BACKGROUND |
At the International Association of Geodesy (IAG) General
Meeting in 1989,
the idea of a global cooperative permanent GPS tracking
network, and associated
data processing service, to support the geodynamic
community was proposed.
On February 1, 1991, a "Call for
Participation" was issued and
more than 100 scientific organisations
(including universities) and governmental
survey agencies announced their
willingness to participate either as an
"observatory" (part of
the globally distributed tracking network),
or as an analysis/data centre.
At the 20th General Assembly of the International
Union of Geodesy &
Geophysics (IUGG) in Vienna, in August 1991, several
workshops and test
campaigns were organised.
The "1992 IGS Test Campaign", June 21 to September 22, 1992, focussed on the routine determination of high accuracy orbits. "Epoch'92" was a two week concentrated campaign in the middle of the three month IGS Campaign for the purpose of densifying the network of tracking stations. Following the success of this test campaign, the IGS Pilot Service was launched in order to bridge the gap between the 1992 IGS Test Campaign and the start of the official service.
The official IGS service started on January 1, 1994.
A globally distributed
network of GPS tracking receivers provides a comprehensive
and robust
source of tracking data which may be processed to yield: (a)
precise GPS
satellite orbits, and (b) a set of accurate station coordinates
that
function as a realisation of a global datum.
The first operational GPS tracking network was established by the U.S. Department of Defense in 1980. By 1985, the network comprised ten stations (Figure below), a combination of U.S. Air Force and Defense Mapping Agency stations which produced data for precise post-processed ephemerides.
Figure 1. U.S. Air Force and
Defense Mapping Agency GPS tracking network.
While the
ephemerides produced by the U.S. DoD ground network were used
by certain
groups in the early 1980's, certain groups of civilian users
began to look
at the GPS technology as a cheaper, more mobile system than
both the
Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry
(VLBI)
techniques, to monitor crustal motion and other ultra precise
applications.
Through the 1980's there was an increasing demand for GPS
receivers and
experimental support. Regional field campaigns began
to mushroom, and
more and more international groups started to collaborate.
By the late 1980's
it was realised that a global distribution of tracking
stations is essential
for such applications.
The conclusion was that a continuous, standardised, precise tracking network was a preferable option, compared to the costly exercise of deploying receivers to remote locations for a matter of just a few weeks -- the average length of campaign experiments. The Cooperative International GPS Network (CIGNET) was an important early activity promoted by the U.S. National Geodetic Survey. The 1989 network shown in Figure 2 was soon augmented by other international partners, to form the core of the initial IGS Network.
Figure 2. Cooperative International GPS Network
(CIGNET) in 1989.
1991 was a crucial year for the development of the global tracking network with the announcement of the "Call for Participation" in the "International GPS Service for Geodynamics". The IGS successfully demonstrated the service during the three month "1992 IGS Test Campaign", June 21 to September 22, 1992, with data from tracking stations being accessed by the seven Analysis Centres within three days of data capture. Precise orbits were made available electronically on the Internet to users within two to three weeks. Figure 3 shows the tracking network configuration during the IGS demonstration campaign in 1992.
Figure 3. Tracking network
configuration during the IGS demonstration campaign,
June 21 to
September 22, 1992.
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© Chris Rizos, SNAP-UNSW, 1999