Delivering real-world ubiquitous location systems

A large number of research and commercial location systems havebeen developed over the past twodecades [7]. In general, these systems have one of two goals: providing highly accurate location estimates, on the order of centimeters, within a small area, or provid-ing lower accuracy over a widecoverage area [8]. Systems with afocus on accuracy typically requireboth extensive infrastructure andrelatively expensive sensors. AT&TCambridge's Active Bats [1] system, for example,employs active ultrasonic badges and requires theinstallation of ceiling-mounted ultrasound receiversevery square meter.