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between stations with a relative accuracy of better than 10 8 order of magnitude
and to establish the skeleton of a national high-precision satellite geodetic network.
This work has laid the foundation for studies concerning crustal movement and
geodynamics.
Being a global GNSS continuously operating station and an integrated service
system, IGS provides users around the world for free with various GPS and Global
Orbit Navigation Satellite System (GLONASS) information, such as satellite pre-
cise ephemeris, rapid ephemeris, broadcast ephemeris, velocities and coordinates of
IGS stations, satellite signals received by IGS stations, and the Earth's rotation rate,
etc. It has supported numerous scientific programs in geodesy and geodynamics,
which include ionosphere, meteorology, reference framework, crustal movement,
precision time-transfer, high-resolution calculation of the Earth's rotation rate and
changes, and so on.
Base stations contributing to IGS are found all over the world and include more
than 380 observation stations that belong to over 100 research institutions, univer-
sities, and governmental organizations in different countries. With the assistance of
IAG, a coordination committee was set up by the agencies that have established the
stations and the government departments of different countries, within which a
board of directors was also formed. Authorized by IGS, this board of directors
manages several Data and Analysis Centers and is in charge of providing services,
launching products, and organizing international collaborative research projects. Its
services and products include:
1. GPS satellite ephemeris service. Its products include broadcast, rapid, and final
precise ephemerides with a precision of 25 cm, 5 cm, and better than 5 cm,
respectively, and with a delay of 0 h, 17 h, and 13 days, respectively. GLONASS
only provides the final ephemeris with a precision of about 30 cm.
2. Service of receiver clock error for GPS satellites and continuously operating
reference systems (CORS). Its products are broadcast, rapid, and final clock
errors with a precision of 5 ns, 0.2 ns, and 0.1 ns, respectively, and with a time
delay of 0 h, 17 h, and 13 days, respectively.
3. Service of the coordinates (including the corresponding framework and epoch)
and movement velocity of the continuously operating stations (tracking sta-
tions). Its products contain the horizontal and vertical positions and movement
velocities of the station. The horizontal and vertical positions are at a precision
of 3 mm and 6 mm, respectively, with corresponding yearly movement veloc-
ities of 2 mm per annum and 3 mm per annum.
4. The Earth rotation parameters service. Its products are rapid and final polar
motion, short-term nutation, LOD (length of day) changes, etc. The accuracy of
the final daily polar coordinates published by IGS is
0.1 milliarcseconds and
0.2 milliarcseconds.
5. Atmospheric parameters service. Its products are final tropospheric parameters,
with the accuracy of zenith delay of 4 mm, issued delay
the corresponding accuracy of the rapid product is
4 weeks. It will further
provide the electron density distribution parameters of the ionospheric grid.
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