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Old 08-02-2014, 02:20   #1
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GPS Receiver Below Decks

I replaced all of my electronics last year, and I did not install a separate GPS receiver. My Zeus Touch plotter at my helm has a built-in GPS/Glonass receiver which I use as my main position data source, and my AIS black box transceiver has its own GPS receiver which feeds the AIS and radios.

But I have not been entirely happy with the arrangement. For one thing, I don't really like leaving the helm plotter switched on all the time, and I need GPS data at anchor or in dock in order to have good weather data from my Maretron DSM-250 (won't give ground wind without position and SOG/COG data).

So I'm thinking about installing a separate NMEA2000 GPS/Glonass receiver somewhere. I don't really want to put it in the traditional position on the pushpit because it's ugly and I'll have to pull a damned long N2K cable -- troublesome and expensive.

Modern GPS receivers seem to be vastly more sensitive than the old ones were. I notice my phone gets a GPS position even indoors, and my AIS black box gets a position without an external antenna, with the black box buried behind my nav table. So I'm thinking -- maybe I can mount it below decks? Anyone done this?

There is a place behind my nav table where there will be nothing above except fiberglass, balsa, and teak decking -- will that degrade the signal and reduce accuracy of the position data?

Alternatively, I could put it behind my scuttle instrument panel -- there's nothing between that position and the sky but a thin layer of plastic. That would be my second choice, because that would have to be on my Network 2. I would prefer to have the GPS antenna on Network 1 in case I prefer to shut down Network 2 (which services engine room and cockpit). These are two N2K networks linked with a Maretron bridge.

What do you guys think?
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Old 08-02-2014, 03:08   #2
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A related problem, I guess - I have two sources of position data on the network as it is - my AIS black box feeds it in, as well as the Zeus. I can manually set up the network to use the position data from the AIS, and this works fine. But the network will not automatically pick up that data source, if the Zeus is offline. Does anyone know of a setting to provide this obvious function - failover to an alternate data source, when the primary source is not present?? This is quite frustrating.
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Old 08-02-2014, 05:29   #3
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Re: GPS Receiver Below Decks

Modern GPS units are incredibly good a will get a position fix in most locations.
However, the quality of fix can be improved with a clear uninterrupted view of the sky.
It always surprises me that people are prepared to accept a poor GPS satellite signal. To me it is the most important navigational instrument, and I want the best quality information possible.

I don't see any any problem with secondary GPS units that are mounted for convenience or ease of installation, providing there is a primary unit providing the best data.
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Old 08-02-2014, 06:10   #4
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pirate Re: GPS Receiver Below Decks

I have a GPS puck that I use for my Notebook CP program.. attaches via USB.. to date the only boat its not worked on below decks has been a steel one when I had to position it under the sloping pilot house windows..
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Old 08-02-2014, 06:19   #5
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Re: GPS Receiver Below Decks

I have my no 2 and 3 backup gps units belowdecks and have never noticed a difference in positions between the outside mounted one. Outside is Garmin, below Delorme and magellan. yes I like redundancy. 3 sources of electronic charts too. Garmin chartplotter, dedicated boat computer and laptop. I feel like kind of a pansy and often wonder how I used to do it back when I only had paper charts and a compass.
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Old 08-02-2014, 06:23   #6
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Both of my GPS receivers are beneath the deck and have never had a problem receiving. The old Garmin is a little slower to get a fix sometimes with the antenna beneath fiberglass but has never failed.
Mine are both beneath the moulded coaming under the dodger, so 3/16" solid glass, sun tells, and some stainless tubes when the angle is wrong.
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Old 08-02-2014, 06:55   #7
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Re: GPS Receiver Below Decks

Quote:
But I have not been entirely happy with the arrangement. For one thing, I don't really like leaving the helm plotter switched on all the time, and I need GPS data at anchor or in dock in order to have goodweather data from my Maretron DSM-250 (won't give ground wind without position and SOG/COG data).
Ground wind and apparent wind are exactly the same if the boat is stationary at anchor or in a dock and a GPS position COG/SOG will not make any difference to the wind data figures displayed. if however you want to set an anchor alarm you could consider a simple handheld GPS with that feature and keep that by your bunk?
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Old 08-02-2014, 07:27   #8
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Re: GPS Receiver Below Decks

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Originally Posted by Robin3 View Post
Ground wind and apparent wind are exactly the same if the boat is stationary at anchor or in a dock and a GPS position COG/SOG will not make any difference to the wind data figures displayed. if however you want to set an anchor alarm you could consider a simple handheld GPS with that feature and keep that by your bunk?
The difference is the direction, and I think his point is that the Maretron will not display it without the COG/SOG input. It probably will, however, using inputs from a heading and knotmeter sensors. In any case, I like having a gps chartplotter display below, and the modern high sensitivity receivers work fine under a fiberglass deck.
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Old 08-02-2014, 07:31   #9
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Re: GPS Receiver Below Decks

Quote:
Originally Posted by Robin3 View Post
Ground wind and apparent wind are exactly the same if the boat is stationary at anchor or in a dock and a GPS position COG/SOG will not make any difference to the wind data figures displayed. if however you want to set an anchor alarm you could consider a simple handheld GPS with that feature and keep that by your bunk?
Obviously, but the bleeding Maretron DSM-250 will not display Ground Wind if it does not have valid COG/SOG data. This is really annoying.

Yes, for an anchor alarm I do, indeed, use a handheld unit
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Old 08-02-2014, 07:32   #10
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Re: GPS Receiver Below Decks

Quote:
Originally Posted by Ziggy View Post
The difference is the direction, and I think his point is that the Maretron will not display it without the COG/SOG input. It probably will, however, using inputs from a heading and knotmeter sensors. In any case, I like having a gps chartplotter display below, and the modern high sensitivity receivers work fine under a fiberglass deck.
Thanks! So below decks it is! Great!

My ideas were formed in the old days when GPS receivers took several minutes to get a fix, and needed a clear view of the sky. Those days are over, I guess, thank God!
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Old 08-02-2014, 07:47   #11
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Re: GPS Receiver Below Decks

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Originally Posted by Dockhead View Post
So I'm thinking -- maybe I can mount it below decks? Anyone done this?
Yes.

All my GPS's are below deck. Even the Beneteau installed GPS for the plotter at the helm is below deck in the lazarette.

By the way, i run a GPS out of the AIS by hard wire into my VHF and into laptop. Why cant you?


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Old 08-02-2014, 08:07   #12
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Re: GPS Receiver Below Decks

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Originally Posted by Dockhead View Post
Obviously, but the bleeding Maretron DSM-250 will not display Ground Wind if it does not have valid COG/SOG data. This is really annoying.

Yes, for an anchor alarm I do, indeed, use a handheld unit
Will it not display apparent wind data since the two are the same, the precise display label is meaningless?

We had 3 plotters and 4 GPS feeds on our last UK boat, a Navman at the chart table with an external antenna mounted on a nearby shelf, another with built in antenna up top and an ancient Raymarine with an antenna on the antenna farm at the stern gantry, all worked equally well. I had spliced the NMEA 183 feed from the exterior Raymrine GPS via a switch so it could be used to power the Navman at the chart table if it's GPS failed plus it also fed the Yeoman paper plotter, the 4th one that I forgot to mention, and the radar. I didn't leave any running 24/7 , but could have done without fear of battery probs whilst connected to shorepower in our berth rather than out at anchor. I never used anchor alarms, I prefer to have good ground tackle that I trust and an inbuilt instinct for subtle changes in sound or motion that might indicate a need to get out of bed and make a proper visual check
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Old 08-02-2014, 08:12   #13
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Re: GPS Receiver Below Decks

I use the same GPS/Chartplotter in the cockpit as well as at the nav station. Just order an additional power supply for your unit and install it at the nav station. When you come inside, move the chartplotter from outside to inside, plug it in and you have a chartplotter inside.
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Old 08-02-2014, 08:21   #14
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Re: GPS Receiver Below Decks

GPS's have gotten VERY good, I was worried when I installed my Garmin 740s in my little boat that I would have to have an external antenna to get good reception. I kept the boat on a trailer in a full metal aircraft hangar. Turning it on, it was receiving and tracking nine satellites with good signal strength, inside of a fully enclosed metal building. I didn't worry about it ever again
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Old 13-02-2014, 01:25   #15
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Re: GPS Receiver Below Decks

I used a Garmin GPS19X NMEA2000 sensor (10HZ) mounted under the foredeck using the standard, supplied with the unit, under-deck mount as my primary GPS receiver in my recent N2K/electronics upgrade. Works perfectly and my Zeus MFDs recognize it as the primary GPS source because it, easily, provides the strongest data to the network. You can also specify the priority of duplicated data on your network manually. I suggest you checkout this antenna. It's the newest technology. B&G also makes a similar product, but I chose the Garmin because of it's lower cost and Garmin is no slouch when it comes to GPS stuff.
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