The Defense Advanced Research Projects Agency (DARPA), writing about GPS, said: “The military relies heavily on the Global Positioning System (GPS) for positioning, navigation, and timing (PNT), but GPS access is easily blocked by methods such as jamming. In addition, many environments in which our military operates (inside buildings, in urban canyons, under dense foliage, underwater, and underground) have limited or no GPS access.” This raises the questions of what DARPA plans as a step up
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DARPA seeks new positioning, navigation, timing solutions
