For most of March, North Korea has been directing a GPS jamming signal across the border, and towards the southern capital, Seoul. A separate jammer has been directed at cell phone traffic. The GPS jamming signal can be detected up to a hundred kilometers south of the DMZ. The usual response to GPS jamming is to bomb the jammers, which are easy to find (jamming is nothing more than broadcasting a more powerful version of the frequency you want to interfere with). But such a response could lead to more fighting, so the south is still considering what to do. The jamming is a nuisance more than a threat, and most military equipment is equipped with electronics and other enhancements to defeat it. The North Korean jamming confirms what was already suspected of them. So now, South Korean and American electronic warfare experts have an opportunity to study the effects of jamming on a large metropolitan area. It is causing intermittent problems for users of GPS devices, and many more cell phone connectivity problems.
Meanwhile, this is old news for the U.S. Air Force, which has spent most of the last two decades developing anti-GPS jamming technology. For years, military aircraft have been equipped with complex, and expensive GPS receivers that will work even if they are being jammed. There are several ways you can defeat attempts to jam GPS signals. Some of the methods are well known, others are classified. No one has successfully used GPS jammers in combat yet, but the potential is there. Now the North Koreans are giving the largest demonstration yet of GPS jamming.
GPS jamming and anti-jamming, technology is very complex. None of the major players (the U.S., Russia, China, Israel and several other industrialized countries) are talking, and for good reason. If you don't know what techniques the other guys is using, you can't deal with them.
China and Russia are both selling GPS jammers. Four years ago China brought to market a powerful, truck mounted, GPS jamming system. These "GPS jamming vans" are meant to create a protective "bubble" over an area the van is in the middle of. Sales have been slow.
A year before the 2003 invasion of Iraq, it was believed that Saddam had bought many GPS jammers, to deal with U.S. JDAM GPS smart bombs. The JDAM has a backup inertial guidance system, so that if the GPS signal is jammed, the less accurate inertial guidance system takes over. The inertial guidance (INS) will land the bomb within 30 meters (92 feet) of the target, GPS will get to within 10 meters (31 feet). The U.S. Air Force does not discuss what, if any, jam-proofing it is doing for its JDAM bombs. The Iraqi GPS jamming efforts had no significant effect on the 2003 campaign.
There are several approaches to defeating GPS jamming, and knowing which one each American GPS guided weapon uses makes it easy to develop a way to jam the "jam-proof" GPS. So the U.S. Air Force is understandably reluctant to discuss what they are doing. Given the cost of jam proofing all existing GPS weapons, it's more likely that jam-proof GPS weapons will only be used against targets where the GPS accuracy is vital. Against most targets, the accuracy provided by the inertial guidance system will do. Also note that you can bomb GPS jammers with a bomb equipped with a guidance system that homes in on a GPS jamming signal. For that reason, it's thought that any use of GPS jammers will involve dozens of jammers in each area so protected. The GPS jamming has no effect on the even more accurate laser guided bombs, and some countries buy smart bombs with both laser and GPS/INS systems.