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Use of GPS in the Military
Before anything else, it's important to understand just how critical GPS technology is to the military. As of now, it's not just used to precisely target missiles and other bombs to avoid excessive civilian casualties and building damage, but also for elaborate troop movement exercises on the ground. Fighting the sort of guerrilla war that is increasingly the name of the game when it comes to warfare requires the ability to quickly navigate the streets of foreign cities and to constantly update in real time small groups of soldiers on where they need to go. Thus, GPS is absolutely critical to modern warfare.
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Who Jams the Signal?
We're not talking about civilian GPS signal jamming, which is typically done out of a want of privacy and doesn't exactly constitute a major threat. GPS signal jamming for military purposes works to jam the particular signal used by the military in an effort to undermine their activities, and thus functions as a serious threat to security. Obviously, there are a lot of legal issues associated with their use.
Who are the signal jammers? Typically, the side of a conflict who doesn't have the technological advantage, and thus has nothing to lose and quite a bit to gain in relative advantage if they manage to jam the GPS signal of their opponent.
Some examples: at the beginning of the Iraq War, the Iraqi army used a Russian-made GPS jamming system to disrupt US GPS-guided missiles. There have been reports of signal jamming on the part of terrorists in Iraq and Afghanistan using it to undermine US military operations since then.
Asides from being used in combat, the marketing of these devices has also been eyed with some suspicion, with reports of North Korea attempting to export signal jammers to countries like Iran and Syria, not to mention worrying their neighbor South Korea. There have also been a build-up of Chinese GPS signal jammers (easily purchased on the Internet) which would pose no disadvantage to China's own use of a GNSS as they currently have a China-only system, Beidou, and are currently working on their own worldwide positioning system, COMPASS.
GPS signal jammer isn't something that needs to be specially manufactured, either. Guides to building GPS signal jammers are widely available on the Internet, and are certainly available for use by any potential terrorists. The concept of how they work is not a particularly complex one, though it does take a little electrical know-how.
While a lot of this constitutes a threat to American military superiority, not everyone views this undermining as a bad thing, as they feel it equalizes the field somewhat. However, whether or whether not this trend constitutes a bad thing comes to subjective views on foreign policy, and will not be discussed in this article.
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Stopping GPS Signal Jamming
That being said, there has been quite a bit of effort on the part of the US to counter these jamming efforts. GPS signal jamming, by its nature, is very conspicuous, and the source of the jamming can quickly be determined on the field. So, while it may prove to be a frustrating disruption, it is not one that is altogether difficult to locate and, after some shenanigans, remove.
There has also been calls to reduce the dependency on GPS, particularly within military operations, and to have back-ups in line. No technology is perfect, and by depending too much on GPS, the US military invites disaster. It's better to be safe than sorry—literally, in the case of the military.
For an excellent piece on GPS military jamming, check out this article from GPS world.