Internal/external GPS aerial?

[PabloPicasso]

I'm finally going to get a chartplotter to site at the navtable on my 26 foot GRP sailing yacht. Will I need an external aerial or will a unit with internal GPS aerial be OK?? it would be easier to fit an internal aerial model and my current old GPS set works fine this way. Anyone got experience of doing this or should I go to the extra hassle of wiring an external aerial on the pushpit?

 

[richardr]

I fitted the antennae for my new Garmin plotter internally but will move it to the pushpit this weekend. Went up to Bucklers Hard on Saturday and the plotter showed my 50m inland. Suboptimal! It does pick up satellites but accuracy isn't sufficient. Maybe just my boat or the location of the antennae.

 

[nickfabbri]

we had a standard horizon cp180i on our first boat and it worked fine below deck. Although it was more useful when mounted next to the companionway

 

[Tranona]

Internal ariels seem to work OK down below. I have had success with a Navman, Garmin Gps and a dongle to a PC - all down below. Probably the only way to find out for certain is to try it!

 

[RobHom]

I have always preferred to have my GPS antenna's out in the open with the least amount of possible obstructions between the antenna and the sky.... The more satellites it connects with....the more exact the positioning fix is.

If you have a solid item or items in between the unit and the sky....you may foul the reception. I find that external antennas on WAAS units give extremely close fixes....down to a couple of meters....and with the waters in Florida having a tendency to be a bit "skinny"....its best to have a strong feel for your position...especially if a marker or buoy is missing....

 

[Goldie]

When my old fixed GPS failed, I fitted a Standard Horizon 300i at the chart table, chosen so that I can move it to a position under the sprayhood for pilotage without needing anything other than a 12v supply. It's in it's first season but 100% for reception to date. Still not a convert to plotters though, maybe I'm missing something!

 

[Danny Jo]

The internal antenna on my Raymarine chartplotter works fine at the chart table under two layers of 1/4" ply.

 

[CAPTAIN FANTASTIC]

I have a Standard Horizon 300i (internal antenna) located above a large chart table on a 32 ft yacht; never had a problem with reception; have been using it for the last 3 years. I have not heard of anyone having any problems with reception with a modern variety of chart-plotters with an internal antenna when used below.

 

[webcraft]

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Slightly off topic, but why do people fit plotters down below? They are far more useful when visible from the helm.

- W

 

[PeterR]

My own GPS is mounted externally but I have chartered a lot of boats with internal arials and most worked OK but with a reduced signal strength compared to my handheld on deck. Some bigger boats don't get an adequate signal at the nav station presumably because of a thicker hull/deck layup.

 

[Cloven]

.
Slightly off topic, but why do people fit plotters down below? They are far more useful when visible from the helm.

- W

Couldn't agree more. You want it where you can see it while you are at the helm.

 

[Danny Jo]

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Slightly off topic, but why do people fit plotters down below? They are far more useful when visible from the helm.- W

So that they can do their passage planning at the chart table rather than hunched in the wind and rain in the cockpit. But I agree - if you have only one display, stick it wear it can be seen from the helm.

 

[JimC]

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Slightly off topic, but why do people fit plotters down below? They are far more useful when visible from the helm.
- W

For planning your route in advance, putting in waypoints etc.

 

[ianabc]

INTERNAL/EXTERNAL ANTENNA power required....

Power for the CP 180 external (with external gps connected) is 0.460 amps vs 0.520 for the internal antenna version.

 

[nimbusgb]

As you have no doubt deduced, the GPS signal will penetrate glassfibre and plywood ( don't try it with a steel or carbon boat though ) but the signal is attenuated and reflections and multipaths will reduce the accuracy.

What happens in the rain or on a day with a good blow? Deck gets wet and now you have completely changed the material above the aerial to one not so ready to allow the passage of radio signals. In addition several layers of cloud and things are getting marginal.

Give the poor thing a decent chance. The GPS satellites orbit 20 000 km up. By the time the signal gets to ground there's not a lot of it left!

Fine for knocking about, but one day in the fog and your GPS is 100m out ..... then what!

 

[Cloven]

For planning your route in advance, putting in waypoints etc.

OK point taken but quite simply - buy a second mounting kit then you can use a plotter with internal aerial in both locations. In fact, all you really need is a second power/data cable and a 12V supply. That is what I have and I can input data in the cabin, at home etc and then use the plotter mounted under the sprayhood when sailing.

 

[lw395]

As you have no doubt deduced, the GPS signal will penetrate glassfibre and plywood ( don't try it with a steel or carbon boat though ) but the signal is attenuated and reflections and multipaths will reduce the accuracy.

What happens in the rain or on a day with a good blow? Deck gets wet and now you have completely changed the material above the aerial to one not so ready to allow the passage of radio signals. In addition several layers of cloud and things are getting marginal.

Give the poor thing a decent chance. The GPS satellites orbit 20 000 km up. By the time the signal gets to ground there's not a lot of it left!

Fine for knocking about, but one day in the fog and your GPS is 100m out ..... then what!

I second these remarks.
GPS works fine until the signal level drops below a certain point, then errors increase very quickly and total loss of lock is not far away.
There is not much margin.
You should be allowing for rain, wet decks, interference and the possibility that the satellite transmit power will fall a little.

You may also not be seeing some satellites at low elevations at some angles relative to the boat. This may limit the accuracy you can get. This may get worse if you go up river or in port near buildings etc. OK, I'm not relying on the GPS in these situations, but it erodes crew confidence if it doesn't work as well as other people's!

OTOH a spare aerial mounted in the dry in case the one on the pushpit gets trashed isn't a bad idea either!

 

[AntarcticPilot]

OK point taken but quite simply - buy a second mounting kit then you can use a plotter with internal aerial in both locations. In fact, all you really need is a second power/data cable and a 12V supply. That is what I have and I can input data in the cabin, at home etc and then use the plotter mounted under the sprayhood when sailing.

And if all the wiring for depth sounder, DSC VHF, Log, Compass and Tiller Pilot all go to the chart table? There are at least three connections to my C80 (power, SeaTalk and NMEA (latter required for connection to DSC VHF)). I've thought about the potential for having a second mount for my C80 in the cockpit, but it isn't really a practical proposition. Also, with tiller steering, there isn't an obvious location in the cockpit where the plotter would always be visible to the helm. Where it is, I can consult it when necessary; I wouldn't dream of using it for detailed navigation in confined waters - I know too much about the accuracy of both charts and GPS to do that. What I can see is much more reliable, providing I've studied the chart (including the electronic chart) and can match what I can see to what I expect. For me, the chart plotter is a convenient means of ensuring I have a chart visible at the navigation station at all times, not a pilotage tool.

 

[lw395]

.........................I wouldn't dream of using it for detailed navigation in confined waters - I know too much about the accuracy of both charts and GPS to do that. What I can see is much more reliable, providing I've studied the chart (including the electronic chart) and can match what I can see to what I expect. For me, the chart plotter is a convenient means of ensuring I have a chart visible at the navigation station at all times, not a pilotage tool.

I feel that you are missing a lot of the usefulness of a good chartplotter.
When pilotage gets a bit hectic, like in the Channel Islands, it enables you to work much faster and is continuous check on your observance of the passage plan.
Yes, there are errors, yes you still need common sense and to keep your eyes open, you still need a plan etc, but it's so much quicker than any other check when you've got 5knots of tide under you.
I just like plenty of opportunities to check what it's telling me!

 

[Danny Jo]

For me, the chart plotter is a convenient means of ensuring I have a chart visible at the navigation station at all times, not a pilotage tool.

Yes, but some chartplotters also display other data (radar, AIS, waypoint bearing etc.,) that is potentially more useful when visible at the helm.