November 23, 2007: China has developed what appear to be powerful GPS jamming systems, that are carried around in trucks. Apparently the "GPS jamming vans" are meant to created a protective "bubble" over an area the van is in the middle of.

For over a decade, the U.S. Air Force has been developing anti-GPS jamming technology. For years, military aircraft have been equipped with complex, and expensive (over $30,000 each) 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.

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.

Five years ago, it was believed that Iraq had bought many GPS jammers, to deal with U.S. 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 will land the bomb within 150 feet of the target, GPS will get to within 30 feet. The U.S. Air Force does not discuss what, if any, jam-proofing it is doing for its JDAM bombs.

There are several approaches to defeating GPS jamming, and knowing which one JDAM 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 JDAMs, it's more likely that jam-proof JDAMs 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.