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    Re: Accuracy of sextant observations at sea
    From: Hewitt Schlereth
    Date: 2010 Nov 29, 11:34 -0400

    Alan-
    
    Are you still there?
    
    From your posts I have the impression you've started right in on
    multi-body fixes, and difficult ones at that. It's easy to get
    glitches with moon sights just from the extra arithmetic of the many
    corrections. With stars and planets you're contending with either a
    fading horizon or dimming stars. Also, your data suggest you're
    relying on single shots of each body.
    
    So, I'm wondering if you've tried comparing single sunlines to your
    GPS position to see what sort of results the intercepts will indicate
    you can expect from any single shot in optimal conditions?
    
    If you've done that, have you tried "taking five and averaging," as
    the one rule of ye olde sextant swingers has it?  Since a run of
    sextant sights tends to contain altitudes above and below the correct
    one, averaging lets the high (+) ones cancel the low (-) ones and get
    you closer to a true one.
    
    If you've done these things, please excuse this post. I just didn't pick it up.
    
    It's good to have you aboard.
    
    Hewitt Schlereth
    
    
    
    
    
    
    On 11/29/10, Andres Ruiz  wrote:
    >
    >
    > For Alan�s data:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > GHA
    >
    >  DEC
    >
    >  HO
    >
    >  BO
    >
    >  LO
    >
    >  LHA
    >
    >  HC
    >
    >  Z
    >
    >  p [�]
    >
    >  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 � =  20� 14.1'
    >
    > Dec = -3.563867 � =  -3� 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: s�bado, 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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