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Digital Camera Cel Nav Follow-up

     Photos of the sun taken at high altitude through a filter can be
augmented to full screen on a laptop for determining minutes of arc
(M.O.A.) per pixel.

Example for 4 power zoom on a Canon PowerShot A560 :

Solar diameter 07/07/2008    31.46' = .291 M.O.A. per pixel
Pixel solar diameter         108

     Using M.O.A. per pixel enables subsequent sun to horizon pixel
counts to be converted to M.O.A. for a designated zoom power.



On Jul 3, 4:31�pm, 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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