Before the Iraq war, many pundits predicted that the Iraqis could easily jam American GPS guided bombs. The Iraqis tried, and their six jammers were quickly taken out, one them via a GPS bomb. Aside from the fact that U.S. JDAM GPS bombs have a (less accurate)  backup inertial guidance system, it was often overlooked that GPS was designed to be jam-proof. The Department of Defense won't discuss anti-jamming technology in detail, for obvious reasons. But it is known that jam-resistant antennas and variable power levels from the GPS satellites has been used. A future enhancement will involve digital signal processing and beam steering (looking for the real signal from where the satellites is, not where the jamming is coming from) to suppress jamming signals. Also, the next generation of U.S. manufactured GPS receivers (stand-alone and built into weapons and equipment) must include a GPS Selective Availability Anti-Spoofing Module (Saasm), and other anti-jamming technology that is not discussed much. It will always be possible, in theory, to jam the GPS signal. But by constantly adding and updating anti-jamming technology to military GPS receivers, such jamming remains difficult and, for most nations, nearly impossible.