Wideband RF Detection of Changes Within Building Structures

In this letter, we develop a means for sensing changes within building structures using wideband RF probe signals with an array of sensors. A unique signature is formed for a given building by plotting the angles of specular return versus distance along a drive-by path in a ldquogramrdquo format, with coloring to indicate the path delay. This signature can be stored as a reference file and differentially compared to subsequent grams to detect significant internal changes. A simulation is presented using a ray-trace program to analyze a specific building structure. When an artificial occlusion is added inside the building, it is clearly detected by observing the differential gram. To establish feasibility in a realistic scenario, this is compared to differential grams corresponding to small changes in the drive path. As a first indication, it appears that changes of 0.1 m in any dimension produce comparable changes to a perfectly reflecting 2-m cube placed in one of the rooms.