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Old 05-04-2014, 05:32   #1
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nmea WPL for APRS , howto show MMSI

Hello,
I am implementing a man over board message in opencpn. Using !GPWPL to show the position.
This works fine, the map shows the position and the text.
But also a MMSI number is shown. Where does that number come from? There must be a certain message to set this?
As wpl only has position and a text field, this puzzles me!

Best regards,
Chris
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Old 05-04-2014, 14:35   #2
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Re: nmea WPL for APRS , howto show MMSI

Chris...
Show us the NMEA feed recorded with the VDR plugin. There of course is no way MMSI could be known if it is not there. My guess would be you have an AIS target at the same position, but it really is just a guess.
BTW, does APRS work fine? I have never heard from anybody testing it in real world since it was implemented.

Pavel
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Old 06-04-2014, 03:38   #3
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Re: nmea WPL for APRS , howto show MMSI

The APRS works fine.
the setup is here on my desk, with some instruments connected. like a gps32 in simulation mode, a gps repeater display, nasa wind, and also some seatalk equipment.
Wifi connection to my laptop with opencpn 3.3.1419 running on there.
All is connected through my own design wifi multiplexer. Ive designed the hardware an d software for that myself.
there is a switch input. That can be selected to be used as a man overboard switch. The multiplexer uses the position in the RMC sentence ( or the position message form seatalk) and sends a !GPWPL message to the laptop.
I dont send other ais messages. The mmsi number that appears is 199397788.
Must be a sort of deault number. Which is fine by me.
but how can a send a valid mmsi? All zeros would me more clear. This must be a different message. And how is it clear that this number is locked to this wpl message?

As opencpn shows it, there must be a way...
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Old 06-04-2014, 03:50   #4
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Re: nmea WPL for APRS , howto show MMSI

Chris...
There are "ways" to do anything. The question is what it is you exactly want to do.
The APRS logic is implemented as an extension to the AIS logic (as is ARPA), that's why it internally uses private, non-existing MMSIs (look at AIS_Decoder.cpp:677)

Pavel


Quote:
Originally Posted by roozeboos View Post
The APRS works fine.
the setup is here on my desk, with some instruments connected. like a gps32 in simulation mode, a gps repeater display, nasa wind, and also some seatalk equipment.
Wifi connection to my laptop with opencpn 3.3.1419 running on there.
All is connected through my own design wifi multiplexer. Ive designed the hardware an d software for that myself.
there is a switch input. That can be selected to be used as a man overboard switch. The multiplexer uses the position in the RMC sentence ( or the position message form seatalk) and sends a !GPWPL message to the laptop.
I dont send other ais messages. The mmsi number that appears is 199397788.
Must be a sort of deault number. Which is fine by me.
but how can a send a valid mmsi? All zeros would me more clear. This must be a different message. And how is it clear that this number is locked to this wpl message?

As opencpn shows it, there must be a way...
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Old 06-04-2014, 03:56   #5
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Re: nmea WPL for APRS , howto show MMSI

Ah Thanks Nohal, there is indeed!
but would it be possible to change the number to anohter number? If it is only internal, there is no reason to show it in my opinion.

It is not really so important, it's just nice to know where it comes from.

At the moment I show the text "MAN OVERBOARD", which is clear enough.
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Old 06-04-2014, 04:44   #6
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Re: nmea WPL for APRS , howto show MMSI

Well, as you see from the code, the part after "1993" is a hash of the text received in the WPL sentence, so to change it you either have to change the algorithm or the name you send.
As to not show the MMSI at all, it is of course doable. To be honest I never really tested this code since I wrote it and generated a bit of artificial data to see if it works, so it probably can be polished in quite a few ways.
You can open a feature request for the improvements in OpenCPN::Tracker All Projects: Tasklist

Pavel
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