**8.2. Location of nodes in WSN**

Another vital aspect in WSN is mutual location of the nodes. Only BS can know its precise location among the other nodes (hence only BS is able to plot its migration vector). Not all WS node have capability to precisely define their position in space. Only a few of nodes have optional (built-in or add-in) GPS device, such nodes with GPS are exceptionally useful for remaining nodes without this capability, they can act as beacons – providing other nodes with their position at the same time enabling to set position on their own. Having three beacons in communicational range gives a node an opportunity to precisely set its position. After this operation such a node becomes an anchor which unfortunately cannot be used for any further nodes location designation. Node's (anchor) location could be determined either by Ad hoc On-Demand Distance Vector (AODV) Routing developed in Nokia Research Center, University of California, Santa Barbara and University of Cincinnati [17] or Angle of Arrival using Received Signal Strength [20, 21]. Node's position can be also relatively precisely determined based on loss of transmitted signal strength. When there are just two beacons and one anchor nodes location designation is still possible, but less precise than before. The least accurate location designation applies to a situation where there are just three anchors in node's proximity. In each case where anchor is used to location designation, it is being recognized as a classifier rather than a positioning element.

A constellation term, known from astronomy as a group of stars involved in a specific area of the celestial sphere that is shown in the relative position on the Earth night sky, could be also applied in WSN field, per analogy celestial bodies (nodes), sky (WSN), Earth (BS). Therefore constellation in WSN can also show only relative position, towards selected, known points. The advantage of constellation is that one need to know neither precise distance between nodes nor having any single beacon in a communicational proximity. One needs only select a reference point and then can calculate distance to it. Main disadvantage of this method is that it accumulates errors, the greater the determined distance the bigger inaccuracy. However these cumulative errors can be partially mitigated by introducing at least one beacon.
