TABLE OF CONTENTS Localization is quite a broad problem domain [80, 185], and the component issues and techniques can be classified on the basis of a number of key questions.
What to localize? This refers to identifying which nodes have a priori known locations (called reference nodes) and which nodes do not (called unknown nodes). There are a number of possibilities. The number and fraction of reference nodes in a network of n nodes may vary all the way from 0 to n ? 1. The reference nodes could be static or mobile; as could the unknown nodes. The unknown nodes may be cooperative (e.g. participants in the network, or robots traversing the networked area) or non-cooperative (e.g. targets being surveilled). The last distinction is important because non-cooperative nodes cannot participate actively in the localization algorithm.
When to localize? In most cases, the location information is needed for all unknown nodes at the very beginning of network operation. In static environments, network localization may thus be a one-shot process. In other cases, it may be necessary to provide localization on-the-fly, or refresh the localization process as objects and network nodes move around, or improve the localization by incorporating additional information over time. The time scales involved may vary considerably from being of the order of minutes to days, even months.
How well to localize? This pertains to the resolution of location information desired. Depending on the application,...
