-- Time difference of arrival, which looks at the amount of time it takes signals from two or more access points (AP) to reach an individual client and does a speed-of-light calculation.

-- Angle of arrival, which measures the angle of a given signal received at multiple APs; a little triangle-based geometry, and we have the location.

-- Signal strength, which is normally an unreliable mechanism for determining relative location, since radio waves fade quite dramatically over distance, and other forms of fading, such as that resulting from multipath, can severely distort a signal's amplitude. But if we take enough samples over a period of time (even a couple of seconds), we can use signal strength to quite accurately determine location.

All these techniques work best when we can calibrate a given environment, using a kind of site survey to take readings at different locations and then build a database of values that aid in navigation, if you will.

Skyhook Wireless Inc. uses a related technique called radio frequency fingerprinting to extend the idea of WLAN-based location and tracking into metropolitan areas. It literally drives through metropolitan areas and notes the pattern of signals present from APs that just happen to be installed in a given area. Note that Skyhook doesn't use the APs for anything other than the signals it sends, and no traffic is sent over these APs. They are used quite literally as beacons: When a Skyhook client is in a given location, the client can use the pattern of AP signals to determine, quite accurately, his location.