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    Re: GPS Anti-Spoof feature request
    From: Peter Monta
    Date: 2019 Jul 31, 03:13 -0700
    Hi Tony,

    how do you plan to arrange that the landmark occurs exactly under the Sun?

    In general the landmark will not be exactly under the Sun; the arc between the two will have some azimuthal component to it---it's something like a lunar distance or a star-star distance, which of course are situated haphazardly on the sky in all orientations.  But that's fine, because this angle is still useful for celestial navigation---it gives a circle of position.

    For a conventional horizon shot, the apparent horizon is merely the limb of the Earth, the large object directly underfoot covering half the sky.  An ordinary Sun-to-horizon shot is thus a limb-limb shot, and once you factor in the Sun and Earth radii, you have an angle measurement from Earth center to Sun center.

    For a landmark shot, instead of to the plumb line, you measure the angle to some other direction.  The particular direction doesn't matter provided it is known.  As with all circles of position, you will need several for a fix, so several landmarks would be good, or else several celestial bodies (e.g. stars), or the same celestial body at different times of day.


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