Compressed sensing is a signal processing paradigm enabling the acquisition and successful reconstruction of a sparse signal from a reduced set of measurements, potentially in violation of the Nyquist sampling criterion. In this paper the results of preliminary investigations into Compressed Sensing applied to the acquisition of wide bandwidth millimeterwave compact radar range data are presented. Primary motivations for application of Compressed Sensing to compact radar range acquisition and imaging include increasing data acquisition speed as well as reducing required data storage. In this work only signal reduction in the frequency domain is examined. Compressed Sensing fully-polarimetric compact range data acquisition and imaging for both a simple canonical target (cylinder) and a complex target (Slicy) are presented as radar cross section (RCS) measurements and interferometric inverse synthetic aperture radar (IFISAR) images. Correlations of compact range data provide a measure of error between the reconstructed and complete data sets as a function of target complexity and sub-sampling rate.

Three-dimensional radar imaging is becoming increasingly important in modern warfare systems, leading to an increased need for deeper understanding of the 3D scattering behavior. Fully polarimetric, three-dimensional radar signature data have been collected using 1/16th scale models of tactical targets in several indoor compact radar ranges, corresponding to radar data at X-band. The high-range-resolution data has been collected through a 2D aperture in azimuth and elevation. This data has been processed into 3D coordinates using a standard 3D Fourier transform. The radar signatures have also been rendered into 3D coordinates using Interferometric ISAR techniques. The results of applying compressed sensing techniques to the analysis will be presented. Mathematical 3D correlation analysis has been used to compare the results of each method of 3D reconstruction.

In 2019, landing in zero visibility conditions remains an unresolved, dangerous problem for commercial aircraft for both regional and transport markets. Common sensor solutions such as Infrared or LIDAR offer selective, limited obscurant penetration (i.e. rain, fog, snow, smoke). Honeywell’s active millimeter wave radar platform presents a sensor solution capable of seeing through all types of weather, while generating a meaningful, actionable image in real time. Results from recent field testing with the active mmW radar on approaches to multiple small municipal airports are presented and discussed, validating the application of this sensor to this need. While smaller airstrips challenge the performance of even high resolution radar sensors given reduced detectable features of prominence, the active millimeter wave radar produces real-beam imagery at the necessary angular resolutions and relevant approach ranges to support real time imaging of runways and a variety of prominent identifying guide-features. Recorded data is combined in an output with a synthetic 3D environment, supplementing the image with available a priori information. An integrated visual result is proffered that can feed into a heads-up display (HUD), with discussion about sensor improvements and additional image processing to perhaps eventually produce a comprehensive solution to the fixed wing, zero visibility landing problem.

This paper describes the design and demonstration of a 1024 element coded aperture subreflector array, implemented with single-bit phase shifters that utilize GaN HEMTs to modulate signals upon reflection. An active reflect-array enables digital beamforming using a single 235 GHz radar transceiver. Wafer level fabrication and assembly allows large arrays to be tiled up while maintaining reasonable costs.

Conventionally, resolution characterization of an imaging radar is performed by means of analyzing the diffraction limited point-spread-function (PSF) pattern of the radar. Such an analysis is straightforward and can easily be implemented at microwave and millimeter-wave frequencies using simple point-scatter targets. However, it poses significant challenges at submillimeter-wave (or THz) frequencies due to the strong scattering response of secondary objects that are used to align the PSF targets for imaging at these frequencies. As a result, the reconstructed PSF patterns suffer from artifacts caused by the secondary objects present in the background. In this work, we present the use of the acoustic levitation principle to obtain the PSF characterization of a 340 GHz stand-off imaging radar. We show that using a water droplet acoustically levitated at the focal point of the 340 GHz imaging radar, high-fidelity PSF characterization of the radar is achieved, revealing the resolution limits of the radar while exhibiting good signal-to-noise ratio (SNR).

We demonstrate a handheld millimeter-wave radar head connected with remote signal synthesizer via an optical fiber, for the realization of a small and handheld nondestructive imaging system. An inertial measurement unit (IMU) mounted on the radar head identifies the direction and orientation of the head to provide three- dimensional point clouds for inspection of building structures. A small lidar system with the IMU is also discussed for construction of 3D point clouds for inner structure mapping in the buildings.

We demonstrate active millimeter wave (mmWave) 3D synthetic aperture radar (SAR) imaging through common building and construction materials. Human-safe mmWaves penetrate many non-metallic construction materials to a surprising degree, allowing for non-destructive structural and utility integrity analysis. We demonstrate that mmWave imaging can be used to locate pipes, electrical wiring, wood and metal studs, rebar, and other structural elements and utility infrastructure embedded in building walls. Millimeter wave imaging can also be used to find contraband items hidden inside walls. In addition, liquid water strongly reflects mmWave energy, allowing the localization of potential leaks. We present K-Band (15 - 26:5 GHz) and V-Band (50 - 65 GHz) 3D SAR images of brick, drywall, tile, wood, and concrete wall test structures. Imaging was performed using a 2D raster scan of a bistatic horn antenna pair, yielding a 3D SAR image with a voxel resolution of approximately 1.5 cm (K-band) and 5 mm (V-band).

It is critically important for the existence of life on Earth to understand and reliably predict how our planet is changing on a long-term basis. Over the past few decades, NASA, and other space agencies, have developed technologies and methodologies to measure Earth science parameters on a global scale with high spatial and temporal resolutions. NASA’s Goddard Space Flight Center (GSFC) has significantly contributed to this cause. In this report, we plan to review the microwave, millimeter wave and optical sensors technology developed, validated, and patented at GSFC for Earth monitoring and analyzing data to create more accurate predictive models. Since its foundation, GSFC has been in the business of launching many spacecraft to explore Earth and the solar system. A number of novel technologies developed at GSFC have been embedded into sensor instruments on these exploring missions. This report will explore some of these innovations, many of which have been patented. GSFC is interested in licensing these patented technologies to U.S. industries and participating in collaborating discussions with global entities.

Millimeter wave imaging systems are a promising candidate for many applications, such as indoor security, non-destructive evaluation, and automotive driver assistance. In this paper, we use an automotive radar sensor for imaging applications. We implement mono-static single-input-single-output (SISO) configuration to perform synthetic aperture radar (SAR) measurements. The reconstructed image of a spiral target by using this real imagery SAR system is compared with computational simulation results. We also present SAR measurements on several targets including: a mannequin with a gun for concealed weapon detection; pipes behind a wall for seeing through wall; and range discrimination of multiple objects. Eventually, we propose a volumetric method for data collection for automotive applications.

Security screening system for large crowded venues such as stadium events and air and rail transportation hubs have been limited to slow, single person scanning. We present a 3D holographic imaging system that operates in the unlicensed 5.8 GHz ISM band and can screen crowds of people for large hidden threat objects. We further present a unique divide-and-conquer algorithm capable of efficiently processing the system’s voluminous data sets to search for suspicious objects. Current RF holographic screening systems have utilized expensive millimeter wave technology that required prolonged (multi-second) single-person scans, making them unsuitable for high throughput scenarios. While moving from millimeter waves to centimeter waves reduces the imaging resolution, it greatly reduces system cost, increases screening range, and increases screening penetration through obscurants (such as clothing) to better detect hidden threats in dense, unstructured crowds. Furthermore, our ad-hoc sensing array elements can be integrated into ceilings, floors, walls, and structural columns to facilitate screening anywhere it is needed. The imaging algorithm quickly locates scattering objects with a large volume by casting the imaging problem as a multi-resolution octree.

Active three-dimensional (3D) microwave and millimeter-wave imaging is useful for a variety of applications including concealed weapon detection, in-wall imaging, non-destructive evaluation, and others. High-resolution imaging is usually performed using a fixed two-dimensional planar or cylindrical aperture that is defined using a two-dimensional array or precise mechanical scanning of a transceiver or sequentially-switched linear antenna array. For some applications, it is more convenient to manually translate a linear array over the scene of interest, or equivalently, move the target in front of the linear array to scan an effective aperture. Manually scanning the array or target creates several challenges for accurately focusing, or reconstructing, an image of the target. The motion of the array or target must be known accurately, typically with precision of 0.05-0.1 wavelengths. Additionally, the image reconstruction algorithm needs to be able to compensate for aperture shapes which are highly non-uniformly sampled, and which are not of a specific canonical shape such as planar or cylindrical. This paper explores high-resolution 3D microwave imaging of a moving target by using optical motion capture to track the moving target and develops highly versatile image reconstruction techniques that account for the irregular motion. Several experimental results are shown for moving targets in front of a fixed linear array.

The radar frequency backscattering behavior of rough ocean surfaces has been investigated using a physical scale modeling approach and a millimeter-wave compact radar range. The method involved fabrication of two 1/16th scale simplified rough ocean surfaces from a material that at 160 GHz behaved as the dielectric equivalent of seawater at 10 GHz. By measuring the backscattering behavior of the physical models in a 160 GHz compact range, the X-band radar frequency scattering behavior was determined. Though the physical models used were static, a scale modeling approach readily offers the ability to examine ocean backscattering phenomenology of a given surface texture over an extended range of look angles and radar frequencies, which otherwise would be challenging in a dynamic ocean environment. Computational electromagnetic modeling of the surface was also performed and compared with compact range measurements. This effort, involving more realistic ocean surfaces, builds on previous ocean modeling work presented by our team that utilized a periodic rough surface. The simulated surfaces studied here represent a natural progression toward our goal of developing reliable methodologies to characterize the backscattering behavior of land and sea clutter using physical scale modeling technologies.

We describe bistatic scattering measurements at 230 GHz, in the 330-490 GHz range and, at 650 GHz on various surfaces. These include a series of eight reference targets constructed from alumina grit embedded in an absorptive epoxy matrix, and a set of conventional outdoor building materials. The samples’ surface topographies were measured by focus-variation microscopy (FVM) and their autocorrelation lengths and RMS roughness levels extracted. All bistatic measurements were performed in the principal plane, at incidence angles of 25^o, 45^o, and 65^o, in s and p polarization. The reference samples’ normalized roughness levels cover the range 0.040 ≤ σ/λ ≤ 0.60, and their normalized autocorrelation lengths cover the range 0.086 ≤ 𝐿/λ ≤ 1.14. The measurements are described in terms of bidirection reflectance distribution function (BRDF) or normalized radar cross section (nRCS), and include regimes of both diffuse scattering and specular reflectance. The reference samples’ measurements are compared to two ab initio scattering theories, the Modified Integral Equation Method (IEM-B) of A. Fung, and the Generalized Harvey-Shack (GHS) model, that have no free parameters. Although there are several individual cases where either the IEM or GHS theory (or both) provide a good match to measurement, their overall agreement across the entire dataset is poor. In addition, the diffuse BRDF in each bistatic scan has been fit to a Lambertian (constant) dependence of scattering angle, and a purely empirical model developed for the dependence of Lambertian scattering on frequency, roughness, polarization, and incidence angle. The empirical model provides the best match to measurement across the full dataset, and can be used for reliable phenomenology studies of submillimeter imaging or wireless telecommunication. Nearly all the outdoor building materials, like the roughest of the reference samples, fall in a regime where L/σ is not large, and therefore where ab initio scattering theories can’t be expected to apply.

The test and evaluation of millimeter-wave imaging systems for explosive detection is facilitated by the substitution of explosive simulants which have an identical response to millimeter-wave illumination. The primary detection feature for millimeter-wave imaging is the dielectric constant (or electrical permittivity), so the approach to developing simulants is to match the complex dielectric constants of explosives to inert simulant materials at frequencies relevant to the imaging system. This paper describes a measurement-based methodology to assure that the simulant is a suitable substitute for the explosive. The methodology is demonstrated by dielectric measurement at 86 GHz to establish a simulant for ethylene glycol dinitrate (EGDN).

The characterization of dielectric materials is of great importance for many applications, being for instance quality control during product fabrication or status control of outside constructions over time. In many outside situations the objects of interest have limited accessibility, and the investigation has to be done without destruction of any part of the object and without any health risks for an operator. Hence remote sensing from stand-off position is often desirable, and the use of microwaves, millimeter-waves or THz waves offers some penetration capability into matter, depending on its chemical and physical decomposition and of course frequency and polarization. Many objects of interest consist of a dielectric coating or enclosure, which can electromagnetically be treated as a dielectric layered structure or a dielectric slab surrounded by air. In former investigations a linearly polarized Ka-band radiometer was connected via an electronically steerable polarization rotator to the antenna of a near-field scanner. Using this setup four linear polarization states were realized in time multiplex and the corresponding brightness temperatures of close objects were measured for each scene point. Then an estimate of the real part of the permittivity was extracted from those values using radiometry specific polarimetric modeling for incident radiation on tilted dielectric surfaces and corresponding retrieval computation. For new experiments the polarizer was replaced by simple waveguide sections providing the four required linear polarization states. Furthermore a far-field scanner was used in order to allow the imaging of large scenes of various interesting content. In this paper the theoretical background of the approach is briefly outlined and the new measurement results are discussed.

As part of the effort to provide navigation of aircraft through terrain and obstacles in a degraded visual environment (DVE) due to fog, smoke, rain, and other whiteout conditions, the U.S. Army Combat Capabilities Development Command Army Research Laboratory (CCDC ARL) is developing a forward-looking 3-D synthetic aperture radar (SAR). The program includes an electromagnetic modeling (EM) study, and the development of a hardware prototype and signal processing algorithms. We propose a 35-GHz SAR system using a 1-D array of antennas combined with the synthetic aperture of the aircraft’s forward motion to provide 3-D SAR imagery for obstacle avoidance. In this paper, we study the baseline performance of this forward-looking Ka-band 3-D SAR imaging for the helicopter landing application, and determine the key challenges and associated signal processing research problems that need to be addressed.

Three-dimensional (3-D) millimeter-wave (mm-Wave) Forward-Looking Synthetic Aperture Radar (FLoSAR) is an emerging technology that images targets in the forward direction of the flight path and holds the promise of high-resolution imaging by exploiting the unlicensed, wide bandwidth available at mm-Wave. This may be beneficial for applications such as self-landing, navigation, and collision-avoidance in a deteriorated vision environment. High angular resolution is difficult to achieve in FLoSAR because terrain points located symmetrically along the flight path have the same range and Doppler history. Additionally, the challenge at mm-Wave FLoSAR is to provide motion compensation because, at mmWave, GPS accuracy is insufficient to obtain platform position at subwavelength level, thereby leading to loss of resolution in reconstructed images. In this work, we investigate the phase gradient autofocus method for 3-D mm-Wave FLoSAR.