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Re: SNO-T tests
From: Jim Hickey
Date: 2006 Apr 12, 11:49 -0400

Alex,

One of my main interests has been to see what the experience of others has been as far as accuracey of sextant sights. Having said that I not collected a pile of data and analyzed it the way I would like to. Lack of time sounds better than lazy but too lazy is closer to the truth!

In response to your posting:

I typically use data from Omar Reis' Navigator program. In my opinion it is a nice program to use and seems to be quite accurate. Next I would use my Palmn T3 running either Celestnav or Astro Navigation. Celestnav is nice since it actualy provides almanac data. Astro Navigator is quite adequate for practical real life navigation situations. I have used the ICE from the hygrographics office and of course last and not least the good old tabular nautical almanac. Every once in a while I come across as sight that seems to be out more than I would expect. If I run the numbers through the various alamancs I find small variations in the alamanac data.

If I do a lunar observation, Frank's online calculator is great although I have taken almanac data and run it through a spreadsheet. I really have not compared what impact different alamanc data has on Lunars although from what I have seen, I am sure it would impact the results to some degree.

I have by this point tried about every type of measurement I could make with a sextant but for the most part, I simply let the conditions guide me.

The following is some data I posted some time ago. It represents a rather poor effort to start collecting some data for further alaysis but I have not really pursued it much more. The data was taken with an Oaklet sextant which is similar to an Astra and using either the PC or th Palm for reduction.

I actually would be interested in working with someone to compare results collected and evaluated in a similar way.

- These are differences in minutes of angle between Hc and Ho, approximate position being GPS position
- Mostly star sights
- Taken at different times over the past year or so
- All taken with an artificial horizon
- All taken under ideal conditions i.e. calm clear
- No averaging or plotting

0.4
0.3
0.4
-0.4
-0.5
0.2
0.1
0.1
-0.1
0
-0.2
0.1
-0.4
0.2
-0.1
0.4
-0.3
0.3
0.1
0
-0.1
0.3
0.1
-0.1
-0.3
0.1
0.4
-0.2

-----Original Message-----
From: Alexandre E Eremenko <eremenko@MATH.PURDUE.EDU>
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Sent: Wed, 12 Apr 2006 10:48:30 -0400
Subject: Re: SNO-T tests

Jim,
What you are saying is very interesting.
When you have time, would you post (or send to me)
some specific data?

> For what it's worth, I have found that under good conditions
> measurements  are typically within 0.7' most of the time (say  90% of the
time) and
> within 0.3' about a third of the time.

What sextant do you use? What sort of measurement you are talking
about (Sun altitudes with real/artificial horizon? Star altitudes?
Lunar or star distances?)

> I have noticed that there can be some significant differences (0.3 at  times)
> depending on the source of the almanac data at times.

I only use two sources: THE Nautical Almanach (on paper, commercial
edition, which is guaranteed to be identical with the Govnmnt edition)
and Frank's web calculator for the Lunars. Couple of times I reduced
my lunars myself using the same Almanach, and the result
(rounded to 0.1') was exactly the same as Frank's calculator gave. So I
trust
Frank's calculator (coupled to an electronic almanach).

Another source of error I encountered was the use of various simplified
reduction
tables. (I suppose HO tables cannot introduce the error of 0.1', but
actually I never checked this. My prefered method is Casio f-calculator
which I trust completely. This calculator was recommended (as the best
calculator
for Cel Nav) in a Russian book of 1980 ! Such a pity that
production of this wonderful
thing was discontinued! My one is 17 years old, and I see nothing as good
on the market today. But I hope it will serve another 17 years unless
I drop it to the sea:-)

Alex.

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