Information Technology Reference

In-Depth Information

devices whose location is known (reference devices). Distances can be

estimated by means of several approaches [3].

Time-of-flight
method estimates the distance between a moving object

and a fixed point by measuring the time a signal takes to travel between

the object and point
P
at a known speed. This method requires accurate

clocks, especially for radio frequency (RF) signals. In this case, a 1-µs error

in timing leads to a 300-m error in distance estimation.

The
attenuation
approach takes into account the fact that the intensity of an

emitted RF signal decreases as the distance from the emission source increases.

Given the signal strength at the emission, it is possible to estimate the distance

from an object to some point
P
by measuring the signal strength at the point
P
.

Of course, a function correlating attenuation and distance is needed.

Once distance has been estimated, the positioning task is mainly

accomplished by the
triangulation
method. With this method, the position of an

object is obtained by measuring its distance from multiple reference positions.

2D triangulation algorithm requires three different measurements from three

non-collinear points, and the mobile device is located at the intersection of

three circles (Figure 1). 3D positioning requires distance estimation from four

non-coplanar points. For example, a global positioning system (GPS) receiver

needs at least four satellites for 3D position estimation.

In the case of RF signal attenuation method, the circles' radius is

evaluated on the measured strength of received signals, exploiting the

relationship between signal strength and distance. RF signals are affected by

a high degree of uncertainty (and consequently estimated distances too), and

actually measured RF signal strength define a cloud-shaped region around the

base station (Figure 2).

Figure 1: 2D positioning by triangulation.