use of $GP sentences to position boat on chart

[getawayarb]

Hello

Technical context:
Using OpenCpn 5.6 under LinuxMint cinnamon 21.
One VHF/AIS RO4800 receiving and transferring GP31 Furuno NMEA data and connected to linux ttyUSB0 port
One USB GPS connected to Linux ttyUSB1 port.

When only VHF is connected to the computer, NMEA window within OpenCpn shows entry of sentences from ttyUSB0.
- Sentences !AIVDM and AIS target are displayed on the map.
- Sentences $GP (GLL and RMC) issued by GP31 via VHF/AIS but OpenCpn doesn't use them to display the boat on the map (OpenCpn windows appears as if no GPS was connected).

When USB GPS is connected too, one can see entry of sentences on ttyUSB1 ($GP RMC,VTG,GGA and GSV) on the NMEA window and the boat appears red on the map.

My question is: which NMEA sentences are used by OpenCpn to position the boat and why doesn't it use the $GPGLL and $GPRMC issued by GP31 which it receives via VHF/AIS in the first config?

In fact I'd like the whole thing to work with only one connection to the computer.

[nohal]

OpenCPN does use GLL na RMC, so your problem likely is either wrong/lacking checksum in them while you have the connection set to check it or your filtering settings. Hard to tell without a sample of your incoming data and without seeing your actual settings.

[getawayarb]

Hi nohal


Thank you for your answer.


I have no filtering settings at all.


Attached, 2 NMEA window screenshots
- first one with VHF/AIS connected to ttyUSB0 along with USB GPS connected to ttyUSB1
-second one with only VHF/AIS connected to ttyUSB0


both screenshots showing $GP sentences input

[nohal]

Your second screenshot shows that the RMC source was switched to ttyUSB0. Then no more NMEA sentence providing position is received.
Unfortunately your screenshot shows only 3 seconds, so we still don't know at all what is happening, but the GPS receivers usually provide the position at at least 1 Hz (= once per second).
What is even more interesting is that there was no RMC or FWIW any other GPS related sentence for 2 whole minutes before that.
If your VHF provides position only once every few minutes, that certainly won't work as you expect.

[getawayarb]

I think you are right and something goes wrong at home...
I give up for today...
May be more to morrow

[sv_pelagia]

Doesn't the Furuno GP31 have a non-fixable date rollover problem?

[getawayarb]

Hi Pelagia


Yes I've heard something about problem of time with GP31, it's part of what could be going wrong here.
Thank you for interest.

[HeywoodJ]

Originally Posted by sv_pelagia

Doesn't the Furuno GP31 have a non-fixable date rollover problem?

Bingo. In the single RMC that is visible from USB0 the date field (near the end of the sentence) is “040903” = September 4 2003. So, OpenCPN does know where your boat was 20 years ago (well, 1024 weeks ago).

If you really want to use that GPS you might be able to massage the sentence with the NMEA plugin.