This room-temperature, all-solid-state active submillimeter imager can be used to detect concealed weapons through clothing.
With this high-resolution imaging radar system, coherent illumination in the 576-to-589-GHz range and phase-sensitive detection are implemented in an all-solid-state design based on Schottky diode sensors and sources. By employing the frequency-modulated, continuous-wave (FMCW) radar technique, centimeter-scale range resolution has been achieved while using fractional bandwidths of less than 3 percent. The high operating frequencies also permit centimeter-scale cross-range resolution at several-meter standoff distances without large apertures. Scanning of a single-pixel transceiver enables targets to be rapidly mapped in three dimensions, so that the technology can be applied to the detection of concealed objects on persons.
The system evolved from a tunable, continuous-wave (CW) 600-GHz vector imager system. The radar’s key components, custom-built for a different application at JPL, are the Schottky-diode multipliers generating transmit powers of 0.3 to 0.4 mW over 576 to 595 GHz and a balanced fundamental mixer exhibiting a double-sideband noise temperature of ≈4,000 K over the same range. Also notable in the design is that residual phase-wander between the locked radio frequency (RF) and local oscillator (LO) K-band source synthesizers is canceled at an intermediate 450 MHz IF stage before final conversion to baseband through an IQ mixer.
To implement the FMCW chirp, a 2–4 GHz low-phase-noise commercial YIG synthesizer is used with a tuning bandwidth of 5 kHz, typically ramping over 350 MHz (subsequently multiplied by 36 to 12.6 GHz) in 50 ms. The chirp signal is up-converted onto the CW synthesizers’ signals before multiplication. De-ramping of the FMCW waveform occurs at the 600 GHz receiver mixer. While high multiplication factors should be generally avoided in FMCW radar systems to minimize the impact of phase noise in the transmitted signal, in this case, the short standoff ranges produce a phase noise floor that lies below the thermal noise except for the brightest, mirrorlike specular targets.
The submillimeter power is transmitted first through a silicon wafer beam splitter and then a plano-convex Teflon lens with a diameter of 20 cm. This lens focuses the THz beam to a spot size of ≈ 2 cm at a standoff range of 4 m. To achieve scanned images, a flat mirror on a two-axis rotational stage deflects the beam in the desired direction.
This innovation is an improvement over an earlier submillimeter high-resolution radar. First, a faster frequency-sweeping method consisting of a wideband YIG oscillator has been implemented. Second, the data acquisition and signal processing software has been updated in order to deal with the faster radar pulse repetition rate.
The improvements mean that the 580-GHz imaging radar can now acquire three-dimensional images of people in about five minutes. It is also feasible to detect objects concealed by clothing. This capability is possible because of the improved speed and functionality of the imaging radar’s hardware and software.
This work was done by Robert Dengler, Ken Cooper, Goutam Chattopadhyay, Peter Siegel, Erich Schlecht, Imran Mehdi, Anders Skalare, and John Gill of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Refer to NPO-45156, volume and number of this NASA Tech Briefs issue, and the page number.