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See Inside October 2010

Getting GPS Out of a Jam

How tiny waves of matter may help missiles stay on track



Nick Koudis Getty Images

GPS, the Global Positioning System we rely on for guiding nuclear missiles and steering tourists to Mount Rushmore, has become a ripe target for enemy attack. In response, U.S. scientists are developing new ways to circumvent blocked GPS signals using matter waves to measure acceleration.

GPS is vulnerable because the radio signals that satellites broadcast to receivers, such as those in smart phones and  in cars, are so weak that even low-power jammers can easily block them. (GPS devices use the signals from several satellites to triangulate their position.) During the past decade, China and other countries have put satellites for their own regional navigation systems into orbit that work on different frequencies, which means that on a battlefield they could block U.S. signals without disrupting their own.

To get around this potential risk, U.S. scientists are developing gadgets that can track an object’s position in the event GPS signals are cut off. These inertial measurement units, or IMUs, determine a target’s location by measuring changes in acceleration since the last GPS reading. Until now such devices, based on a variety of technologies from mechanical to laser-based, have often been bulky and prone to error after prolonged use. By taking advantage of the quantum-mechanical properties of matter, however, engineers have come up with gadgets that could prove 1,000 times more accurate.

These “cold atom” devices use lasers and magnets to confine clouds of atoms into a very narrow range of energies, explains Werner J. A. Dahm, the U.S. Air Force’s chief scientist. (Such constraints make them “cold” in a quantum-mechanical sense, not in temperature.) Under these conditions, scientists can detect matter behaving like a wave. The devices split these matter waves in two and send each part in opposite directions before bringing them back together. If the device moves while the waves are split apart, one wave will experience acceleration slightly before its counterpart. The laser detects this change when the waves recombine. Because the waves have very small wavelengths—billionths of a meter in size—scientists can use them for ultraprecise measurements of acceleration. The gadgets might be ready for wide-scale use within a decade.

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