For a comprehensive explanation of the NMEA 0183 protocol please read
NMEA Revealed by Eric S. Raymond
Please note the 2008 Obsolete Nmea Sentences list in the Eric Raymond document.
Other References
Note: OpenCPN does not use or recognize NMEA 2000
If HDG message also contains the variation and E/W flags then O will use that. If not then it will look in RMC message for a variation. If nothing there it will use WMM plugin unless user has selected the variation manually in options.
To be sent by a navigation receiver when a destination waypoint is active.
''
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1 2 3 4 5 6 7 8 9 10 11 12 13| 15
| | | | | | | | | | | | | | |
$--RMB,A,x.x,a,c--c,c--c,llll.ll,a,yyyyy.yy,a,x.x,x.x,x.x,A,m,*hh<CR><LF>''
Field Number:
Note: Some autopilots, Robertson in particular, misinterpret “bearing from origin to destination” as “bearing from present position to destination”. This is likely due to the difference between the APB sentence and the APA sentence. for the APA sentence this would be the correct thing to do for the data in the same field. APA only differs from APB in this one field and APA leaves off the last two fields where this distinction is clearly spelled out. This will result in poor performance if the boat is sufficiently off-course that the two bearings are different.
Field Number:
For more details on how OpenCPN interprets all recognized sentences, look at the “AIS_DecoderA.cpp” file in the source code. For the Dashboard plugin, look at the different files in the “nmea0183” directory.
This message is obsolete and NMEA recommends to use XDR transducer messages instead.
However, old instruments might still sending this message.
OpenCPN reads the second value “Barometric pressure, bars”, but only if the number provided is between 0.8 and 1.1
All other values are ignored.
''
$--MDA,x.x,I,**x.x,B**,x.x,C,x.x,C,x.x,x.x,x.x,C,x.x,T,x.x,M,x.x,N,x.x,M*hh<CR><LF> \\
| | | | | | | | | | | \\
| | | | | | | | | | Wind speed, meters/second \\
| | | | | | | | | Wind speed, knots \\
| | | | | | | | Wind direction, degrees Magnetic \\
| | | | | | | Wind direction, degrees True \\
| | | | | | Dew point, degrees C \\
| | | | | Absolute humidity, percent \\
| | | | Relative humidity, percent \\
| | | Water temperature, degrees C \\
| | Air temperature, degrees C \\
| **//Barometric pressure, bars// ** \\
Barometric pressure, inches of mercury''
1 2 3 4 n \\
| | | | | \\
* $--XDR,a,x.x,a,c--c, ..... *hh<CR><LF> \\
Measured Value | Transducer Type | Measured Data | Unit of measure | Transducer Name ------------------------------------------------------------------------------------------------------ barometric | "P" pressure | 0.8..1.1 or 800..1100 | "B" bar | "Barometer" air temperature| "C" temperature | 2 decimals | "C" celsius | "TempAir" or "ENV_OUTAIR_T" pitch | "A" angle |-180..0 nose down 0..180 nose up | "D" degrees | "PTCH" rolling | "A" angle |-180..0 L 0..180 R | "D" degrees | "ROLL" water temp | "C" temperature | 2 decimals | "C" celsius | "ENV_WATER_T" -----------------------------------------------------------------------------------------------------
See details for AIVDO above in list.
NMEA version 2.3.added a mode indicator to many sentences to indicate what
kind of fix the receiver has. The value can be one of
A=autonomous
D=differential
E=Estimated
N=not valid,
S=Simulator.
Sometimes there can even be a null value as well.
The A and D are the only vales that will indicate an Active and reliable Sentence.
This mode character has been added to the end of RMC, RMB, VTG, and GLL sentences.
Optionally, to some others as well, including the BWC and XTE sentences.