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    Re: Visual satellite position fix
    From: Frank Reed
    Date: 2013 Aug 13, 16:31 -0700

    Peter Monta, you wrote:
    "Crazy idea: it would be a good joke to use GPS in optical mode."

    It's not a crazy idea. It's quite straight-forward! While the GPS satellites themselves are too faint for manual observation (at sea), they're within the range for specialized automated cameras. The real downside for the GPS satellites is that they're so high up that you don't get much shift in angular position from a change in the observer's ground position. Those satellites in Low Earth Orbit provide much better sensitivity.

    You also wrote:
    "Frank mentions ISS, and of course that's great for detection and
    photography, since it's so incredibly bright."

    Actually Tom Sult mentioned the ISS. I don't normally recommend it for this sort of visual satellite fix because it's simply not available for observation at a decent altitude all that often. At a given location, you can easily go two weeks with no decent ISS passes. Also some orbital changes are not announced until after the fact. In any case, there are at least a hundred other satellites that can be used.

    And you wrote:
    "But he rightly points out that since it's an LEO object, the orbital prediction degrades beyond a few weeks."

    Orbit decay is generally predictable for many satellites, at least well enough to be useful for visual satellite navigation. But orbital BOOSTS are unpredictable. They fire the rockets when they decide to, and the delta-v is only within some range. So every three or four weeks there is a sudden upward jump in orbital altitude for the ISS and a corresponding decrease in the orbital period. There's a nice graph of that, updated on a daily basis, here: http://www.heavens-above.com/IssHeight.aspx.

    Setting aside the brilliant ISS, there are PLENTY of good satellites that will work for this. The Iridium constellation has nice properties. And there are hundreds of derelict upper stages (like the endless "Cosmos" satellites which you mentioned) many of which are second and third magnitude so quite easy to spot even during nautical twilight.

    Let me be very clear on this: you can do visual satellite position-fixing NOW, TODAY ...no new science or analysis methods are required. You don't need a sextant. A pair of common binoculars will do. This method works all night long since no horizon is required. With relatively casual observations, a fix within five nautical miles is easy. It does take some practice learning how to spot the satellites and identify useful pairs of stars. You need to know the constellations in somewhat better detail than for traditional celestial navigation. But the "reduction" of such observations is literally trivial, under the assumption that a computer (or smartphone) is available and functioning. You simply adjust the observer's lat/lon until the satellite path matches the observations. And if the satellite has an up-to-date set of orbital elements, and if you have recorded the time of its passage by a star to the nearest second, you can get a fix from a SINGLE observation.

    So here's a challenge for you all. Visit a satellite tracking site, like the very popular heavens-above.com (http://www.heavens-above.com/). Set it up for some location near your actual location, maybe 20 or 30 miles away. Then go out and observe a half dozen satellites. How well can you get your longitude? And how about latitude (a little harder, as it turns out, since so many satellites are in polar orbits)? Which ones work for you? How often do you find that satellites are significantly late on their predicted paths? Which satellites don't seem to work at all (various surveillance satellites maneuver actively on a regular basis). Which satellites can you spot visually without binoculars?

    There is tremendous fun and challenge in traditional celestial navigation by sextant. But if the "real" purpose of navigation is to get a fix, then observing satellites visually is considerably easier, works all night long (but unfortunately, not at all in daylight!) and yields good quality fixes whose accuracy can be increased almost to GPS levels in a "scalable" manner. There is a huge amount of navigational information that we are all simply ignoring right now. If the defining principle of good navigation is "use all available navigational data", then surely it is high time we started getting visual satellite fixes every clear night.

    -FER


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