a uniform time coordinate consistent with the International System of Units and is used alongside international standards and observatories for high-precision navigation, space missions, and astronomical almanacs.
Terrestrial Time serves as the defined theoretical time coordinate for ephemerides referenced to Earth's geoid and replaces earlier standards like Ephemeris Time and Universal Time. It is specified relative to International Atomic Time and tied to the SI second as realized by national metrology institutes such as National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, and National Physical Laboratory (United Kingdom). The purpose of Terrestrial Time is to provide a uniform temporal argument for solar system dynamical theories used by organizations including the International Astronomical Union, the International Bureau of Weights and Measures, and space agencies such as National Aeronautics and Space Administration, European Space Agency, and Roscosmos.
The development of Terrestrial Time followed debates at meetings of the International Astronomical Union and recommendations from the International Bureau of Weights and Measures in the 1970s and 1980s, succeeding Ephemeris Time which dated back to work by Simon Newcomb and observational programs at observatories like Royal Greenwich Observatory. It was formalized in resolutions of the International Astronomical Union and adopted in conjunction with revisions to the Astronomical Almanac used by institutions including the US Naval Observatory and the Royal Observatory, Edinburgh. Influential figures and bodies in its history include astronomers and committees associated with International Earth Rotation and Reference Systems Service, International VLBI Service for Geodesy and Astrometry, and metrology efforts at Bureau International de l'Heure predecessors.
Terrestrial Time is defined as a linear transformation of the TAI scale with a fixed offset established to preserve continuity with Ephemeris Time values at the epoch defined by astronomical conventions. The realization of the SI second for TT relies on ensembles of primary frequency standards such as caesium fountain clocks at National Institute of Standards and Technology, ENEA (Italy), and BIPM comparisons coordinated by the Comité International des Poids et Mesures and reported via International Atomic Time and the k=37 offset convention. Realizations employ techniques including two-way satellite time and frequency transfer used by Global Positioning System, GLONASS, Galileo (satellite navigation), and BeiDou constellations, and long-baseline methods such as Very Long Baseline Interferometry and Satellite Laser Ranging.
Terrestrial Time relates to multiple coordinate and civil time scales: it is offset from International Atomic Time by a constant to match historical ephemerides, and it differs from Coordinated Universal Time by a non-constant number of leap seconds prescribed by International Telecommunication Union and proclaimed by national timing centers like US Naval Observatory. It connects to the barycentric coordinate time system Barycentric Dynamical Time through relativistic transformations described by the General Theory of Relativity and implemented in planetary ephemerides by teams at Jet Propulsion Laboratory, Institut de Mécanique Céleste et de Calcul des Éphémérides, and the Institute of Applied Astronomy of RAS.
Terrestrial Time is the independent variable in published planetary and lunar ephemerides produced by organizations such as the Jet Propulsion Laboratory, Institut de Mécanique Céleste et de Calcul des Éphémérides, IMCCE, and the Minor Planet Center. Astronomical almanacs issued by the United States Naval Observatory, the Royal Greenwich Observatory successors, the HM Nautical Almanac Office, and the International Astronomical Union working groups provide positions, apparent places, and phenomena times expressed in TT. Spacecraft navigation conducted by NASA Deep Space Network, European Space Operations Centre, and mission design teams at Jet Propulsion Laboratory and Roscosmos use TT for modeling spacecraft ephemerides and tracking via radiometric data analyzed with software frameworks like SPICE (spacecraft navigation), SOFA (Standards of Fundamental Astronomy), and NOVAS.
The accuracy of Terrestrial Time depends on the stability of the underlying atomic frequency standards and the precision of time transfer techniques. Current realizations achieve uncertainties at the level of 10^-16 to 10^-15 in rate via primary standards at institutions such as Physikalisch-Technische Bundesanstalt, National Metrology Institute of Japan, and National Research Council (Canada). Limitations arise from systematic biases in clock ensembles, geophysical effects addressed by International Earth Rotation and Reference Systems Service products, and relativistic modeling approximations used by ephemeris teams at Jet Propulsion Laboratory and Observatoire de Paris. Research in optical clocks at organizations like NIST, PTB, and SYRTE aims to improve realizations and reduce these limitations.
Terrestrial Time values are disseminated indirectly through published differences from International Atomic Time produced by the BIPM and via timing signals from global navigation satellite systems operated by United States Department of Defense, Roscosmos State Space Corporation, European Union Agency for the Space Programme, and China National Space Administration. Time transfer and synchronization use facilities and techniques at Observatoire de Paris, US Naval Observatory, Herstmonceux Observatory, and international services including International GNSS Service, International Laser Ranging Service, and Global Geodetic Observing System. Software libraries and standards from International Astronomical Union working groups such as Standards of Fundamental Astronomy (SOFA) and mission toolkits at Jet Propulsion Laboratory provide implementations for converting between TT and other scales for users at observatories, laboratories, and space agencies like European Southern Observatory and SpaceX.
Category:Time scales