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    Re: Accuracy of sextant observations at sea
    From: Antoine Couëtte
    Date: 2010 Nov 25, 02:18 -0800

    Dear Jeremy,


    Nice to read some true world observational data from you again !!!

    1 - DATA PROCESSING

    Just a bit of computation results here, in which I am indicating additionnal figures which obviously do not have any practical sense. I think that everybody will admit that, and will feel comfortable with this preliminary caution.

    1.1 - From your values, and assuming that they all have the same "weight" or importance), I am finding that - FROM YOUR DR POSITION - your "Observed Position" is in Azimuth 039.36° and at a distance of 0.06 NM. Standard deviation of these LOP's - i.e. a number which measures how much the LOP's are "gathered" (or better : "scattered" ???) around the "Observed Position" - is equal to 0.35 arc minute.

    1.2 - Then, IF - ad only IF - we assume that there WOULD BE be a constant and systematic error in your measurements, such error would amount to 0.29 arc minute. If so, through reworking all data (i.e. removing 0.29 Arc minute to all your intercepts), we would get a "systematic error free" OP in Azimuth 355.78° and at a distance of 0.18 NM from your DR Position.

    2-DISCUSSION OF RESULTS


    2.1 - About Point 1.1 : the resulting distance from 1.1 indicates a .06 NM fix error - and that is essentially what counts !!! It is extremely close - if not identical - to the "0.0 fix error" you have indicated. The minor difference here might be due to different number crunching algorithms. These results speak for both the high quality of your observations and your professionnal habit of shooting bodies all around the horizon.


    2.2 - About Point 2.2 : I am always reluctant to derive statistical results from "assumptions", especially with such a limited number of samples. I am not even comfortable that my assumption here is valid. So, I would not give priority to the results of Section 1.2, over the ones of Section 1.1. As a conclusion ... YOU STILL ACED IT Buddy !!!


    3 - ADDITIONNAL COMMENTS


    I think that your observations - once again - are excellent ! especially since you keep observing from a quite significant elevation (106 FT ???). Since the QUALITY OF THE HORIZON is essential, when there is some choice of a lower elevation it is certainly recommended to go for the lowest one, as long as the swell or even the waves themselves do not "destroy" he horizon line. This brings me to the following point, where we can still get (very) good results even with "irregular swell/waves horizons" : mutiples shots on one same body.

    Are your individual data single shots results, or are they averaged values ? After some experiment I have observed that if I am to take 20 shots, I am (almost) always getting much better fixes - at least in Latitude because Longitude can only be as good as your Chronometer ... - if I shoot 4 bodies (5 measures on each) than if I shoot 20 (**) different bodies. (** : not quite actually, with my record being 17).

    There is an UNQUESTIONABLE SUPERIORITY OF AVERAGING SIGHTS, and it has also been very well and clearly addressed here by Frank not long ago.

    Well ... nothing more to say here, but that I am again very impressed at the quality of you observations. Attaboy, Mate !

    Last but not Least ... Which Sextant are you using ? Half View / Full View mirror, Telescope size ???

    *******

    Joker question : how about shooting a Moon-Venus Lunar in just a few days (even to-morrow morning) and publishing your observational data ? If you can give the the following : carefully recording Moon + Venus heights, and everything else (All Observation times, Temp, Press, Speed and Course) we will even go for a "full fix" just from the Lunar and its associated heights.

    NOTE TO DOUGLAS DENNY : I am certainly one of the Members who keep higly regretting your now on-going silence on Lunar Matters in which you have so nicely contributed in the past month of October. I really enjoyed how much enthusiam you have shown in this field. My earlier comments were only aimed at some improvement - not even in your observations, but essentially in your lunar computations - and I trust that you felt that way about them. If you decide to publish you next Venus-Moon Lunar (the one you might be intending to shoot to-morrow morning also), it will be a real pleasure for me to study it. Thanks, and Best Friendly Regards to you Douglas.

    *******

    Thank you for your Kind Attention and Best Friendly Regards to you too Jeremy.


    Antoine


    Antoine M. "Kermit" Couëtte


    PS : I now understand why you were in Diego Garcia earlier this year. You are on a US NAVY Military Sealift Command Vessel. It explains all ... including the story of your Skipper who always wanted to stay within reach of TV broadcast.
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