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<BODY TEXT="#000000" BGCOLOR="#FFF0F0" LINK="#FF0000" VLINK="#800080" ALINK="#0000FF"><tt>From:</tt> <tt>
John Birch <sunstone@idirect.com></tt>
<br><br>
Hi Sanders;
<P>We have a Garmin 120, now upgraded to the 128 model with external patch
antenna.
<P>Mount the antenna on the pushpit - clear of life rings, BBQ, etcetera
with whatever brand you use.
<BR>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.
<P>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.
<P>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.
<P>Personally I wouldn't spend the extra coin on the Differential System
- but many do, to each their own.
<P>There my nickel's up. <B>; )</B><B></B>
<P>Cheers,
<P>John, Sunstone KC-65
<P>Timothy C. Lackey wrote:
<BLOCKQUOTE TYPE=CITE>From: "Timothy C. Lackey" <tlackey1@maine.rr.com>
<P>All GPS--whether differential or not--provide very relaible and accurate
<BR>SPEED OVER GROUND readings. They do not provide speed through
the water as
<BR>a knotmeter does. Differential GPS provides a correction to the
satellite
<BR>signal to correct for the government's random downgrading of the original
<BR>signal and therefore may provide more accuracy (assuming that the original
<BR>signal is even downgraded, which it probably isn't much of the time;
it's a
<BR>random (called "selective availability" or SA) manipulation of the
signal,
<BR>and the amount, if any, varies from nothing to within a reasonable
limit).
<BR>However, it is only speed over the ground--useful readings, to be sure,
but
<BR>different than what your knotmeter will tell you.
<P>For navigational purposes, speed over the ground is what is important--it
is
<BR>what will tell you how long you have to go, etc. It invisibly
takes into
<BR>account all external forces--current, set, drift, etc. and is more
accurate
<BR>in the long run than a knotmeter. For your purposes, Sanders,
it WILL work
<BR>as a very accurate knotmeter, because it does measure the speed over
ground
<BR>instead of simply speed through the water. Example: Say
you're heading due
<BR>east at 5 knots through a 3 knot westerly current. You're only
making 2
<BR>knots over the ground, and this is the value your GPS will give you.
If you
<BR>calculate ETA based on your knotmeter, and not knowing the exact current,
<BR>you'd probably come up with a time far earlier than you will really
arrive.
<BR>The GPS speed given on the screen would be dead-on accurate at all
times,
<BR>however.
<P>Get a GPS--you won't be disappointed. Like all electronics, it
has its
<BR>limitations, but it is an excellent, accurate, reliable system.
<P>Tim
<P>----- Original Message -----
<BR>From: <sandersm@aol.com>
<BR>To: <alberg30@onelist.com>
<BR>Sent: Saturday, February 26, 2000 1:50 PM
<BR>Subject: Re: [alberg30] GPS mount
<P>> From: sandersm@aol.com
<BR>>
<BR>>
<BR>> In a message dated 2/26/00 1:29:02 PM, mgrosh@shore.intercom.net
writes:
<BR>>
<BR>> << For what it is worth, for the last several months I have
been using a
<BR>DGPS
<BR>>
<BR>> (Differential) which is the Coast Guards patch to restore accuracy
to the
<BR>>
<BR>> DOD system (I know, I know, but this is the US Gov't we are talking
here
<BR>>
<BR>> :) ) and it's accuracy is impressive. >>
<BR>>
<BR>> Does the DGPS unit provide reliable speed readings? I must
confess
<BR>(having
<BR>> undue confidence in my piloting and dead reckoning skills) that I
was
<BR>totally
<BR>> uninterested in GPS until the yard told me what a help it would be
as a
<BR>> knotmeter. I'm not interested in holing my hull for a knotmeter,
but the
<BR>> thought of having a way to measure speed and pinpoint location with
one
<BR>> no-hassle instrument does appeal to me. Bob's remarks about
GPS's
<BR>> uselessness as a knotmeter lead me back to my original inclination
to
<BR>avoid
<BR>> all electronics. Unless, perhaps, the DGPS solves the problems
Bob
<BR>mentions.
<BR>> Does it?
<BR>>
<BR>> Sanders McNew
<BR>> WILD ELF (# 297)
<BR>></BLOCKQUOTE>
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