User's Manual Part 3

7 Troubleshooting
124 GeoExplorer 2008 Series User Guide
Recommended GPS settings for maximum precision
The following table lists some of the factors that affect the precision of your data, and
describes how to minimize the effect of atmospheric interference and poor satellite
geometry.
B
Tip – To quickly find the recommended values shown here, move the GPS slider in the GPS
Settings form to the middle (default) setting.
Factor Description To Maximize Precision
Number of
visible satellites
The quality of your data increases with the
number of satellites being used to calculate
the position.
You need at least four satellites to calculate
an accurate 3-dimensional position. Trimble
data collection software only logs GPS
positions when four or more satellites are
visible. Tracking more satellites can help to
lower DOP values.
Multipath Multipath is when GPS satellite signals are
reflected off nearby objects, such as buildings
or cars, causing an erroneous signal to be
received by the GPS antenna. This can cause
errors of several meters.
To reduce multipath, collect data in an open
environment away from large reflective
surfaces and with a clear view of the sky. In
high multipath environments, record velocity
data and use velocity filtering when
postprocessing the data.
Weak satellite
signals
Signal-to-Noise Ratio (SNR) is a measure of
the strength of the satellite signal relative to
the background noise. GPS quality degrades
as the signal strength decreases. Weak signals
may be caused by signals coming through
vegetation, multipath signals, or low satellite
elevation.
Set the GPS field software to ignore satellites
with a weak SNR.
Trimble recommends a minimum SNR setting
of 39 dBHz.
Poor satellite
geometry
Dilution of Precision (DOP) is a measure of the
quality of GPS positions, based on the spread
(geometry) of the satellites in the sky that are
used to compute the positions. When
satellites are widely spaced relative to each
other, the PDOP value is lower, and position
accuracy is greater. If the view of the sky is
partially blocked, or if all of the satellites are
in one area of the sky, the geometry and DOP
may be poor.
Set the GPS field software to ignore positions
with a poor DOP value.
You can choose to filter positions based on
PDOP (Position Dilution of Precision) or HDOP
(Horizonal Dilution of Precision). PDOP is a
measure of the horizontal and vertical quality
of the GPS positions, whereas HDOP is just a
measure of the horizontal precision (x and y
coordinates).
Select HDOP rather than PDOP if you want to
ensure positions are accurate horizontally,
and when vertical accuracy is less important
Trimble recommends a maximum PDOP
setting of 6, or a maximum HDOP setting of 4.
Satellite
elevation
When a satellite is low on the horizon,
satellite signals must travel farther through
the atmosphere. This results in a lower signal
strength and delayed reception by the GPS
receiver, which can cause errors in calculating
the position.
Set the elevation field in the GPS field
software to ignore satellites that are low in
the sky.
Trimble recommends a minimum elevation
setting of 15°.
Occupation time
at a point
Occupation time is the time spent at a point
logging GPS positions.
For point features, remain at the feature and
log a number of GPS positions to obtain an
averaged position.
When collecting line and area features,
collect them using averaged vertices.