The Air Force deployed four global positioning satellites into space this past year — the most it has put up in a single year in more than three decades.
Within each satellite is a secondary payload called the Nuclear Detonation Detection System, Air Force 2nd Lt. Christopher Phillips of the 2nd Special Operations Squadron, Schriever Air Force Base, Colorado, said in a statement released on Dec. 7.
“NDS performs an important mission,” he said. “It helps verify the Limited Test Ban Treaty of 1963. It makes sense. The whole reason the GPS constellation has these nuclear detonation sensors is because it provides global coverage. We can see every part of the planet.”
... I didn't. Like the article says, it makes sense though. And it's not like some future bad guys aren't already going to do their best to shoot them down anyway during the next big conflict (which we hope never happens, but humans being humans, and governments being governments...)
But makes you wonder what else might be on them [Putting tin hat on now!]