5.3 Space-Time Reference Based on Newtonian Mechanics
295
The orientation of ITRS is determined by the CTP and Zero Meridian of the BIH at
epoch 1984.0, and its specific position on the Earth is reflected by the coordinates of
the sites distributed globally. Theoretically, as long as the position coordinates of any
three sites on the Earth are known, a conventional terrestrial coordinate system can
be determined uniquely. However, due to the observation errors, there will be errors
for the initial meridian plane, coordinate origin and Z-axis direction, and thus more
than three reference sites are usually used for orientating the terrestrial coordinate
system. Moreover, due to the influence of the Earth’s plate motion and local crustal
deformation, the coordinates of the reference sites always change dynamically. The
ITRF is a geocentric coordinate system derived from the observation data of the global
sites, a four-dimensional geocentric coordinate reference frame. Besides giving a
high-precision space rectangular coordinate form of the sites, it also provides the
drift rates of the sites. At present, the site’s coordinate accuracy gets to the level of
millimeter. By using multiple observation means like the VLBI, SLR, LLR, GNSS
and DORIS, and combining with the solution results from multiple data analysis
center, the ITRF composed of a series of the sites’ coordinates and velocities relative
to a reference epoch can be obtained. After establishing the ITRF, in order to get
the ITRF coordinates of an undetermined point, it is only necessary to measure the
position coordinates of the point relative to the ITRF sites.
As early as 1984, an initial terrestrial reference frame, known as the BIH Terrestrial System (BTS84), was established by the BIH using the observation data from
the VLBI, LLR, SLR and Transit, and then the series of BTS85, BTS86 and
BTS87 were completed sequentially. In 1988, the IUGG and IAU jointly created
the IERS, and then the ITRF is established by the IERS. At present, there are thirteen
ITRF versions released by the IERS, including ITRF88, ITRF89, ITRF90, ITRF91,
ITRF92, ITRF93, ITRF94, ITRF96, ITRF97, ITRF2000, ITRF2005, ITRF2008 and
ITRF2014, in which the number of year following “ITRF” denotes the data of the next
year used for establishing the reference system. For example, the ITRF97 denotes the
reference frame with the sites’ positions and velocities established in 1999, by using
all available IERS data up to 1998. The observation techniques, reference epochs and
plate motion models used in the versions from the ITRF88 to ITRF2014 are listed
in Table 5.1.
With the increasing number of the observation sites and co-location observation
sites distributed all over the world, as well as the improvement of performance of
various measurement devices, the ITRF will be refined constantly. For example,
the ITRF88 has about 100 stations and 22 VLBI/SLR/LLR co-location sites, the
ITRF2000 includes 500 stations and 101 co-location sites, and the ITRF2014 includes
1499 stations and 975 co-location sites. Meanwhile, with constant improvement of
the data processing ways for various IERS analysis centers, the difference between
the coordinate and velocity solutions of ITRF sites will become smaller and smaller.
Currently, the ITRF accuracy has reached the millimeter level. The transformation
parameters between the ITRF versions can be obtained from the websites of the IERS
and International GNSS Service (IGS).
295
The orientation of ITRS is determined by the CTP and Zero Meridian of the BIH at
epoch 1984.0, and its specific position on the Earth is reflected by the coordinates of
the sites distributed globally. Theoretically, as long as the position coordinates of any
three sites on the Earth are known, a conventional terrestrial coordinate system can
be determined uniquely. However, due to the observation errors, there will be errors
for the initial meridian plane, coordinate origin and Z-axis direction, and thus more
than three reference sites are usually used for orientating the terrestrial coordinate
system. Moreover, due to the influence of the Earth’s plate motion and local crustal
deformation, the coordinates of the reference sites always change dynamically. The
ITRF is a geocentric coordinate system derived from the observation data of the global
sites, a four-dimensional geocentric coordinate reference frame. Besides giving a
high-precision space rectangular coordinate form of the sites, it also provides the
drift rates of the sites. At present, the site’s coordinate accuracy gets to the level of
millimeter. By using multiple observation means like the VLBI, SLR, LLR, GNSS
and DORIS, and combining with the solution results from multiple data analysis
center, the ITRF composed of a series of the sites’ coordinates and velocities relative
to a reference epoch can be obtained. After establishing the ITRF, in order to get
the ITRF coordinates of an undetermined point, it is only necessary to measure the
position coordinates of the point relative to the ITRF sites.
As early as 1984, an initial terrestrial reference frame, known as the BIH Terrestrial System (BTS84), was established by the BIH using the observation data from
the VLBI, LLR, SLR and Transit, and then the series of BTS85, BTS86 and
BTS87 were completed sequentially. In 1988, the IUGG and IAU jointly created
the IERS, and then the ITRF is established by the IERS. At present, there are thirteen
ITRF versions released by the IERS, including ITRF88, ITRF89, ITRF90, ITRF91,
ITRF92, ITRF93, ITRF94, ITRF96, ITRF97, ITRF2000, ITRF2005, ITRF2008 and
ITRF2014, in which the number of year following “ITRF” denotes the data of the next
year used for establishing the reference system. For example, the ITRF97 denotes the
reference frame with the sites’ positions and velocities established in 1999, by using
all available IERS data up to 1998. The observation techniques, reference epochs and
plate motion models used in the versions from the ITRF88 to ITRF2014 are listed
in Table 5.1.
With the increasing number of the observation sites and co-location observation
sites distributed all over the world, as well as the improvement of performance of
various measurement devices, the ITRF will be refined constantly. For example,
the ITRF88 has about 100 stations and 22 VLBI/SLR/LLR co-location sites, the
ITRF2000 includes 500 stations and 101 co-location sites, and the ITRF2014 includes
1499 stations and 975 co-location sites. Meanwhile, with constant improvement of
the data processing ways for various IERS analysis centers, the difference between
the coordinate and velocity solutions of ITRF sites will become smaller and smaller.
Currently, the ITRF accuracy has reached the millimeter level. The transformation
parameters between the ITRF versions can be obtained from the websites of the IERS
and International GNSS Service (IGS).