NBC News reports that four U.S. government sources tell them that Russians are jamming American radio-controlled military drones in Syria. "GPS receivers in most drones can be fairly easily jammed," said Dr. Todd Humphreys, the director of the Radionavigation Laboratory at the University of Texas at Austin. "It's like shooting at them with radio waves instead of bullets." Blocking or scrambling a drone's reception of a signal from a Global Positioning System satellite could cause them to malfunction or crash. "At the very least it could cause some serious confusion" for the drone operator who might read an incorrect position and get lost. Humphreys says the Russian military began jamming drones in eastern Ukraine four years ago, during the invasion of Crimea. U.S. military officials are mum about what the Russian signal-blocking is doing to the smaller surveillance drones. "The U.S. military maintains sufficient countermeasures and protections to ensure the safety of our manned and unmanned aircraft, our forces and the missions they support," said Pentagon spokesman Eric Pahon.