Location Sensing Techniques

We define the term lateration to mean for distance measurements what angulation means for angles. Lateration computes the position of an object by measuring its distance from multiple reference positions. Calculating an object’s position in two dimensions requires distance measurements from 3 non-collinear points as shown in Figure 1. In 3 dimensions, distance measurements from 4 non-coplanar points are required. Domain-specific knowledge may reduce the number of required distance measurements. For example, the Active Bat Location System measures distance from indoor mobile tags, called Bats, to a grid of ceiling mounted ultrasound sensors [4]. A Bat’s 3-dimensional position can be determined using only 3 distance measurements because the sensors in the ceiling are always above the receiver. The geometric ambiguity of only 3 distance measurements can be resolved because the Bat is known to be below the sensors and not in the alternate possible position on the next floor or roof above the sensor grid. ∗This technical report is a companion to Location Systems for Ubiquitous Computing, an article appearing on pp. 57-66 of the August 2001 issue of IEEE Computer magazine.