User's Manual

Using GPS
GeoExplorer 2005 Series Getting Started Guide 31
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 your GPS application 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 maxiumum
PDOP setting of 6, or a maximum
HDOP setting of 4.
Weak
satellite
signals
Signal-to-Noise Ratio (SNR) is a
measure of the strength of the
satellite signal relative to the
background noise. Accuracy
degrades as the signal
strength decreases. Weak
signals may be caused by
signals coming through
vegetation, multipath signals,
or low satellite elevation.
Set your GPS application to ignore
satellites with a weak SNR.
Trimble recommends a minimum SNR
setting of 39 dBHz.
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 your data
collection 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.
Factor Description To Maximize Accuracy