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ComNav Vector G2B Manual

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ComNav Vector G2/G2B Installation & Operation Operation
Document PN 29010078 V2.0 - 61 -
DRAFT #3 – 29 May 2009
DGPS Source
The factory-default DGPS correction source in the Vector G2 is NONE; the default source
in the G2B is BEACON. This may be changed using the $JDIFF command (see Table 15
on page 55).
To select SBAS signals for the DGPS data, use a $JDIFF,WAAS comm
and.
However, this should only be done in areas where the visible SBAS satellites are sending
data that is valid for the area where the G2 or G2B is currently located.
• For example, the “WAAS” correction data is provided by the FAA in the U.S., so it
is thus only officially valid for the continental U.S., Alaska, Hawaii & Puerto Rico.
Unofficially, it is valid for much of southern Canada (in a strip along the 49
th
Parallel), and northern Mexico.
See the GPS Technical Reference manual for a more detailed description of the
capabilities and limitations of the SBAS technology.
Note: although the parameter in the $JDIFF command which enables
the G2 to use SBAS DGPS corrections is the word “WAAS”, that word
in fact tells the G2 to use whatever valid source of SBAS signals is
visible in the geographic location the G2 is currently at. Thus, in
Europe, “WAAS” means “use EGNOS” – and similarly in other regions
which may have SBAS service.
The Beacon receiver in the G2B is always running, no matter which DGPS mode is in
use. The receiver is by default running in full-automatic tuning mode; it may be changed
to manual tuning, using the GPMSK command (see the GPS Technical Reference).
If neither SBAS nor Beacon corrections are available, an external source of RTC SC-104
differential corrections may be selected ($JDIFF,OTHER).
Baud Rates & Data Bandwidth
If you are going to change the enabled sentences and/or update rates that the G2 is
outputting (on one or both ports), you may need to change the Baud rate, with the
JBAUD command
20
.
The factory-default Baud rate for both Ports A & B in the G2 is 4800 Baud; this complies
with the NMEA 0183 Standard. The G2’s serial data is always formatted 8-N-1, which
means that there are 10 bits sent per byte, and so the factory-default transmit/receive
data bandwidth is 480 bytes/second, on each port.
In any serial communications system, the total number of bytes sent per second must not
exceed the data bandwidth. A good rule of thumb when setting up serial communications
systems is that the total bytes/sec should be less than ~90% of the bandwidth:
• The NMEA & proprietary sentences enabled in the factory defaults fit within the
data bandwidth of the factory-default 4800 Baud: 417.4 bytes/sec on Port A, and
445 bytes/sec on Port B.
If the Baud rate is too low – i.e., bandwidth less than the total bytes/sec that the G2 is
configured to transmit – the data will be corrupted, in random & unpredictable ways. The
result will typically be “Invalid Data” errors on the equipment that is connected to the G2.
On the next page is a screen shot of a “bandwidth estimation” calculation (a simple
MS Excel® worksheet) for the G2. In your own calculations, you should use the lengths
of the data in the example sentences of the screen shot, to check that any changes you
wish to make do not exceed the bandwidth at the Baud rate(s) the G2 is configured to. If
they do, you will have to change the G2’s Baud rate accordingly, or else set up a
different, lower-bandwidth, combination of enabled sentences & update rates.
20
If you change Port A’s rate using HyperTerminal or PocketMAX, you will have to change their Baud rates too, right
after the G2’s rate is changed. The Navigator G2 automatically detects the G2’s Baud rate, in contrast.
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… 63 …


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