Harperdog writes "This article details how GPS can help detect secret nuclear tests, giving the US more reason to ratify the Comprehensive Test Ban Treaty. Here's a quote about the 2009 North Korea test: 'At the time, however, the May 25 bomb also sent a different signature, this one into the atmosphere. It did not release radioactive gas or dust, as would be the case for a bomb detonated on the Earth's surface. Rather, it released a shockwave — a bubble of disturbed air that spread out from the test site across the planet and high into the ionosphere. ... We quickly gathered data from 11 GPS receivers — six belonging to the South Korean GPS network and five belonging to the International GNSS Service and scattered around Eastern Asia. The data indicated a sudden spike in atmospheric electron density just after the underground test.'"