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    Old LOP's and the Celesticomp V
    From: Richard B. Emerson
    Date: 2000 Aug 05, 8:16 AM

    I posted a question about advancing or retiring LOP's over 12 hours
    old relative to the current fix time.  I also said there were some
    other circumstances involved in the problem that developed from doing
    so.  Here we go...
    
    Briefly restating the situation, on Tuesday, 20 June I did a noon sun line
    and an afternoon sun line and a morning sun line on 21 June.  Using a
    Celesticomp V to advance Tuesday's LOP's to Wednesday morning, I
    consistently come up with a 30-35 mile error in position (verified by
    GPS and a bearing off Montauk Point, Long Island).  An error of a
    third to half of that would have been acceptable (if somewhat
    disappointing).
    
    In trying to recreate what went wrong, it appeared that the
    Celesticomp was introducing the error.  Reductions with a StarPilot
    and plotting the LOP's manually both gave a more reasonable position,
    within about 5 miles of our GPS position.  I then wrote the author of
    the Celesticomp about the matter and received an explanation of what
    happened.
    
    In the Celesticomp manual, on pages 19 and 34, there is a comment:
    "...you can use any sight within 12 hours of fix time..." which I took
    to be merely an admonition to avoid using older lines.  Not so; in
    fact the program reduces older (i.e., age +/- 12 hours from current
    fix time) LOP's , actually the sight series, with the fix time's
    Greenwich hour angle (GHA) and declination instead of the GHA and
    declination for time of the original sights.  This introduces,
    depending on a number of factors, an error on the order of a few tens
    of miles.
    
    Now, before anyone writes "this calculator is trash", *that is not the
    case!*  The real problem lies in the manual which doesn't make this
    circumstance clear.  I have been using the Celesticomp in connection
    with plotting sheets and was trained by people who do the same thing,
    all with the expected accuracy *IF* each round of sights is plotted
    immediately and all running fixes, etc. are done on the sheets.
    
    Had I relied solely on the mathematical reduction of two or more
    LOP's, I would have gotten my due reward.  Blind faith in numbers
    displayed on a screen or LCD panel is not justifiable.  The result
    must be tested for reasonableness and cross checked by all means
    possible.  When I advanced the LOP's down the DR track manually, I got
    the result confirmed by direct observation (a bearing from Montauk Pt,
    in this case, as well as a GPS fix).
    
    My point in writing is to simply point out a lack of an important
    warning in the Celesticomp's manual.  I continue to believe the
    Celesticomp V is a useful navigation tool.
    
    Rick
    S/V One With The Wind, Baba 35
    

       
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