From a cursory search (gps jamming hacking military) it looks like screwing with the signals would be a piece of cake for those sophisticated in electronic warfare.
http://gpsd.berlios.de/gps-hacking.htmlHow GPS Works
First, the basics of how GPS works. It depends on the fact that satellite orbits are very predictable. A GPS sensor is a specialized computer that knows about the orbits of GPS satellites, and in particular can predict exactly where each satellite will be at any given time with respect to the fixed Earth. (For those of you who enjoy such details, what they actually predict is each satellite's position with respect to an imaginary ellipsoid called the "WGS 84 geoid" which closely fits the mean sea level of Earth.)
There are presently 28 dedicated GPS satellites, 11,000 miles up in high-inclination orbits so that each one's trajectory wraps around the Earth like a ball of yarn as the planet spins beneath them. The inclinations are tuned to guarantee that about twelve will be visible at any given time from anywhere on Earth (coverage falls off a little at high latitudes). Additional GPS coverage is provided by a couple of maritime navigation satellites parked in geosynchronous orbits over the middle of the Atlantic and Pacific oceans.
You can look at a very nifty simulation of GPS satellite orbits. (Also includes GLONASS, the Russian military equivalent of GPS.) You can also look at pictures of GPS satellites and the control system.
Each satellite broadcasts identification pulses, each one including the clock time it was sent. A GPS receiver, picking up one of these pulses, can compare it to an internal clock and know the time it took to arrive. Multiplying by lightspeed gives the distance to the satellite. This starts to be useful when the GPS can get accurate timings to three or more satellites; at that point, computing the GPS's exact position with respect to the satellites becomes a relatively simple if tedious exercise in spherical trigonometry (which, fortunately, the GPS's firmware does for you).
That's the theory. In practice, the system has important limits. Anything, natural or artificial, that messes with the signal timings will degrade the accuracy of your position fix. Until it was abolished by Presidential decree in 2000, the most important limit was artificial, the so-called 'Selective Availability' feature. The satellites were programmed to introduce patterned timing jitter into the signals. The U.S. military knew the pattern, but nobody else did (or, at least, nobody who was admitting it).
http://www.hacking-gps.com/gps-notes/archives/2005/04/lowcost_gps_jam.phpTechnical article on building a portable GPS jammer that affects the L1 carrier signal. This means that it would have no effect on the military L2 carrier or the P-code that that carries. It does however cast a huge cloud of doubt over the use of GPS in tracking tagged criminals.
http://www.computerworld.com/securitytopics/security/story/0,10801,77702,00.htmlJanuary 17, 2003 (Computerworld) -- Government officials and communications experts are assessing the public safety and security implications of a newly posted online article that provides directions for making cheap devices that can jam Global Positioning System (GPS) signals.
Information in the article that appears in the current issue of the online hacker magazine Phrack potentially puts at risk GPS devices used for commercial navigation and military operations, authorities said.
The Phrack article provides a detailed guide to building a low-cost, portable GPS jammer out of components that can be easily obtained from electronics supply houses. According to the article, the "onslaught of cheap GPS-based navigation (or hidden tracking devices) has made it necessary for the average citizen to take up the fine art of electronic warfare." Electronics and GPS experts who read the article this week called it technically competent and said amateurs with a certain amount of technical skill could build a GPS jammer from the plans.
Although the article said the jammer is designed to work only against civil-use GPS signals broadcast on the frequency of 1575.42 MHz and not the military frequency of 1227.6 MHz, James Hasik, an Atlanta-based consultant and author of the book The Precision Revolution: GPS and the Future of Aerial Warfare, disagreed. (...)
http://www.fuhs.de/en/news/sathack.shtmlPress Release 03.04.2003
'Coalition Guidance Systems Compromised'
The explosion of an alleged allied forces so-called smart bomb on a market place in Baghdad where about 50 civilians were killed could have been caused by the deliberate jamming of the guidance system by a GPS jammer. This is the opinion of the internationally well-known security consultant Howard Fuhs. ‘This incident is similar to the bombing of the Chinese embassy during the Kosovo conflict’, Fuhs said. ‘It is a safe assumption that the error was caused by GPS jamming transmitters and not through navigational errors.’
Fuhs maintains that not only is the jamming of GPS signals a threat to allied forces and civilians in Iraq, but there is also the danger of the enemy, even terrorists, hacking into satellite communication channels. Such hacking and jamming may explain the level of civilian casualties and attack from 'friendly' fire.
'I have seen not only ex-Soviet hand-held GPS jamming devices for sale on the internet, but details of how to make jammers out of cheap components. The threat of jamming is widely known, but rarely publicised'. See:
www.computerworld.com
www.ac11.org
www.space.com
What is even more disturbing is the possibility of satellites actually being taken over and their communication channels used for ill. 'Satellite security is at the same stage as computer security was ten years ago, with the 'owners' in denial. Usually, encryption is either weak or non-existent, in the belief that little-known modulation types are sufficient security. This is palpably untrue. There is no modulation type that cannot be decoded with money to buy the equipment. In some cases, sufficient wattage is all that is needed to open up the controls. (...)
http://www.computerworld.com/mobiletopics/mobile/story/0,10801,65096,00.htmlPentagon is probably jamming GPS in Afghanistan, experts say
Bob Brewin
October 26, 2001 (Computerworld) -- The U.S. Defense Department has probably been selectively jamming signals from the Global Positioning System (GPS) in Afghanistan since the start of the air campaign earlier this month, according to nonmilitary GPS experts.
The experts emphasized that the jamming in Afghanistan will have no effect on civilian users, including airlines, which increasingly rely on GPS for transoceanic navigation. Signals from the GPS satellite system available to civilian users provide an accuracy of 36 meters or better, while separate, encrypted military signals used to guide so-called smart bombs in Afghanistan provide accuracy within 6 meters, according to Richard Langley, a professor of geodesy and precision navigation at the University of New Brunswick. Langley's Web site plots the GPS military signal over Kabul as of Oct. 11.
Langley said the Pentagon has developed the capability to jam civilian GPS signals within a specific targeted area and could easily deny the 36-meter-accuracy civilian signal to the Taliban forces without interfering with users in other areas of the world. Depending on whether the Pentagon, which developed and operates the 28-satellite GPS constellation, uses airborne or ground jammers, this could deny the signal to the Taliban over a wide area, with some of the jamming potentially spilling over into Pakistan.
http://www.schneier.com/blog/archives/2005/04/universal_autom.htmlGPS, my behind. Who needs to hack those tamperproof big brother GPS boxes when it's much easier to jam the weak satellite signals? Couple of hours of soldering and assembling, top, for an amateur.
Oh, and that will also screw up GPS navigation for everyone unlucky to be close by.
The law of unintended consequences strikes again.
Police comedy, the next round: the hunt for GPS jammers. "What's that in your pocket, boy?"
Where this silliness is going to end? EMP guns?
http://www.dailywireless.org/2006/10/08/satellite-jam/Ground segments and communications links remain the most vulnerable components of space systems, susceptible to attack by conventional military means, computer hacking, and electronic jamming. A number of intentional jamming incidents targeting communications satellites have been reported in recent years and Iraq’s acquisition of GPS-jamming equipment for use against US GPS-guided munitions during Operation Iraqi Freedom in 2003 suggests that jamming capabilities are proliferating.
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