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    From: Joel Jacobs
    Date: 2004 Jul 26, 07:49 -0400

     GO: http://www.seamanship.co.uk/deck/navigator/ASNAv/ASNAv%20Site/
    Joel Jacobs
    ASNAv is an astronomic navigation program written by a Merchant Navy
    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
    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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