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    Re: Position from a photo
    From: Frank Reed
    Date: 2012 Dec 27, 12:38 -0800

    Geoffrey, you wrote:
    "Could you take a casual looking photo which would convey enough information to make a fix within a few nm of where the photo was taken?"


    1) Is there a horizon or equivalent and does the camera record exact time stamps on photos (we have to assume it's been set properly)? If so, then a couple of photos of the Sun, properly exposed, and/or Moon separated by 20 or 30 degrees in azimuth would provide a standard two-body fix.
    2) Do we have time but no horizon? If the terrain is flat enough, even perspective clues should provide enough info to estimate the horizon within maybe 10 or 20 minutes of arc. That's not the "few nm" you're looking for but it would be enough to get us in the right general area and then do landscape matching.
    3) No time data. This, of course, is the tough one, It's the longitude problem. The photos from (1) can give us latitude, but for longitude we need one or two more photos. They should show the Moon with some bright stars or planets nearby (a photo taken in nautical twilight would work well enough). This gives us the date and UT within a few minutes. Then to narrow things down to a fraction of a mile, we need a photo of the International Space Station passing overhead, again with some bright stars nearby. This could be just a short streak in the background of some casual vacation pic in late twilight. Other satellites would also work and with a pair we could get a very exact fix if there's some other photo (Moon, e.g.) for approximate UT, but photographing two satellites seems a little too obvious and would require careful preparation. But if you do that, you could also eliminate the Sun photos.

    Just a reminder for anyone who hasn't heard this before: you can get position fixes more accurate than traditional celestial navigation by observing artificial satellites with nothing more than a pair of binoculars and a decent star chart. No sextant required. No horizon needed. You do need rather detailed advance predictions of positions, but that's easy today.

    -FER


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