SXBlue II GPS Series Technical Reference Manual
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Space-Based Augmentation Systems broadcast an ionospheric map on a periodic basis that
may take up to 5 minutes to receive upon startup. The SXBlue II GPS uses the GPS broadcast
ionospheric model until it has downloaded the SBAS map, which can result in lower
performance as compared to when the map has been downloaded. This will be the case for
any GPS product supporting SBAS services.
Caution – When the map has been downloaded, you may observe a position jump due to
the potential difference between the GPS ionospheric model and the ionospheric SBAS
map. To minimize the impact of this issue on your use of the SXBlue II GPS, you may
wish to wait up to five minutes before using the SXBlue II GPS or issue the
$JQUERY,GUIDE<CR><LF> message to ‘ask’ the SXBlue II GPS if it feels performance
will be sufficient for operation.
To improve upon the ionospheric map provided by SBAS, the receiver extrapolates information
from the broadcast ionospheric coverage map, extending its effective coverage. This allows the
receiver to be used successfully in regions that competitive products may not. Please note that
the process of estimating ionospheric corrections beyond the WAAS broadcast map would not
be as good as having an extended SBAS map in the first place. This difference may lead to
minor accuracy degradation.
Figure 2-1 shows sample grids for WAAS and EGNOS systems. The inner areas are actual
broadcast grids by the satellites and the outer areas are the extrapolation the SXBlue II GPS
achieves.
Figure 2-1 Sample Extrapolated Ionospheric Grids
2.3 DGPS RadioBeacon
Many marine authorities, such as Coast Guards, have installed networks of radiobeacons that
broadcast DGPS corrections to users of this system. With the increasing utility of these
networks for terrestrial applications, there is an increasing trend towards densification of these
networks inland.
