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• GEO Ambassador

  Jaybird December 5, 2013 at 6:08pm

  Ty Olinghouse is definitely the guy to get deep with on this subject..  He wrote a book once called "Redneck's Guide to Celestial Observation"..  just run a search on the network and you'll find him...just thought I'd put that out there...
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    Ty Olinghouse > Jaybird January 27, 2014 at 8:54am

    The ITRS definition fulfills the following conditions:

    1. It is geocentric, the center of mass being defined for the whole earth, including oceans and atmosphere.
    2. The unit of length is the metre (SI). This scale is consistent with the TCG time coordinate for a geocentric local frame, in agreement with IAU and IUGG (1991) resolutions. This is obtained by appropriate relativistic modelling.
    3. Its orientation was initially given by the BIH orientation at 1984.0.
    4. The time evolution of the orientation is ensured by using a no-net-rotation condition with regards to horizontal tectonic motions over the whole earth.

    
    See the IERS Conventions (2010), especially  Chapter 4 (pdf/661 KB), for a detailed description of the ITRS.

    The ITRS is realized by estimates of the coordinates and velocities of a set of stations observed by VLBI, LLR, GPS, SLR, and DORIS. Its name is International Terrestrial Reference Frame (ITRF).

    General documentation on terrestrial reference systems and frames is available at the ITRS Centre of the IERS.

    The ITRS can be connected to the International Celestial Reference System (ICRS) by use of the IERS Earth Orientation Parameters (EOP).

    
    Text provided by the former Central Bureau, modified.
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      Ty Olinghouse > Ty Olinghouse January 27, 2014 at 8:55am

      The International Celestial Reference System (ICRS)

      At its 23rd General Assembly in August 1997, the International Astronomical Union (IAU) decided that, as from 1 January 1998, the IAU celestial reference system shall be the International Celestial Reference System (ICRS), in replacement of the FK5 (Fricke et al. 1988). 
      The consequences of this new situation for accuracy needs more stringent than 0.05" are summarized by Feissel and Mignard (1997).

      By Reference System it is meant the set of prescriptions and conventions together with the modeling required to define at any time a triad of axes.

      The ICRS is accessible by means of coordinates of reference extragalactic radio sources, the International Celestial Reference Frame (ICRF).

      The ICRS complies with the conditions specified by the 1991 IAU Recommendations. Its origin is located at the barycenter of the solar system through appropriate modelling of VLBI observations in the framework of General Relativity. Its pole is in the direction defined by the conventional IAU models for precession (Lieske et al., 1977) and nutation (Seidelmann, 1982). Its origin of right ascensions was implicitly defined by fixing 
      the right ascension of 3C 273B to the Hazard et al. (1971) FK5 value transferred at J2000.0. See Arias et al. (1995) for more details.

      The Hipparcos star positions and proper motions and the JPL Solar System ephemerides are expressed in the ICRS.

      The directions of the ICRS pole and right ascensions origin are maintained fixed relative to the quasars within +/- 20 microarcseconds. 
      Thanks to the fact that the Hipparcos catalogue includes all the FK5 stars, the location of the FK5 pole and right ascensions origin is known with 
      an uncertainty of a few mas (Mignard and Froeschlé 1997). Using a state of the art precession-nutation model, the analysis of long VLBI series 
      of the observed motion of the celestial pole allows to derive the coordinates of the mean pole at J2000.0 in the ICRS: 17.3 +/- 0.2 mas in the direction 12 h and 5.1 +/- 0.2 mas in the direction 18 h.(IERS 1997). Comparing VLBI and LLR earth orientation and terrestrial reference frames, Folkner et al. (1994) estimated the frame tie between the IERS celestial system and the JPL planetary ephemeris, and concluded that the mean equinox of J2000.0 is shifted from the ICRS right ascension origin by 78 +/- 10 mas (direct rotation around the polar axis).

      The ICRS is realized by VLBI estimates of equatorial coordinates of a set of extragalactic compact radio sources, theInternational Celestial Reference Frame (ICRF).

      The ICRS can be connected to the International Terrestrial Reference System (ITRS) by use of the IERS Earth Orientation Parameters (EOP).