Due to concerns over privacy, efficiency, and accuracy, security screening at airports and other public places is constantly being improved. At NASA's Jet Propulsion Laboratory, a 675-GHz imaging radar with the potential to perform such screenings from a distance has been proposed by Ken B. Cooper, Robert J. Dengler, Nuria Llombart, Bertrand Thomas, Goutam Chattopadhyay, and Peter H. Siegel. This radar can conduct rapid "frisk" or "pat-down" types of searches of persons as far away as 25 m via a focused, low-energy terahertz beam.
The team picked the 675-GHz band because it benefits from low atmospheric attenuation. At the same time, it provides sufficiently high spatial resolution for a favorable tradeoff between antenna size and standoff range. To achieve sub-centimeter-range resolution, the radar relies on the frequency-modulated-continuous-wave (FMCW) radar technique in combination with a bandwidth of nearly 30 GHz. To optimize the radar's range resolution, a software-calibration procedure compensates for signal distortion from radar waveform nonlinearities.
The radar achieves low-noise, high-dynamic-range detection with a combination of a heterodyne RF architecture, low-noise chirp source, and 675-GHz transceiver. With its quasi-optical design, it allows low-distortion, fast beam scanning for single-pixel imaging. The portable laboratory prototype operates in FMCW mode over a 28.8-GHz bandwidth, currently centered at 676.7 GHz. With peak output power below 1 mW, it is well within health safety limits. See "THz Imaging Radar for Standoff Personnel Screening," IEEE Transactions On Terahertz Science And Technology, Sept. 2011, p. 169.