2.2. Positioning 15Figure 2.4 Positioning with triangulation techniques. The length of edges of thetriangle is determined by ratio of the RSSI values of between the transmitters T1and T2.related measure. The position of the receiver is assumed to be the position of thetransmitter. If multiple transmitters are available, a choice has to be made betweenthem. This choice is guided by using the maximum signal strength, and thereforeintroducing a range related information.Explicit usage of range or angle related information is made by techniques thatrely on triangulation for positioning. An example of such an approach is calledLateration . Lateration uses the ratio between the RSSI values of two transmitters forfinding the two locations in a 2D space that have the property to be the third point oftriangle that includes both transmitters as the other points. The triangle is definedby the property that the variable edges between the transmitter and the unknownlocation of the receiver have the same ratio as the RSSI values. The approach canalso be generalized to either multiple transmitters or other information sources likeTDOA, then called Hyperbolic localization, or AOA, then called Angulation.Another group of techniques can be gathered under the topic of fingerprinting. Theseapproaches rely on knowledge of the surrounding of the transmitters. These knowl-edge has either to be empirically collected or analytically modelled and has to bestored for efficient access during the localization phase. In the localization processthe collected information is compared with the stored data and the best matchinglocation is chosen.The empirical construction of a fingerprinting map makes it mandatory to rebuildthis map if the environment changes. By employing analytical models, like raytracinggenerated signal strength maps, it is easier to adapt to environment changes. But theinitial costs of such models can be higher. For example in the case of the raytracer,a 3D Map containing probably unknown materials is needed.