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Subject: Re: accuracy of automatic celestial navigation
From: Dan Allen (danallen46@XXX.XXX)
Date: Sat Dec 07 2002 - 01:18:52 EST
On Friday, December 6, 2002, at 04:05 PM, George Huxtable wrote:
> For those of us that sail our small craft out at sea, my opinion is
> that if we can achieve a precision of 2 min, we are doing pretty well.
> What do others think?
I would agree. 2 nmi at sea is very decent; on land we should be able
to get to 1/2 mile or better.
On land I recently took a string of shots using a Tamaya Jupiter
sextant with a 7X scope. I shot the sun mid afternoon over a period of
20 minutes. (This was on the coast of Oregon at a beachhouse on the
water.) I used a Garmin GPS in averaging mode to determine my actual
position to 15 feet or so. The resulting intercepts in nmi are
interesting to analyze. Because my assumed position is known to within
15 feet, these can be taken as errors in my sights. Here they are in
order:
0.91
-0.06
0.59
-0.05
0.22
0.55
0.22
-0.28
-0.53
-0.57
-0.77
-0.03
-0.19
-0.11
-0.43
-0.62
0.27
0.24
0.44
0.25
The mean of the intercepts is a mere 0.0025 nmi -- very good -- but the
individual readings varied over a 1.68 nmi range -- not so good.
When we are "warming up" and we haven't used our sextants in a while I
find that the first shots are often in greater error than later sights.
This is in fact the case with this set of shots shown above: the worst
error came from the very first sight. Therefore I have found that it
is wise throw out the the first reading or two, or at least to discount
them.
Other experiments I have done:
I took five shots in my backyard using the same sextant and an
artificial horizon and got a 1.27 nmi range of errors and a mean error
of 0.644 nmi.
I took fourteen shots in my backyard using a Tamaya Venus (not nearly
as powerful of scope) and an artificial horizon and got a 4.3 nmi range
of errors but a mean error of -0.45 nmi because of the greater number
of sights taken.
And for my finale, one afternoon I used six different sextants in my
backyard and I took a total of 46 sights using an artificial horizon.
I got an 11.2 nmi range of errors with a mean error of
-1.23 nmi.
From this the statistical nature is shown to be very useful: although
any one shot could be in error up to almost a mile, a series of shots
averaged does indeed improve accuracy to very good levels; STELLA
appears to do this, and thus an automated system is very appealing.
Dan
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