On Sun, 27 Jan 2008 12:08:31 -0000, JohnW
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

JohnW wrote in
:

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.
-

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.

Boat GPSes are NOT GPS surveying instruments like the Geodetic Survey
little Japanese guy who comes to my house to check the fault line I live
on for movement in mm every month. God help any of you that think that
cheap piece of crap in the plastic box is gonna put you within 5 ft of
the bouy in the fog. It's just NOT accurate to inches.....EVER.

Here, test it at the dock. Turn it on and clear its bread trails.
Leave it on sitting dead still at the dock in perfectly flat water until
tomorrow. See if it stays within 5 ft for a day sitting still. It
won't, but you need to know and NOT trust it so much. If you live in a
metro area with an airport, the aluminum clouds flying by will make it
really go crazy over the course of a day, suddenly jumping way down the
dock, then jumping back as the aluminum clouds move around. GPS works
on the phase relationships between precisely pulsed microwave signals
from 3 or more overhead birds. If you change the PATH from the birds to
the GPS, huge errors are introduced into the GPS phase relationships.

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.

All this terror over the motion of the mast is just crazy! The mast,
itself, and all your rigging to any GPS antenna on the deck is causing
multipath signals from the overhead birds....and screwing up the timing.

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

In article ,
larry 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

--
remove bye and from mercial to get valid e-mail
http://www.heusser.com

On Sun, 27 Jan 2008 15:15:32 +0000, larry wrote in
:

JohnW wrote in
:

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

On Mon, 21 Jan 2008 09:51:04 -0800 (PST), Rob
wrote:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob

Either is fine, as long as the GPS antenna is clearly outside of the
radar beam. (Although on a previous boat, an ancient GPS
antenna/receiver was mounted about a foot from a 4 KW radar scanner,
right in the beam, with no apparent problems.)


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

On Mon, 21 Jan 2008 09:51:04 -0800 (PST), Rob
wrote:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob

Disregard my previous post (although it is correct about keeping the
GPS antenna out of the radar beam). As others have said, the GPS
antenna should be mounted fairly low on the boat, and in a location
where it has a clear view of the sky in all directions.

Mounting the GPS antenna on the mast will cause it to see spurious
movements as the boat rocks. In addition, if the GPS antenna is
immediately below the radar scanner, the scanner will block the GPS
satellite signals from a fair portion of the sky.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

Is this a sailboat? I assumed powerboat with an electronics mast. If
it is sail, get it low and clear (as all the other posters suggested).
If power and you only have the mast space then separate as far apart
and I prefer BELOW the beam of the radar. I have 3 on my hardtop, all
mounted within 24" of an open array 4KW but all just below the beam. No
SNR issues at all.



Rob wrote:
I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob

Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob


Wrong attitude. The GPS antenna has no need for ALTITUDE. Aboard
Lionheart, I got tired of them bumping into both the Raystar little dome
and the Garmin active antenna, so I mounted them INSIDE the overhead
cabinet in the galley, behind the helm (just forward of the bulkhead-
mounted wheel on an Amel 41 Sharki ketch). Fiberglass and plastic is
transparent to RF energy (especially RF tearing up the damned HF/SSB). The
signals work perfectly.

On a big center console fishing boat, they were looking for a good place to
mount the GPS so the fishing could tear it off and were looking way up
high, which is crazy. I said, "Let's just lay it INSIDE the overhead
cabinet behind the GPS and radios under the plastic bimini top and see how
it does across the harbor." During my free boat ride..(c;...we noted the
GPS worked perfectly with strong signals in all directions. So, when we
got back to the dock, having run completely out of beer, we mounted the GPS
antenna in the back corner of the INSIDE of that electronics cabinet, out
of the way of the other gear and wires. It's worked perfect there ever
since. If he had had a sheet metal top, of course, that wouldn't have been
possible. Noone fishing can get to the GPS antenna to rip it off, now.
They'll have to pull the overhead electronics console apart to even find
it!

larry wrote:
Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob


Wrong attitude. The GPS antenna has no need for ALTITUDE. Aboard
Lionheart, I got tired of them bumping into both the Raystar little dome
and the Garmin active antenna, so I mounted them INSIDE the overhead
cabinet in the galley

Belowdecks is the way to go in a fiberglass boat.

My friend held the antenna alternately in the clear abovedecks, and
belowdecks hard up against the coachroof in various spots. I watched the
signal strength indicator on the chartplotter as she moved the antenna
between the abovedecks and belowdecks positions. In many spots
belowdecks there was no difference in signal strength from the
satellites with the antenna abovedecks or belowdecks.

My GPS antenna is now mounted in a good spot belowdecks, out of the
weather, away from running rigging and flying feet.

Moonshadow wrote:
larry wrote:
Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?

Belowdecks is the way to go in a fiberglass boat.

My friend held the antenna alternately in the clear abovedecks, and
belowdecks hard up against the coachroof in various spots. I watched the
signal strength indicator on the chartplotter as she moved the antenna
between the abovedecks and belowdecks positions. In many spots
belowdecks there was no difference in signal strength from the
satellites with the antenna abovedecks or belowdecks.

My GPS antenna is now mounted in a good spot belowdecks, out of the
weather, away from running rigging and flying feet.

Better yet, get an internal antenna version of the unit, it
has optimal connectivity with no cabling. My Garmin GPSMAP
172C is mounted inside a pilothouse and always hits on any
satellite shown above the horizon, without problem.