This PDF file contains the front matter associated with SPIE Proceedings Volume 12111, including the Title Page, Copyright information, Table of Contents, and Committee Page.

VIPR (Vapor Inside-cloud Profiling Radar) is a differential absorption radar operating over 155-175 GHz, covering a portion of the lower frequency flank of the 183 GHz water vapor absorption resonance. Transmitting at a ~1.9 mm wavelength, VIPR is highly sensitive to scattering from small particles comprising clouds and precipitation. Variation of VIPR’s cloud and precipitation echo power with frequency is often dominated by water vapor absorption, allowing humidity profiles to be retrieved along the radar’s beam path. One confounding effect in practical measurements is phase noise carried by the radar’s transmit signal, which can result in strong range sidelobes extending from bright targets and obscuring the echo signals of more weakly scattering clouds. Here we show that the magnitude and shape of phase-noise induced clutter from extended, bright cloud signals can be accurately modeled using a combination of empirical measurements of surface-echo phase noise and an analytic model based on the phase-noise sidelobe magnitude from a theoretical point target. This improved understanding of a potential clutter source in millimeter-wave imagine radar can lead to better performance modeling, and it provides motivation to improve the phase noise of very high frequency local oscillators.

Volcanic plumes pose a global risk to aviation, due to fine ash that can be dispersed over a global scale. Hazard mitigation relies on forecasts of plume evolution over time. However, the main sources of uncertainty in plume dispersion modelling remain the accurate quantification of the eruption source parameters, known as ‘source term’, describing the plume characteristics and informing the dispersion models. These parameters include particle size distribution (PSD) and concentrations of ash particles injected into the atmosphere. Estimation of the source term of eruptive plumes by reflectivity measurements using a single frequency radar depends upon assumption of a how PSD and concentrations are combined. The chosen assumptions are however ambiguous as, for example, a high concentration of smaller particles can produce the same reflectivity as that of a low concentration of larger particles. R4AsH is a triple frequency laboratory FMCW radar that simultaneously measures a controlled column of airborne ash at three frequencies: 10, 35 and 94 GHz. Coincident optical measurement of the PSD within the column are also taken to inform analysis. The aim of the R4AsH experiment is to develop a triple-frequency inversion algorithm to enable simultaneous retrieval of the ash PSD and particle concentration by combining radar reflectivity data across the Rayleigh – Mie scattering regime. Following on from our previously reported system design, we will present a review of the radar system performance and preliminary testing for the R4AsH experiments scheduled for the spring/summer of 2022.

The range resolution capability of the aircraft altimeter is one of the key parameters during the precise height measurements. During the landing of the rotary-wing aircraft, it is vital to detect the obstacles such as power cables, especially in poor visibility conditions like snow (White-out) and dust clouds (Brown-out) to assist a safe landing.

For lower height measurements, Frequency Modulated Continuous Wave (FMCW) radars are used to achieve a better resolution with the aid of higher bandwidth. However, this comes with two challenges: (1) the design complexity of the Radio Frequency Integrated Circuit (RFIC) (2) the high rate of atmospheric attenuation caused by the gases and aerosols.

Using the frequencies in the neighbourhood of 94 GHz for the transmission significantly reduces the atmospheric attenuation and provides an atmospheric transparency window.

In this paper, the W Band FMCW altimeter radar developed by ARRALIS is described. ARRALIS comes with the solution of both integrated circuits and modules used in a transmitter/receiver chain designed for the W Band. This provides a complete FMCW radar system working in the frequency range of 92-96 GHz with the aid of commercial off the shelf analog to digital converters and Digital Signal Processing (DSP) evaluation boards. Thus, by achieving a 4 GHz bandwidth at the centre of 94 GHz, a theoretical range resolution of 3.75 cm is achieved, which is then degraded by the windowing function factor during converting the signal into the frequency domain. The FMCW radar system uses a triangular waveform by default, which then can be converted to other waveforms as well.

Millimeter-wave (mm-wave) multiple-in-multiple-out (MIMO) imaging systems have been explored to use more and more complicated radar waveforms to achieve advanced multiplexing and high-performance imaging. As the complexity of radar waveforms increases, conventional radars inevitably suffer from higher design difficulty and cost. This work presents a software-defined mm-wave (SDMMW) multistatic radar design by making use of cost- effective commercial software-defined radios (SDRs) and additional radio-frequency/mm-wave modules. Due to the great baseband flexibility of the SDRs, the efficient space-time-coded (STC) orthogonal frequency-division multiplexing (OFDM) is developed as the radar waveform to achieve simultaneous MIMO transmission at the same time and frequency. The radar is designed at 83:5 GHz with a frequency bandwidth of 4:8 GHz, where an 8-by-8 waveguide array is specially designed to form 64 virtual channels with a small element-separation of 9 mm for three-dimensional imaging applications. To further enhance the sensing capacity and improve the imaging resolution of the SDMMW radar, the spatial coding created by the compressive reflector antenna is combined with the efficient multiplexing based on the STC OFDM. Preliminary experimental results show good imaging performance, giving great potential for developing future high-performance mm-wave imaging systems with advanced multiplexing.

In this contribution, a portable freehand millimeter-wave imaging system based on a commercial highly integrated radar-on-chip is presented. The portable imager, whose position is tracked during its operation, is moved by hand over the inspected area, enabling versatile screening capabilities with a compact device. The movement of the scanner is leveraged to create a synthetic aperture, resulting on an increased lateral resolution of the system, which comprises up to 400 independent radiofrequency channels. The performance of the proposed freehand imaging system, which is capable of providing high-resolution images at a fast acquisition rate, is illustrated through measurements of different targets.

We present a portable 3-D millimeter wave imaging system operating in the K-band (24 GHz). This imaging system consists of a multiple input, multiple output (MIMO) array of 32 transmit elements and 32 receive elements that illuminates a scene with millimeter wave energy and processes the reflected signals to reconstruct a 3-D image. To achieve an acceptable image resolution from this sparse array, the system combines multiple measurements while the sensor is moved relative to the scene being imaged. For ease of portability, the imaging system uses a single Ethernet cable to power the sensor and transfer the captured raw data to a laptop computer. A graphics processing unit (GPU)-optimized image reconstruction algorithm transforms the raw data to a 3-D image with approximately 1 cm voxel resolution, which is rendered in 3-D in a Web browser based user interface. We present measured test images and demonstrate an achieved dimensioning accuracy of ±1 − 3 mm when the system is used to detect and dimension objects hidden behind opaque building materials such as drywall, plywood, ceramic tile, vinyl flooring, and cement.

We present the experimental results of a submillimeter-wave standoff imaging system based on a frequency-diverse hologram and image reconstruction via machine learning at 220-330 GHz. The imaging system operates in a single-pixel, monostatic configuration consisting of a transceiver together with a frequency-diverse phase hologram to interrogate the region of interest with quasi-random field patterns. The spatial reflectivity distribution in the region of interest is embedded in the wide-band frequency spectrum of the back-reflected signal and the images are acquired without mechanical or electrical scanning. Images from a visible-light camera are used as the ground truth of the target elements. The targets are scanned in the region of interest, while the wide-band reflection spectrum for the target is measured. The collected image-signal pair data are used to train a deconvolutional neural network for image reconstruction with the submillimeter-wave reflection spectra as input. In experiments, a corner-cube reflector and a complex test target made of copper foam were imaged in a 28-degree field of view at a distance of 600 mm from the imaging system. The effect of bandwidth on image quality is evaluated using 10-40 GHz bandwidths centered at 275 GHz to image the copper foam target. The resolution in the image predictions was estimated from fitted point-spread functions to be from 12 mm to 30 mm, with the highest resolution at the broadest bandwidth. We have correlated the measured field patterns at the region of interest with the mean squared error (MSE) of the predicted corner-cube images to analyze the effect of field characteristics on imaging accuracy. The results demonstrate increased accuracy in locations with high electric field amplitude and variation over the imaging bandwidth.

Active incoherent millimeter-wave (AIM) imaging is a recently introduced imaging technique that combines the benefits of passive and active millimeter-wave imaging by using incoherent noise illumination to mimic the properties of thermal radiation. In this work, we investigate the performance of a video-rate AIM imager in an outdoor scenario. The use of active illumination overcomes challenges in other modalities such as sky reflections and other environmental signals. We use a 38 GHz active incoherent millimeter-wave camera with multiple noise transmitters to demonstrate imaging in outdoor scenarios at ranges of more than 9 m.

CNT has a very light density of about half that of aluminum, but its strength is about 20 times that of steel, its current density resistance is more than 1000 times that of copper, and it also has higher thermal conductivity than copper. Although it is an excellent material that is expected to be used in various fields, its usage is special and it was difficult for everyone to handle it easily.

We have succeeded in developing a flexible and stretchable CNT paint with the aim of applying this excellent material to DEA electrodes. This CNT paint can be easily applied to various materials by using a spray or other methods and since it can expand and contract, it can be applied to easily deformable materials such as polymers, wood, paper, and resin. It is possible to add the characteristics of CNT to various materials. One of the characteristics of CNTs that has been attracting attention in recent years is the radio wave absorption effect.

In the later part of 2021, based on the results obtained from early that year, the shielding effect and absorption rate were measured and verified for the radio wave absorption effect in the millimeter wave bands. This research was conducted since the band is being increasingly used for systems like the automobile collision prevention radar system or 5G.

In this experiment, we also conducted a demonstration experiment using an in-vehicle millimeter-wave radar system for the purpose of verifying its practicality. The purpose of this experiment was to prevent the target from being detected by applying SWCNT paint using the evaluation radar system used in the development of the in-vehicle millimeter-wave radar system.

Pyroelectric lithium tantalate (LT) wafers were integrated with sub-wavelength resonant absorbers for spectral sensing at THz frequencies. Devices were designed using electrodynamic simulation. A periodic surface pattern of gold resonators was patterned on the surface of thin LT wafers by photolithography, using a Ti sticking layer. Reflectivity was characterized using vacuum-bench Fourier transform reflectance spectroscopy down to 0.3 THz using a globar source, mylar pellicle beamsplitters, and liquid helium-cooled bolometer at 4 K. Photoresponse was measured using a blackbody and a tunable mm-wave source, with the detector thermally isolated in a vacuum box with polyethylene window. The spectra reveal a pair of absorption bands separated by 0.30 to 0.45 THz. The maximum absorption varies between 30 and 70 % as a function of design parameters. The resonances are insensitive to incidence angle or polarization. Experimental results agree with design predictions. The sticking layer used for gold adhesion was found by the simulation to shift the resonance frequencies by up to 7%, to decrease the maximum absorption, and to broaden the resonances. The LT thickness of 50 micron was chosen to be thin enough to have low thermal mass but thick enough to be handled during processing. However, some of the responses can be attributed to Fabry-Perot resonances when the wavelength in LT becomes comparable to the LT thickness, so a more spectrally pure response would be achieved by avoiding those thicknesses.

The Electromagnetic Signatures of Explosives Laboratory at the Transportation Security Laboratory supports the test and evaluation of millimeter wave imaging systems through the measurement of the dielectric properties of materials of interest. The EMXLAB also actively works towards identification of materials using wideband millimeter wave imaging. Accurate complex permittivity values are necessary to discriminate materials, model their response in MMW imaging systems and create simulant materials.

This work describes the simulation, development and implementation of a resonant waveguide cavity measurement system designed to operate between 75 GHz and 110 GHz and to measure the complex permittivity of sub-microliter volumes of liquid. Unlike other resonant waveguide cavity systems, the liquid is not placed at the center of the cavity in a fixture. Instead, the liquid is placed on the opposite cavity wall to the coupling aperture without a fixture. This requires the standard perturbation theory equations to be modified to allow the sample placement on the cavity wall.

The resonant waveguide cavity measurement system was to used characterize a liquid threat material and in the development of a liquid simulant material for a millimeter wave imaging system of interest. Data will be presented along with modeling results showing excellent agreement between the liquid threat material and its liquid simulant material for the imaging system of interest.

Measurement of reflectivity in free space is a useful technique to quantify the refractive index and associated dielectric constant of materials at millimeter-wave frequencies. The configuration of the target and the incident and scattered radiation fields are often idealized, and the analytic models can be studied numerically to improve measurement precision. A finite element (FE) 2D model of a free space dielectric measurement system at 60-90 GHz is presented with an example target consisting of a partially transparent “disk” on metal with assigned characteristic dielectric values. The simulated data for the reflected signal at the antenna are compared with the predicted reflection spectra and are analyzed to derive the “measured” dielectric constant. The efficacy of using an aperture in the measurement system to control the beam and reduce the required target dimensions is demonstrated. A method to use an array of antenna elements to reduce the illuminated spot size is also explored.

This paper proposes a simple design method for a multi-static aperiodic array to achieve 220 GHz sparse imaging, and a corresponding image reconstruction algorithm based on Fast Fourier Transform (FFT) and sparse data recovery. The proposed aperiodic sparse array originates from the linear sparse periodic array (SPA), it can further save the number of sampling data, transceivers and system cost compared to SPA imaging system. Low rank matrix recovery technique with principal component pursuit by alternating directions method (PCPADM) is used to recover the missing data caused by the sparse sampling. In order to achieve fast image reconstruction, FFT-based matched filtering method is used in which multistatic-to-monostatic conversion and interpolation are applied for data pre-processing. The proposed imaging scheme has been verified in experiments. An imaging resolution of 6 mm resolution is achieved at 1.4 m with 192 mm × 300 mm field of view, with a significantly reduced reconstruction time in comparison to the generalized synthetic aperture focusing technique (GSAFT).

An approach to designing multiple waveforms in a multiple-input multiple-output (MIMO) system is presented so that the full capacity of the transmitting and receiving antennas can be utilized at the same time. On the transmitter-side, the antenna elements are classified into different groups according to their specific signal. On the receive-side, we use a multi-resolution analysis to retrieve the signals of each channel. Due to the superior characteristics of the FMCW signal, especially in terms of sampling, in the proposed approach, an FMCW radar is considered. To adapt the introduced system to multistatic near-field imaging, we use more accurate models than the effective phase center principle. This contributes to the successful reconstruction of the scene image by efficient Fourier-based image reconstruction methods. The performance of the proposed approach is confirmed by numerical simulations.

Active microwave and millimeter-wave imaging is commonly used for security screening at airport checkpoints and other protected facilities. This paper explores an imaging system concept that may improve screening convenience, reduce cost, and enable alternative operational concepts by allowing a person to walk naturally through the system. Millimeter-wave imaging systems require data to be acquired over a 2D spatial aperture to form a high-resolution image. This requirement is usually met using mechanical scanners or large antenna arrays that provide a 2D aperture and provide strict control over the position of the array in relation to a motionless target. The new concept explored in this paper replaces the mechanical scan with motion of the passenger. The complex motion of the passenger is expected to be optically tracked as he or she passes by stationary linear vertical millimeter-wave arrays and can be modeled using skeletal animation. Multiple linear arrays illuminate the passenger from a wide variety of angles to provide full coverage of the body. The radar data are then correlated with the skeletal animation model by employing generalized synthetic aperture focusing or back-projection techniques. These methods accurately reconstruct the image by integrating the measured response multiplied by the conjugate of the expected response from a point scatterer anywhere within a 3D image volume. This process yields an optimally focused image and can be applied to situations involving complex target motion. This paper describes this concept in detail and provides numerous simulation-based imaging results to explore the effectiveness of the proposed methods.

The simplicity and cost factor of millimeter-wave hardware restrict the implementation of near-field radar imaging systems in many applications. These drawbacks are exacerbated when using multiple antennas, each being connected to a dedicated sampling chain. To significantly alleviate these constraints, the use of analog multiplexing techniques appears inevitable. The choice of a suitable solution for a given application can be challenging. Among the available technologies, this study focuses on frequency beamsteering and space-frequency random scanning, comparing the characteristics and performances of these techniques in a radar imaging scenario in the 92-96GHz frequency band.

The relative benefits of an offset Dragonian reflector compared to equivalent off-axis parabolic (OAP) mirrors for feeding collimated beam to a scanned beam imaging system are investigated. Physical-optics simulation of the Dragonian are performed at 500 GHz. The input is a Gaussian beam with a frequency dependent waist radius fit to the output of a standard Pickett-Potter horn. The collimated output beam properties are characterized, including cross-polarization, beam waist radius, Gaussicity, and M-squared parameter. Next, by sweeping the parameters of an OAP reflector (parent focal length and incidence angle) in the physical-optics simulations, we find the geometry in which the properties of the output beam best match the Dragonian geometry. This reflector is found to be an OAP with 108.22 mm parent focal length and 30◦ incidence angle. An additional OAP reflector is also considered in these simulations, which is the most often used 90◦ OAP. The parent focal length is 56.95 mm for this mirror, so that we have a similar beam waist radius in the detector plane. Finally, physical optics simulations reveal that the Dragonian geometry produces much smaller cross-polarization in the detector plane (−23 dB at the beam waist) in comparison with OAP reflector (being −14 dB and −8 dB for 30° and 90° off-axis mirrors, respectively). The 30° OAP reflector is able to produce similar beam quality in terms of phase variation, Gaussicity, and beam waist radius at the detector plane.