Trimble Outdoors aggps 332 用户手册

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2          Overview
1 2     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 
.
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+