I. Introduction
Distributed sensor networks consisting of a large number of low-cost and low-power sensors have found many applications in military surveillance, homeland security, traffic surveillance, environment monitoring, etc., for collecting observations and processing information. The use of distributed nodes with multiple sensing modalities can significantly enhance the robustness and the accuracy of the decision making process in such environments [1]. Data collected by nodes are fused at various levels and in different ways in order to make useful inferences at the decision-making center. The inferences made based on the fused information range from distinguishing among different event (or threat) types, estimating important parameters such as location or velocity of an object, target tracking, as well as detecting the presence or absence of events with certain “frequency” or “spectral” characteristics [2].