5.4 Space-Time Reference Based on General Relativity
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and nutation. The new precession-nutation model contains the geodetic precession
and nutation. In this way, the coordinate transformation between the ITRF and ICRF
can be achieved by using the new precession-nutation model, CIP, CIO, TIO and
ERA.
The introduction of the new instantaneous reference system will change the theory
and practice for astrometry. The WGRF of IAU is defining a set of systematic theoretical methods and standardized terms. For example, the right ascension is usually
calculated from the vernal equinox. If the terms of the right ascension and declination continue to be used in the new instantaneous reference system, their specific
meanings must be redefined and explained. Of course, during the transitional period,
the ICRS will coexist at the same time with the new reference system by which the
theoretical model, basic parameter, observational data and commonly used term will
be defined, and gradually introduced into the relative articles in the future.
5.5 Theory of Pulsar Timing System
Space and time are the basic forms of material existence. Space-measuring system
depends on the realization, establishment and maintenance of time system. In astrometry research and space technology application, the time system is the datum to
accurately describe the positions of celestial bodies and spacecrafts, and the connections between them and ground TT&C stations. One of the core tasks to achieve the
XPNAV is to construct and maintain a space-ground integrated time system with
pulsar time as datum.
5.5.1 Expressions of Pulsar Time
It is well known that time is usually measured according to the motion law of celestial
bodies. The UT1 and ET are the timescales established, respectively, by using the
rotation and revolution of the Earth, and the corresponding mean solar second and
ephemeris second had ever been used to define the length of second in the SI units.
The UT1 is obtained by observing the rotation of the Earth, which is proportional to
the ERA around its axis. Due to the instability of the Earth’s rotation, the uncertainty
of the mean solar second is about 10
−8 s/s. The ET is achieved by calculating and
observing the orbit of the Earth’s revolution around the Sun, and it is also not a
uniform time standard, with the uncertainty of about 10
−9 s/s. The atomic time using
the electronic transition radiation period to define the length of a second has a very
stable frequency. The uncertainty of TAI reaches 10
−15 s/s, which is obviously better
than the traditional astronomical timescales. Nevertheless, if it is further expected to
improve the long-term stability of the atomic time and establish and maintain largescale time reference, then there are some difficulties in both theory and practice.
331
and nutation. The new precession-nutation model contains the geodetic precession
and nutation. In this way, the coordinate transformation between the ITRF and ICRF
can be achieved by using the new precession-nutation model, CIP, CIO, TIO and
ERA.
The introduction of the new instantaneous reference system will change the theory
and practice for astrometry. The WGRF of IAU is defining a set of systematic theoretical methods and standardized terms. For example, the right ascension is usually
calculated from the vernal equinox. If the terms of the right ascension and declination continue to be used in the new instantaneous reference system, their specific
meanings must be redefined and explained. Of course, during the transitional period,
the ICRS will coexist at the same time with the new reference system by which the
theoretical model, basic parameter, observational data and commonly used term will
be defined, and gradually introduced into the relative articles in the future.
5.5 Theory of Pulsar Timing System
Space and time are the basic forms of material existence. Space-measuring system
depends on the realization, establishment and maintenance of time system. In astrometry research and space technology application, the time system is the datum to
accurately describe the positions of celestial bodies and spacecrafts, and the connections between them and ground TT&C stations. One of the core tasks to achieve the
XPNAV is to construct and maintain a space-ground integrated time system with
pulsar time as datum.
5.5.1 Expressions of Pulsar Time
It is well known that time is usually measured according to the motion law of celestial
bodies. The UT1 and ET are the timescales established, respectively, by using the
rotation and revolution of the Earth, and the corresponding mean solar second and
ephemeris second had ever been used to define the length of second in the SI units.
The UT1 is obtained by observing the rotation of the Earth, which is proportional to
the ERA around its axis. Due to the instability of the Earth’s rotation, the uncertainty
of the mean solar second is about 10
−8 s/s. The ET is achieved by calculating and
observing the orbit of the Earth’s revolution around the Sun, and it is also not a
uniform time standard, with the uncertainty of about 10
−9 s/s. The atomic time using
the electronic transition radiation period to define the length of a second has a very
stable frequency. The uncertainty of TAI reaches 10
−15 s/s, which is obviously better
than the traditional astronomical timescales. Nevertheless, if it is further expected to
improve the long-term stability of the atomic time and establish and maintain largescale time reference, then there are some difficulties in both theory and practice.