gps antenna placement

[Leonard Lookner]

Most marine GPS sensors today are combined antenna/receiver units. If the catalog calls it a sensor (as opposed to an antenna), then
you can be pretty sure it's one of these. Also it does require a 12 volt supply if it is independently mounted.
The Garmin 4208 which was originally suggested as the instrument of use comes with a GPS 17 poll mounted unit which is the one I referred to as being not an antenna but a GPS. You can buy this unit, a Garmin GPS 17, and use it like I have for 100.00 dollars

[Joe CD MS 300]

Maybe it's just semantics and I'm kind a Garmin guy so I'm basically only familiar with their descriptions. Below is the link to Garmin's installation instructions for the 4208 Chartplotter. Page 6 of the pdf file gets into the installation of the GPS 17 Antenna. The mushroom looking thing. By the way for Mark the instruction recommends for the antenna installation (in this case the GPS 17) a clear unobstructed view of the sky in all directions. I think that antenna's are also sometimes referred to as a receivers. When I'm referring to a GPS it is the the unit with the processing electronics and screen. Joe



http://www8.garmin.com/manuals/1675_Ins ... ctions.pdf

[SurryMark]

Most marine GPS sensors today are combined antenna/receiver units. If the catalog calls it a sensor (as opposed to an antenna), then
you can be pretty sure it's one of these. Also it does require a 12 volt supply if it is independently mounted.
The Garmin 4208 which was originally suggested as the instrument of use comes with a GPS 17 poll mounted unit which is the one I referred to as being not an antenna but a GPS. You can buy this unit, a Garmin GPS 17, and use it like I have for 100.00 dollars

Leonard,
I drive down to the Freeport area now and then, and can go the Camden way. Do you think I could drop in for a more detailed explanation of what you're saying?

This is one big-time helpful forum to belong to.

Mark

[Maine Sail]

The higher you place the GPS antenna the more it will move about and the less accurate it will be.

Can you please point us to references for this statement?

Let's look at my automobiles GPS system for a moment. I can drive 80 MPH and never have my GPS skip a beat. How are the "slow" righting moments and damping movements of a boat at 2-3 MPH going to make my pole mounted antenna "less accurate"? My cars GPS system, which works off the same satellites, and is made by the same manufacturer, can operate at 5-80 MPH all day long and rarely if ever show me not exactly where I am. The only time I ever lose reception is when I am in a big city with tall buildings or when under a bridge. The speed I drive does not seem to affect the GPS one iota..

My Garmin GPS 17 is mounted on the stern radar pole about eight feet above deck level. Six times out of ten my 3205 is seeing and picking up MORE satellites than my GPS MAP 176, with internal antenna, which is mounted at the helm.




Mark,

Here's an option for the antenna that is neat, clean & professional looking. Snaking & installing this this took me all of about 30 minutes. The mount can be purchased from Hamilton. Use a punch to set your drill point and go slow so as not to work harden the stainless. Clean up any burrs with a Dremel and install grommets to prevent any chafe.

[SurryMark]

Some day I may do a study of the scientific method and the internet. In this case the scientific method means looking at repeatable evidence, and the internet means saying any old thing. Garmin itself says that masthead mounting is less accurate, and so it's easy to take that as gospel, as in fact it may be. But aside from Maine Sail's 80mph question, I was wondering how inaccurate can masthead mounting be? If my masthead is more than 30' laterally from my butt, I'll be scrambling too fast to be checking my location. That doesn't matter though. The evidence shows that mounting the antenna (gps or whatever) on the stern rail, or on the cabin top, or under the cabin top, can be perfectly accurate, depending on the unit.

Back to the internet study: I think that this forum would rate near the top on the use-your-head scale. Nobody looking at this, or the spinning-or-locked prop issue, would imagine that you could pass on revealed truth without a struggle.

[The Patriot]

... Garmin itself says that masthead mounting is less accurate, and ... aside from Maine Sail's 80mph question, I was wondering how inaccurate can masthead mounting be? ...

The higher you place the GPS antenna the more it will move about and the less accurate it will be.

Can you please point us to references for this statement? ...

I am afraid my cutting and pasting may not accurately attribute certain posts, but even so I'll give it a shot.

A while back Practical Sailor evaluated several GPS units using GPS antennae at the mast truck. The highly simplified conclusion (and I am relying on only very dim recollections) was that while the masthead location works well enough for lat/lon display, the placement is inappropriate for determining "derived" information such as SOG and COG. Evidently, even though the gross movement sustained by antennae at the masthead is not large when compared, say, with distances in automotive applications, the movement is indeed significant when the application is a slow sailing boat. The movements become overpowering factors in algorithms used for the derived calculations.

It's certainly possible that gear has improved since the tests I am trying to recall, and this situation may not prevail today. In fact even the least expensive units these days are probably 12 channel jobs with rapid updating, which may not have been the case during the referenced tests. My first unit in 1993 was marketed as a "revolutionary 2 channel unit."

This issue reminds me of another situation from "sailboat electronics history" in which certain manufacturers marketed wind direction accessories for electronic autopilots that also were intended for masthead mounting. It happened that the changes encountered at the masthead were large enough to render the wind direction input to the pilot as useless. The devices were constantly "hunting" and operated with large and unsatisfactory course corrections. As I recall even the wind direction devices for pilots that were carried at the stern rail were never considered entirely satisfactory. A slightly reversed version using a small pilot to adjust the trim tab on mechanical steering vanes is used by quite a few folks I know who report satisfactory results.

[Joe CD MS 300]

Maine Sail,

I'm not exactly sure what Parfait's Provider's reasoning is or whether he has read something about this issue but just based on my own antidotal experience with GPS use both on boats and cars I think there could be some reason for what he suggests. I assuming in the software used by GPS units there is usually some type of averaging of the data that the unit gets from it's antenna whether it be internal or external to comput both speed, location and direction readouts that it gives you. None of the units that I use either on the boat or in the car jump around a lot either with speed, location or direction indications. when moving at any significant speed. They do seem to sometimes get "confused" particulary as to direction when going very slowly and or stoped.

I could see how a unit (depending on its software) on a slow moving boat might be happier with an antenna mounted close to the intersection of it's fore/aft, port/starboard and vertical pivot point (Would this be the center of gravity or effort?) than with one mounted at the top of the mast. I'm thinking that the top of a 50' foot mast could be swinging 10' in a variety of directions when you are in light winds, slow moving with lumpy seas. I think I can even remember this happing with the direction indication when moving very slowly on my boat. Come to think of it the accuracy circle that a lot of Garmin units can display tend to increase as speed decreases or as direction becomes irratic.

The software for automotive units and marine units must also vary somewhat as the auto units will lock you onto the road it assumes you are on up to a point. (Ever use a GPS on an access road and the unit thinks you are on the adjacent highway?) Another indication of this differences between auto and marine units is apparent when you look at a track log of cars vs. boats. The tracks for cars don't jump around usually even when stopped. A track for a boat tied up at a slip will bounce around to some extent.

I don't know if the difference would be significant between the top of the mast and lower such as on the stern rail but I'm thinking that closer to the pivot pt, center of gravity or what ever it is would be more accurate to some degree, especially when moving slow and bouncing around.

Joe

[Maine Sail]

I just don't get why anyone would mount one, or consider mounting one at the top of a mast to begin with. It requires more cable, is less accessible and the GPS signals are not like VHF so it's not necessary at all.

The speed thing is reasonable at a mast head but my antenna on my radome pole always matches my GPS mounted lower and at the helm. I am always running two GPS units, one for data and the other for radar & plotter though it too displays SOG and I can easily compare the two so i know 8 feet makes no difference in SOG. Of course 60 feet probably wood in rough seas..

There is no real point in anyone trying to mount a GPS antenna at the top of their mast and dealing with snaking more wires and splicing more wire onto the 30' supplied by most antenna makers.

As far as "less accuracy" is concerned I was thinking specifically about Lat/Long and not small speed differences. When I think accuracy I think location. With Lat/Long you are never going to see, physically on the screen, or be able to plot on the screen, even at the 20 foot range, 3 feet of movement at the mast head for locations sake. Speed perhaps, but location of that movement, 3 feet, is very doubtful that you could make your plotter display it. I can't and I just did it on my deck!

For grins I just went out on my deck with my GPS Map 176 and held my arm out as far as it would go and made a complete circle when zoomed into the 20 foot scale. I could not even get one additional cookie dot to display on the screen doing this. I know the GPS picked up my movements but it was unable, at the 20 foot scale, to display it.


When my boat swings on her anchor I can often read .1 knot SOG so they do react to slow speeds well. Even at less than one knot my little triangle corrects itself and orients in the direction we are moving..

[Joe CD MS 300]

Maine Sail,

Sorry I might be confused. I thought you were questioning Parfit Providers remark (when you quoted his comment) about NOT putting the GPS Antenna/Receiver at the top of the mast and were suggesting that there was no reason not to put it there. I think most of us are in agreement that the top of the mast is NOT the place for a GPS antenna.

I just did a little looking at some of the Garmin installation manuals for GPS antennas. Some of Garmin's instructions specifically recommend NOT putting it on the mast because of the movement issues up there and recommend it be placed as close to the water line as possible.

Most of the newer antennas are pretty sensitive these days and provide good reception even when partially covered.

Joe

[Troy Scott]

This is late, and it may also be covered elsewhere on the board. Anyway, the antenna needs to be where there is not so much motion. This is because the position sensing and relative motion computing will be far more accurate if the system isn't confused by the pitching and swaying at the masthead.