SXBlue GPS Reference Manual
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The SXBlue GPS currently ships with a SBAS (WAAS) application and the e-Dif application.
Chapter 8 describes the $JAPP command used to change between the two SXBlue GPS
applications. Within the WAAS application, there are two differential modes, being WAAS
and external correction source. Chapter 8 describes the $JDIFF command used to change
between the two differential modes.
4.5 e-Dif Operation
Operation of the SXBlue GPS unit with the optional e-Dif application requires manual
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 8 provides detailed information on the commands supported by the e-Dif feature.
4.5.1 Start-up
When you turn the SXBlue GPS on with the e-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 GPS stationary for this process, but you should ensure that the
SXBlue GPS maintains acquisition on the satellites available. We refer to this process of
gathering information for the e-Dif application as calibration.
4.5.2 Calibration
Calibration is the process of zeroing the increasing errors in the e-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.
4.5.3 e-Dif Performance
The positioning performance of the SXBlue GPS unit is dependant upon the rate at which
the environmental modeling of e-Dif and the environmental errors diverge. The more that e-
Dif is able to model the errors correctly, the longer that e-Dif will provide reliable, accurate
positioning. As there’s no way in real-time to know the rate of divergence, a rule of thumb is
to set your maximum age of differential to either 30 or 45 minutes, depending on how much
error your application is able to tolerate. Testing, a sample of which is shown in Figure 1-4
has shown that accuracy will often be better than 1.0 m 95% after 30 minutes of e-Dif
operation.
We suggest that you perform your own testing at your location to determine the level of
performance that you would expect to see on average. When testing this feature, it’s a good
idea to look at a number of e-Dif cycles per day, and monitor performance against a known
coordinate. This should be done over a number of days with different states of the
ionosphere.
You can monitor the energy level of the ionosphere based upon the amount of solar flare