Geography Reference
In-Depth Information
signals from at least four GPS satellites are needed if no other information is
available. If the elevation of the GPS receiver is known, only two satellites are
needed to determine position.
With the variability of the atmosphere, the movement of the GPS
receiver, and possible obstructions in the local environment of the GPS
receiver, the accuracy of GPS positioning may be limited. The factors that
reduce the accuracy are summarized in the measurement called the posi-
tional dilution of precision (PDOP). Larger values indicate less accurate GPS
positioning. If the values are greater than 8, than the positional location pro-
vided by the receiver is very inaccurate. Values less than 4 are a good indica-
tion of high accuracy.
Various other factors impact the positional accuracy of GPS measures.
The accuracy of most GPS receivers is less than 3 m under ideal conditions.
The accuracy is often even around 1 m. If the PDOP value is less than 4, the
positional values may even be accurate down to 1 or 2 m. Most GPS receivers
take atmospheric interferences into account by using information about the
atmosphere at a given time. More accurate receivers compare the speeds of
the GPS satellite signals to calculate the reduction in positional accuracy.
Most receivers also take the reflection of GPS satellite signals off of the
ground and buildings (multipath error) and changes in satellite orbits into
account. A receiver can reduce error by choosing satellite signals based on
the characteristics that introduce error into the position locations and using
satellites that help produce the mathematically most accurate results. The
Finding position with GPS.
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