http://www.bbc.com/news/world-asia-35940542

I bring this to your attention not due to my interest in navigation, and my distrust of modern digital “tech” and its vulnerability to crime, vandalism and enemy action; but because our armed forces have become so dependent on it. Not only are ships, aircraft and ground units navigated by GPS, but most of our precision “smart” munitions won’t work without it. I realize the Armed Forces no doubt have taken precautions and countermeasures, but experience has shown us that sooner or later a smart hacker can crack it. And training on alternative methods is just not realistic. Traditional navigation requires in-depth training and constant practice. Its just not going to happen if you can just press a button and get a lat-lon.

In time of war, the GPS system will almost certainly come under attack, either by jamming or by direct fire. Even if an enemy fails to disable the system prior to the start of a conflict, will we be willing to consider a deliberate attempt to do so an act of war and respond accordingly? Will an American President start a war just because someone is shooting at our satellites?