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SXBlue II GPS Series Technical Reference Manual
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Type 5
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Type 6
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Type 7
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Type 9
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Type 16
To return to using SBAS as the correction source, send the following command to the
SXBlue II GPS:
$JDIFF,WAAS<CR><LF>
You will find detailed information on NMEA messages supported by the SXBlue II GPS
in Chapter 4 - Complete NMEA 0183 Interface.
3.4.4 Auto-Dif in Autonomous and Base Station Operations
Operation of the SXBlue II GPS unit with the Auto-Dif application requires sending of
NMEA messages. These commands may be automatically issued through customized
software or a simple terminal interface running on a PDA or data logger. Chapter 4
provides detailed information on the commands supported by the Auto-Dif feature.
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The Auto-Dif autonomous mode requires only that a reference point be set with
the $JRAD,1,x command.
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The Auto-Dif in base station mode also requires the $JRAD,2 command be also
sent (after the $JRAD,1) to tell the receiver to use the entered reference point for
base station operation. In addition, RTCM message output should be activated
on one of the ports using the $JASC,RTCM,1,PORTx
The remainder of this sub-section covers the autonomous mode of Auto-Dif operation.
3.4.4.1 Start-up
When you turn the SXBlue II GPS on with the Auto-Dif application running, it will require
a minimum of a few minutes to gather enough satellite tracking information to model the
errors for the future (up to 10 minutes may be required depending on your environment).
You do not have to keep the SXBlue II GPS stationary for this process, but you should
ensure that the SXBlue II GPS maintains acquisition on the satellites available. We refer
to this process of gathering information for the Auto-Dif application as Calibration.
3.4.4.2 Calibration
Calibration is the process of zeroing the increasing errors in the Auto-Dif modeling
process. Calibration can be performed either in a relative or absolute sense, depending
on your positioning needs. Relative positioning will provide positions that are accurate to
one another, however, there may be some offset compared to truth. Calibrating for
relative positioning is easier than for absolute since any arbitrary position can be used.
Calibrating for absolute positioning mode requires that you perform this task with the
antenna at a known reference location.
3.4.4.3 Auto-Dif Performance
The positioning performance of the SXBlue II GPS unit is dependant upon the rate at
which the environmental modeling of Auto-Dif and the environmental errors diverge.