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    Re: GPS jammers?
    From: C.L. Holm
    Date: 2003 Mar 31, 09:46 -0500
    RE: GPS jammers?

    An excellent concern, but some have already considered the problem.

    Some factors to consider:

    - GPS has excellent long term position accuracy and stability when values are averaged or smoothed (i.e., no drift), but poor short term stability (i.e., position jumps around somewhat randomly)

    - INS has excellent short term position accuracy and stability without the need to average values (i.e., no random position change), but poor long term stability (i.e., has drift)

    - Mechanical gyro INS used to be very expensive, big and prone to failure.
    - Costs for an INS using a ring-laser gyro without any moving mechanical components have reached very reasonable values with excellent reliability

    - Kalman filtering of GPS and INS can provide the best of both without the negatives. 
    - For a typical long range munition, in the cruise phase, smoothed GPS would be used to eliminate drift in the INS
    - In the terminal guidance phase, INS would provide the majority of the final guidance and correction, with almost no reliance on GPS.

    - Jamming effectiveness is primarily a function of jammer power and distance between jammer and receiver
    - GPS signals are radiated from space and therefore most receiver antenna are mounted on the top
    - A effective jammer is a beacon that radiates and can itself be targeted. 
    - Although astro-trackers are good at high altitude, it would be difficult to put one into a precision munition, and probably not effective at low altitude

    - Few midship'n would want to ride along with a one-way munition to take star shots.

    Chris

    -----Original Message-----
    From: Joe Shields [mailto:jshields{at}POST-GAZETTE.COM]
    Sent: Monday, March 31, 2003 7:24 AM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: GPS jammers?


    It was in the news a while back, that the Russians were selling Iraq GPS
    jammers.  Does anybody know anything about these?  What sort of range do
    they have?  Can they be selective so that only your enemies receivers are
    jammed?
    In this "smart bomb" era, I would think a network of GPS jammers that covers
    your cities and extends well out over your coastal borders would provide a
    pretty good safety unbrella for neutralizing a lot of expensive weopons the
    U.S. now employs. Could make GPS-guided missles the next "Maginot Line" type
    boo boo.  If I was Commandant of the U.S. Naval Academy, it might make me
    re-think taking Celestial Navigation out of the cadets curriculum.
    -- Joe Shields
    40d34' lat, 80d03' long


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