FAQ: How Google Latitude locates you

Multiple wireless technologies help Google's new Latitude service pinpoint your location

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I thought you had to have GPS to work out your location. How does Google get around that? The technique that Google uses to work out your location is actually the same one that any GPS device uses: triangulation.

Here's how it works: When Latitude turns on, it automatically tries to reach any available GPS satellite, cell tower or Wi-Fi access point (AP). Once it establishes three or more links, it starts working out your location.

It does this essentially by figuring out, for example, that if you're two blocks from the cell tower at the church, and you're right under the Wi-Fi AP at the coffee shop, and you're at x distance from a GPS satellite, you must be at Buster's Coffee Shop. Typically, devices can use up to 24 reference points to work out your location.

Now, by itself your mobile device doesn't have the CPU horsepower to work that out. It takes the raw data and transmits it via a GSM, CDMA or Wi-Fi link to an assistance server. This technique is called A-GPS (assisted GPS). Your mobile device or computer works together with the server to plot out your location.

And this is the important part for Latitude: Since the assistance server has the results of the calculation, it's easy to share your position with anyone else who uses Latitude and has your permission to see your location.

How does the system know where Wi-Fi APs and cell towers are? Google uses a database of Wi-Fi AP and cell tower locations. Together, these tens of millions of fixed locations give Latitude the grid it needs to work out your location.

How accurate is Latitude? It depends. If all you're working with is Wi-Fi APs, as would be the case with an iPod Touch or most PCs, it can work out your location only within about 200 meters. If you're using multiple cell towers -- say you're in a city -- you can get it down to a 100-meter circle. In the country, you may be as far out as 300 meters. And with GPS, you can lock down your location to a few meters. If you combine systems, you can be within GPS's accuracy range.

Many variables can interfere with your accuracy, however. Unfortunately, there are no hard and fast rules on determining how accurate Latitude or any other LBS application will be at any given location. For example, if you're inside a large building, you probably won't get a GPS signal. On the street, you may get the GPS, but you'll lose the Wi-Fi signals.

One way or another, though, we're entering an age where you can always keep track of where you're at, who's near you and what businesses are close by. The flip side, of course, is that others can also track you.

Steven J. Vaughan-Nichols has been writing about technology and the business of technology since CP/M-80 was cutting-edge and 300bit/sec. was a fast Internet connection — and we liked it! He can be reached at sjvn@vna1.com.

Editor's note: This story has been modified since it was originally posted. It includes additional information from Google public relations.

Copyright © 2009 IDG Communications, Inc.

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