Coordinate Systems for Time Scales
Barycenter
- center of mass of the solar system
- a system of barycentric space-time coordinates for the solar system within the framework of General Relativity with metric tensor specified by the IAU 2000 Resolution B1.3
- Formally, the metric tensor of the BCRS does not fix the coordinates completely, leaving the final orientation of the spatial axes undefined
- However, according to IAU 2006 Resolution B2, for all practical applications, unless otherwise stated, the BCRS is assumed to be oriented according to the ICRS axes
- the original name for the Celestial Intermediate Origin (CIO) given in the IAU 2000 resolutions
- used 1984 to 2003 with IAU 1980 Theory of Nutation as reference pole for nutation and polar motion
- axis of figure for mean surface of a model Earth in which the free motion has zero amplitude
- This pole was originally defined as having no nearly-diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system and being realized by the IAU 1980 nutation. It was afterwards determined by using VLBI observations of celestial pole offsets. It is now replaced by the CIP, which is defined by IAU 2000 Resolution B1.7
- origin for right ascension on the intermediate equator in the Celestial Intermediate Reference System
- is the non-rotating origin in the GCRS that is recommended by the IAU 2000 Resolution B 1.8, where it was designated the Celestial Ephemeris Origin
- name Celestial Intermediate Origin was adopted by IAU 2006 Resolution B2
- CIO originally set close to GCRS meridian and throughout 1900-2100 stays within 0.1 arcseconds of this alignment
- geocentric equatorial pole defined by IAU 2000 Resolution B1.7 as being the intermediate pole, in the transformation from the GCRS to the ITRS, separating nutation from polar motion
- replaced CEP on 1 January 2003
- Its GCRS position results from (i) the part of precession-nutation with periods greater than 2 days, and (ii) the retrograde diurnal part of polar motion (including the free core nutation, FCN) and (iii) the frame bias
- Its ITRS position results from (i) the part of polar motion which is outside the retrograde diurnal band in the ITRS and (ii) the motion in the ITRS corresponding to nutations with periods less than 2 days
- motion of the CIP realized by IAU precession-nutation plus time-dependent corrections provided by the IERS
- geocentric reference system related to the GCRS by a time-dependent rotation taking into account precession-nutation
- defined by intermediate equator (of the CIP) and CIO on specific date (IAU 2006 Resolution B2)
- similar to system based on true equator and equinox of date, but equatorial origin is at the CIO
- system of geocentric space-time coordinates within the framework of General Relativity with metric tensor specified by the IAU 2000 Resolution B1.3
- 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 spatial orientation of the GCRS is derived from that of the BCRS, that is (cf. IAU 2006 Resolution B2), unless otherwise stated, by the orientation of the ICRS
- 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
- adopted by IUGG 2007 Resolution 2 and replaces Conventional Terrestrial Reference System
- the original name for the Terrestrial Intermediate Origin (TIO) given in the IAU 2000 resolutions
- origin of longitude in the Intermediate Terrestrial Reference System
- is the non-rotating origin in the ITRS that is recommended by the IAU 2000 Resolution B1.8, where it was designated Terrestrial Ephemeris Origin
- The TIO was originally set at the ITRF origin of longitude and throughout 1900-2100 stays within 0.1 mas of the ITRF zero meridian
- geocentric reference system defined by the intermediate equator of the CIP and the TIO (IAU 2006 Resolution B2)
- related to the ITRS by polar motion and s’ (TIO locator)
- related to the Celestial Intermediate Reference System by a rotation of ERA around the CIP, which defines the common z-axis of the two systems