[alberg30] GPS mount

[Timothy C. Lackey]

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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>
>


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