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    Re: Anti-spoof app
    From: David C
    Date: 2020 Apr 11, 19:45 -0700

    Cloud cover meant that I could not take a noon sight. I decided instead to observe the sun hanging. I placed my phone and tablet side by side. The app resolution is 0.1'. Every time the  display on the tablet incremented I un-paused and then paused the phone. When the  altitude on the tablet decremented by 0.1'  I paused the tablet. My reaction time was probably about  one second. The results are:

    Start  (phone)   0.1° T   79° 44.0'   002001 UT

    End (tablet)  359.9° T     79°  39.9'   002038   UT

    With a 0.1' resolution the sun  hung for 37 seconds.  With clear skies and a sextant with bertter optics than mine  that should be ample time to take an accurate noon sight.

    Note the slight difference in GPS positions. Immediately before taking the photo I had moved both devices to get better light and maybe the two devices settled down at a different rate. If my maths is correct 0.001' of latitude is approximately 6 ft - say 2m.  The displayed latitude difference between the two devices is 6 * 2 = 12m. This s consistant with the errors quoted by the two devices  -  8m and 16m.  In case you are wondering the difference between  01 39 33 UT ad 00 20 01 UT was my lunchbreak.

    I was using the AH setting so the maximum observed  altitude was 39° 52'.   From the Altitude correction table in the 1973 Nautical Almanac the SD + refraction correction is  +14.9'. The actual altitude is therefore 40° 7'. The Sun Facts Pro app on my phone gives the maximum altitude as 40.2°  or 40° 12'.  Rounding error?

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