"Electromagnetic trackers [Raab:79] can determine object locations and orientations to a high accuracy and resolution (around 1 mm in position and 0.2° in orientation), but are expensive and require tethers to control units. Furthermore, electromagnetic trackers have a short range (generally only a few meters) and are sensitive to the presence of metallic objects.

Optical trackers are very robust, and can achieve levels of accuracy and resolution similar to those of electromagnetic tracking systems. However, they are most useful in well-constrained environments, and tend to be expensive and mechanically complex. Examples of this class of positioning device are a head tracker for augmented reality systems [Wang:90] , and a laser-scanning system for tracking human body motion [Sorensen:89].

Radio positioning systems such as Global Positioning System (GPS) and LORAN [Sonenberg:88] are very successful in the wide area, but ineffective in buildings because of the reflections of radio signals that occur frequently in indoor environments. Inbuilding radio positioning systems do exist (e.g., the work of Feuerstein and Pratt [14]), but offer only modest location accuracies of around 50 cm or more.

Location information can also be derived from analysis of data such as video images, as in the MIT Smart Rooms project [Pentlab:95] . Accurate object locations can be determined in this way using relatively cheap hardware, but large amounts of computer processing are required. Furthermore, current image analysis techniques can only deal with simple scenes in which extensive features are tracked, making them unsuitable for locating many objects in cluttered indoor environments."[Ward:97]