[alberg30] GPS mount

[Robert Kirk]

From: Robert Kirk <kirk at neptune.gsfc.nasa.gov>

At 01:50 PM 2/26/00 -0500, sandersm at aol.com wrote:
>[...]
>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?


GPS is impressively accurate and reliable in speed measurement.

GPS speed is calculated (by all the manufacturers, now, I believe) from the 
satellites' doppler and not by differentiating their positions. So, to 
first order, SA is not a factor in the speed calculation once you get 
underway with more than a knot or two of speed. There may be minor 
differences amongst the manufacturers in the implementation of the doppler 
algorithms and the degree of smoothing but the newer units are all very 
good. If  I remember, the older Magellans did a terrible job and used 
position differentiation, averaged over a selectable interval. But they've 
been superseded. Even my antediluvian Motorola uses doppler. (You know, 
like the cop's radar).The bottom line is that you can expect 1/10 
knot  absolute speed accuracy from a moving boat -  pretty good compared to 
the log.

However, you might not want to throw away the old Pit Log yet. GPS 
gives  you speed over the ground, which is great for navigation, but if 
you're racing you might want to know speed through the water, too.  Anfd 
you can use the two of them in combination to calculate the current set & 
drift with a simple maneuvering board. Also, the log is analog and so is 
superior in picking up the quick trends of minor sail adjustments which 
might disappear in the noise of any digital calculation from GPS if you're 
racing.

Anyway, the GPS is revolutionary; you could throw away your  compass, pit 
log, & sextant.

Cheers,
Bob Kirk
Isobar #181





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