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    Re: GPS jammers?
    From: Nels Tomlinson
    Date: 2003 Mar 31, 07:18 -0900

    I like your last point especially.  Kids, nowadays!
    
    Seriously, there is another flaw in the original idea, if you look at it
    from the other side, from the jammer's perspective: all the jammers can
    acomplish is to make the bombs land NEAR the target, instead of ON the
    target.  If the civilian population is of no concern to the government,
    this might seem a good tradeoff.  That's probably why the Iraqis tried it.
    
    For a civilized country, using jammers wouldn't seem a good idea.  They
    couldn't be used to protect targets in populated areas, since they'd
    increase civilian casualties, and wouldn't help in isolated areas where
    the attacker could use nukes and get close enough without any guidance
    for the last thousand miles.
    
    So, it wouldn't help (your point), and most countries wouldn't want to
    accept the cost of using it even if it did (my point).
    
    I suppose that a city with no military targets might find some sense of
    security from GPS jammers, but terrorists probably wouldn't care if they
    got the wrong skyscraper or the wrong block, so I suspect that it would
    be a false sense of security.
    
    Nels
    
    C.L. Holm wrote:
    > 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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