GPS antenna location

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Docktor

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What is the best location for an external GPS antenna on a steel boat. Planned location 2m away from VHF antenna and 1.5 from passive UHF tv antenna.

GPS antenna is Garmin 17x - NMEA 2k - drop cable length approx 3m to feed into NMEA backbone.

I expect no receiving problems but wanted to know if anyone has suggestions on the best possible location for GPS antenna location in general - can't find anything in previous threads.....
 
Middle of the boat, not too high, where it could be "waved around", but able to see maximum sky.
To get maximum advantage of GPS sensitivity, you don't want to have to dampen it all out with menu settings.
 
If you talk to Garmin support they will tell you it works perfectly well indoors. We have three and they are all installed under the cockpit coaming, in the lazarette or at the chart table.
 
Ex-SolentBoy said:
If you talk to Garmin support they will tell you it works perfectly well indoors. We have three and they are all installed under the cockpit coaming, in the lazarette or at the chart table.
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Quite possibly not in a steel boat though.

Just pick a convenient place on deck, which will depend on your particular boat. On the pushpit is quite common but I don't really like this as they can be vulnerable to snagging with mooring lines etc. Mine happens to be flat on the deck (it's like an inch-thick hockey puck) just aft of the mast and slightly to one side.

High up (which it sounds like you have in mind talking about VHF antennas?) is not good for them, as the rolling of the boat means they're moving from side to side instead of being able to settle on one position.

Pete
 
lw395 said:
Or in extreme rain etc.
May as well give it the best chance of working as well as possible.
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Well, I don't see 3mm of fibreglass attenuating the signal all that much, so I'd happily mount an antenna under a cockpit coaming etc. Just that the OP's boat is steel which is considerably less transparent to radio waves.

Pete
 
Thanks everyone - it seems to be the case that the antenna should be sited so as to have maximum "exposure" to the satellites - 12 in the case of the Garmin 17x.

Pete's right - my concern, as well as adjacent antennas, is that my boat is steel. I am going to locate the antenna on the coach house roof just above the lower/inside helm - and use the "mushroom" adapter on an existing antenna mount - this should allow me to change the antenna in the future if I change my Garmin network without being committed to the footprint of the 17x.
 
prv said:
Well, I don't see 3mm of fibreglass attenuating the signal all that much, so I'd happily mount an antenna under a cockpit coaming etc. Just that the OP's boat is steel which is considerably less transparent to radio waves.

Pete
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3mm of fibreglass can have no effect, or lots, the devil is in the detail as we say.
3 mm of grp with a good film of salt water running over it can be in a different league.

GPS is a digital system. It works up to a certain level of signal degradation then fails catastrophically.

Personally, I think the pushpit is a good compromise.
 
lw395 said:
3mm of fibreglass can have no effect, or lots, the devil is in the detail as we say.
3 mm of grp with a good film of salt water running over it can be in a different league.

GPS is a digital system. It works up to a certain level of signal degradation then fails catastrophically.

Personally, I think the pushpit is a good compromise.
Click to expand...

I am not a scientist so I can only speak from experience.

Satellite signals come from above, so who knows if a steel hull makes and difference at all. Is the OP's boat steel coach roof as well?

Based on Garmin advice our aerials are under GRP and a whole load more than 3mm probably more like 10mm.

We have never had a problem, either in rough seas, torrential rain, or even with a couple of inches of snow.

Surely the pragmatic thing to do is to temporarily install the aerial somewhere convenient near the plotter or whatever else it is serving. If it works fine then fix it permanently. If it doesn't find a better place.

Why drill holes in your boat and run long cables if there is no need?
 
Ex-SolentBoy said:
Satellite signals come from above, so who knows if a steel hull makes and difference at all. Is the OP's boat steel coach roof as well?
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Well, I assumed it was, they generally are.

If in fact it's GRP (a very odd combination) or, say, plywood (slightly more plausible) then I agree the hull material isn't relevant.

Pete
 
Last week we were in our motorhome on the Igoumenitsa - Ancona ferry. Sitting in the van aboard the ship waiting to disembark I turned on the Tomtom gps and was astonished when it gave our position after a short wait. We were on deck five, about midships, so six or seven steel decks above us.
 
Ex-SolentBoy said:
I am not a scientist so I can only speak from experience.

Satellite signals come from above, so who knows if a steel hull makes and difference at all. Is the OP's boat steel coach roof as well?

Based on Garmin advice our aerials are under GRP and a whole load more than 3mm probably more like 10mm.

We have never had a problem, either in rough seas, torrential rain, or even with a couple of inches of snow.

Surely the pragmatic thing to do is to temporarily install the aerial somewhere convenient near the plotter or whatever else it is serving. If it works fine then fix it permanently. If it doesn't find a better place.

Why drill holes in your boat and run long cables if there is no need?
Click to expand...

The trouble with this approach is that you never know if you have tested with the worst level of atmospheric attenuation you might reasonably expect.
Also, the GPS satellites are currently putting out signals well above the stated minimum. As the satellites age, or for other reasons, that could reduce significantly.

We do get people on here with unexplained GPS outages.
In my view, I can envisage situations where anything that delays GPS position after say a power interruption might be a very bad idea.
 
vyv_cox said:
Last week we were in our motorhome on the Igoumenitsa - Ancona ferry. Sitting in the van aboard the ship waiting to disembark I turned on the Tomtom gps and was astonished when it gave our position after a short wait. We were on deck five, about midships, so six or seven steel decks above us.
Click to expand...

That is the trouble with these discussions, the modern receivers are amazingly good up to a point. The GPS in my phone will sometimes work deep within buildings, sometimes not.
I used to use a tomtom in a Mondeo with a heated screen. 99.9% fine, but just occasionally it would lose the plot under a few trees. Driving in central London it struggled more when surrounded by tall buildings, compared to doing the same journey in another car. It used to think I was going sideways down the wrong side of the road at some points.
GPS is RF at about 1.575GHz, that's about 20cm wavelength, so will go through doorways, companionways etc.
So maybe any tests need to be done with the washboards in and people slinging salt water around in a merry fashion.

Seems easier just to mount it outdoors and know you've done a proper job.
 
vyv_cox said:
Last week we were in our motorhome on the Igoumenitsa - Ancona ferry. Sitting in the van aboard the ship waiting to disembark I turned on the Tomtom gps and was astonished when it gave our position after a short wait. We were on deck five, about midships, so six or seven steel decks above us.
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Strange isnt it.

When we installed the kit on our boat it was still in the factory. So the aerial was inside the boat, inside a metal factory. Still got a perfect signal though.
 
Kit has definitely improved. My dad bought a Magellan handheld in the early 90s, and that wouldn't see the satellites unless you stood in the middle of the garden.

Pete
 
GPS reception works best with line-of-sight to the satelites. The antenna can see through GRP and most plastics unless they are coated with a metallic film which will degrade the signal or block it completely.

The GPS first recieves an almanac from the satelites, this tells it where all the satelites are and which ones it should be able to see - sometimes this takes a few seconds to aquire if the system has been off for a long time.

When the individual satelite signals reach the ground they are bounced around by buildings, metal surfaces etc. The effect of this is that they still provide a fix, just not as accurate as one where it is direct line-of-sight. A bit like laser range-finding on an object while standing in a hall of mirrors.

Car navigation systems also deploy map-matching on top of the GPS position (i.e. they assume you are on a road) which with reflected signals can lead to your position jumping around or being false. A gyro records your path and then roads in the vacinity are checked for a match so sometimes the vehicle is placed on a road which is not exactly where the GPS position is.

The advice to trial the system with the antenna in as good a position as you can find (not below a metal deck or in the shadow of metallic objects) is sound. The plotter should have a screen displaying the no. of satelites and the signal strength and fix accuracy. Use this to find a suitable location. Our antenna is directly below the coaming at the rear of the cockpit seats and provides a good fix accuracy and sees almost all of the 'visible' satelites with good signal strength.

I wouldn't be too concerned about the effect of rolling and pitching unless there is a gyro included in the GPS reciever, the GPS signals travel at light speed - in cars it works fine up to 320kmh (I haven't managed to test them any faster ;))
 
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Much to my surprise, my e7 plotter works fine with its inbuilt aerial in my GRP boat.

You also need to take into account the possibility of lines and so on getting caught up on the aerial, which was occasionally a problem when we had ours on the pushpit. They also make very tempting hand-holds for the ignorant on other craft coming alongside.
 
johnalison said:
You also need to take into account the possibility of lines and so on getting caught up on the aerial, which was occasionally a problem when we had ours on the pushpit. They also make very tempting hand-holds for the ignorant on other craft coming alongside.
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Agree - I don't really know why this is such a common place when there are so many downsides and no real benefits. Flat to the deck in a place un-walked-upon is much better.

I do mean flat though - I was once on a boat with an old system whose antenna looked like a golf ball on top of a tee, but scaled up to about four or five inches in diameter. It was on the coachroof above the chart table, so just in front of the spray hood. A better shape for catching lines or being kicked would be hard to come up with on purpose. How we didn't rip it off its stalk just in the one week I have no idea.

Pete
 
vyv_cox said:
Last week we were in our motorhome on the Igoumenitsa - Ancona ferry. Sitting in the van aboard the ship waiting to disembark I turned on the Tomtom gps and was astonished when it gave our position after a short wait. We were on deck five, about midships, so six or seven steel decks above us.
Click to expand...

lw395 said:
That is the trouble with these discussions, the modern receivers are amazingly good up to a point. The GPS in my phone will sometimes work deep within buildings, sometimes not.
I used to use a tomtom in a Mondeo with a heated screen. 99.9% fine, but just occasionally it would lose the plot under a few trees. Driving in central London it struggled more when surrounded by tall buildings, compared to doing the same journey in another car. It used to think I was going sideways down the wrong side of the road at some points.
GPS is RF at about 1.575GHz, that's about 20cm wavelength, so will go through doorways, companionways etc.
So maybe any tests need to be done with the washboards in and people slinging salt water around in a merry fashion.

Seems easier just to mount it outdoors and know you've done a proper job.
Click to expand...


A GPS in a situation such as Vyv and lw395 mention may well work - but be seriously inaccurate. The problem here is that you don't know the pathway by which the GPS signal is reaching your receiver, and there may (probably will) be all sorts of reflections and waveguide effects. As GPS depends on measuring tiny differences in arrival time of the precisely timed signals, this increases the error budget manyfold. This may not matter in a lot of cases - the error will still be small compared with dead reckoning, astro, three-point fixes and so on - but if you were relying on 10m accuracy and you've actually got 50m accuracy, and you're in confined water....

Tall buildings etc. give the same effect, which is why GPS can go silly amongst trees or buildings.

As others have said, mount it outside (mine is on the pushpit, like most others) and you remove a potential source of error or failure.
 
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