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Old 22-10-2017, 10:44   #1
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NMEA sentence basics

Hi all

Im playing around with open cpn
Generating nmea sentences to play around with wind position, boat headings and things of that nature
Ive read the nmea2000 white paper on sentences but there are so many

I was just wondering could someone rattle me off a few key sentences to get me up and running

I have the boat spinning through the HDT sentence

Im looking for a sentences that moves the boat in a given direction at a particular speed??
thanks
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Old 22-10-2017, 11:17   #2
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Re: NMEA sentence basics

To start with, there is no NMEA sentence that moves the boat, the boat is moved by engine sails, current or whatever else. The NMEA sentences you receive from various sensors contain information about the boat movement and other stuff.
There are two NMEA standards in use today, the older, more understandable and well documented NMEA 0183, which is what OpenCPN uses and you obviously talk about. And NMEA2000, the proprietary standard available only to the members of the NMEA consortium, irrelevant for us.

Let's assume you are interested in data from a GPS sensor, then from the variety of NMEA sentences that exist and cover this kind of info, you are probably interested in RMC sentence, but the same information can be established also from GGA or GLL, for example.

Pretty much all of the NEMA 0183 sentences and their meaning are described in NMEA Revealed

Pavel
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Old 22-10-2017, 16:21   #3
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Re: NMEA sentence basics

Quote:
Originally Posted by nohal View Post
To start with, there is no NMEA sentence that moves the boat, the boat is moved by engine sails, current or whatever else. The NMEA sentences you receive from various sensors contain information about the boat movement and other stuff.
I would have thought it fairly obvious that the OP was talking about moving the boat icon in OpenCPN, not his actual vessel.
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Old 23-10-2017, 11:36   #4
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Re: NMEA sentence basics

On the internet you will find files containing NMEA-0183 data of actual boats under sailing conditions. A member here on CF provides a nice file on his web site: http://navmonpc.com/downloads.html

NavMonPC can feed NMEA data into OpenCPN. It can also do lots of other things.
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Old 23-10-2017, 17:44   #5
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Re: NMEA sentence basics

Quote:
Originally Posted by new_to_cuiser View Post
Hi all



Im playing around with open cpn

Generating nmea sentences to play around with wind position, boat headings and things of that nature

Ive read the nmea2000 white paper on sentences but there are so many



I was just wondering could someone rattle me off a few key sentences to get me up and running



I have the boat spinning through the HDT sentence



Im looking for a sentences that moves the boat in a given direction at a particular speed??

thanks


I gather you are trying to manually feed nema sentences to simulate using a boat in the Nav software.

Sending sentences about heading and boat speed or speed over ground won’t “move” the boat on the chart. It will display the numbers on and “instruments” page but the boat will remain at whatever lat/long it last received. Only way to “move” the boat is to change the lat/long.
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Old 27-10-2017, 07:06   #6
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Re: NMEA sentence basics

OpenCPN gets information about the boat's position from NMEA sentences sent by the GPS receiver updating latitude and longitude. Feeding a series of position sentences to OpenCPN will make the boat's charted position appear to move.

Fabbian
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Old 27-10-2017, 07:41   #7
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Re: NMEA sentence basics

You can generate a series of position nmea sentences in a spreadsheet and save them in a file. This file played back will make the boat move in the plotter.

You must use a plotter that allows you to disable checksums. You cannot generate correct checksums from an xls generator unless you are quite skilled in programmin VB and willing to spend heaps of time writing the macro.

There is a simple alternative though: go sailing, dump your gps data to file. Then use this file to play it back on a plotter. This way you get a file with correct checksums on each line.

Etc.

Cheers,
b.
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Old 27-10-2017, 07:59   #8
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Re: NMEA sentence basics

Quote:
Originally Posted by barnakiel View Post
You can generate a series of position nmea sentences in a spreadsheet and save them in a file. This file played back will make the boat move in the plotter.

You must use a plotter that allows you to disable checksums. You cannot generate correct checksums from an xls generator unless you are quite skilled in programmin VB and willing to spend heaps of time writing the macro.
Maybe not a bad idea, use a spreadsheet to create random incremental positions.

Node red will create the checksum for you and read the file with a limit on how fast.

Quote:
var nmea = msg.payload;
var checksum = 0;

for(var i = 0; i < nmea.length; i++) {
checksum = checksum ^ nmea.charCodeAt(i);
}
checksum = checksum.toString(16); //convert to hex
nmea = '$' + nmea + '*' + checksum; //make the full nmea sentence again
msg.payload = nmea+"\r\n";
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Old 27-10-2017, 08:47   #9
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Re: NMEA sentence basics

Quote:
Originally Posted by new_to_cuiser View Post

(...)
Im looking for a sentences that moves the boat in a given direction at a particular speed??
thanks
Yep. As others said. A series of RMC or alt.

Standard NMEA-0183 sentences description

RMC file dump is nice as it contains speed and heading in one file to play with.

It is easy to write a simple RMC file generator in Android that will generate a file for any start stop point speed and heading.

Just that doing a dump while walking the dock with your smartphone in your pocket is twice as simple and effective. Many apps do the dump.

Cheers,
b.
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Old 29-10-2017, 13:07   #10
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Re: NMEA sentence basics

Look at these posts:
Feature Requests

Celestial Navigation Plugin Redux
Copy and rename CN-bos-boat-pos.txt.doc to CN-bos-boat-pos.txt and run with VDR_pi.

You may want to change the lat-long in the file with a text editor.
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