<HTML><tt>From:</tt> <tt>
"Mr. Kelly McDaniel" <kelly.mcdaniel@gov.calgary.ab.ca></tt>
<br><br>
Anyone contemplating "throwing out" your compass, pit log or sextant please
contact me direct as I could definitely put these to practical use.
<P>Kelly _/)
<P>Robert Kirk wrote:
<BLOCKQUOTE TYPE=CITE> <TT>From:</TT> <TT>Robert Kirk <kirk@neptune.gsfc.nasa.gov></TT>
<P><TT>At 01:50 PM 2/26/00 -0500, sandersm@aol.com wrote:</TT>
<BR><TT>>[...]</TT>
<BR><TT>>Does the DGPS unit provide reliable speed readings? I must
confess (having</TT>
<BR><TT>>undue confidence in my piloting and dead reckoning skills) that
I was totally</TT>
<BR><TT>>uninterested in GPS until the yard told me what a help it would
be as a</TT>
<BR><TT>>knotmeter. I'm not interested in holing my hull for a knotmeter,
but the</TT>
<BR><TT>>thought of having a way to measure speed and pinpoint location
with one</TT>
<BR><TT>>no-hassle instrument does appeal to me. Bob's remarks about
GPS's</TT>
<BR><TT>>uselessness as a knotmeter lead me back to my original inclination
to avoid</TT>
<BR><TT>>all electronics. Unless, perhaps, the DGPS solves the problems
Bob mentions.</TT>
<BR><TT>> Does it?</TT>
<BR>
<P><TT>GPS is impressively accurate and reliable in speed measurement.</TT>
<P><TT>GPS speed is calculated (by all the manufacturers, now, I believe)
from the</TT>
<BR><TT>satellites' doppler and not by differentiating their positions.
So, to</TT>
<BR><TT>first order, SA is not a factor in the speed calculation once you
get</TT>
<BR><TT>underway with more than a knot or two of speed. There may be minor</TT>
<BR><TT>differences amongst the manufacturers in the implementation of
the doppler</TT>
<BR><TT>algorithms and the degree of smoothing but the newer units are
all very</TT>
<BR><TT>good. If I remember, the older Magellans did a terrible job
and used</TT>
<BR><TT>position differentiation, averaged over a selectable interval.
But they've</TT>
<BR><TT>been superseded. Even my antediluvian Motorola uses doppler. (You
know,</TT>
<BR><TT>like the cop's radar).The bottom line is that you can expect 1/10</TT>
<BR><TT>knot absolute speed accuracy from a moving boat - pretty
good compared to</TT>
<BR><TT>the log.</TT>
<P><TT>However, you might not want to throw away the old Pit Log yet. GPS</TT>
<BR><TT>gives you speed over the ground, which is great for navigation,
but if</TT>
<BR><TT>you're racing you might want to know speed through the water, too.
Anfd</TT>
<BR><TT>you can use the two of them in combination to calculate the current
set &</TT>
<BR><TT>drift with a simple maneuvering board. Also, the log is analog
and so is</TT>
<BR><TT>superior in picking up the quick trends of minor sail adjustments
which</TT>
<BR><TT>might disappear in the noise of any digital calculation from GPS
if you're</TT>
<BR><TT>racing.</TT>
<P><TT>Anyway, the GPS is revolutionary; you could throw away your
compass, pit</TT>
<BR><TT>log, & sextant.</TT>
<P><TT>Cheers,</TT>
<BR><TT>Bob Kirk</TT>
<BR><TT>Isobar #181</TT>
<BR>
<BR>
<P>
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