Source Of Gps Signal Errors - Garmin GPS Manual

09/25/02 GPS_field_guide 8:19 A9/P9
1.3 Sources of GPS signal errors
Even if today's GPS receivers are extremely accurate, certain atmospheric factors and
other sources of error can affect the accuracy of GPS receivers.
If most of the sources of error are unavoidable, it is important for the user to be aware of
the ones that he can influence and be prepared to take steps to reduce their impact.
The greatest source of error is connected to the position of the satellite in the sky when
taking the measurement. The spread of the satellites in the sky is called the Positional
Dilution of Precision (PDOP). A good PDOP is obtained when the satellites are located
at wide angles relative to each other (Figure 6 a). In the contrary, a poor PDOP result
from satellites being located in a line or in a tight grouping (Figure 6 b).
a) b)
Figure 6: a) Good PDOP b) Poor PDOP
The second source of error comes from the infrastructure (buildings, bridges) and
particular landform (mountains) that are located around the receiver. These objects can
block the reception of the signal (Figure 7), causing position errors or possibly no
position reading at all.
a
a
b
b
Figure 7 - Example of good visibility (a)
and bad visibility (b) of satellites due to obstacles
GPS units typically will not work indoors, underwater or underground.
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