By Tom Simonite, Technology Review
A new antenna design being tested by the U.S. Air Force could make GPS significantly more reliable and able to function in dense urban areas where GPS accuracy is weak. It might even allow the technology to work indoors in some cases. Good GPS readings are hard to get in cities because of the multipath phenomenon: signals from positioning satellites bounce off buildings and other structures. That confuses GPS receivers, which calculate their location by knowing exactly how long it took for signals to arrive from satellites overhead. A signal that has bounced takes longer to arrive than it would if it had traveled directly, muddying a receiver’s math and sending location readings off by tens or hundreds of meters. Smartphones and in-car GPS units often have to work out their true location by analyzing maps and by getting a series of readings over time.
http://www.technologyreview.com/news/519811/a-cure-for-urban-gps-a-3-d-antenna/
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