Overview

Wireless sensor networks are fundamentally intended to provide information about the spatio-temporal characteristics of the observed physical world. Each individual sensor observation can be characterized essentially as a tuple of the form <S,T,M>, where S is the spatial location of the measurement, T the time of the measurement, and M the measurement itself. We shall address the following fundamental question in this chapter: How can the spatial location of nodes be determined?

The location information of nodes in the network is fundamental for a number of reasons:

1. To provide location stamps for individual sensor measurements that are being gathered.

2. To locate and track point objects in the environment.

3. To monitor the spatial evolution of a diffuse phenomenon over time, such as an expanding chemical plume. For instance, this information is necessary for in-network processing algorithms that determine and track the changing boundaries of such a phenomenon.

4. To determine the quality of coverage. If node locations are known, the network can keep track of the extent of spatial coverage provided by active sensors at any time.

5. To achieve load balancing in topology control mechanisms. If nodes are densely deployed, geographic information of nodes can be used to selectively shut down some percentage of nodes in each geographic area to conserve energy, and rotate these over time to achieve load balancing.