Global Positioning System Reference
In-Depth Information
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provided by the Jet Propulsion Laboratory (JPL). Relative positioning has thus far
been the backbone of most positioning with GPS and will still remain very important
in the future. The burden of processing for relative positioning has been lessened be-
cause of Internet processing services provided by, e.g., the National Geodetic Survey
(NGS). Ambiguity fixing, with emphasis on least-squares ambiguity decorrelation
adjustment (LAMBDA), is dealt with in a separate section because of its importance
to achieving centimeter-accurate relative positioning. The nonlinear pseudorange po-
sition solutions are given for point positioning and relative positioning.
7.1 THE IGS AND ITS PRODUCTS
The IGS is a response to a call by international users for an organizational structure
that helps maximize the potential of GPS. It is a globally decentralized organization
that is self-governed by its members and is without a central resource of funding. The
support comes from various member organizations and agencies around the world,
called contributing organizations. The IGS was formerly established by the Interna-
tional Association of Geodesy (IAG) in 1993 and officially began its operations on
January 1, 1994.
A governing board sets the IGS policies and exercises broad oversight of all IGS
functions. The executive arm of the board is the central bureau, which is located at
the JPL and is sponsored by NASA. There are nearly 300 globally distributed perma-
nent GPS tracking sites (Figure 7.1). These stations operate continuously and deliver
data hourly or daily to data centers. There are currently three global data centers,
five regional data centers, and twenty-three operational data centers. This scheme
of data centers provides for efficient access and storage of data, data redundancy,
[22
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Jan 2 16:10:25 2003
Figure 7.1
IGS permanent tracking network in 2002.
(Courtesy NASA/JPL/Caltech.)
