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    Problems with AstronavPC
    From: George Huxtable
    Date: 2004 Feb 15, 15:15 +0000

    Problems with AstronavPC.
    
    Do any listmembers possess the book (with CD), "AstronavPC and compact data
    2001-5" or an earlier edition? Or perhaps another version of the same
    thing, published in the UK as "NavPac and compact data 2001-2005". What
    follows will be of interest only to those that do.
    
    Even better, have any listmembers attempted to implement the procedures
    given, in the chapter "Compact Data-Explanation"; in sections 7.4,
    "Position from intercept and azimuth by calculation", and/or 7.5,
    "Estimated position error"? Better still, does anyone claims to understand
    those procedures?
    
    I'm not asking whether the built-in program gives the right answer when you
    ask it to "display : plot of position lines". I've no reason to think it
    doesn't, though unable to try it out because it doesn't work on my Mac. All
    I an questioning is the written explanation.
    
    1. Go to section 7.5, "Estimated position error". This shows how a
    "confidence ellipse" can be calculated, with major axis a, minor axis b,
    and orientation of the major axis "theta". Near the end of the section is
    stated "The ideal situation is to produce a circular distribution of
    errors, with A = B and C = 0, so that the errors are the same in all
    directions." This seems to me to be wrong: to produce a circular
    distribution of errors, I suggest that what's needed is instead for A = C
    and B = 0. Only then would the ellipse become a circle; (a = b, and theta
    becomes immaterial). Does anyone agree (or, more interesting, disagree)?
    
    2. A few lines down from the start of 7.5 is stated "The standard
    deviations sigma(L) and sigma(B) in longitude and latitude are given by-"
    and so on. I don't question the value for latitude, but I suggest that for
    longitude, the value given is for standard deviation of the East-West
    displacement, in nautical miles (that is, the "departure"), rather than in
    longitude (which is always expressed in terms of angle).
    
    3. Toward the end of 7.4 is stated- "If d exceeds about 20 nautical miles
    set L(F) = L(I), B(F) = B(I) and repeat the calculation until d, the
    distance between the position of the previous estimate and the improved
    estimate, is less than about 20 nautical miles".
    
    In the sentence above, "repeat the calculation" must first require, for
    each body sighted, the following-
    
    [Using the newly updated values for B(F) and L(F) as Lat and Long, rework
    the calculated altitudes and azimuths, in 7.2.2, 7.2.3, and 7.2.4, and
    obtain a new intercept p from (observed alt. - calculated alt.). For this
    to be possible, the original values for dec., GHA, and observed altitude of
    the body must have been retained.]
    
    It's a pity that there's no specific mention of that necessary part of the
    iteration procedure, given in square brackets
    
    Then the new values Z and p are collected together for all these bodies in
    the expressions for A to G in 7.4. This allows new values for dB and dL to
    be calculated for a further iteration.
    
    In 7.4 is the statement- "Additional observations may be included in the
    solution by simply adding the extra terms to the summations A,B,C,D,E, and
    F and calculating dL and dB again." To me, that seems to be rather futile,
    in that it would be impossible to work any further iterations using those
    added terms unless their original values for dec, GHA, and observed
    altitude had been preserved.
    
    ===============
    
    I'm tempted to submit these as suggestions for the next revision of this
    publication by HM Nautical Almanac Office (which must be due soon), but
    before making a fool of myself in front of them by getting things wrong, I
    would rather risk making a fool of myself in front of Nav-L readers. Any
    comments would be most welcome.
    
    George.
    
    
    
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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