Surveillance of strategic sub-surface sanctuaries

Electromagnetic (EM) imaging techniques are being developed to survey strategic subsurface sanctuaries. The overall goal of this study is to develop and demonstrate techniques for sub-surface profiling from ground based and/or airborne (or even space) platforms. This surveillance scheme combines and utilizes bistatic RCS measurement techniques, broadband GPR antenna technologies, and far-field SAR remote sensing techniques. The combined RCS/GPR/SAR surveillance technique is used to extract target signatures concealed in measured RCS data and to remove thermal nose and ground clutter at the earth/air interface from the SAR data. The combined RCS/GPR/SAR surveillance process utilizes ground contact and airborne transmitting (TX) and receiving (RX) antennas. Small, but efficient, ultra-wideband (100:l bandwidth) conformable GPR antennas are being designed and developed to operate over the HFNHF bands. Planar wire-grid bowtie antennas are being developed as broadband GPR radar antennas. These antennas are 2D approximations to frequency independent, i.e., ultra-wideband, 3D solid biconical antennas. The antennas are truncated to finite lengths, which reduce the bandwidth to a finite range that is adjusted to cover the HFNHF bands. 2D cross-sectional versions of the bowtie antennas were built and tested at the AFRL/RRS sub-surface antenna range and were compared to an adjustable standard-gain half-wave dipole antenna.