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    Re: Fwd: Re: GPS "spoofing"
    From: Don Seltzer
    Date: 2013 Jul 29, 11:01 -0700

    Frank Reed wrote:
    "Is celestial immune, though? It could be spoofed with enough
    resources. Deploy a number of UAVs around the ship. Some are
    equipped with small screens, which would be interposed between the
    real stars and the user's sextant."

    Sure. You could just drop a big planetarium dome over the vessel, and then you've got 'em right where you want 'em!! ;) I would say that spoofing the stars is too preposterous even for the most diabolical super-villain.

    Too complicated. All you need to do is spoof the horizon. Hire a banner towing aircraft to fly parallel to the ship, pulling a long banner with a false horizon.

    Back to GPS spoofing, I think that larger vessels could detect spoofing with two GPS systems, with antennas mounted fore and aft. The delta T between the reception of a particular satellite signal at the two sites will be dependent upon the altitude and azimuth with respect to the vessel's orientation at that moment. There is no way that I can see that a spoofing transmitter located on shore can accurately reproduce that delta T.

    For a simple case, suppose the real satellite is directly at the zenith at a particular moment. Its signals will be received simultaneously by the two antennas. The spoofing transmitter can only duplicate that if it happens by chance to be directly abeam of the vessel. Any other direction and there will be a measurable difference. A few minutes later, the satellite will be at another orbital location, and the deltaT will be different. And of course, there are another four or five satellites being monitored in the same way, each in a different direction from the vessel.

    Don Seltzer
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