Leica Geosystems GPS Basics - Manual (Page 14)

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GPS Basics -1.0.0en How GPS works
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3.1.3 Error Sources
Up until this point, it has been assumed
that the position derived from GPS is very
accurate and free of error, but there are
several sources of error that degrade the
GPS position from a theoretical few
metres to tens of metres. These error
sources are:
1. Ionospheric and atmospheric
delays
2. Satellite and Receiver Clock
Errors
3. Multipath
4. Dilution of Precision
5. Selective Availability (S/A)
6. Anti Spoofing (A-S)
1. Ionospheric and Atmospheric delays
As the satellite signal passes through
the ionosphere, it can be slowed down,
the effect being similar to light refracted
through a glass block. These atmo-
spheric delays can introduce an error in
the range calculation as the velocity of
the signal is affected. (Light only has a
constant velocity in a vacuum).
The ionosphere does not introduce a
constant delay on the signal. There are
several factors that influence the amount
of delay caused by the ionosphere.