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Jim Donohue wrote:
"Steven Shelikoff" wrote in message ... On 20 Dec 2004 07:58:48 -0800, wrote: It does get locally turned off once in a while for tests, military exercises, etc. Usually they give plenty of warning. For instance, here's an AP line about an outage back in June for the Carolinas to Florida: ---------- Military testing system designed to jam GPS signals off coast WILMINGTON, N.C. (AP) - Military officials are testing a system for jamming signals from navigational satellites - an experiment that could disable boaters' GPS devices and force them to rely on dead reckoning. The test began Friday and will run through June 20, as part of a NATO training exercise along the Atlantic Coast. The Coast Guard said GPS signals will be unreliable or unavailable in waters up to 60 miles off North Carolina and along the coast of northern and central Florida. Officials said the testing may affect cell phones as well as GPS navigation devices in boats, airplanes or cars. Coast Guard stations are trying to get word out about the interference. "What we kind of foresee happening is some confusion on the mariners' end," said Petty Officer 1st Class David Christensen. ---------- And mo Would anyone want to be in a commercial or private airplane when making an initial approach in low ceiling weather to landing and having GPS suddenly go off? This may well be an "end." No, that won't be the end either... unless the plane doesn't have a pilot and it's being flown automatically, directed by GPS. Steve Actually Steve I think the military jammed the signal rather than turning it off or degrading it. I also believe that there are antennas that would make Right. That's what the article says, that the military was jamming GPS signals from 60 miles off North Carolina and along the coast of northern and central Florida. Of course, it doesn't really matter why it's unusable ... whether it's jammed or turned off, it's still unusable. it difficult or impossible to jam a GPS particularly from a boat a few miles away from the jamming site. I doubt the military will talk about it but If you increase "few" miles to a hundred miles or more, I'd agree. That also depends on how they jam it. I.e., whether it's from the air or ground. You can possibly get directional antennas and point the null in the direction of the jammer. But that assumes there's only one and that it's not somewhere above you. There's also ways of decreasing the effectiveness of a jammer via signal processing. Of course, most boaters don't go out equipped to handle possible jamming. reasonably simple shielded antenna with deliberate limitation of the sky viewed can probably provide pretty good reception even in the face of heavy jamming. It's got to be a little better than that. It has to have a pretty good null specifically in the direct of the jammer. That's because GPS signals are so weak (actually, they're below the level of background noise and signal processing is needed to dig them out) that just about any interference can cover them up. On the other side jamming GPS in a relatively close in situation like a harbor or a city is a piece of cake. I am sure the hacker community already has a couple of desings on the internet. You want a link? I've seen several make your own designs, complete with schematics and parts lists with sources, design considerations, etc. To take out GPS up to maybe 50 miles or more you only need about 4 watts ERP, same as a CB walkie talkie. If you're equipped to handle the jamming, you can overcome that and limit the range of the jammer to maybe only a few miles. But again, most boaters aren't equipped to deal with jamming and it would render the GPS of most boaters useless. None of this in any way applies that the gov can't easily screw around with the signal to any desired extent. I would also point out it would be relatively trivial for the gov to take out Russian or European sattellites as well. Probably construable as an act of war but why else would the gov. do it? It's just as trivial for another sophisticated government, one with space capability, to take out our GPS satellites. Also probably an act of war. Steve |
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