ComNav Vector G2 & G2B Installation & Operation Appendices
Document PN 29010078 V2.0 - 87 -
DRAFT #3 – 29 May 2009
8) Here is a typical power-on sequence from Port B (captured simultaneously with the
Port A sequence above):
• the first second is all Null data:
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPVTG,,T,,M,,N,,K,N*2C
• after ~35 seconds more, the VTG sentence begins to have valid speed & COG data:
$GPHDM,,M*1B
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPVTG,29.03,T,10.95,M,0.27,N,0.50,K,D*23
• and after a few seconds more, the RTK-based heading data begins to appear, in the
HDM & HDT sentences:
$GPHDT,,T*1B
$GPHDM,,M*1B
$GPHDT,263.44,T*02
$GPHDM,245.37,M*02
Note: as you can see in the example outputs above, the ”position”,
“heading” & other output data is initially Null values – i.e., the
respective fields in the NMEA sentences are empty.
Yet the sentences still have proper structure and format, including
correct checksums.
Some NMEA equipment may accept these sentences as “valid”, and
not generate any ”invalid data”, “lost heading” or similar alarms or
warnings.
Other equipment may react to them with such alarms &/or warnings!!
L CAUTION! Although the examples above were captured during a
power-on sequence, the same “Null heading” (empty fields) behaviour
occurs whenever the G2 encounters a GPS signal blockage; for
example, when passing under a bridge, or if the vessel is in a narrow
channel with high sides that block most of the sky view, or similar).
The Null heading condition can also occur, randomly, when there are
strong multi-path reflections (see page 41).
Note that if the Gyro sens
or is ena
bled in such circumstances, the
Null heading condition will not occur for 3 minutes after loss-of-signal
(see page 44).
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