For something we rely on so heavily, GPS is pretty rubbish. In the open, it is accurate to within a few metres, but in built-up areas it frequently cannot even locate the correct city block. Now a new type of antenna promises to improve its accuracy in urban areas – even indoors.
The problem is multipath interference, the curse of all radio-based location systems. When signals from satellites are reflected off buildings navigation systems get confused.
"Multipath is the dominant source of interference for GPS in cities and indoors," says consultant David Last, former president of the UK's Royal Institute of Navigation. "Nothing else causes as much serious error."
Now VRay, a new type of antenna that eliminates multipath interference, has been developed by the US Air Force Institute of Technology and Locata Corporation of Canberra, Australia. It is based on Locata's previous work with ground-based positioning technology.
VRay is a "beam-forming" antenna that combines the signal from an array of receiving elements to identify and subtract multipath and other interference to extract a clean signal. The antenna scans for millions of virtual beams a second, which helps it differentiate the valid GPS signal from the rest of the clutter, says Locata's CEO Nunzio Gambale.
Previous beam-forming antennae have been large and expensive, with a separate radio receiver for each antenna element. VRay has a single receiver that switches at high speed between dozens of elements.
"The concept of beam-steering for GPS is well known. However, its use has been mainly confined to the military due to the high cost. Locata's breakthrough is to time-multiplex the receiver," says Last.
This design means VRay will be much smaller, cheaper and simpler than previous designs. Early VRay prototypes are still bulky, but the plan is to build antennas that are flush with airframes and vehicle roofs, and to create a portable version within a helmet.
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