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Old 08-06-2013, 20:55   #1
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Problem? CPN or GPS

The attached PDF illustrates an interesting situation. As you can see from the timing of the screen shot, bottom right, the incident had taken place some 30 minutes earlier. The track clearly shows that shortly after an accurate fix, GPS plus a couple of bearings on land silhouettes, the ship, steering on autopilot, took off on a 60 degree lurch to port followed by a starboard turn of about 120 degrees before coming back on the original course.

It didn't happen!

The weather was fine, the wind too light to sail and we were doing around 6 knots through the water with about 1 knots of fair tide pushing us along. We had about 30 metres under the keel and the bottom is smooth. I caught site of the plot about halfway through the turn to port, and raced about the ship, checking the autopilot compass, the magnetic steering compass and searching for any possible interference. Autopilot and steering compass both confirmed 'no change of course' and I could find no obvious source of electronic interference. I also closed down CPN, switched off the laptop and rebooted. The plot restarted seamlessly and continued as shown.

As I said, the conditions were perfect but what if this had happened in dirty weather with poor visibility in restricted conditions?

Can anyone throw any light on this?
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Old 08-06-2013, 21:58   #2
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Re: PROBLEM? CPN OR GPS

No attachment...

Dave
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Old 08-06-2013, 23:24   #3
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Re: PROBLEM? CPN OR GPS

Third try - seems to be here now but have abad connection
Attached Files
File Type: pdf GPS or CPN.pdf (52.4 KB, 88 views)
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Old 09-06-2013, 00:33   #4
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Re: PROBLEM? CPN OR GPS

Occasionally my track will show a spike, usually to a 0,0 pos off africa. Sometimes it only goes only a short distance. It happens so infrequently I never worried about it, only surmised a hiccup in a data connection. I never had a nice curve like you have though.
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Old 09-06-2013, 02:20   #5
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Re: PROBLEM? CPN OR GPS

galleyslave

A scenario that is consistent with your story and the track, is that your boat passed through the center of a localized, current or tidal stream, eddy.
I don't know if this is what happened and I'm not familiar with the coast of Malaysia. Just a possibility.....

Thomas
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Old 09-06-2013, 04:40   #6
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Re: PROBLEM? CPN OR GPS

That thought did occur to me but the bottom is pretty smooth at that point with no marked variations in depth and nothing suggesting a possible physical cause. Also, the way in which the track came back to a continuation of the course before the occurence exactly would seem to point to an electronic source rather than a physical one
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Old 09-06-2013, 04:54   #7
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pirate Re: PROBLEM? CPN OR GPS

Maybe it was just the 'Masters' playing with the satellites.... again
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Old 09-06-2013, 05:08   #8
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Re: PROBLEM? CPN OR GPS

How was your autopilot steering ? e.g. heading mode, track to waypoint, wind angle ?

Was your autopilot actually using GPS signal to steer, or if in heading mode just the compass ?

Over what time period did this event take place, e.g. how many minutes/seconds did this deviation occur ?

And did you consider magnetic anomalies - are they known or charted in this area ? Did you pass anything that might affect your compass ?
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Old 09-06-2013, 18:26   #9
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Re: PROBLEM? CPN OR GPS

Autopilot in compass mode - not connected to a GPS
Total time scale was around 10 minutes
There was no actual change of course - the whole incident took place on the laptop screen. Therefore, it would appear to be either caused by the GPS feeding the position data or the programme itself. Unfortunately, in my scurry to try and locate possible causes, I didn't think to look at the incoming data stream until too late.
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Old 09-06-2013, 18:45   #10
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Re: Problem? CPN or GPS

galleyslave....

From the look of the track, there was not just one isolated invalid GPS position report, but a whole series, creating a nicely curved track line.

OpenCPN will not artificially generate such a curved trackline to a single or multiple outlying point. A single bad point will show as a jump out and jump back, with sharp (often impossible) course changes.

So, I am left with the supposition that the GPS receiver fed these points to OpenCPN as valid points, and O plotted diligently.

Sure wish we had the VDR recording...

Dave
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Old 09-06-2013, 19:44   #11
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Re: Problem? CPN or GPS

It is most likely that this single event was caused by a meteor ionizing the Earth's atmosphere in the signal path between one or more GPS satellites. The strong daytime Arietids meteor shower peaked on June 7. GPS signals passing through a heavily ionized patch of atmosphere will be slightly delayed. This type of delay can cause an error in the position calculated by the GPS receiver.

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