Glossary
geocenter (geocentre)
center of mass of the Earth including the atmosphere and oceans.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
geocenter motion
the motion, on the level of a few mm, of the mass center of the entire Earth system (solid Earth, ocean and atmosphere) relative to the origin of the ITRF. It is opposite in sign from the origin translation vector defined in Chapter 4.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Geocentric Celestial Reference System (GCRS)
a system of geocentric space-time coordinates within the framework of General Relativity with metric tensor specified by the IAU 2000 Resolution B1.3. The GCRS is defined such that the transformation between BCRS and GCRS spatial coordinates contains no rotation component, so that GCRS is kinematically non-rotating with respect to BCRS. The equations of motion of, for example, an Earth satellite, with respect to the GCRS will contain relativistic Coriolis forces that come mainly from geodesic precession. The spatial orientation of the GCRS is derived from that of the BCRS, that is (c.f. IAU 2006 Resolution B2), unless otherwise stated, by the orientation of the ICRS.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
Geocentric Coordinate Time
Acronym: TCG
In French: Temps-coordonnée géocentriqueTCG is the coordinate time at the center of mass of the Earth.
TCG is computed by TCG = TT + LG × (MJD - 43144) × 86400 s, where LG is defined to be 6.969290134 × 10-10.
Related terms:
geocentric terrestrial reference system (GTRS)
a system of geocentric space-time coordinates within the framework of General Relativity, co-rotating with the Earth, and related to the GCRS by a spatial rotation which takes into account the Earth orientation parameters. It was adopted by IUGG 2007 Resolution 2. It replaces the previously defined Conventional Terrestrial Reference System.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
geoid
the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: http://www.ngs.noaa.gov/GEOID/geoid_def.html
Global Positioning System (GPS)
The Global Positioning System (GPS), the U.S. component of the Global Navigation Satellite System (GNSS). The GPS satellites, at an altitude of 20000 km, transmit down to the Earth carrier signals at two L-band frequencies (1:227 and 1:575 GHz) which are modulated by a pseudo-random noise code. When four satellites are in view, the user has enough information to solve for the station position and the clock offset from GPS time.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Greenwich Mean Sidereal Time (GMST)
Greenwich hour angle of the mean equinox defined by a conventional relationship to Earth Rotation Angle or equivalently to UT1.
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
Greenwich Sidereal Time (GST)
Greenwich apparent sidereal time (GAST), the hour angle of the true equinox from the Terrestrial Intermediate Origin (TIO) meridian (Greenwich or International meridian).
Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
