NMEA 0183 question

Discussion in 'Boats, Motors, Trailers and Towing Rigs Forum' started by sixohfour, Sep 20, 2020.

  1. sixohfour

    sixohfour Member

    I recently took the time to replace a non functioning vhf on the bridge and to add a second chartplotter station. While I was at it, I decided to crest a NMEA o183 network so that position data shows up on the vhf screen and DSC data shows up on chartplotter screen.

    After a fair bit of reading, I thought I had it figured out. Ordered the parts I needed and went at it. Happily, on the first try, I had position data showing on the vhf screen. Unfortunately, the position data on the vhf differs from the chartplotter position data by 60s N and 92s W. Although this is likely better info than I could communicate in an emergency situation, I find it curious that it is different. Has anyone experienced this? Do I have one of the wired crossed or something? Would love to fix it if possible.

    thanks.
     
  2. sixohfour

    sixohfour Member

  3. RGS

    RGS New Member

    First you do have it wired correctly or you would not get any data at all on the screen.

    On the offset you mentioned it is not 60 seconds but .060 minutes. Not sure why the offset but maybe it could be due to datum offset. The datum offset is used to correct for position offset on charts https://en.wikipedia.org/wiki/Geodetic_datum. I think that the position sent out of the GPS via 0183 is the raw data from the satellites and what is displayed on the chart plotter has the datum offset that is selected applied but not sure.
     
    ericl likes this.
  4. sixohfour

    sixohfour Member


    Thanks RGS for pointing out that those numbers are decimal minutes, not seconds. Good to know that the wiring is correct, I had assumed that only one configuration would result in data showing in vhf, so good to have that confirmed. I will look into the datum offset you mentioned, that seems to make some sense as well, though it is not very interesting reading!

    Cheers.
     
  5. RGS

    RGS New Member

    Yes, reading about datum's would be a good sleep aid.:) I would not worry about it. If you change the datum on the chartplotter your chart would be off. the difference is only a few hundred feet so would put any rescue effort in close to where you are.
     
  6. Chasin' Dreams

    Chasin' Dreams Well-Known Member

    Do you have your chart plotter set to show the GPS coordinates of the cursor and not the vessel? If so the VHF may be showing actual GPS antenae location while your plotter may be showing the cursor gps location. Try putting your cursor right on your vessel and see if it then is closer to the GPS coordinates the VHF is showing.
    I have mine like that cause I like to have my plotter set on my cursor gps instead of vessel.
     
  7. noluck

    noluck Well-Known Member

    there may have a deviation affecting one of them if you are not using a heading senor that is common to both
     
  8. sixohfour

    sixohfour Member

    Good question. I will read up on this tonight and check tomorrow when I am at the boat.

    Cheers.
     
  9. sixohfour

    sixohfour Member

    The only location data going to the vhf is from the chartplotter, so aren’t they both using the heading sensor (gos?) from the chartplotter?
     
  10. Sir Reel

    Sir Reel Well-Known Member

    What model vhf and what model chartplotter?
     
  11. sixohfour

    sixohfour Member

    Icom M-504 and Lowrance HDS 9 carbon.
     
  12. RGS

    RGS New Member

    It looks like you where tied up but one other thing on NEMA0183 is that the data is only transmitted every 2 sec. I am not sure how often the chart plotter is updated with the internal GPS.
     
  13. Sir Reel

    Sir Reel Well-Known Member

    Your baud rate should be highest it can be which is 38,400. Default is 4800 and as RGS said it may just be the refresh delay causing a difference in your position.
     
    sixohfour likes this.
  14. sixohfour

    sixohfour Member

    Interesting, I will look at the GPS refresh rates. Yes, both those pics were taken while tied up. I have not had the chance to take the boat out on the water with the new set up yet.

    Cheers.
     

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