Description:
OBJECTIVE: Construct an instrument to measure the millimeter wave polarimetric bidirectional reflectance distribution function of objects. DESCRIPTION: Millimeter-wave (mmw) RADAR techniques may exhibit advantages over other imaging methodologies for aiding navigation in degraded visual environments, providing high-resolution terminal missile guidance, and detecting wires and small-caliber threats. Surprisingly, there is very little information about the bidirectional reflectance distribution function (BRDF) of objects in the mmw region and how it depends on waveband or polarization. The transition from Ka-band to W-band to G-band involves more than just shorter wavelengths and their commensurate technological challenges; it also involves a change in the nature of the scattering problem. Millimeter-sized structures and imperfections significantly change the RCS of objects that would appear smooth at longer wavelengths, so when targets must be separated from clutter, different wavebands and polarizations will discriminate different features. Contributions from surface roughness and millimeter-sized features increasingly dominate the BRDF of an object, producing a combination of Lambertian and specular scattering. Lambertian scattering from man-made structures may be caused by surface corrugation, abrasion, rust, or corrosion, for example, while Lambertian scattering from terrain, grass, and foliage may also depend on environmental conditions (esp. hygroscopicity and wind). Specular reflection may come from surface imperfections like cracks, dents, fasteners, facets, corners, and edges that act as antennas or retroreflectors in a manner that may or may not be sensitive to the object"s orientation and the RADAR"s polarization. Measurements of the polarimetric BRDF of potential targets and unintentional scatterers (clutter) are required at these wavelengths to identify unique target signatures and ascertain their detectability above ambient clutter from naturally occurring scatterers. Having a common platform in which multiple mmw wavebands and polarizations (both co- and cross-aligned) may be measured would allow a direct comparison of the BRDFs and the identification of unique signatures. BRDF measurements can take hours to days to acquire, placing severe stability and reproducibility requirements on both the heterodyne transmit/receive (Tx/Rx) hardware and the gonioreflectometer. The rapid maturation of heterodyne mmw Tx/Rx modules has made it possible for such long-term measurements to be performed, and verifiably accurate imaging techniques developed in other spectral regions may be adapted for alternative approaches to mechanical scanning by a gonioreflectometer. The deliverable will be a working gonioreflectometer-mounted heterodyne Tx/Rx system (or equivalent) configurable for mono-static or bi-static measurements, plus an analysis package that can autonomously obtain the four-dimensional polarimetric BRDF of test objects at least 30 cm x 30 cm x 30 cm in size in each of the following wavebands: Ka-band (35 GHz), V-band (60 GHz), W-band (94 GHz), D-Band (140 GHz), and G-band (220 GHz). The instrument, which will be delivered to AMRDEC at the end of Phase II, must measure and analyze BRDFs in a large (nominally 20"x 20"x 20") anechoic chamber in a manner that can constrain or validate increasingly sophisticated models of RADAR cross sections for each waveband and polarization combination. PHASE I: Design an instrument that can measure the mmw polarimetric BRDF of test objects at least 30 cm x 30 cm x 30 cm in size for at least a week of continuous operation. The heterodyne Tx/Rx module mounted on the gonioreflectometer or equivalent instrument must coherently measure amplitude and phase information and construct a BRDF with user-specified angular precision in each of the following mmw bands: Ka-band (35 GHz), V-band (60 GHz), W-band (94 GHz), D-Band (140 GHz), and G-band (220 GHz). An ideal instrument will require only trivial modifications to change operating waveband and polarization so comparative BRDF measurements may be easily made. The Phase I deliverable is a detailed construction plan with performance estimates based on specified, available hardware with the required stability, power, and sensitivity to maximize signal to noise ratio (SNR) and minimize dwell time at each angle. PHASE II: Using this plan, construct and deliver to AMRDEC an instrument that can measure the mmw polarimetric BRDF of test objects at least 30 cm x 30 cm x 30 cm in size for at least a week of continuous operation. The heterodyne Tx/Rx module mounted on the gonioreflectometer or equivalent instrument must coherently measure amplitude and phase information and construct a BRDF with user-specified angular precision in each of the following mmw bands: Ka-band (35 GHz), V-band (60 GHz), W-band (94 GHz), D-Band (140 GHz), and G-band (220 GHz). An ideal instrument will require only trivial modifications to change operating waveband and polarization so comparative BRDF measurements may be easily made. The Phase II deliverable will have the required stability, power, and sensitivity to maximize SNR and minimize dwell time at each angle. It will also have a user-friendly analysis and graphical user interface to render the BRDF in a variety of user-specified formats that facilitate comparative analyses as a function of angle, waveband, and polarization. PHASE III: Dual Use Application: Expand the instrument so that it may estimate the RCS of larger military or civilian targets in the presence of ambient clutter. Millimeter wave RCS measurements will prove very useful in the growing need for remote detection of concealed anti-personnel terrorist threats in civilian venues like airports, courthouses, stadiums, and other large public gatherings. The value of an instrument for measuring the mmw BRDF is that it can help identify new methodologies (e.g. polarimetric, multispectral) for detecting such threats with greater reliability and a lower false detection rate. REFERENCES: 1) K.B. Cooper et al., IEEE Microwave and Wireless Components Letters, Vol. 18, p. 64 (2008). K.B. Cooper et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 56, p. 2771 (2008). 2) W.L. Chan et al., Reports on Progress in Physics, Vol. 70, p. 1325 (2007). W.L. Chan et al., Applied Physics Letters, Vol. 93, p. 121105 (2008). 3) B. Gorshunov et al., International Journal of Infrared and Millimeter Waves, Vol. 26, p. 1217 (2005). 4) J.C. Dickinson et al., Terahertz for Military and Security Applications IV, Proc. Of SPIE, Vol. 6212, p. 62120Q-1 (2006). 5) A. Semenov et al., Terahertz for Military and Security Applications VI, Proc. Of SPIE, Vol. 6949, p. 694902-1 (2006). 6) http://www.saic.com/products/software/xpatch/ 7) Yasuhiro Mukaigawa et al.,"Rapid BRDF Measurement Using an Ellipsoidal Mirror and a Projector"IPSJ Transactions on Computer Vision and Applications, Vol. 1, 21-32 (Jan. 2009).
