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    Re: Accuracy of sextant observations at sea
    From: Andrés Ruiz
    Date: 2010 Nov 29, 10:15 +0100

     

    For Alan´s data:

     

     

     

     

    GHA

     DEC

     HO

     BO

     LO

     LHA

     HC

     Z

     p [º]

     p [nm]

    79.062

    8.9008

    64.2067

    34.655

    -77.06

    2.002

    64.1804

    184.5451

    0.0262

    1.572

    38.5189

    -29.5635

    16.2217

    34.655

    -77.06

    321.4589

    16.2051

    145.6407

    0.0165

    0.99

    20.235

    -3.5639

    24.4833

    34.655

    -77.06

    303.175

    24.4502

    113.4106

    0.0331

    1.986

     

    Greg, Alan provided Ho, observed, not Hs, sextant altitude. And for Jupiter the correct data is:

    23/10/2010

    22:54:37 UT1

     

    ALMANAQUE NAUTICO - NAUTICAL ALMANAC

    Planet Jupiter

    GHA = 20.234995 º =  20º 14.1'

    Dec = -3.563867 º =  -3º 33.8'

    SD  = 0.399345 '

    HP  = 0.035627 '

     

     


    De: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] En nombre de Greg Rudzinski
    Enviado el: sábado, 27 de noviembre de 2010 1:27
    Para: NavList@fer3.com
    Asunto: [NavList] Re: Accuracy of sextant observations at sea

     

    Alan,

    You need to include two important pieces of info- Height of Eye and Index Error.

    I went ahead and worked out the observations for a height of eye of seven feet and zero index error using the GPS position provided.

    Altair (Dec N 8° 54.0' SHA 62° 10.1' GHA Aries 16° 53.6')
    Intercept 1.5' away
    Azimuth 184.5°

    Formalhaut (Dec S 29° 33.8' SHA 15° 25.8' GHA Aries 23° 05.3')
    Intercept 5.0' away
    Azimuth 145.6°

    Jupiter (Dec S 3° 34.0' GHA 20° 14.6')
    Intercept 3.0' away
    Azimuth 113.4°

    Since all your intercepts are away it looks as if you may have about 3 minutes of index error off the arc which means that 3' can be added to each observation to improve the plot. You will also notice that this puts your fix outside the original triangle. When a round of observations differ in azimuth by less than 180° then it is possible that a constant error will place the fix outside the triangle (or cocked hat).

    Greg Rudzinski

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