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There has been increased interest in applying radio frequency (RF) imaging to solving problems in surveillance, situational awareness, and security screening. This has been brought about by recent advances and new programs in component development from 100 GHz to 1 THz. While no new phenomenology has been uncovered in this region, the potential to finally realize systems that can begin to approach the resolution of optical imaging while operating under adverse conditions of weather and obscuration has prompted new investments. Several of the issues, beyond mere component availability, that must be considered when developing these systems are reviewed. Additionally, previous analysis of two applications proposed to operate in the region above 100 GHz is reviewed: a landing aid system and a standoff security screening imager, both of which are greatly influenced by atmospheric effects.

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Abstract-This study, consisting of three complimentary topics, examines of the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90 , corresponding to grazing angles of 20 to 0 . The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient 0 , particularly near grazing incidence. In this paper, we provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for 0 is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle , the surface relative dielectric constant , and the surface roughness ks, where k = 2= and s is the surface root mean square (rms) height.