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Ambiguity resolution tends to fail when moving
and base receivers are more than a few tens of kilometers apart,
because of the poorly known effect of the ionosphere on the
signals. This depends on time of day, season, latitude, and
the sunspot cycle. Very large area mapping surveys with altimeters,
SAR, InSAR, side-scanning sonar, etc., may depend on finding
the precise location of the sensors with GPS over much longer
distances (hundreds to thousands of kilometers). There is a
way around, known as "floating" the ambiguities, that combines
signals received at the two GPS frequencies to eliminate the
effect of the ionosphere, but treats the phase ambiguities as
mere real-valued error states. It is the preferred approach
in very long baseline geodetic surveys, and also works quite
well in navigation, but at a price. It can take quite long for
the navigation Kalman filter to converge to the desired level
of precision, particularly with the kinematic approach. This
approach very wisely ignores the usually complex vehicle dynamics,
and so it works with any vehicle. The speed of convergence matters
both in real-time and in post-processing, and the trick is how
to increase it without giving up the versatility of kinematic
positioning. In this talk, the convergence problem and its possible
solutions will be illustrated with results from actual tests.
Material from Talk pdf (157KB) postscript (558KB)
Back to Mathematical Challenges in Global Positioning Systems (GPS)
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