SX BLUE GPS iSXblue II+ GNSSTM, User Manual

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SXblue Series User Manual 49
2.2.1 Automatic Tracking and Ranging
The SBAS demodulator featured within the SXblue receiver will automatically scan
and track the SBAS satellite signals. At time of print, SBAS signals available are
WAAS (USA and covering Central and North America), EGNOS (Europe), MSAS
(Japan) and GAGAN (India). This automatic tracking allows the user to focus on
other aspects of their application rather than ensuring the receiver is tracking
SBAS correctly.
The SBAS demodulator features three-channel tracking that provides an
enhanced ability to maintain acquisition on a SBAS satellite in regions where more
than one satellite is in view. This redundant tracking approach will result in more
consistent acquisition of a signal when in an area where signal blockage of either
satellite is possible.
Furthermore, the SXblue receiver is set by default with the SBAS ranging feature
activated. This feature uses the SBAS satellites in the position computation. In
other words, the geosynchronous SBAS satellites, besides providing with the
differential correction and when broadcasting ranging information, are also used
as “extra” GPS satellites. As an example, WAAS satellites provide with ranging
information whereas EGNOS’ do not.
2.2.2 SBAS Performance
The performance of the SBAS receiver is described in terms of a differential LED.
SBAS requires a line of sight to the SBAS satellites in order to acquire the signal.
The BER number indicates the number of unsuccessfully decoded symbols in a
moving window of 2048 symbols. Due to the use of forward error correction
algorithms, one symbol is composed of two bits. The BER value for both SBAS
receiver channels is available in the RD1 NMEA data message described in detail
in chapter 4.
A lower BER indicates that data is being successfully decoded with fewer errors,
providing more consistent throughput. The bit error rate has a default, no-lock
value of 500 or more. As the receiver begins to successfully acquire the signal, it
will result in a lower bit error rate. For best operation, this value should be less than
150 and ideally less than 20.
Space-Based Augmentation Systems broadcast an ionospheric map on a periodic
basis that may take up to 5 minutes to receive upon start-up. The SXblue receiver
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.
The SureTrack feature is also dependent on the SBAS ionospheric map and the
GLONASS satellites will not be included in the solution before the SBAS map is
downloaded.