Global positioning system (GPS) technology—now found in everything from cars to wristwatches—has become increasingly popular over the past few years for tracking location. But it has its limits—most notably, roofs, walls and floors that shield satellite signals and keep them from locating GPS receivers indoors.

Enter the indoor positioning system (IPS), a budding technology that IPS manufacturers envision as one day tracking the movement of firefighters battling blazes inside burning buildings, patients in hospitals and even retail merchandise swiped from store shelves. Although this has sparked invasion-of-privacy fears in some, the technology itself is designed to deliver useful locator services that pick up where GPS leaves off.

Why can IPS go where GPS cannot? GPS technology relies on signals from multiple satellites and employs a triangulation process to determine physical locations with an accuracy of about 33 feet (10 meters); the most common forms of IPS, both in use and under development, employ radio, ultrasound or infrared signals to home in on enclosed locations.

Radio signal–based systems that rely on wireless local area networks (WLANs) and Wi-Fi signals have several advantages over indoor positioning systems designed to rely on ultrasound or infrared, one former IBM researcher says. "The biggest advantage for wireless LANs is [that] the technology is relatively cheap and available in a lot of places," says Jin Chen, a PhD student researching distributed systems and autonomic computing at the University of Toronto, who in 2003 as a researcher with IBM in China co-wrote a paper that examined the use of WLANs for indoor positioning systems.

Many businesses and homes already have wireless networks for connecting laptops, PDAs and mobile phones, and these devices could be tracked simply by adding enabling software, Chen says. WLAN-based systems also cover larger areas than other types of indoor positioning systems and could even work across multiple buildings.

Companies that make ultrasound-based IPS say that sound waves can more accurately pinpoint people and objects than radio-frequency waves, which can be picked up by multiple sensors, making it difficult to figure out the exact proximity of a particular object to a given sensor. "If you have an RF [radio-frequency] tag, it is emitting radiation through its antenna," says Wilfred Booij, chief technology officer of Sonitor Technologies, AS, based in Oslo, Norway. The accuracy of RF waves is diminished within buildings, where the waves reflect off of metallic or ceramic objects. "If you have a very open area, you can have very good accuracy with RF—between five and 10 meters [16 and 33 feet]," he says. "But in complex buildings like a hospital, accuracy is more like 15 meters [49 feet]."

Ultrasound is detected by microphones placed in rooms where the tracking is to be done. When ultrasound signals—which have short wavelengths—are emitted, the walls and doors confine the signals to that room. Sonitor is trying to improve the accuracy of its ultrasound system by shaping the sensitivity of its detectors to create "subzones."

"With ultrasound, we have much better control over signal strength," Booij says. "A microphone can be designed to be more sensitive in a particular direction. We can shape the sensitivity of our detectors so that rather than picking up all the signals in a room, they pick up a specific signal that can be specific to a particular doctor or patient."

Sonitor so far has installed its technology in 20 hospitals in the U.S. and Europe, where physicians and staff use the ultrasound systems to track patients and medical equipment. Among them, the University of Pittsburgh Medical Center (U.P.M.C.) since October has been testing different IPS technologies to create a "smart room" that detects a doctor or nurse who has entered it and displays patient information on bedside monitors.

U.P.M.C. is using Sonitor technology to identify to patients the different doctors, nurses and staff they encounter during their stays. The goal is to help patients keep track of their caregivers, something not easy to do if the patients require round-the-clock care from multiple doctors working different shifts. Each worker wears a tag smaller than a pager that emits a sound the IPS can detect when he or she enters a smart room. An ultrasound detector there reads the tag and identifies the staffer by name and job title, displaying the information on a flat-screen monitor at the foot of the patient's bed.

In this pilot phase, tags have been assigned to doctors, nurses, nursing assistants and phlebotomists as well as dietary hosts and hostesses. With about 5,000 physicians affiliated with U.P.M.C., including more than 2,300 staff physicians, IPS is expected to introduce efficiency, accuracy and familiarity in an environment where patients are often ill-informed and overworked doctors are prone to make mistakes.