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Re: GPS backup/emergency navigation
From: Rick Emerson
Date: 1998 Nov 25, 7:12 AM

```A friend wrote the following comments in response to a discussion
about the need for sight reductions with the Nautical Almanac and
sight reduction tables of some sort (e.g., HO 249) versus doing the
reductions by solving the navigation triangle "by hand".

[...]
> No you don't, i.e. need an almanac or HO 249.  Well, you need an almanac,
> but the perenial one in Bowditch that's about 3 or 4 pages long will
> suffice for the sun and stars.  It's based on the fact that the almanac
> nearly repeats every 4 yrs. and a fudge factor is added.   A few basic trig
> formulas and you don't need 249.  So, volume is minimal and nothing goes
> out of date.  So, it's perfect for the lifeboat, as long as you practice
> with it occasionally.  The Celesticomp or just a programmable scientific
> calculator will certainly make things easier, but I'd like to be dependant
> on no technology except the watch and the sextant.  Yes the time factor is
> in your favor.
>
> When I find time, I'd like to try working the same sights several ways:
>
> 1)  With yearly almanac and HO 249
> 2)  Perennial almanac and 249
> 3)  Per. alm. and calculator for direct sight reduction.
> 4)  Per. alm. and trig. tables and figure by hand.
> 5)  Per. alm. and abbreviated trig. tables, e.g. sine and cosine just for
> whole degrees, and then extrapolate in between as needed.
>
> I figure that if the error is less than 5 miles, it's adequate for backup
> navigation.  And satisfying in it's KISS philosophy, i.e. doing the most
> with the least.
[...]

As I told Mark, I can try to check the assumption by reducing fixes
with data 4 years from the correct date but if anyone has done
research on the techniques listed above, we'd like to know about
quantitative results.  No point in re-inventing or re-examining the
wheel!

Rick
S/V One With The Wind, Baba 35
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