[alberg30] GPS mount

[John Birch]

Hi Sanders;

We have a Garmin 120, now upgraded to the 128 model with external patch antenna.

Mount the antenna on the pushpit - clear of life rings, BBQ, etcetera with
whatever brand you use.
You will likely have trouble with signal loss if mounted with an internal
antenna at the old compass location, particularly with a dodger and people near
by.

If you want an accurate knot meter, buy one (knot meter, that is). There are
several relatively inexpensive models available - suggest mounting the impeller
forward as close to the centreline as reasonable and where you can get at it
inside.  It was a mistake when Whitby mounted the Sumlogs aft on the keel side,
guaranteeing inaccuracy on one tack due to turbulence.

A GPS is very useful and a good aid to navigation but its very short term speed
accuracy for incremental trim changes is questionable.  It is much more accurate
measuring speed/distance and course overtime, when the randomness of SA is
cancelled out.

Personally I wouldn't spend the extra coin on the Differential System - but many
do, to each their own.

There my nickel's up. ; )

Cheers,

John, Sunstone KC-65

Timothy C. Lackey wrote:

> From: "Timothy C. Lackey" <tlackey1 at maine.rr.com>
>
> All GPS--whether differential or not--provide very relaible and accurate
> SPEED OVER GROUND readings.  They do not provide speed through the water as
> a knotmeter does.  Differential GPS provides a correction to the satellite
> signal to correct for the government's random downgrading of the original
> signal and therefore may provide more accuracy (assuming that the original
> signal is even downgraded, which it probably isn't much of the time; it's a
> random (called "selective availability" or SA) manipulation of the signal,
> and the amount, if any, varies from nothing to within a reasonable limit).
> However, it is only speed over the ground--useful readings, to be sure, but
> different than what your knotmeter will tell you.
>
> For navigational purposes, speed over the ground is what is important--it is
> what will tell you how long you have to go, etc.  It invisibly takes into
> account all external forces--current, set, drift, etc. and is more accurate
> in the long run than a knotmeter.  For your purposes, Sanders, it WILL work
> as a very accurate knotmeter, because it does measure the speed over ground
> instead of simply speed through the water.  Example:  Say you're heading due
> east at 5 knots through a 3 knot westerly current.  You're only making 2
> knots over the ground, and this is the value your GPS will give you.  If you
> calculate ETA based on your knotmeter, and not knowing the exact current,
> you'd probably come up with a time far earlier than you will really arrive.
> The GPS speed given on the screen would be dead-on accurate at all times,
> however.
>
> Get a GPS--you won't be disappointed.  Like all electronics, it has its
> limitations, but it is an excellent, accurate, reliable system.
>
> Tim
>
> ----- Original Message -----
> From: <sandersm at aol.com>
> To: <alberg30 at onelist.com>
> Sent: Saturday, February 26, 2000 1:50 PM
> Subject: Re: [alberg30] GPS mount
>
> > From: sandersm at aol.com
> >
> >
> > In a message dated 2/26/00 1:29:02 PM, mgrosh at shore.intercom.net writes:
> >
> > << For what it is worth, for the last several months I have been using a
> DGPS
> >
> > (Differential) which is the Coast Guards patch to restore accuracy to the
> >
> > DOD system (I know, I know, but this is the US Gov't we are talking here
> >
> > :) ) and it's accuracy is impressive.  >>
> >
> > Does the DGPS unit provide reliable speed readings?  I must confess
> (having
> > undue confidence in my piloting and dead reckoning skills) that I was
> totally
> > uninterested in GPS until the yard told me what a help it would be as a
> > knotmeter.  I'm not interested in holing my hull for a knotmeter, but the
> > thought of having a way to measure speed and pinpoint location with one
> > no-hassle instrument does appeal to me.  Bob's remarks about GPS's
> > uselessness as a knotmeter lead me back to my original inclination to
> avoid
> > all electronics.  Unless, perhaps, the DGPS solves the problems Bob
> mentions.
> >  Does it?
> >
> > Sanders McNew
> > WILD ELF  (# 297)
> >
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