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Old 27-08-2016, 03:47   #1
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Change GPS receiver or not

Hi!

Mounting new VHF and AIS and soon new plotter with internal GPS receiver.

Bundled with the True Heading AIS was a GPS receiver that fits its input.

No GPS receiver bundled with the VHF.

Already got at least 3 GPS receivers on this boat. Old.

With the very old plotter we've noticed we have been 20 m or more in the wrong place.

Is this likely to be because of
A. Old GPS receivers
B. Old plotter
Or both?

Asking as we want to reuse one of the old GPS receivers for AIS and VHF (is splitting ok?) rather than having many many GPS units.

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https://drive.google.com/open?id=0By...TRDeVBHSTVQMVE

Cheers
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Old 27-08-2016, 04:16   #2
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Re: Change GPS receiver or not

Another possibility for the 20 m error. The chart data could be off. Depending on where in the world you are, the charts may be accurate as far as the depths, contours, compass headings, etc but be off in how the lat/lon on the chart is referenced to the lat/lon reference used in the GPS.
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Old 27-08-2016, 04:33   #3
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Re: Change GPS receiver or not

Ok thanks. Can I use the old GPS receivers or should I upgrade?

What about splitting one receiver to multiple units, problematic?

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Old 27-08-2016, 04:41   #4
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Re: Change GPS receiver or not

Quote:
Originally Posted by skipmac View Post
Another possibility for the 20 m error. The chart data could be off. Depending on where in the world you are, the charts may be accurate as far as the depths, contours, compass headings, etc but be off in how the lat/lon on the chart is referenced to the lat/lon reference used in the GPS.
Very true.

Is the discrepancy consistent in size and direction?

I've seen errors of hundreds of meters in some areas resulting from this.

Where are you and what charts does your chartplotter use?

And do you mean GPS receivers or GPS antennas? You can buy splitters to send the signal received from a single antenna to multiple devices ("receivers").
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Old 27-08-2016, 07:36   #5
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Re: Change GPS receiver or not

Read all white Papers. They say use bundled GPS receiver. Will follow advice.

There is a diagnostics program with this True Heading AIS. If I get all green buttons and 3D fix = ok with the GPS receiving unit inside the boat. Is it ok to mount it inside?

Don't want to drill any holes...

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Old 27-08-2016, 09:25   #6
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Re: Change GPS receiver or not

There are advantages to redundancy. Having multiple GPS receivers, old or new, give you this. Have you compared the Lat/Long readings for each instrument to confirm they're all giving you the same data? If they are, it is likely a charting error, as pointed out by Skipmac.
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Old 27-08-2016, 10:00   #7
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Re: Change GPS receiver or not

Quote:
Originally Posted by skipmac View Post
The chart data could be off. Depending on where in the world you are, the charts may be accurate as far as the depths, contours, compass headings, etc but be off in how the lat/lon on the chart is referenced to the lat/lon reference used in the GPS.
Lord knows you don't have to go that far. Here in the Tampa Bay area my Humminbird, with the latest Florida/Bahamas chip, sometimes shows my little fishing boat plowing a furrow through someone's yard!
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Old 27-08-2016, 10:03   #8
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Re: Change GPS receiver or not

See if your charts are based on WGS84, the first world wide GPS survey, and that your receivers are set on it as well. Everything before that was based on a patchwork of grids based on local benchmarks taken with a sextant. Really messed up on the west coast of Africa and the Cayman Islands. Another possible source of error may be that your older receivers are not WAAS enabled.
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Old 27-08-2016, 10:15   #9
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Re: Change GPS receiver or not

PS: Let's see. I've got four GPS antennas: OPCN, autopilot (with fluxgate as well), backup GPS, and AIS. Oh, and the older chartplotter on the backup GPS does indeed plot the boat to be in the marsh, not the channel, but the Lat/Lon coming out is dead on the nav GPS. NOTHING wrong with redundancy.
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Old 27-08-2016, 10:42   #10
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Re: Change GPS receiver or not

The ais needs its own antenna, this is law, it will not share from others. Install the one that came with it to ais.

Use new plotter to feed GPS to VHF, probably via nmea183. Maybe 2000 depending on radio. If new plotter has issues getting GPS where it is mounted , add new external for that system (probably n2k). Get rid of all old useless ones. Put into garbage.
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Old 27-08-2016, 11:49   #11
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Re: Change GPS receiver or not

Most GPS units have a menu to get you to a place where they show you how accurate they are at that moment. Odds are the charts were off, but an older unit without WAAS could be that far off. Remember when we were happy being within a few hundred feet of where we thought we were (I always figured maybe within 10 miles using celestial).

Also not sure why anyone buys a plotter any more, unless they need a weatherproof one at the helm. For a fraction of the cost you can get a couple of netbook computers, hockey puck GPS receivers and free OpenCPN with all US charts free. I bet there is a weatherproof computer touchscreen that you could put in an exposed area for much less than a plotter.

For backup, how many smartphones do you have on board?

Another possible issue with GPS receivers is that they are very weak signals and easy to block, especially with a body. The frequency used does not penetrate hu,am bodies.

Back to original question, any GPS receiver that is WAAS enabled should be fine. New ones don't do anything more than old ones. Only issue may be the interface, but there are easy converters.

Bill
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Old 27-08-2016, 13:03   #12
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Re: Change GPS receiver or not

Are your old GPS units eqpd for DGPS & not WAAS?
DGPS is being shut down.
Nine differential GPS sites closed - Ocean Navigator - Web Exclusives 2016

Compare the Lat/Long on all your displays. It should be identical except for the useless 3rd decimal number (6ft ).
Then plot that LL on a paper chart & compare.

Many electronic charts are wrong-compare yours to the boat next to you.

I recommend ext. mounted antennae-the 1/4" hole required may be much smaller than the hole in your hull resulting from GPS error.

Three sats will give a rough position. Four sats is minimum acceptable for < 20m accuracy IMHO.

I believe in redundancy in case of failure,but there is not much sense in using two or three units that never agree.
Record the LL (& compass course) of your favourite spots & Wpts. as a backup to electronic chart display.

Fair winds / Len
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Old 27-08-2016, 16:14   #13
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Re: Change GPS receiver or not

usually 50 mtrs accuracy/line of sight/dgps 10 mtrs accuracy but patchy/ WAAS enabled 10 mtrs accuracy/ais gps receiver factory matched to instrument/some plotters work on variable board rate and will use different receivers/best to use what is bundled especially when upgrading/most cheap gps receivers run on 12 band/ trackers run on 20 band and remember the position of sat orbits/I use the ais gps on open cpn there is an option in gps settings under the spanner/vhf will usually pair to any receiver with compatible board rate/some new gps receivers are 50 band they track 50 sats at once therefore are more reliable/are you lost yet
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Old 29-08-2016, 06:57   #14
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Re: Change GPS receiver or not

Quote:
Originally Posted by Moontide View Post
I bet there is a weatherproof computer touchscreen that you could put in an exposed area for much less than a plotter.
No, actually, that's the problem. A weatherproof, daylight readable monitor will cost you at least as much as, if not more than, a standalone chartplotter. Not sure why that's so. It doesn't really make sense. But it is what it is.
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Old 29-08-2016, 07:54   #15
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Re: Change GPS receiver or not

Quote:
Originally Posted by SV Bacchus View Post
Lord knows you don't have to go that far. Here in the Tampa Bay area my Humminbird, with the latest Florida/Bahamas chip, sometimes shows my little fishing boat plowing a furrow through someone's yard!
But the important point, did you catch anything?
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