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    Re: accuracy of automatic celestial navigation
    From: Herbert Prinz
    Date: 2002 Dec 8, 06:34 +0000

    Glenn Geers wrote:
    >  The algorithms take the motion of the vessel into account and can provide very
    > high (30m) accuracy.
    Well, the algorithms can do much better. In his article "The Motion of the
    Observer in Celestial Navigation", published in "The Navigator's Newsletter" of
    the Foundation for the Promotion of the Art of Navigation, Issue 51, Spring 1996,
    George H. Kaplan compares several algorithms which account for the motion of the
    observer. Two are rigorous, one is an approximative method. Clearly, the two
    rigorous methods yield identical results of absolute accuracy. The approximative
    method yields a result that is typically within 0.05 arcminutes of the rigorous
    solution, degenerating with increasing length of track of the observer between
    individual observations. The two rigorous methods remain correct regardless of the
    observation span. To avoid any potential misunderstanding, Kaplan concludes the
    analysis with the explicit warning: "In any event, the degree of equivalence among
    these procedures indicates only mathematical precision and should not be mistaken
    for navigational accuracy."
    Therefore, the question that George Huxtable raised in a post of Dec 6,
    >  If that claim related simply to the accuracy of the calculations, then it
    > would be easy to accept. It's rather harder to understand if it relates to
    > the overall precision of the complete system, which would correspond to
    > measuring star altitudes, with respect to the vertical, to 0.5 to 1 second
    > of arc.
    must be answered affirmatively. The envisaged accuracy of 1" is indeed that of the
    entire navigation system and not just that of the algorithm. Since the algorithm
    is strict, the 1" error is entirely observational.
    This is also confirmed by a statement of G. H. Kaplan in a letter to the
    Navigation Foundation in 1996, saying "[...] we decided to anticipate the
    possibility that at some time in the future automated shipboard  star trackers
    might provide the basic observational data. Since these devices are capable of
    arcsecond-level measurement precision, our software was designed with one
    arcsecond (30 meter) or better precision as a specification".
    The system specification of 1" precision was obviously not classified information
    in 1996. Whether it has been met or not (or even exceeded) may be classified
    information; I don't know. Where the accuracy of the star tracker is concerned,
    Kaplan speaks of a fact, not of a specification.
    Herbert Prinz

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