In order to position a device, WPS acquires the access point information which the device scanned, then compared with the information of central reference network. Then it compute the location of the device with the information about the access points whose location is in the reference network. While doing this, the data base in the reference network is updated with new information. Moreover, Skyhook has a fleet of data collection vehicles to update the reference network.
For more information: http://www.skyhookwireless.com/howitworks/wps.php