I think this might have been mentioned in a CP thread sometime back. At least it should have ;-)
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Date: Fri, 9 Jan 1998 22:29:14 +0000
From: "Marcus L. Rowland"
Subject: GPS Jamming
*New Scientist* (8 Jan 1998, http://www.newscientist.com) included an
article saying that a Russian company called Aviaconversia was offering a
4-watt GPS/Glonass jammer for less than $4000 at the September Moscow Air
Show. It says that it could stop civilian aircraft locking onto GPS signals
over a 200 Km radius; military aircraft would be harder to jam, but a more
powerful unit could be built.
The risks (terrorism etc.) are fairly obvious, and it's mentioned that it
would probably be easy to build one even if this company's product is
somehow removed from the market.
Marcus L. Rowland http://www.ffutures.demon.co.uk/
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GPS uses a wideband Direct Sequence Spread Rectum technology. DSSS, as implemented in GSP, is excellent for distance an direction finding due to its inherent, very accurate, time base. In order to conserve satellite power and provide a reasonable signal level at small (e.g., handheld) receivers, it spreads a low data rate signal over wide band, achieving a 63dB 'process gain' (equivalent to about a 2 million fold increase in receive signal level). DSSS signals (in general) can be jammed by very narrow (e.g., CW or continuous wave) carriers, especially pulsed, with relatively modest power. GPS includes significant provisions for anti-jam and although I haven't done a detailed analysis, I'd be surprised if a 4-watt transmitter would render a 200 km radius unfit for GPS navigation.
See "Spread Spectrum Communications Handbook," ISBN 0-07-057629-7.
--Steve
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