Location Systems for Ubiquitous Computing

57 onal, nor equally applicable to every system, the classification axes we present do form a reasonable approach to characterizing or evaluating location systems. The Global Positioning System is perhaps the most widely publicized location-sensing system. GPS provides an excellent lateration framework for determining geographic positions. The worldwide satellite constellation has reliable and ubiquitous coverage and, assuming a differential reference or use of the Wide Area Augmentation System, allows receivers to com-T o serve us well, emerging mobile computing applications will need to know the physical location of things so that they can record them and report them to us: What lab bench was I standing by when I prepared these tissue samples? How should our search-and-rescue team move to quickly locate all the avalanche victims? Can I automatically display this stock devaluation chart on the large screen I am standing next to? Researchers are working to meet these and similar needs by developing systems and technologies that automatically locate people, equipment, and other tan-gibles. Indeed, many systems over the years have addressed the problem of automatic location sensing. Because each approach solves a slightly different problem or supports different applications, they vary in many parameters, such as the physical phenomena used for location determination, the form factor of the sensing apparatus, power requirements, infrastructure versus portable elements, and resolution in time and space. To make sense of this domain, we have developed a taxonomy to help developers of location-aware applications better evaluate their options when choosing a location-sensing system. The taxonomy may also aid researchers in identifying opportunities for new location-sensing techniques. A broad set of issues arises when we discuss and classify location system implementations. These issues are generally independent of the technologies or techniques a system uses, as described in the " Location-Sensing Techniques " sidebar. Although certainly not all orthog-When attempting to determine a given location, we can choose from three major techniques: • Triangulation can be done via lateration, which uses multiple distance measurements between known points, or via angulation, which measures angle or bearing relative to points with known separation. • Proximity measures nearness to a known set of points. • Scene analysis examines a view from a particular vantage point. Location system implementations generally use one or more of these techniques to locate objects, people, or both. A report describing these techniques in detail can be found at