ARLA/CLUSTER: VRay: Antena revolucionária promete resolver o "multipath interference"

[João Costa > CT1FBF]

Uma antena que pretendem resolver um dos grandes problemas que afecta em
muito as comunicações em meios densamente urbanizados, em entre outros
sistemas, na precisão dos GPS´s e que é a recepção em simultâneo de sinais
multi tranjectória (multipath interference), onde os sinais que chegam à 
antena do GPS não são só os diretos, assim como outros vindos dos satélites
mas que antes foram reflectidos por dezenas de estruturas envolventes em
meios urbanos,  comprometendo em muito a precisão em dezenas ou mesmo
centenas de metros.

[image: Signal received and understood<i> (Image: Locata)</i>]

Basketball antenna filters out
noise<http://www.youtube.com/watch?v=HEmhFxdo2iM&feature=youtu.be>

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.
<http://transition.fcc.gov/bureaus/pshs/advisory/csric3/CSRIC_III_WG3_Report_March_%202013_ILTestBedReport.pdf>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
<http://www.afit.edu/>and Locata Corporation <http://www.locatacorp.com/>of
Canberra, Australia. It is based on Locata's previous work with ground-based
positioning technology.
<http://www.newscientist.com/article/mg21728985.600-new-positioning-technology-could-compete-with-gps.html>

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.

Fonte: New Scientist
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