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2 Overview
20 DSM 132/132RS Receiver User Guide
23.2 DGPS accuracy
Submeter accuracy from the DSM 132 receiver utilizing differential
correction is best achieved under the conditions described in
Table 2.2.
Table 2.2 DGPS accuracy
Condition Optimum
Value
Description
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.
The number of visible satellites constantly changes and is
typically in the range 5–9. The DSM 132 receiver can track
up to 12 satellites simultaneously.
Note – To see when the maximum number of GPS satellites are
available, use the Quickplan utility and a current ephemeris
(satellite history) file. Both files are available free from:
www.trimble.com/support_trl.asp?Nav=Collection-3627
Maximum
PDOP
< 4 Position Dilution of Precision (PDOP) is a unitless, computed
measurement of the geometry of satellites above the
receiver’s current location.
Note – In some agricultural applications that do not require high
accuracy, a maximum PDOP of 12 or more can be used.
Signal to Noise
Ratio
> 6 Signal-to-Noise ratio (SNR) is a measure of the satellite
signal strength against electrical background noise. A high
SNR gives better accuracy.
Minimum
Elevation
> 7.5 Satellite that are low on the horizon typically produce weak
and noisy signals and are more difficult for the receiver to
track. Satellites below the minimum elevation angle are not
tracked.
Multipath
environment
Low Multipath errors are caused when GPS signals are reflected
off nearby objects and reach the receiver by two or more
different paths. The receiver incorporates the EVEREST
multipath rejection option.
RTCM-
compatible
corrections
These corrections are broadcast from a DSM 132RS or
equivalent reference station.