SX BLUE GPS iSXblue II+ GNSSTM, User Manual

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SXblue Series User Manual 78
This operating mode has been added to the SXblue II-B GPS in order to be
compliant with the specification IEC 61108-4 for ship borne DGPS maritime radio
beacon receiver equipment. The basic operation is outlined below.
1. The receiver will determine the 10 closest stations after computing a valid
position. This list can be accessed using the command $PCSI,3,2.
2. The primary channel tries to tune to the closest available station, using the
frequency and bit rate specified in the station database.
3. The background channel tunes to the frequency of the closest station using
an alternate bit rate.
4. The primary channel retunes to the alternate bit rate if lock is achieved on
the background channel (with acceptable station health and WER).
5. The background channel continually searches for a closer station using the
station database once a lock is achieved on the primary channel.
6. The primary channel will remain tuned to the same station unless one of the
following occurs:
•
Word error rate (WER) drops below 10%
•
Station becomes unhealthy or unmonitored
•
Background channel finds a closer station
•
Position changes enough to affect station list
•
Performance in Database Mode
In most installations, Database mode will result in faster initial acquisition and
better GPS accuracy compared to Auto Mode. In some installations, Database
Mode may not work as well as Auto Mode for the following reasons:
1. The closest station is not in the station database and the SXblue II-B GPS
has not yet received a Type7 Almanac message. Most stations now
broadcast the Almanac message every 10 minutes. Assuming the SXblue
II-B GPS can tune to a surrounding station and receive a Type7 message,
it will update the station database and automatically retune to the closest
station.
2. Signal quality in the area is poor. IEC61108-4 requires the receiver to switch
away from a station when WER rises above 10%. For installations that don’t
need to comply with IEC61108-4, this threshold can be increased as usable
corrections can be obtained for word error rates up to 50%.
3.3.4 Evaluating Performance
The SXblue II-B GPS calculates a Signal to Noise Ratio (SNR), measured in dB
(Decibels), which indicates the receiver’s performance. The SNR is height of the
signal above the noise floor. The higher the SNR, the better the internal beacon
receiver is demodulating the signal. By monitoring the SNR, you can determine the