In the concrete canyons of city centres, GPS satellite positioning systems often fail because high buildings block the signals they rely on. But an unlikely back-up for GPS is emerging: Wi-Fi. A Wi-Fi based positioning system developed in the US and the UK works best where GPS fails: in cities and inside cavernous complexes like shopping malls. And because cheap Wi-Fi technology is already appearing on a raft of gadgets like PDAs, cellphones and laptops faster than more expensive GPS receivers are, the developers predict that Wi-Fi could become central to new location-based applications.
They say emergency services in particular could find the system an essential back-up. Wi-Fi allows people to connect devices wirelessly to the internet. Base stations are springing up in coffee bars, libraries, universities, airports, phone booths and other public places. Each base station broadcasts a radio signal to announce its presence to devices within a range of around 100 metres. This signal incorporates a unique network address code that identifies the base station. Anthony LaMarca of the University of Washington in Seattle, and colleagues at Intel’s research labs in both the US and the UK, have developed software that constantly records the radio signal strengths from nearby base stations. It can identify the origin of the signal from a database giving the location of 26,000 Wi-Fi base stations in the US and the UK. Using the signal strength from at least three base stations, it can then triangulate the user’s location. "This is a poor man’s GPS," says team member Bill Schilit of Intel Research in Santa Clara, California.
At the moment, the new system, called Place Lab, is not as precise as GPS. It can provide accuracy to within 20 to 30 metres, whereas the GPS average is 8 to 10 metres. But with improved algorithms that take into account, say, the height of the base station above the ground, or the building materials in the vicinity, LaMarca says "we could get on a par with GPS" in an area as densely served with Wi-Fi as downtown Seattle. The growth of Wi-Fi means all urban areas should one day have similar blanket coverage. Once a user has Wi-Fi they won’t have to buy extra hardware to use Place Lab, and the software can be downloaded for free from http://www.placelab.org. Increasingly, laptops, cellphones and PDAs are being sold with Wi-Fi capability already installed for around an extra $30. "This is not the case with GPS," LaMarca points out.
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