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    Re: Something Of Interest
    From: Henry Halboth
    Date: 2004 Aug 3, 11:11 -0400

    Checked out the referenced site and found it to be well put together and
    most interesting - I did find the lack of co-incidence, i.e., the
    so-called "cocked hat", to be, in my opinion, somewhat over-emphasized.
    In my experience, varying horizon conditions are the bugaboo of celestial
    navigation and the cause of most variation in sight results. As
    previously posted, and ruling out otherwise correctable errors, including
    instrument error and inexperience, "cocked hats" are to be opined
    resultant upon poor sights amongst those taken and not resolvable by
    technology, but rather by experienced evaluation. Lecky discusses this
    subject to an extent and, if my recollection is correct, quantifies AM/PM
    sight differentials up to 8-miles, as between AM/PM observations - true,
    I believe his discussion referred to the Red Sea, where refraction
    differences can be enormous, and where I myself noted as much as 12-mile
    differentials, apparently refraction associated. I have no doubt that
    celestial navigation will be relegated to the dustbin in the future, just
    a Lunars have been, as technology becomes more trustworthy, as well as to
    an extent oversold - back-up systems upon back-up systems also will seek
    to insure reliability under conditions so often cited for and against
    against celestial navigation, - however, until that happens, experience
    seems the qualifier of any of the older forms of navigation, a little
    calculation notwithstanding.
    On Mon, 26 Jul 2004 07:49:59 -0400 Joel Jacobs 
    >  GO: http://www.seamanship.co.uk/deck/navigator/ASNAv/ASNAv%20Site/
    > Joel Jacobs
    > ASNAv is an astronomic navigation program written by a Merchant Navy
    > officer.
    > ASNAv calculates the user position by means of stars observations. A
    > nautical almanac is not required.
    > The user enters only his estimated position and sextant altitudes of
    > heavenly bodies (stars, planets, sun, moon). The program computes
    > and plots
    > (see example) the most probable position and an ellipse of
    > probability
    > around it, together with the lines of position (LOPs).
    > ASNAv is something new in the world of celestial navigation
    > programs.
    > Because ASNAv is designed by a seaman for seamen.
    > The program aims to be a practical option as backup of the today
    > omnipresent
    > GPS. This means that ASNAv is trying to decrease the additional
    > workload to
    > a minimum. There is nearly no time consumed outside the time
    > required for
    > the observations themselves.ASNAv is written by a Merchant Navy
    > chief
    > officer and is user-friendly from a seaman point of view.
    > Because ASNAv is definitely smarter that the existing
    > astronavigation
    > programs.
    > Indeed, a statistical analysis is performed:
    >   a.. each observation can include some small errors due to a bad
    > horizon,
    > clouds or the ship's rolling during the measure. It is difficult to
    > estimate
    > oneself the quality of a given observation because the small errors
    > can
    > counterbalance each other. That's why ASNAv gives a certain weight
    > to each
    > observation according to its reliability in the normal law model.
    >   b.. very often an observer is repeating a same constant error for
    > each
    > observation (uncorrected sextant index error and personal error).
    > This
    > possible systematic error of the observer can be computed and
    > eliminated;
    >   c.. the program can also correct the assumed course and speed if
    > enough
    > observations are provided (exactly the same way the GPS is able to
    > give the
    > course and speed of the vessel if enough satellites are visible).
    > This last feature can greatly improve the accuracy of the results
    > compared
    > with a traditional method if the exact course and speed are not
    > known
    > (under- or overestimated drift). However, you need at least 8
    > observations
    > during 3 hours in order to make course and speed correction
    > possible. So,
    > it's a good idea to combine a star fix and a number of sun
    > observations.
    > See an example of the ASNAv skills.

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