This PDF file contains the front matter associated with SPIE Proceedings Volume 11411, including the Title Page, Copyright information, and Table of Contents.

The Jet Propulsion Laboratory has developed a 170 GHz airborne radar for cloud and humidity remote sensing. Called VIPR (Vapor Inside-cloud Profiling Radar), the system uses differential absorption at closely spaced frequencies near the 183 GHz water vapor resonance to obtain range-profiling measurements of absolute humidity inside clouds, and partial-water- column measurements in clear skies. VIPR transmits around 300 mW over 167-174.8 GHz, has a system noise figure of 8 dB, and uses a 60-cm diameter aperture. The radar has been deployed both on the ground pointing toward zenith, and from an aircraft with nadir pointing. Based on architectures originally developed for submillimeter-wave security imaging, VIPR uses ultra-high-isolation transmit/receive duplexing with a single primary antenna. This approach achieves thermal noise limited sensitivity even while using frequency-modulated continuous-wave ranging methods, and even when the radar is mounted in an aircraft with its beam emerging from an open-air viewport. Here we present a validation measurement of VIPR’s ability to sense humidity in clear skies using ground reflection magnitudes at different altitudes and frequencies. These results have also motivated a new investigation of using a higher-frequency 557 GHz differential absorption radar for water vapor sensing in the low pressure, cold, and dry conditions on Mars. We have developed a 552-558 GHz RF source with several mW of output power that could be used for making local humidity measurements on Mars out to several kilometer ranges.

The use of radar systems is limited in some applications by spatial constraints or special thermal and environmental conditions. The spatial separation of the sensitive electronics and the more robust antenna by a flexible connection therefore opens up new applications. A 160-GHz radar system with a mechanically flexible front end fulfilling these requirements is proposed in this paper. The flexible front end is an extremely low loss dielectric waveguide feeding a dielectric elliptical lens antenna (28 dBi gain). The dielectric waveguide has dielectric losses of 4.5 dB/m at 160 GHz and is very flexible, allowing bending radii of down to 1.5 cm with negligible losses. The dielectric waveguide is fed by a 160-GHz radar monolithic microwave integrated circuit (MMIC), which allows bandwidths of up to 20 GHz for a high range resolution. The transition between MMIC and dielectric waveguide is realized with a rectangular-waveguide interface. The radar back end consists of a phased-locked loop (PLL) with standard components, an intermediate frequency (IF) signal conditioning part, and a Xilinx Zynq 7030 System-On-Module (SOM) with an FPGA and an ARM-based processor. The sampled signal is processed directly on the FPGA with a 2D Fourier transform and is available as a UDP stream with an update rate of up to 15 Hz. In addition, a camera image is taken for each radar measurement. The presented system is used to detect and measure the flight behavior of honey bees. The electronics are housed in a building whereas the flexible dielectric waveguide allows the antenna to be placed anywhere around the beehive, where it is exposed to environmental conditions.

This paper presents a millimeter-wave code-modulated interferometric imaging system, which is a lens-less approach to realizing imagers using repurposed phased arrays. To use a phased array as an interferometer, incoming signals are code modulated using phase shifters, multiplexed using a power combiner, and processed through a shared receiver chain. An interference pattern is then obtained by a squaring operation, from which complex visibilities can be demodulated. Here, a four-element 60-GHz phased array chip is packaged with slot antennas, and a single 60-GHz output is measured using a power detector. This scalar measurement is then demodulated to obtain the interferometric visibilities. The four-element phased array is thinned to obtain a 13-pixel image and the system is demonstrated through a point-source detected at different locations.

Microwave imaging systems allowing the real-time scanning of short-range objects are difficult to implement on a large scale due to their complexity and cost. In this paper we introduce a new ultra-wideband multiple-input multiple-output radar using microwave photonic components in reception. These components permit ultra-fast time division multiplexing of all receiving signals and hence their measurement with a single acquisition channel. This architecture makes possible to decrease the time of acquisition compare to architecture with a sequential reception.

A 94 GHz electronic scan radar concept is presented that includes measured results of key 94 GHz hardware technical advancements that are required to bring to fruition a low cost millimeter wave radar front end. Described will be a system concept that uses a line array of 0.61λ spaced receive element radiators at 94 GHz enabling wide scanning digital beam forming and Doppler beam sharpening techniques to be used for high spatial resolution ground and obstacle mapping modes. Key hardware challenges in the area of millimeter wave radiator implementation and receiver front end integration need to be solved in order to bring this low cost millimeter wave solution to fruition. Measured results are presented for the evolutionary development of a batch processed, printed wiring board based W-band radiating element that overcomes the manufacturing tolerance and grid spacing constraints that have prohibited this from being practical in the past with current design and low cost manufacturing methods. The design enables a surface mount W-band MMIC down converter receiver package to be assembled onto the radiator board behind each element to minimize feed losses. Measured beam patterns for a proof of principle 8 element, 0.61λ spacing, waveguide radiator line array that includes surface mounted 94 GHz MMIC down converters in each channel will be presented. Measured 94 GHz results for other fabricated structures across the board will be presented to evaluate performance yield across large PWB board area.

Advanced Imaging Technology (AIT) uses millimeter-waves for airport passenger screening. The Identification of Explosives (IDX) technique addresses secondary screening of detected anomalies by analyzing the reflectivity data across the frequency band of the imaging system to probe the electrical permittivity of the potential threat. To be practical, IDX must apply to targets that are not configured for free-space metrology, but have poorly defined surfaces and are imaged with non-ideal boundaries. In particular, the magnitude of the reflection coefficient, which is key to free-space measurement, cannot be obtained with any confidence. The detection is accomplished from frequency-dependent features in the interference spectrum, which provide material identifying information in the form of the dielectric loss tangent.

This work describes the characterization of liquid explosive materials and the development of a simulant material. A Keysight coaxial probe was used to characterize the complex permittivity of the liquid explosive of interest and the simulant material formulations as a function of frequency. Data was collected over the frequency range of 500 MHz to 50 GHz. The frequency range overlaps several existing millimeter wavelength imaging systems. Complex permittivity data was processed using a Fresnel reflection/transmission model which produces an effective reflection coefficient for a sample of material as a function of frequency. The model accounts for sample thicknesses and backing materials such as skin and air. An ensemble average across the bandwidth of the millimeter wave system of interest is then applied to model the response of the material to the imaging system of interest. Complex permittivity data will be presented along with model results showing excellent agreement between the explosive material and its paired simulant material for MMW imaging systems of interest.

Millimeter wave (MMW) imaging systems have the capability of detecting anomalous objects, which can include explosive threats and other prohibited items concealed on persons in airport checkpoints and other facilities requiring personnel screening. Benign materials that simulate explosives and other threats are used to test Advanced Imaging Technology (AIT) systems when live threats cannot be placed on human subjects. While laboratory dielectric measurements are used to formulate candidate simulants, it is useful, and sometimes necessary, to independently validate a simulant for an AIT system of interest. An imaging phantom has been fabricated using standard vacuum hardware and thin plastic films for containing samples of interest. The phantom’s design allows for simultaneous imaging of threats and candidate simulants in a fixed, repeatable fashion. The phantom was self-validated with deionized water using a criterion for resolving two overlapped distributions. Results obtained from a subsequent study of a flammable liquid versus its candidate simulant are presented, validating the use of the simulant for use with the target AIT system.

The Pacific Northwest National Laboratory (PNNL) has recently developed an active 3D microwave/millimeter-wave shoe scanner. This system is designed to detect threats concealed within the soles of common footwear. The system was designed in response to the security incident involving Richard Reid, known as the “Shoe Bomber”. The system operates over the 10-40 GHz frequency range. Waves in this band readily pass through common shoe materials, such as leather, rubber, plastics, foams, and synthetic and natural cloth materials. The shoe scanner system consists of a linear array positioned underneath a low loss dielectric window that the person is directed to stand upon. The linear array is positioned so the antenna propagation is vertical, and the array axis is horizontal across the width of the shoes. A linear mechanical scan translates the arrays along the length of the shoes. A frequency-modulated continuous wave (FM-CW) transceiver is used to collect the signal scattered from the scene. The data collected from the system is fully 3D covering two spatial and one frequency dimensions. The system presents several challenges for efficient image reconstruction, including the dielectric window, multi-row linear arrays, and focusing close to the antenna elements. The dielectric window presents a significant challenge for image reconstruction since the waves will travel through an inhomogeneous layered media. In this paper, an efficient back-projection reconstruction algorithm is presented that overcomes these challenges. Experimental imaging results are shown that demonstrate high-resolution imaging performance for this new scanner.

We present a new method to carry out localization based on distributed beamforming and neural networks. A highly dispersive hologram, is used together with a terahertz spectrometer to localize a corner-cube reflector placed in the region of interest. The transmission-type dielectric hologram transforms input pulse from the spectrometer into a complex pattern. The hologram causes complicated propagation paths which introduce delay so that different parts of the region of interest are interrogated in a unique way. We have simulated the emitted pulses propagating through the hologram. The hybrid simulation combines the finite-difference and physical optics methods in time domain and allows for evaluating the dispersion and directive properties of the hologram. The dispersive structure is manufactured of Rexolite and it has details resulting in varying delay from 1 to 19 wavelengths across the considered bandwidth. The spectrometer is configured in reflection mode with wavelets passing in to the region of interest through the hologram. A data-collecting campaign with a corner-cube reflector is carried out. The effective bandwidth for the localization is from 0.1 THz to 2.1 THz, and the measured loss is 57 dB at minimum. The collected data is used to train a fully-connected deep neural network with the known corner-cube positions as labels. Our first experimental results show that it is possible to predict the position of a reflective target in the region of interest. The accuracy of the prediction is 0.5-0.8 mm at a distance of 0.17 m.

In this paper, we describe the recent development of new algorithms applied to short-range radar imaging. Facing the limitations of classical backpropagation algorithms, the use of techniques based on Fast Fourier Transforms has led to substantial image computation accelerations, especially for Multiple-Input Multiple-Output systems. The necessary spatial interpolation and zero-padding steps are still particularly limiting in this context, so it is proposed to replace it by a more efficient matrix technique, showing improvements in memory consumption, image computation speed and reconstruction quality.

Active three-dimensional (3D) microwave and millimeter-wave imaging techniques have been extensively developed for concealed threat detection at the Pacific Northwest National Laboratory (PNNL), most notably the cylindrical millimeterwave imaging method currently in use for airport screening. Typically, a linear array is mechanically scanned over a cylindrical or planar aperture in order to form a high-resolution 3D image. A linear array mounted on a low-cost encoderdriven rail system was desired for rapid data collection and evaluation of concealed threat detection on a stationary target. A rail system to sweep out a planar aperture was quickly developed, however, due to the low-cost implementation of the rail system and encoder, resulting images were lower quality than expected. It was determined that the position information provided by the rail system encoder was not accurate enough to generate an image of the desired quality. Instead of using a traditional encoder wheel with the rail system, optical motion tracking was used to record 3D position information of the linear array synced with the radar as it was manually scanned over a nominally planar aperture. While optical motion tracking can provide position information with sub-millimeter level accuracy, it doesn’t guarantee that the scanned aperture is strictly planar or uniformly sampled. Reconstruction techniques necessary to incorporate 3D position information and compensate for an irregular imaging aperture are developed. Experimental results showing the benefit of precise optical motion tracking for a manually scanned linear array are presented.

Millimeter-wave imaging provides a promising option for long-range target detection through optical obscurants such as fog, which often occur in marine environments. Given this motivation, we are currently developing a 150 GHz polarization-sensitive imager using a relatively new type of superconducting pair-breaking detector, the kinetic inductance detector (KID). This imager will be paired with a 1.5 m telescope to obtain an angular resolution of 0.09° over a 3.5° field of view using 3,840 KIDs. We have fully characterized a prototype KID array, which shows excellent performance with noise strongly limited by the irreducible fluctuations from the ambient temperature background. Full-scale KID arrays are now being fabricated and characterized for a planned demonstration in a maritime environment later this year.

Millimeter wave frequencies has been proven to be able to penetrate clothes and luggage allowing the detection of concealed threats. Previous work showed the high performances of the passive Millimeter wave Imager developed by MC2-Technologies. The system can detect threats through clothes in Real-Time up to 16 images/s with a large field-of-view. This paper introduces a patented scanning technique to simultaneously measure radiated wave from two different scenes while providing high performances and video rate images. The proposed approach is integrated in a global solution so-called SACOP and allows the drastically reduction of the staff and material resources for implementation in real environment. Imaging has been conducted at millimeter-wave frequency with conclusions on the merits. The results demonstrate the potential of the new scanning technique for people screening in a challenging indoor situation.

We present a fully-staring THz video camera prototype intended for security screening. The camera utilizes so-called kinetic inductance bolometers to detect THz radiation in the bandwidth of 0.3-1 THz. The imaging distance is 2.5 m with the field-of-view being 2 m × 1 m. The camera is equipped with a kilo-pixel detector array, large field-of-view optics, intermediate-scale cryogenics operating at 6 K, and low-noise electronics to read out the whole detector array. The imaging capabilities of the system are demonstrated through radiometric performance characterization and actual imaging experiments.

The U.S. Army Combat Capabilities Development Command Army Research Laboratory (CCDC ARL) is investigating using 3-D millimeter-wave synthetic aperture radar (SAR) technology to provide navigation for aircraft through terrain and obstacles in a degraded visual environment (DVE). We have previously identified the key challenges and associated signal processing research problems that need to be addressed. In this paper, we focus on the first problem—the computationally prohibitive challenge associated with image formation. We present a fast version of back projection 3-D SAR image formation. The fast algorithm is implemented and benchmarked using both a CPU and GPU for real-time operation. We also suggest the operating parameters and demonstrate 3-D SAR image visualization of a large and realistic scene from near- to far-range using electromagnetic (EM) simulation data that illustrates the advantage and key features of 3-D SAR imagery.

Free-form two-mirror antenna for millimeter wave imaging has been modeled using a proprietary software for optical systems design PODIL. Optimization procedure is based on adaptive Cauchy differential evolution method, the principal features of which allow efficient global optimization. The system of two mirrors has the 300 mm aperture stop, and both mirrors have 380 mm the larger size. Their profiles are described by the 7th order Zernike polynomials. Pick-to-valley height difference is 18.037 mm and 35.60 mm for primary and secondary mirror correspondingly. The system allows 20- degree field of view. The RMS of the focal plane spot diagram is balanced with variations within 0,54 … 0.64 mm. 90% of energy in the focal plane over the total field of view is contained within the quadratic zones with the sides 1.6 mm. The antenna can have a focusing function.