Making Things Talk

HOW TO LOCATE (ALMOST) ANYTHING 277

In order to locate it within a two- or three-dimensional

space, though, you need to know more than distance. The

most common way to do this is by measuring the distance

from at least three points. This method is called trilatera-

tion. If you measure the object’s distance from two points,

you get two possible places it could be on a plane, as

shown in Figure 8-7. When you add a third circle, you have

one distinct point on the plane where your object could

be. A similar method, triangulation, uses two known points

and calculates the position using the distance between the

two known points and the angles of the triangle formed by

those points and the position you want to know.

Determining Position Through Trilateration

Distance ranging tells you how far away an object is from your measuring point in one

dimension, but it doesn’t define the whole position. The distance between your position

and the target object determines a circle around your position (or a sphere, if you’re

measuring in three dimensions). Your object could be anywhere on that circle.

Figure 8-7

Trilateration on a two-dimensional plane. Knowing the distance from

one point defines a circle of possible locations. Knowing the distance

from two points narrows it to two possible points. Knowing the

distance from three points determines a single point on the plane.

The Global Positioning ...