In order to improve upon automated sensor performance for secu- rity applications in public and private settings, numerous alternative sensor designs have been developed to provide affordable and effective detection and identification performance. Radio frequency (RF) sensors offer a balanced approach to system design for a wide variety of geometries and threat targets. These threat targets include persons carrying weapons and explosives, portable containers with contraband including cargo boxes, suitcases, and briefcases and fixed structures including building or underground facilities harboring criminals, terrorist or enemy combatants, fn order to achieve the resolution required for the detection and identification of threat targets, separation of interference from the target response is essential. High bandwidth offers a conventional approach to high resolution sensing of the threat. An alternative approach, one based upon wide angular bandwidth (spatial diversity), is presented here. This chapter addresses the issue of spatial diversity in radar applications. There has been an increased need for information via radio frequency (RF) detection of airborne and ground targets while at the same time the electromagnetic spectrum available for commercial and military applications has been eroding. Typically, information concerning ground and air targets is obtained via monostatic radar. Increased information is often equated with increased bandwidth in these monostatic radar systems. However, geometric diversity obtained through multi-static radar operation also affords the user the opportunity to obtain additional information concerning these targets. With the appropriate signal processing, this translates directly into increased probability of detection and reduced probability of false alarm. In the extreme case, only discrete Ultra Narrow Band (UNB) frequencies of operation may be available for both commercial and military applications. As such, the need for geometric diversity becomes imperative.
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