330
5 X-ray Pulsar-Based Navigation: Theories and Experiments
the artificial separation of the axial-precession and nutation, while the axialprecession and nutation models are used separately in the frames based on the
vernal equinox.
(6) The astronomical constants of IAU1976 cannot meet the requirements for
high-precision astrometry. Therefore, the optimal estimates of the defining
constants and primary constants were given by the IAU, IERS and IUGG
working groups, and then the latest astronomical constant values obtained.
The new and more accurate observational data is used to improve the planetary
and lunar ephemerides. For example, the JPL DE/LE405 ephemeris is used
to replace the JPL DE/LE200 introduced in 1984. The new ephemeris table
is consistent with the ICRS, and the corresponding constant values are also
given.
The new instantaneous reference frame of motion is defined and realized on the
basis of the ICRF, with three different forms: the first is the old system in which
the vernal equinox and CEP, as well as the axial-precession, nutation and constants
introduced in 1984 are used, with low accuracy; the second is the system which
still adopts the old concept, definition and vernal equinox, but in which the axialprecession, nutation, constants and celestial reference pole are improved; the third is
the new system in which the new precession-nutation model and the latest constants
are used, with the CIP as the pole and with the CIO as the starting point of the right
ascension. The third form of reference frame has the advantage of high precision and
does not depend on the vernal equinox. It should be pointed out that the vernal equinox
cannot be measured accurately by the modern observation technologies. In order to
define the ICRS strictly, the IAU introduces two space-fixed reference systems, the
BCRS and the GCRS. Under the general relativity, the coordinate transformation
between the two reference systems can be achieved by using the metric tensors
and post-Newtonian approximations. The orientations of the two reference systems
at their respective origins can be determined through the ICRS, and there is no
rotation between them. However, the timescales used in the two reference systems
are different and are, respectively, the TCB and TCG.
In the new precession-nutation model, the expansion of the axial-precession is
not given, but the IAU1976 precession formula is still used, and only the specific
values, such as the precession rate of the Sun and Moon at the J2000.0 ecliptic
longitude as well as the obliquity-rate between the moving equator and the fixed
ecliptic, are improved. A new and more accurate precession-nutation theory is being
studied, whose nutation series will give 678 solar and lunar terms and 687 planetary
terms, with the periods of greater than 2 days and the amplitude of greater than
15 μs, and it will provide the celestial polar coordinates with an accuracy of 0.2
milliarcseconds in the GCRS. The GCRS moves around its center of mass and has a
fixed direction relative to the BCRS and extragalactic radio sources. However, under
the general relativity, the GCRS is originally as a non-rotating geocentric reference
system in the sense of dynamics, but with a relative rotation. The long period part of
the biggest components of the GCRS is the geodetic precession and the short period
is the geodetic nutation. It is completely different in effects from the usual precession
5 X-ray Pulsar-Based Navigation: Theories and Experiments
the artificial separation of the axial-precession and nutation, while the axialprecession and nutation models are used separately in the frames based on the
vernal equinox.
(6) The astronomical constants of IAU1976 cannot meet the requirements for
high-precision astrometry. Therefore, the optimal estimates of the defining
constants and primary constants were given by the IAU, IERS and IUGG
working groups, and then the latest astronomical constant values obtained.
The new and more accurate observational data is used to improve the planetary
and lunar ephemerides. For example, the JPL DE/LE405 ephemeris is used
to replace the JPL DE/LE200 introduced in 1984. The new ephemeris table
is consistent with the ICRS, and the corresponding constant values are also
given.
The new instantaneous reference frame of motion is defined and realized on the
basis of the ICRF, with three different forms: the first is the old system in which
the vernal equinox and CEP, as well as the axial-precession, nutation and constants
introduced in 1984 are used, with low accuracy; the second is the system which
still adopts the old concept, definition and vernal equinox, but in which the axialprecession, nutation, constants and celestial reference pole are improved; the third is
the new system in which the new precession-nutation model and the latest constants
are used, with the CIP as the pole and with the CIO as the starting point of the right
ascension. The third form of reference frame has the advantage of high precision and
does not depend on the vernal equinox. It should be pointed out that the vernal equinox
cannot be measured accurately by the modern observation technologies. In order to
define the ICRS strictly, the IAU introduces two space-fixed reference systems, the
BCRS and the GCRS. Under the general relativity, the coordinate transformation
between the two reference systems can be achieved by using the metric tensors
and post-Newtonian approximations. The orientations of the two reference systems
at their respective origins can be determined through the ICRS, and there is no
rotation between them. However, the timescales used in the two reference systems
are different and are, respectively, the TCB and TCG.
In the new precession-nutation model, the expansion of the axial-precession is
not given, but the IAU1976 precession formula is still used, and only the specific
values, such as the precession rate of the Sun and Moon at the J2000.0 ecliptic
longitude as well as the obliquity-rate between the moving equator and the fixed
ecliptic, are improved. A new and more accurate precession-nutation theory is being
studied, whose nutation series will give 678 solar and lunar terms and 687 planetary
terms, with the periods of greater than 2 days and the amplitude of greater than
15 μs, and it will provide the celestial polar coordinates with an accuracy of 0.2
milliarcseconds in the GCRS. The GCRS moves around its center of mass and has a
fixed direction relative to the BCRS and extragalactic radio sources. However, under
the general relativity, the GCRS is originally as a non-rotating geocentric reference
system in the sense of dynamics, but with a relative rotation. The long period part of
the biggest components of the GCRS is the geodetic precession and the short period
is the geodetic nutation. It is completely different in effects from the usual precession