The Jet Propulsion Laboratory's 675 GHz, 25 m standoff imaging radar can achieve >1 Hz real time frame rates over
40x40 cm fields of view for rapid detection of person-borne concealed weapons. In its normal mode of operation, the
radar generates imagery based solely on the time-of-flight, or range, between the radar and target. With good clothing
penetration at 675 GHz, a hidden object will be detectable as an anomaly in the range-to-surface profile of a subject.
Here we report on results of two modifications in the radar system that were made to asses its performance using
somewhat different detection approaches. First, the radar's operating frequency and bandwidth were cut in half, to 340
GHz and 13 GHz, where there potential system advantages include superior transmit power and clothing penetration, as
well as a lower cost of components. In this case, we found that the twofold reduction in range and cross-range resolution
sharply limited the quality of through-clothes imagery, although some improvement is observed for detection of large
targets concealed by very thick clothing. The second radar modification tested involved operation in a fully polarimetric
mode, where enhanced image contrast might occur between surfaces with different material or geometric characteristics.
Results from these tests indicated that random speckle dominates polarimetric power imagery, making it an unattractive
approach for contrast improvement. Taken together, the experiments described here underscore the primary importance
of high resolution imaging in THz radar applications for concealed weapons detection.