GPS antenna location

Nicholas Walsh wrote:
> Why do people persist in putting their GPS antenna on the stern rail. Is
> it not one of your most important instruments? Do you want it to be
> yanked off by some clumsy git climbing aboard from a dinghy or clipped
> off by a shoreline? I have always mounted mine on the stern but directly
> on the deck where it out of everyone's way. It also gets a perfect view
> of the sky without the pendulum movement of a mast mounting. This is on
> my third boat and I have never had one damaged. How many people keep a
> spare GPS aerial for these eventualities?
>

Mine's coachroof mounted 'limpet' style on our Contessa 26 with excess
cable shortened and a new BNC plug put on. It's predecessor used to be
mounted on a stancheon supporting the mainsheet horse and that was a
nightmare. If it didn't get knocked by the mainsheet, someone would
lean back over it and block the signal, or a warp would abrade its cable
etc. I give the new one a gentle polish a couple of times a season to
keep the water beading up and running off and I get a *far* better
signal. I've replaced the cable into the old one and tested it for a
spare just in case though.


--
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'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.
 >> Stay informed about: GPS antenna location 

On Mon, 21 Jan 2008 19:52:29 -0000, JohnW <invalid RemoveThis @earlsway.invalid>
wrote:

>Mounting it high up on a sailing boat also makes it subject to
>position reporting error due to heel, not to mention confusing
>it when rolling.

I was under the impression that GPS results are referenced to the
location of the antenna. So what does heel have to do with it?

Casady
 >> Stay informed about: GPS antenna location 

Richard Casady, in article <47b6d18e.1524399671
@news.east.earthlink.net>, says...

>I was under the impression that GPS results are referenced to the
>location of the antenna. So what does heel have to do with it?

When you are heeled over, the antenna, if up the mast, will be
over to the side somewhere, some distance from the boat
centerline where it will be giving an incorrect position
report for the boat. Since heel isn't constant, the error
introduced by heel would be variable.

Not that you should be using the position information reported
by GPS to that level of accuracy anyway However, there
have been several GPS assisted collisions with fixed landscape
features, so perhaps that isn't true anymore?

If you are pitching and rolling, the antenna will be moving
relative to the boat so the GPS will include that motion in
with the boat's forward velocity in its speed calculation.
---
One problem with mounting the antenna at deck level, under the
pushpit, is that from a dinghy, it looks too much like a
handle to help getting on deck. If on the pushpit, it can get
knocked. I have mine under the GPS structure which has no
reported signal strength implications. It also isn't a
visible "I've got a GPS available for stealing" signal...

--
JohnW.
Replace the obvious with co.uk in 2 places to mail me.
 >> Stay informed about: GPS antenna location 

On Sun, 27 Jan 2008 12:08:31 -0000, JohnW <invalid DeleteThis @earlsway.invalid>
wrote:

>When you are heeled over, the antenna, if up the mast, will be
>over to the side somewhere, some distance from the boat
>centerline where it will be giving an incorrect position
>report for the boat. Since heel isn't constant, the error
>introduced by heel would be variable.

Well yeah. I dismissed that kind of thing as too trivial to worry
about.
>
>Not that you should be using the position information reported
>by GPS to that level of accuracy anyway

I think that when feet matter, eyes should be on something else, the
world, the sonar, the radar, something. Maybe even an occasional
glance at the engine gauges. Basically GPS gives position. Mariners
used to find that out once a day, with the sextant, to an accuracy of
no better than half a mile. How soon we forget. Soon third world
despots will be able to disappear the system. I am hanging on to my
sextant, just in case. Iran with ASAT?

Casady
 >> Stay informed about: GPS antenna location 

In article <Xns9A3268E4AA8CBnoonehomecom RemoveThis @208.49.80.253>,
larry <noone RemoveThis @home.com> wrote:

> Y'all give a cheap boat GPS WAY too much credit for position fixes!
> It's only good to about 3 feet, on a sunny day, with no reflecting
> airplanes making multipath signals, far out away from any land.

I guess that is still very optimistic - 15-20 m, ie 50-60 ft are more
like it.
If you use SDGPS with corrections by satellites, it might come down to 3
m, or 10 ft.
No way navigating a channel with 3 m leeway on each side by GPS (even
SDGPS). Tested! In perfect conditions ...

HTH

Marc

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 >> Stay informed about: GPS antenna location 

On Sun, 27 Jan 2008 17:05:39 +0100, Marc Heusser
<marc.heusser.RemoveThis@byeheusser.commercialspammers.invalid> wrote in
<marc.heusser-C0D350.17053127012008.RemoveThis@news.uzh.ch>:

>In article <Xns9A3268E4AA8CBnoonehomecom.RemoveThis@208.49.80.253>,
> larry <noone.RemoveThis@home.com> wrote:
>
>> Y'all give a cheap boat GPS WAY too much credit for position fixes!
>> It's only good to about 3 feet, on a sunny day, with no reflecting
>> airplanes making multipath signals, far out away from any land.
>
>I guess that is still very optimistic - 15-20 m, ie 50-60 ft are more
>like it.
>If you use SDGPS with corrections by satellites, it might come down to 3
>m, or 10 ft.
>No way navigating a channel with 3 m leeway on each side by GPS (even
>SDGPS). Tested! In perfect conditions ...

I've done considerable testing of my modest Magellan Sportrak Color, and
with a clear view of the sky it's repeatable to within 10-20 feet, even
in major metro areas, quite capable of navigating real world narrow
channels, albeit not as narrow as your hypothetical case of 3 m on each
side.

--
Best regards,
John Navas <http:/navasgroup.com>
 >> Stay informed about: GPS antenna location 

On Sun, 27 Jan 2008 15:15:32 +0000, larry <noone DeleteThis @home.com> wrote in
<Xns9A3268E4AA8CBnoonehomecom DeleteThis @208.49.80.253>:

>JohnW <invalid DeleteThis @earlsway.invalid> wrote in
>news:MPG.220684998873fe4f989877@news.aaisp.net.uk:
>
>> If you are pitching and rolling, the antenna will be moving
>> relative to the boat so the GPS will include that motion in
>> with the boat's forward velocity in its speed calculation.

Plus interference with direction over ground calculations due to rocking
from side to side.

>If you have a handheld GPS, carry it into the burger joint on a busy
>road and let it bread trail on close range. The signal can't get
>through the roof so what the GPS receives are signals bouncing off
>objects outside, like passing vehicles and stationary (we hope)
>buildings through the big windows. Let it run an hour and its fix will
>cover the whole shopping center....many hundred feet! This same effect
>happens in a HARBOR or the ICW! Signals bouncing off nearby conductive
>objects, especially overhead bridges, just eats it alive. Anywhere near
>shore a GPS fix gets wider and wider in accuracy because of multipath,
>the same signal bouncing that tears up a UHF TV signal on an old analog
>TV with "ghosts", signals arriving later than the main signal which
>ALWAYS make ghosts to the RIGHT of the main signal, because they arrive
>later...we scan from left to right, top to bottom like reading a page in
>a book....except every other line, called interlacing to make it flicker
>less.

I record NMEA output from my Magellan Sportrak Color GPS on my laptop,
and I'm not seeing that kind of error -- my tracks are quite accurate
when checked on the charts on my laptop.

>Ever wonder why it only updates every second? It's trying to average
>out the MULTIPATH MOVEMENT ITS MEASURING!

It's actually feeding valuable real-time data to my laptop, which is
automatically computing and displaying target speed polars in real time.

--
Best regards,
John Navas <http:/navasgroup.com>
 >> Stay informed about: GPS antenna location 

On Sat, 26 Jan 2008 18:44:41 -0000, "Nicholas Walsh" <nmwalsh.DeleteThis@iol.ie>
wrote in <LcLmj.24284$j7.450406@news.indigo.ie>:

>Why do people persist in putting their GPS antenna on the stern rail. Is it
>not one of your most important instruments? Do you want it to be yanked off
>by some clumsy git climbing aboard from a dinghy or clipped off by a
>shoreline? I have always mounted mine on the stern but directly on the deck
>where it out of everyone's way. It also gets a perfect view of the sky
>without the pendulum movement of a mast mounting. This is on my third boat
>and I have never had one damaged. How many people keep a spare GPS aerial
>for these eventualities?

In my experience the stern pulpit rail is safer -- I've seen too many
people kick an antenna mounted at deck level.

I always have at least two hand-held units to back up the boat GPS.

--
Best regards,
John Navas <http:/navasgroup.com>
 >> Stay informed about: GPS antenna location 

John Navas wrote:
> On Sun, 27 Jan 2008 15:15:32 +0000, larry <noone RemoveThis @home.com> wrote in
> <Xns9A3268E4AA8CBnoonehomecom RemoveThis @208.49.80.253>:
>
>> JohnW <invalid RemoveThis @earlsway.invalid> wrote in
>> news:MPG.220684998873fe4f989877@news.aaisp.net.uk:
>>
>>> If you are pitching and rolling, the antenna will be moving
>>> relative to the boat so the GPS will include that motion in
>>> with the boat's forward velocity in its speed calculation.
>
> Plus interference with direction over ground calculations due to rocking
> from side to side.
>
>> If you have a handheld GPS, carry it into the burger joint on a busy
>> road and let it bread trail on close range. The signal can't get
>> through the roof so what the GPS receives are signals bouncing off
>> objects outside, like passing vehicles and stationary (we hope)
>> buildings through the big windows. Let it run an hour and its fix will
>> cover the whole shopping center....many hundred feet! This same effect
>> happens in a HARBOR or the ICW! Signals bouncing off nearby conductive
>> objects, especially overhead bridges, just eats it alive. Anywhere near
>> shore a GPS fix gets wider and wider in accuracy because of multipath,
>> the same signal bouncing that tears up a UHF TV signal on an old analog
>> TV with "ghosts", signals arriving later than the main signal which
>> ALWAYS make ghosts to the RIGHT of the main signal, because they arrive
>> later...we scan from left to right, top to bottom like reading a page in
>> a book....except every other line, called interlacing to make it flicker
>> less.
>
> I record NMEA output from my Magellan Sportrak Color GPS on my laptop,
> and I'm not seeing that kind of error -- my tracks are quite accurate
> when checked on the charts on my laptop.
>
>> Ever wonder why it only updates every second? It's trying to average
>> out the MULTIPATH MOVEMENT ITS MEASURING!
>
> It's actually feeding valuable real-time data to my laptop, which is
> automatically computing and displaying target speed polars in real time.
>
I have had trucks travel all over Europe with gps tracking
to a laptop, and I could consistently see, on which side of the highway
those trucks traveled.
No problems with cars/trucks being around, passing trafficlights, etc.
Only tunnels broke the track
And also very bad weather(high thunder clouds/extremely heavy rain).
Also we used them in harbours for the british navy, in a blind course
guidance experiment. Worked like a charm.
Only place were we had trouble was for the same experiment inside a
helicopter. Those rotorblades dont treat GPS kindly.
 >> Stay informed about: GPS antenna location