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5 X-ray Pulsar-Based Navigation: Theories and Experiments
milliarcsecond; the observation numbers are more than 20; the coordinate deviations
in several test calculations are less than 3-sigma or 0.5 milliarcseconds; the structural
changes and motions of the sources are very small. Since January 1, 1998, the ICRF
has officially replaced the FK5 catalogue, applied to the research fields, such as
astrometry, astro-geodynamics and space exploration.
The IERS uses mathematical models to maintain the stability of the ICRF, mainly
including the elimination of local deformation (local system error) and the orientation keeping. The ICRF does not depend on the Earth’s rotation, or on the ecliptic,
but is only affected by the observation accuracy. According to the radio source catalogues provided by every analysis center each year, the IERS synthesizes them by
mathematical methods to produce the combined radio source catalogue, which is
used to define and realize the ICRF and maintain its stability.
The ICRF is considered to be non-rotational, but it has been found that some
selected compact extragalactic radio sources, such as 3C273 and HC39.25, are still
the extended sources in fine structure, and their structures vary at the order of milliarcseconds. Therefore, it is necessary to continually observe the extended source’s structures and calculate the influence of structural changes on time delay, and then obtain
the source’s structural indicators according to the influence degree. By adding the
correction of source’s structural change to the time delay and considering the apparent
proper motion of the radio sources resulted from the change of cosmic matter distribution, the stability of the established extragalactic radio reference frame can generally
reach the order of microarcseconds.
5.4.5.4 New Instantaneous Reference Frame
In practice, when the ICRF is taken as a space-fixed reference frame, there are still
many factors that affect the Earth’s motion. For the Earth orientating, it is necessary
to establish the instantaneous reference frame of motion, which is a kind of instantaneous Earth’s equatorial coordinate system in strict sense. For this reason, the IAU
decided to initiate a new instantaneous reference frame of motion from 2003. The
new reference frame has nothing to do with the dynamical reference frame, and it is
determined by the new precession-nutation model, Celestial Intermediate Pole (CIP)
and Celestial Intermediate Origin (CIO). The CIO does not depend on the theoretical
model, and its long-term prediction accuracy reaches the order of milliarcseconds.
Due to the CIO associated with the non-rotating origin, a new definition of the
Universal Time (UT1) measured by the Earth rotational angle (ERA) is introduced.
There is only a linear relationship between the ERA and UT1.
Just as the FK5 system realized with the J2000.0 reference frame is replaced by the
ICRS realized by the ICRF, the instantaneous reference frame of motion, established
on the basis of IAU 1980 nutation theory, Celestial Ephemeris Pole (CEP) and true
equinox, is also replaced by the new reference frame. The basic features of the new
instantaneous reference frame are as follows:
5 X-ray Pulsar-Based Navigation: Theories and Experiments
milliarcsecond; the observation numbers are more than 20; the coordinate deviations
in several test calculations are less than 3-sigma or 0.5 milliarcseconds; the structural
changes and motions of the sources are very small. Since January 1, 1998, the ICRF
has officially replaced the FK5 catalogue, applied to the research fields, such as
astrometry, astro-geodynamics and space exploration.
The IERS uses mathematical models to maintain the stability of the ICRF, mainly
including the elimination of local deformation (local system error) and the orientation keeping. The ICRF does not depend on the Earth’s rotation, or on the ecliptic,
but is only affected by the observation accuracy. According to the radio source catalogues provided by every analysis center each year, the IERS synthesizes them by
mathematical methods to produce the combined radio source catalogue, which is
used to define and realize the ICRF and maintain its stability.
The ICRF is considered to be non-rotational, but it has been found that some
selected compact extragalactic radio sources, such as 3C273 and HC39.25, are still
the extended sources in fine structure, and their structures vary at the order of milliarcseconds. Therefore, it is necessary to continually observe the extended source’s structures and calculate the influence of structural changes on time delay, and then obtain
the source’s structural indicators according to the influence degree. By adding the
correction of source’s structural change to the time delay and considering the apparent
proper motion of the radio sources resulted from the change of cosmic matter distribution, the stability of the established extragalactic radio reference frame can generally
reach the order of microarcseconds.
5.4.5.4 New Instantaneous Reference Frame
In practice, when the ICRF is taken as a space-fixed reference frame, there are still
many factors that affect the Earth’s motion. For the Earth orientating, it is necessary
to establish the instantaneous reference frame of motion, which is a kind of instantaneous Earth’s equatorial coordinate system in strict sense. For this reason, the IAU
decided to initiate a new instantaneous reference frame of motion from 2003. The
new reference frame has nothing to do with the dynamical reference frame, and it is
determined by the new precession-nutation model, Celestial Intermediate Pole (CIP)
and Celestial Intermediate Origin (CIO). The CIO does not depend on the theoretical
model, and its long-term prediction accuracy reaches the order of milliarcseconds.
Due to the CIO associated with the non-rotating origin, a new definition of the
Universal Time (UT1) measured by the Earth rotational angle (ERA) is introduced.
There is only a linear relationship between the ERA and UT1.
Just as the FK5 system realized with the J2000.0 reference frame is replaced by the
ICRS realized by the ICRF, the instantaneous reference frame of motion, established
on the basis of IAU 1980 nutation theory, Celestial Ephemeris Pole (CEP) and true
equinox, is also replaced by the new reference frame. The basic features of the new
instantaneous reference frame are as follows: