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Old 07-04-2015, 14:17   #1
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GPS- Minimum SOG for COG ?

Hi everybody

I have a OEM GPS (like this: FV-W9U ~ SAN JOSE TECHNOLOGY, INC.) connected to a laptop with OpenCPN, all running fine.

But sometimes, the GPS freezes the heading output, sending out the same value, when the boat is running at speeds below 2 knots.

Increasing the speed, the GPS start refreshing the COG again and the readings are good.

Before moving to other GPS, I'd like to know if this behavior is common to other GPSs, or if there is a known minimun SOG among devices for getting a reliable COG.

Regards, David
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Old 09-04-2015, 15:06   #2
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Re: GPS- Minimum SOG for COG ?

David,
I have a Garmin GPS 152 (only connected to the DSC VHF radio) and it displays a value for the COG even when the SOG is less than 0.1 knot. However, I don't know what is sent over the NMEA-0183 link.

Maybe your GPS receiver is set to send data to an AIS transceiver, which doesn't update the position often if the speed is low. I hope this helps.

Alain
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Old 09-04-2015, 16:44   #3
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Re: GPS- Minimum SOG for COG ?

Thank you very much Alain, I will look for AIS related settings (if any), or I'll move to a better GPS if the problem is not there.

Regards, David
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Old 09-04-2015, 17:02   #4
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Re: GPS- Minimum SOG for COG ?

One thing to check would be the DOP if it is available on that unit. Some units when the DOP is too high and speed is too low can't compute an accurate COG so they freeze at the last known good value.

If you can't see the DOP then take it outside for a bit where it has a good view of the sky with no obstructions. If the COG starts working at lower speeds that's probably because the DOP got a lot better due to the stronger GPS signals.
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Old 09-04-2015, 18:40   #5
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Re: GPS- Minimum SOG for COG ?

Maybe you can update the software (inside the unit) or maybe you can get a newer PC (if you are on a PC) driver.

Have you tried?

Is
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Old 09-04-2015, 18:47   #6
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Re: GPS- Minimum SOG for COG ?

Sailing in increasing current?
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Old 09-04-2015, 19:44   #7
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Re: GPS- Minimum SOG for COG ?

Quote:
Some units when the DOP is too high...
I can see right now (at home) the GPS reports a PDOP value 0.9 indoors, I'll check this value on the boat next time, and see what happens when freezes, thanks for the tip.
Quote:
Maybe you can update the software
It's an OEM module, for now I don't see firmware update capabilities, I'll search info about this topic, may be there is something there.
Quote:
Sailing in increasing current?
Yes indeed, sailing in the Paraná River 2-3 knots against

Thank you all for your advices
David
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