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 17:05:39 +0100, Marc Heusser
d wrote in
:

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 ...

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

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 Sat, 26 Jan 2008 18:44:41 -0000, "Nicholas Walsh"
wrote in :

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

John Navas wrote:
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.

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.