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space geodetic techniques, such as GNSS, VLBI, SLR, and DORIS, that can
provide three-dimensional coordinates, rather than classical geodetic methods
(i.e., triangulation, traversing, leveling, etc.), which cannot achieve such results.
3.3.1 The Three-Dimensional Coordinate Datum
In traditional geodetic surveys, horizontal and vertical coordinates are referred to
different datums, which are inconsistent with each other, generating many problems
in practical use. For example, research on geodynamic issues such as satellite and
aircraft technology, the Earth's rotation, and plate movement must refer to the
three-dimensional datum. Space geodetic techniques can realize the three-
dimensional geodetic datum by determining or calculating the three-dimensional
coordinates of surface points.
Global Three-Dimensional Coordinate Datum
The development of space geodetic techniques such as GNSS, VLBI, SLR, and
DORIS has created conditions for establishing a global three-dimensional datum,
which is realized by connecting the above-mentioned space geodetic stations
distributed worldwide together into a corresponding global network. Each space
technique is organized, coordinated, and managed by its corresponding interna-
tional agency such as IGS, IVS (International VLBI Service for Geodesy and
Astrometry), ILRS (International Laser Ranging Service), IDS (International
DORIS Service), and so on. After the combined adjustment of the geodetic net-
works realized by each single space geodetic technique, a comprehensive three-
dimensional datum, i.e., the International Terrestrial Reference Frame (ITRF) is
formed.
Global IGS Network
In 1991, IAG decided to establish a global IGS observation network (originally
known as the “International GPS Service” and changed into “International GNSS
Service” in March, 2005). During the period from June to September, 1992, IAG
implemented the first campaign for connection. Taking this opportunity, China held
a nationwide “China 1992 GPS campaign” which was a cooperative endeavor
involving many organizations. Major cities such as Shanghai, Wuhan, Taibei,
Kunming, Xi'an, and Urumchi all participated in the IGS observation network.
China's involvement in the global IGS cooperation aims to determine the high-
precision geocentric coordinates nationwide and establish a new generation geo-
centric reference frame, as well as its transformation parameters with the national
coordinate system. In addition, it also intends to determine the baseline vector
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