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    Re: Digital Camera Celestial Navigation
    From: Hewitt Schlereth
    Date: 2008 Jul 3, 20:40 -0400

    Hello Greg Rudzinski -
    
    This is extremely interesting to me as I tried this with a Polaroid
    back in the late 70s, but it was just too clumsy - image too small
    mainly, but you've got a handle on that.. Anyway, I never followed up
    on it and it never occurred to me that digital technology could make
    it possible - hangover of being of the slide-rule generation, I guess.
    
    Anyway, i hope you'll pursue this. It would be neat if you could have
    a hand-held 'camera' that would take and measure and give you an Hs
    readout. Hell, with the technology available today ti ought to be
    possible for the gizmo to read out an intercept and bearing.
    
    Keep going!
    
    Hewitt Schlereth
    
    On 7/3/08, Greg Rudzinski  wrote:
    >
    >      A timed digital photo of the sun at low altitude provides enough
    >  information to generate a line of position. For this task I use a
    >  Canon PowerShot A560 set at 3 power optical zoom, 7 mega pixel, and
    >  ISO 800. The high ISO setting helps reduce camera shake on a moving
    >  platform.
    >      The digital image of the sun (using a shade filter held out in
    >  front of the camera covering the sun) is downloaded to a laptop,
    >  cropped, rotated, and enlarged so that the sun has at least a 20
    >  millimeter screen diameter on the laptop. This limits altitudes to
    >  about 6 degrees. Measurements are made using a ruler directly on the
    >  laptop screen. The almanac diameter of the sun is a given at
    >  approximately 32 minutes of arc. Knowing this will allow the ratio of
    >  the laptop screen measurements to yield an altitude of the sun's lower
    >  limb above the horizon in minutes of arc. Convert minutes of arc to
    >  degrees and minutes. Correct for refraction, dip, semidiameter
    >  (temperature and pressure if necessary). Perform normal sight
    >  reduction for the GMT of the sun's photo to get an azimuth and
    >  intercept.
    >      Preliminary trials from the beach for altitudes between 4 degrees
    >  and 6 degrees show the average LOP within 4 nautical miles of GPS.
    >  Precision is limited by rounding to the nearest millimeter on laptop
    >  screen measurements.
    >
    >  EXAMPLE:
    >
    >  Semidiameter 15.73' (by navigation computer)
    >  Full diameter 31.46'
    >  GPS Latitude 34 09.8'N  Longitude 119 14.0'W
    >  Height of eye 8 ft.
    >  GMT 02H 44M 00S Date 07/03/08
    >
    >  Laptop measurements in millimeters:
    >
    >  Horizon to lower limb 226mm
    >  Sun diameter 27mm (measured horizontal to horizon to avoid refraction
    >  differences between upper and lower limb)
    >
    >  (226)(31.46) = 263.3' = 4 degrees 23.3' (hs) uncorrected
    >     27
    >
    >  Corrected and reduced to an azimuth 294.8 and intercept 0.4' away (by
    >  navigation computer)
    >  (not corrected for temperature and pressure)
    >
    >  The Nautical Almanac A4 table shows an additional +0.5' correction for
    >  a pressure of 29.9 in. and a temperature of 70 F
    >
    >  iPHOTO was used to enlarge and crop sun images.
    >
    >  >
    >
    
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