7.2.3 Basic Modelling Concepts
GPS DATUM DEFINITION ISSUES
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The GPS satellite datum enters the GPS phase solutions through the definition
of the known components of the range eqn (7.1-19)
and the Figure below, that is:
- the known satellite coordinates Xi, Yi, Zi,
and
- the fixed reference station coordinates -- this defines the origin
and orientation of the local Cartesian coordinate system.
The datum in which the satellite coordinates are expressed is nominally
the WGS84 system. However, the accuracy of the broadcast ephemerides
is perhaps at the one or two dekametre level. The fixed station coordinates
should therefore also be defined in the same (WGS84) reference system at
this dekametre accuracy level. If the results are required in the local
geodetic datum, the coordinates must be transformed using an appropriate
procedure (Chapter 11).
The geometric range: receiver to satellite.
In GPS satellite positioning, the following datums are involved:
- World satellite datums such as
WGS84 -- a geocentric reference system with axes aligned parallel
to the earth's primary axes. The GPS orbit information transmitted by the
satellites is referenced to this system, consequently GPS point positioning
gives coordinate results in this system. It is also the system implicit
in the local Cartesian system for GPS relative positioning.
- In the case of the relative positioning mode, use is made of
a local Cartesian system parallel to the global
GPS satellite datum. This system has its origin at the fixed station
of a baseline or network. It is a block-shifted version of WGS84, or whatever
satellite datum is being used. (For example, if post-processed GPS orbits
are being used, the datum is no longer WGS84, but a satellite datum such
as the International Terrestrial Reference System.)
- In general, station coordinate information is available in
the conventional geodetic system.
To integrate GPS results with terrestrial geodetic results requires the
relationships between all the (nearly parallel) local and global Cartesian
systems to be known, or determined.
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© Chris Rizos, SNAP-UNSW, 1999