Bombs using GPS bombs can, in theory, be easily misled by jamming. Using readily available components worth less than a hundred bucks, you can make a device that will jam all GPS signals for a hundred miles. If this is so, why is the U.S. upping producing of its JDAM GPS guided bombs (to over 200,000 bombs)? Simple, the jamming can be defeated. Talking openly about how this is done is counterproductive. If the bad guys know how you are prepared to defeat their jamming, they can adjust their jamming equipment to be more effective. For example, if you use a Digital Excision Temporal Filter (DETF) to defeat the jammer the enemy may know that DETF cannot defeat a shorter range wideband barrage jammer (but can handle the cheaper, and longer range, CW, pulse CW and swept CW types.) You can defeat barrage jammers using beamsteering and locking-on techniques. Like any other form of electronic warfare, no counter-measure stays effective for once the enemy has experienced it.