Product specifications

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The SBAS demodulator features two-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.
SBAS P ERFORMANCE
The performance of the SBAS receiver is described in terms of a bit error rate
(BER). 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 6.
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 startup. The
Crescent Vector 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 Crescent, we may wish to wait up to five minutes before using the Crescent
or issue the $JQUERY,GUIDE<CR><LF> message to ‘ask’ the Crescent if it
feels performance will be sufficient for operation.
COAST™ T ECHNOLOGY
The Crescent module incorporates Hemisphere GPS COAST technology that allows it
to operate with old correction data for up to 30 to 40 minutes or more without
significant accuracy degradation. The feature’s performance is attributed to
sophisticated algorithms that are able to anticipate how errors change during a
period of correction loss.
Traditional receiver technology would experience an increasing degradation with
increasing age of corrections, resulting in less than adequate performance over a