Microsoft has embarked on an ambitious plan to create a ground-based alternative tracking system to the satellite-based GPS. Microsoft already operates cars in many U.S. and UK cities; these cars trawl the streets to locate WiFi signals. Microsoft has also used light aircraft to take detailed aerial pictures of cities and suburbs across the country. The company says it has already built a database containing the whereabouts of "millions" (the company would not say how many millions) of WiFi networks.

Microsoft says it is collecting only the unique identifier, or MAC address, that each WiFi network broadcasts. The MAC address cannot be traced to a street address or an individual user. By recording the position of each MAC address on a giant map, Microsoft has created a positioning system which should make it possible for anyone with a WiFi-enabled laptop computer to identify their location to within 30 meters or so. Privacy advocacy groups say that the combination of powerful tracking methods wedded to accurate maps available on the internet pose new threats to privacy.

For more on Microsoft's latest plan
-see this MSNBC report
-Microsoft's release about its Virtual Earth location finder
-Microsoft's Virtual Earth U.S. map
-see this Michael Kanelos's c|netnews report on a similar effort by Intel

PLUS: Wherify Wireless has some interesting ideas about the design -- and audience -- for its GPS locater phone. Release.