5.4 Space-Time Reference Based on General Relativity
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celestial sphere is called mean north celestial pole, and the corresponding celestial
equator and vernal equinox are called mean celestial equator and mean equinox,
respectively. Under the influence of the gravity from the Sun, Moon and planets, the
instantaneous north celestial pole will rotate around the mean north celestial pole,
and its trajectory will be approximately elliptical, with the semi-major axis of about
9.2 arc-seconds and the period of about 18.6 years. In astronomy, this phenomenon is
called nutation, as shown in Fig. 5.6b. Under the influence of the axial-precession and
nutation, the orientation of the axis in the instantaneous celestial coordinate system
is always changing, which no longer meets the fundamental condition keeping the
datum point and plane unchanged for establishing the inertial system. In the noninertial system, the Newtonian mechanics cannot be directly used to study the motion
laws of celestial bodies and spacecrafts.
In order to establish a reference system close to the inertial system, a certain
moment is generally selected as a standard epoch t 0 , and at epoch t 0 , the instantaneous
Earth’s rotation axis pointing to the north celestial pole and the direction from the
Earth’s center to the instantaneous vernal equinox are taken, respectively, as the Z-axis
and X-axis after the corrections of the instantaneous axial-precession and nutation,
so as to establish a space-fixed coordinate system, which is called the Conventional
Celestial Reference System (CCRS) for the standard epoch. From January 1, 1984,
the IAG and IAU decided to initiate the CCRS, whose coordinate-axial orientations
are defined by the equator and equinox at 12:00 TDB on January 1, 2000, with the
J2000.0 epoch as the standard one.
The CCRS is defined by the pure theory associated with the space-time and gravitational model, and its specific realization, the Conventional Celestial Reference
Frame (CCRF), is usually established by kinematical or dynamical method. The
CCRF is achieved through the optical star catalogue, pulsar catalogue containing the
radio and X-ray pulsars, solar system planets and lunar ephemeris. Therefore, the
establishment and maintenance of the CCRF depend on the natural celestial bodies,
such as the millisecond pulsars, extragalactic radio sources, quasars, stars, planets
and Moon, as well as the man-made Earth satellites. The observational technologies, such as the optics, radio, X-ray, Very Long Baseline Interferometer (VLBI),
Very Large Array (VLA), radar-ranging, Lunar Laser Ranging (LLR), Satellite Laser
Ranging (SLR) and satellite-positioning, are usually used to establish and maintain
the CCRF, and the observational equipment may be placed on the ground or in space,
as shown in Fig. 5.7.
Currently, the establishment and maintenance of CCRF are mainly achieved
by the ground-based observations. The astrometry on the ground is conducive to
the construction of large-diameter telescopes, and the long-term management and
maintenance cost of the equipment are relatively low. Nevertheless, the groundbased observation conditions are greatly affected by atmospheric refraction, and it
is difficult to realize the all-sky observation by single equipment. The advantages
of space-based astrometry are there is no atmospheric refraction problem, and thus
the observed star image is a fixed diffraction image, which is more accurate than the
star image observed on the ground; there is not the effect of mechanical moment,
so that the position of the star image at the center of the FOV does not change with
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celestial sphere is called mean north celestial pole, and the corresponding celestial
equator and vernal equinox are called mean celestial equator and mean equinox,
respectively. Under the influence of the gravity from the Sun, Moon and planets, the
instantaneous north celestial pole will rotate around the mean north celestial pole,
and its trajectory will be approximately elliptical, with the semi-major axis of about
9.2 arc-seconds and the period of about 18.6 years. In astronomy, this phenomenon is
called nutation, as shown in Fig. 5.6b. Under the influence of the axial-precession and
nutation, the orientation of the axis in the instantaneous celestial coordinate system
is always changing, which no longer meets the fundamental condition keeping the
datum point and plane unchanged for establishing the inertial system. In the noninertial system, the Newtonian mechanics cannot be directly used to study the motion
laws of celestial bodies and spacecrafts.
In order to establish a reference system close to the inertial system, a certain
moment is generally selected as a standard epoch t 0 , and at epoch t 0 , the instantaneous
Earth’s rotation axis pointing to the north celestial pole and the direction from the
Earth’s center to the instantaneous vernal equinox are taken, respectively, as the Z-axis
and X-axis after the corrections of the instantaneous axial-precession and nutation,
so as to establish a space-fixed coordinate system, which is called the Conventional
Celestial Reference System (CCRS) for the standard epoch. From January 1, 1984,
the IAG and IAU decided to initiate the CCRS, whose coordinate-axial orientations
are defined by the equator and equinox at 12:00 TDB on January 1, 2000, with the
J2000.0 epoch as the standard one.
The CCRS is defined by the pure theory associated with the space-time and gravitational model, and its specific realization, the Conventional Celestial Reference
Frame (CCRF), is usually established by kinematical or dynamical method. The
CCRF is achieved through the optical star catalogue, pulsar catalogue containing the
radio and X-ray pulsars, solar system planets and lunar ephemeris. Therefore, the
establishment and maintenance of the CCRF depend on the natural celestial bodies,
such as the millisecond pulsars, extragalactic radio sources, quasars, stars, planets
and Moon, as well as the man-made Earth satellites. The observational technologies, such as the optics, radio, X-ray, Very Long Baseline Interferometer (VLBI),
Very Large Array (VLA), radar-ranging, Lunar Laser Ranging (LLR), Satellite Laser
Ranging (SLR) and satellite-positioning, are usually used to establish and maintain
the CCRF, and the observational equipment may be placed on the ground or in space,
as shown in Fig. 5.7.
Currently, the establishment and maintenance of CCRF are mainly achieved
by the ground-based observations. The astrometry on the ground is conducive to
the construction of large-diameter telescopes, and the long-term management and
maintenance cost of the equipment are relatively low. Nevertheless, the groundbased observation conditions are greatly affected by atmospheric refraction, and it
is difficult to realize the all-sky observation by single equipment. The advantages
of space-based astrometry are there is no atmospheric refraction problem, and thus
the observed star image is a fixed diffraction image, which is more accurate than the
star image observed on the ground; there is not the effect of mechanical moment,
so that the position of the star image at the center of the FOV does not change with