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    Re: Fwd: Re: GPS "spoofing"
    From: Don Seltzer
    Date: 2013 Jul 29, 12:42 -0700

    Lu Abel wrote:
    There are GPS "compasses" that detect direction using the phase difference between satellite signals as received by a triangular array of antennas with the triangle about 1/2 meter on a side.

    Following up on Don's thoughts, I wonder if a GPS compass would show a radical shift in the ship's course when the spoofer is turned on. Or, alternatively, easily detect a signal whose apparent direction does not match the orbital data for the satellite.

    Very much the same idea, the difference being the baseline used and the resolution required.
    I was thinking of two antennas spaced about 100 feet apart, bow and stern, which would detect the time difference between reception, the basic raw information used by all GPS systems to calculate position. The GPS compass uses the phase difference, a finer resolution measurement, requiring a much smaller baseline.

    Let us imagine an oil tanker steaming due north past Block Island, heading for Narragansett Bay. A sophisticated terrorist on Conanicut Island at the entrance to the bay wishes to misdirect the tanker onto Brenton Reef. He spoofs the signal from a GPS satellite that is currently low on the SE horizon.

    The GPS compass antenna array can immediately spot the deception. The radio signal is clearly coming from the north, but the actual satellite is to the SE.

    For the method that I proposed, the antenna in the bow records the signal about 100 ns before the stern antenna. This is also clearly wrong. Based upon the satellite's known location, the aft antenna should be receiving it about 70 ns before the bow antenna. a 170 ns difference. The deception is detected almost immediately. The tanker anchors offshore, and a local resident with celestial navigation skills is ferried out to assist.

    Don Seltzer
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