Imaging in UWB Sensor Networks

Sensor networks consist of a number of spatially distributed nodes. These nodes perform measurements and collect information about their surrounding. They transfer data to neighboring nodes or to a data fusion center. Often measurements are performed in cooperation of several nodes. If the network consists of Ultra-Wideband (UWB) radar sensors, the network infrastructure can be used for a rough imaging of the surrounding. In this way bigger objects (walls, pillars, machines, furniture) can be detected and their position and shape can be estimated. These are valuable information for the autonomous orientation of robots and for the inspection of buildings, especially in case of dangerous environments (fire, smoke, dust, dangerous gases). Applications of UWB sensor networks are summarized in Thomä (2007). In this article basic aspects of imaging in UWB sensor networks are discussed. We start with a brief description of two types of UWB radar devices: impulse radar and Noise/M-sequence radar. Network imaging is based on principles of Synthetic Aperture Radar (SAR). Starting from SAR some special aspects of imaging in networks are explained in section 3. Sections 4 and 5 form the main part of the article. Here two different imaging approaches are described inmore detail. The firstmethod ismultistatic imaging, i.e. the measurements are performed in cooperation of several sensor nodes at fixed places and onemobile node. The second approach is imaging by an autonomous mobile sensor, equipped with one Tx and two Rx units. This sensor uses a network of fixed nodes for its own orientation. Part of the described methods have been practically realized in a laboratory environment. Hence, practical examples support the presentation. Conclusions and references complete the article.