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Old 17-10-2013, 07:02   #1
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LifePart2's Avatar

Join Date: Sep 2010
Location: ex Canada - now on the boat
Boat: Leopard 42 catamaran
Posts: 162
NMEA / Chartplotter Diagnosis

I am getting some funny results on my chartplotter. I believe I am getting some dodgy input from the GPS, can anyone help me diagnose?

Here is the scoop. I have a Standard Horizon chartplotter with its own GPS attached. I have also output the NMEA sentences by bluetooth so I can receive them on OpenCPN.

When I run this setup, my chartplotter flashes between two times. As I write this, one shows approx 03:30 and the other 12:15. The latter is the correct local time for GMT +3. I have no idea where that 03:30 comes from. On OpenCPN the only time displayed on the dashboard clock is a steady 00:00:00.

On my PolarCom terminal I see the time 03:30 GMT +3

Here is the NMEA output for this period:

$GPRMC,002254,A,3614.490,N,02923.920,E,4.2,219.0,0 00000,0,W,A*16
$GPRMC,002256,A,3614.490,N,02923.910,E,4.0,216.0,0 00000,0,W,A*1A
$GPRMC,002258,A,3614.490,N,02923.910,E,4.0,216.0,0 00000,0,W,A*14

Now, if I turn on my AIS which has its own GPS antenna, and multiplexes its NMEA into the mix, then on my chartplotter, OpenCPN and PolarCom I get the correct time, viz 12:15

(all times are approximate as I am writing this over the space of a few minutes.)

The NMEA output I am now getting is here:

$GPRMC,092943.00,A,3613.44339,N,02923.07443,E,3.01 8,205.86,101013,,,D*6F
$GPRMC,092944.00,A,3613.44262,N,02923.07391,E,3.04 4,206.93,101013,,,D*61
$GPRMC,092945.00,A,3613.44183,N,02923.07343,E,3.09 0,204.81,101013,,,D*6B

Incidentally, I believe I have set the ShipModul to treat the AIS GPS as primary, and thus to suppress the chartplotter GPS.

The main difference I see between the two are the $GPGBS and $GPRMC . Both of these seem to show the correct time (092945 at the moment - as I said, a number of minutes since the previous capture). whereas in the other output there is no $GPGBS at all, and the $GPRMC says 002258, Now, here is where there is another discrepancy. While the $GPRMC is saying 002258, the chartplotters are all saying 03:30 - which doesn't seem to match up at all. Is there some other sentence in there that is providing spurious times?

So, can anyone shed any light on this?

Bottom line, does it look like my primary GPS is kaput?

Thanks in advance.
Noel Swanson

Life is too short to live in ugly places.
LifePart2 is offline   Reply With Quote

chartplotter, nmea

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