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.

Improved algorithms

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 will not have to buy extra hardware to use Place Lab, and the software can be downloaded for free from 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.

Researchers at the University of Glasgow in the UK are planning to test Place Lab's positioning ability on volunteers equipped with Wi-Fi enabled PDAs during 2004's Edinburgh Festival in August - despite the city's patchy Wi-Fi coverage. And in 2005, new software for the PDAs will help people find their way around Edinburgh and keep track of their friends.

Location aware

Place Lab is not the first software to use Wi-Fi for positioning but - belying its name - it is the first to bring the idea out of the lab and into the real world.

"Microsoft once set up arrays of access points in a lab, but we are using the existing Wi-Fi infrastructure," LaMarca says. Place Lab has already been tested in Berkeley, San Diego, Seattle, Manhattan and Cambridge in the UK, among other cities.

The Place Lab team have other applications in the works. Jason Hong, a computer scientist at the University of California, Berkeley, is developing a "location-aware" version of Microsoft's Instant Messenger software. This will allow users to transmit their Place Lab data so that "buddy lists" will not only reveal when friends are online, but also where they are.

It could also be used to direct a user to the nearest petrol station or coffee bar, remind them their books are overdue as they pass the library, or warn them of traffic congestion ahead.

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