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2 Overview
12 AgGPS 332 GPS Receiver User Guide
24.4 Sources of Error in GPS Positioning
The GPS positioning method influences the accuracy of the GPS
position that is output by the AgGPS 332 receiver. The factors described
in Table 2.2 also affect GPS accuracy.
Table 2.2 Factors that influence the accuracy of GPS positions
Condition Optimum
value
Description
Atmospheric
effects
GPS signals are degraded as they travel through the
ionosphere. The error introduced is in the range of 10 meters.
The error is removed by using a differential or RTK positioning
method.
Number of
satellites used
> 5 To calculate a 3D position (latitude and longitude, altitude,
and time), four or more satellites must be visible. To calculate a
2D position (latitude and longitude, and time), three or more
satellites must be visible. For RTK positioning, five satellites are
needed for initialization. Once initialized, four or more
satellites provide RTK positions. The number of visible satellites
constantly changes and is typically in the range 5 through 9.
The AgGPS receiver can track up to 12 satellites simultaneously.
Note – To see when the maximum number of GPS satellites are
available, use the Trimble Planning software and a current
ephemeris (satellite history) file. Both files are available free
from the Trimble website at www.trimble.com.
Maximum PDOP < 4 Position Dilution of Precision (PDOP) is a unitless, computed
measurement of the geometry of satellites above the current
location of the receiver. A low PDOP means that the
positioning of satellites in the sky is good, and therefore good
positional accuracy is obtained.
Signal-to-noise
ratio
> 6 Signal-to-noise ratio (SNR) is a measure of the signal strength
against electrical background noise. A high SNR gives better
accuracy.
Normal values are:
• GPS 6
• WAAS 3+
• Beacon 12+
• OmniSTAR XP/HP/VBS 6+