5.4 Space-Time Reference Based on General Relativity
329
(1) The CIP is the reference pole determined by the new precession-nutation model
(IAU 2000), used to replace the original CEP and taken as the pole of new
instantaneous reference frame. The motion that the period of CIP is less than
two days is regarded as the polar wander. In this way, for the terrestrial reference frame, the residuals between observations and models, the high-frequency
variation of the Earth-orientating parameters, and the diurnal term caused by
the defects of nutation model can be absorbed by the polar wander.
(2) Since the ICRF has nothing to do with the vernal equinox, it is unnecessary that
the orientation of the X-axis of the instantaneous reference frame of motion is
related to the equinox. By introducing the Celestial Ephemeris Origin (CEO)
to replace the vernal equinox, the confusion between the star catalogue and
dynamical equinoxes can be eliminated. In 2003, the IAU decided to change
the name of the CEO to the CIO. The CIO is determined by the non-rotating
origin, its specific position on the equator can be calculated by the precessionnutation model and observational data, and its hour angle is the ERA, similar
to the sidereal angle. Only by selecting the CIO as the starting point of right
ascension will the ERA’s formula not relay on the axial-precession and nutation.
(3) The ITRS is defined by the IUGG and realized by the ITRF. At present,
ITRF2014, the latest reference frame issued by the IERS, contains the position
and velocity vectors of 1499 stations. The non-rotating origin of the Earth is
used in the ITRF to give the Terrestrial Ephemeris Origin (TEO) of the CIP
on the equator. The 25th IAU general assembly decided to change the name of
TEO to Terrestrial Intermediate Origin (TIO). The TIO is an exact definition
of longitude starting point to meet the requirement for the UT1’s accuracy.
(4) The ERA is an angular distance between the CIO and TIO, around the orbit of
the CIP, and replaces the Greenwich Apparent Sidereal Time (GAST), whose
starting point is the vernal equinox. Since the equator moves relative to the
ecliptic each other, the relation between the GAST and UT1 contains the terms
caused by the axial-precession and nutation, and the relation between the ERA
and UT1 does not depend on the axial-precession and nutation. The ERA
represents the stellar rotation of the Earth around the CIP axis, and thereby it is
also called sidereal angle. The UT1 can be gotten from its linear relation with
the ERA. The non-rotating origin is introduced to make the time derivative of
the UT1 proportional to the instantaneous angular velocity of the Earth. The
relation between the Greenwich Mean Sidereal Time (GMST) and UT1 can be
derived from that between the ERA and UT1.
(5) The new precession-nutation model is used to replace the IAU1980 nutation
theory and IERS 1996 precession-nutation model. Using the new precessionnutation model, the new instantaneous reference frame of motion can be determined, and transformed to the ICRF. There are two versions for the new model:
IAU2000A and IAU2000B, in which the accuracy of the former is 0.1 milliarcseconds and that of the latter is 1 milliarcsecond. The new model includes the
deviation between the reference frames, geodetic precession and geodetic nutation, but does not include free core nutation. The new model is used to avoid
329
(1) The CIP is the reference pole determined by the new precession-nutation model
(IAU 2000), used to replace the original CEP and taken as the pole of new
instantaneous reference frame. The motion that the period of CIP is less than
two days is regarded as the polar wander. In this way, for the terrestrial reference frame, the residuals between observations and models, the high-frequency
variation of the Earth-orientating parameters, and the diurnal term caused by
the defects of nutation model can be absorbed by the polar wander.
(2) Since the ICRF has nothing to do with the vernal equinox, it is unnecessary that
the orientation of the X-axis of the instantaneous reference frame of motion is
related to the equinox. By introducing the Celestial Ephemeris Origin (CEO)
to replace the vernal equinox, the confusion between the star catalogue and
dynamical equinoxes can be eliminated. In 2003, the IAU decided to change
the name of the CEO to the CIO. The CIO is determined by the non-rotating
origin, its specific position on the equator can be calculated by the precessionnutation model and observational data, and its hour angle is the ERA, similar
to the sidereal angle. Only by selecting the CIO as the starting point of right
ascension will the ERA’s formula not relay on the axial-precession and nutation.
(3) The ITRS is defined by the IUGG and realized by the ITRF. At present,
ITRF2014, the latest reference frame issued by the IERS, contains the position
and velocity vectors of 1499 stations. The non-rotating origin of the Earth is
used in the ITRF to give the Terrestrial Ephemeris Origin (TEO) of the CIP
on the equator. The 25th IAU general assembly decided to change the name of
TEO to Terrestrial Intermediate Origin (TIO). The TIO is an exact definition
of longitude starting point to meet the requirement for the UT1’s accuracy.
(4) The ERA is an angular distance between the CIO and TIO, around the orbit of
the CIP, and replaces the Greenwich Apparent Sidereal Time (GAST), whose
starting point is the vernal equinox. Since the equator moves relative to the
ecliptic each other, the relation between the GAST and UT1 contains the terms
caused by the axial-precession and nutation, and the relation between the ERA
and UT1 does not depend on the axial-precession and nutation. The ERA
represents the stellar rotation of the Earth around the CIP axis, and thereby it is
also called sidereal angle. The UT1 can be gotten from its linear relation with
the ERA. The non-rotating origin is introduced to make the time derivative of
the UT1 proportional to the instantaneous angular velocity of the Earth. The
relation between the Greenwich Mean Sidereal Time (GMST) and UT1 can be
derived from that between the ERA and UT1.
(5) The new precession-nutation model is used to replace the IAU1980 nutation
theory and IERS 1996 precession-nutation model. Using the new precessionnutation model, the new instantaneous reference frame of motion can be determined, and transformed to the ICRF. There are two versions for the new model:
IAU2000A and IAU2000B, in which the accuracy of the former is 0.1 milliarcseconds and that of the latter is 1 milliarcsecond. The new model includes the
deviation between the reference frames, geodetic precession and geodetic nutation, but does not include free core nutation. The new model is used to avoid