Joe Katzman discovered an article which made him afraid that our GPS-guided weapons could be defeated with a simple commercial jammer produced in Russia which can be bought for $39.95. I'm not too concerned.
A normal civilian GPS receiver uses an omnidirectional whip antenna, so that any signal from any direction is received equally. I would be very surprised if JDAMs do so. I think it's pretty obvious that any jamming would be coming from near the ground, and that the true GPS signal would be coming from the sky, and it isn't very difficult to design an antenna which selects signals from only one direction or set of directions, and physically rejects signals from all others. Since the bomb that a JDAM is attached to will always assume a standard orientation when it's falling (nose down) it would be quite easy to incorporate a directional antenna into the JDAM, and that alone would go a long way towards solving the problem.
CPO Sparkey works on this stuff and thus can't comment, except to say, "Come on Saddam, just stand still with all the dinky little Internet jammers you can buy just piled around you, please, oh please, oh please!”. It probably should be pointed out that a jammer is a prime target for HARM or equivalent weapons, and by its nature it cannot hide. If a high value target surrounded itself with jammers like this, it would be like painting a target on the site.
I don't on military hardware, but I've worked on related civilian technologies, so perhaps I can offer some speculation.
The original GPS system uses a form of direct-sequence spread spectrum (DSS) for its signals. The CDMA cell phones I used to work on use a much more sophisticated version of the same thing. What has to be understood is that DSS is designed to ignore substantial amounts of environmental noise. The signal in DSS is scrambled using what's known as "pseudo-noise", or PN, which is a carefully crafted sequence of bits known to both transmitter and receiver, and because of how the receiver works, it will very strongly reject anything which is not encoded with exactly the right PN sequence. (Or even anything which is encoded with that PN sequence but at a different phase.) The main reason our design in CDMA was more sophisticated than the original GPS system was that CDMA was designed later and the state of the art had improved. What we were doing required hardware in the receiver which wasn't technically possible when GPS was originally designed. This is important for CDMA because each phone operating on a given frequency looks like a jammer to all the others, but each phone uses a different PN sequence, so the ability to ignore any PN sequence except the one intended for you makes it so that your call will still get through to you. (I wrote an explanation of the basic concepts involved here.)
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