In this article, a polarimetric millimeter-wave FMCW radar operating in the 222-228 GHz frequency range is proposed for mapping applications in indoor environments. Equipped with a mechanically scanning fan-beam reflector antenna with 360° field of view, the radar can generate 2D images of its scene, offering LiDAR-like azimuthal resolution of 0.3°. The system’s performance for indoor mapping as well as Line-of-Sight (LOS) and None-Line-of-Sight (NLOS) obstacle detection tasks is demonstrated in building corridors. The measurement scenarios include various wall types and wooden doors with metallic frames. The overall corridor map is created by taking multiple radar images and combining them through post-processing. Given the strong presence of ghost targets in the radar images, an algorithm is developed for NLOS target detection and localization. As a foundation for this investigation, the wave reflection coefficients of painted drywall for both vertical and horizontal polarizations, ΓVVand ΓHH, are measured. Based on these results, a novel technique for NLOS target identification using polarization discrimination is proposed and experimentally verified. This technique is further tested in a realistic scene, where an L-shaped corridor is mapped using the polarimetric radar measurement, and NLOS targets are detected and localized using a mirroring transformation.

This paper reports on polarimetric radar backscatter responses of different types of surfaces at high incidence angles (80° - 89°) at J-band frequencies (221-231 GHz). This study is motivated by the desire to understand the radar phenomenology of target scenes for the next-generation automotive radars. Measured data were collected for paved and unpaved surfaces under different conditions (dry, wet, ice- and snow-covered). At these frequencies, dry road surfaces are electrically rough and sizes of the subsurface aggregates are comparable to the signal wavelength giving rise to significant surface and volume scattering. The measured backscattering coefficients of all surfaces exhibit over 30 dB dynamic range across all data sets and significant depolarization (between -12 dB and -4 dB). For paved surfaces, the dynamic range narrows to 16 dB and the depolarization ranges between -9 dB and -4 dB. The data represent the first reported observations of radar response of surfaces at high incidence angles at J-band frequencies. They demonstrate the radar’s potential for discriminating between different types of road surfaces needed for improved safety of driverless cars. Also, based on the measured data, a semi-empirical model for the backscattering coefficients of the paved surfaces at high incidence angles is presented.

This paper presents a thorough phenomenological study of the responses of human subjects to millimeter-wave radars operating at J-band (220-325 GHz). In highly automated vehicles, identifying vulnerable road users such as pedestrians and bicyclist on the road and its vicinity is crucial. This paper presents various characteristics of the human body response to J-band radars and possible detection and identification techniques.

A mechanically scanned high-resolution 2-D imaging radar system with full 360° Field-of-View (FoV) is presented. The system is composed of a multi-function 222-228 GHz frequency modulated continuous wave (FMCW) radar and a moment of inertia optimized 3-D printed offset reflector antenna. The reflector geometry is made from anElliptic Toroidalsurface which is shown to produce a high-gain fan-beam with an 11:1 aspect ratio. The reflector is rotated at speeds up to 10 Hz for high throughput imaging. The wobble generated by the high speed rotation is suppressed by optimizing the mass distribution of the reflector body. It is fabricated in-house using a low-cost table-top Fused Deposition Modelling (FDM) 3-D printer. The reflecting surface is metallized with a silver coated copper paint and processed using simple steps to ensure smoothness and accuracy. The reflector achieves beamwidths of 4.6° in elevation and 0.42° in azimuth, with a peak gain of 38.7 dBi. The 3 dB azimuthal resolution of the radar two-way beam is measured to be 0.3°. The system is tested in real-world scenarios and the resulting images and videos are of unprecedented quality due to the large area of coverage, high resolution, high speed and excellent dynamic range. To the best of our knowledge, this is the first demonstration of a compact radar system that is comparable in resolution, speed and angular coverage to lidars, while offering higher dynamic range and immunity to severe weather and lighting conditions.

Radars operating at the higher region of the millimeter-wave frequencies are proposed to applications in new generation of automotive radars. This work is conducted to provide the base knowledge required to optimize the design and operation of such radars. Scattered signals by road surfaces provide useful information in recognizing the road condition and assessing the navigation. The radar polarimetric responses at near-grazing incidence angles of various road surfaces are characterized experimentally. Asphalt and concrete, being the most popular surfaces, are carefully measured and characterized in dry conditions. Road surface that are wet, ice-covered, or snow-covered are also examined.

This article presents a comprehensive technique for calibrating wideband frequency-modulated continuous-wave (FMCW) fully polarimetric radars employing linear FM (LFM) signal chirps. A detailed system distortion model is developed that accounts for both polarimetric distortions and distortions caused by nonlinearities in the transmitted and received radar chirps over multiple polarization channels. Using the radar responses of two point targets at known distances from the radar, the calibration technique is able to estimate the range-dependent nonlinearities in phase and correct them for all targets at all ranges. This allows the radar to achieve its ideal range resolution. In addition, the calibration technique corrects for channel imbalances in the polarimetric radar using a metallic sphere of known diameter and any depolarizing target. Both numeric simulations and actual polarimetric radar measurements using a wideband FMCW radar operating over 76.5–82 GHz were used to validate the calibration technique.

This paper presents novel detection and target identification techniques for a polarimetic J-band millimeter-wave automotive radars. Three major target categories are considered, and extensive experimental and numerical data have been generated to find unique features of different target types. The response of vehicles is experimentally studied to build scattering maps for different kinds of cars. The radar backscatter response level from road surfaces is investigated and possible applications such as lane detection and road condition recognition are discussed. Sample results for each target type is presented to illustrate the suggested detection techniques. A method for micro-Doppler spectrum measurements of a moving human body is suggested as a very useful and unique feature to detect pedestrians in traffic scenes.

This paper proposes radar sensors operating at J-band (220-320 GHz) for automotive applications. Operating at J-band, at about three times higher frequency than the currently available automotive radars operating at 77 GHz, will result in three-fold improvement in cross-range resolution for the same antenna size. Since the characterization of the radar backscatter from different objects on the traffic scenes is necessary for optimizing the design of automotive sensors, the work reported in this paper is the start of a broader study that attempts to fill the lack of knowledge on radar backscatter behavior of road environment at this frequency band. Considering vehicles as the most important object for automotive radars, this paper investigates the response of vehicles from different aspects. A combination of outdoor high-resolution synthetic aperture radar imaging experiments and real-aperture imaging measurements of vehicles are performed using a 222-GHz polarimetric instrumentation radar. The aim of these experiments is to identify the scattering centers on different vehicle bodies and to determine the statistics of the radar return. The results show that significant scattering is due to a limited number of fixtures on the vehicle's outer surfaces facing the radar. The strongest scattering phase-centers observed are due to specular reflections and, hence, have a strong dependence on the relative look angle. The statistics associated with backscatter from vehicles are found, in most cases, to best fit the Weibull distribution.

This paper summarizes the results of an extensive study to model the statistics of the polarimetric backscatter response of vehicles at 222 GHz. Radars operating at high millimeterwave frequencies provide the necessary high azimuth resolution envisioned for many advanced applications of automotive radars, including autonomous vehicles. The data of many polarimetric measurements of vehicles are used here to extract the statistical behavior of radar scattering from vehicles. It is found that the radar statistics for a given vehicle are different as the percentage of the illuminated area of the vehicle is changed. The Weibull distribution is found to be the best fit to all measured data, with its shape parameter adjusted for different types of vehicles, radar polarization, and percentage illumination area. The results reported in this paper should provide valuable information for developers of such radars to optimize the performance of their detection and classification algorithms.

This paper presents an L-band Single Antenna Polarimetric Active Radar Calibrator (SAPARC) designed and constructed at the University of Michigan in collaboration with the suborbital radar group of Jet Propulsion Laboratory. The SAPARC is intended for radiometric and polarimetric calibration of both airborne and spaceborne L-band radars, including the NASA/JPL Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) and NASA-ISRO Synthetic Aperture Radar (NISAR). The presented Polarimetric Active Radar Calibrator (PARC) is designed to have a specific scattering matrix response (equal entries) and a very large and stable radar cross section (RCS) value which is significantly higher than its passive counterparts for providing a much higher signal-to-clutter ratio. The specific scattering matrix response enables the PARC to evaluate all the radar channels' radiometric calibration constants as well as cross-talk and channel imbalances, which cannot be estimated by the passive calibration targets. The much higher signal-to-clutter ratio provided by the PARC further reduces the residual errors of calibration and allows them to be easily deployed at different target scenes during radar data acquisitions.

This paper presents a new single-antenna polarimetric active radar calibrator (PARC) intended for the polarimetric and absolute radiometric calibration of the NASA's soil moisture active passive (SMAP) radar. The PARC receives and retransmits the SMAP signal through a dual-polarized horn antenna with 17-dB gain at the center frequency of 1.26 GHz with 100 MHz of bandwidth. The transmit and receive polarizations are perpendicular and are isolated from each other using a precision orthomode transducer (OMT) specially designed for this application. The antenna is rotated 45° in the plane perpendicular to the direction of incidence, so that the scattering matrix of the PARC with respect to SMAP polarization coordinates has equal entries that enable radiometric calibration of all four channels simultaneously. As SMAP radar resolution is coarse (1 km), a point target with a very large radar cross section (RCS) is required to provide a high signal-to-clutter ratio. The proposed PARC can provide RCS values as high as 80 dBsm to achieve a 30-dB signal-to-clutter ratio. The PARC is controlled by a microcontroller to autonomously start minutes before the SMAP radar is expected to scan the area, stabilize the amplifier gain, record the magnitude of the pulses transmitted from the radar, and transmit these data to a base station. The design procedures of the OMT and the antenna, as well as control and RF circuits, are discussed, a number of leakage cancelation techniques are introduced to increase the isolation between the ports, the RCS of the fabricated PARC is fully characterized, and the image of the PARC as seen by SMAP is presented.

The paper reports on an extensive set of outdoor measurements aimed at characterizing the polarimetric radar backscatter response of vehicles at 222 GHz. This effort supports the future application of high frequency radars operating around 230 GHz in autonomous vehicle navigation and collision avoidance. It is observed that radar backscatter is primarily from the outer surfaces of the vehicle and that few strong scattering centers dominate the radar return. Derived radar statistics from reported data may be used by researchers in optimizing detection.

This paper reports on a set of radar experiments conducted at 222 GHz in support of envisioned application of radars to autonomous vehicles. The measured radar backscatter data of road surfaces at incident angles between 80 and 88oare presented. The polarimetric radar response level from different surfaces encountered in roads and highway environments and the angular dependency are discussed. The results are helpful in developing models for different surfaces and in the design process and assessment of autonomous vehicles sensors.

Subterahertz frequencies between 100 and 300 GHz remain an untapped portion of the frequency spectrum for many radar- and radiometer-based remote sensing applications. This can be attributed in part to the lack of knowledge of the phenomenology of signal interaction with terrain at these frequencies. This paper examines recently acquired polarimetric radar backscatter data of different types of surfaces using a newly constructed polarimetric instrumentation radar operating at 222 GHz. At this frequency bare surfaces such as asphalt and dirt are electrically rough with subsurface aggregate sizes comparable with the wavelength resulting in substantial volume and surface scattering. The data show strong backscatter responses from bare surfaces with angular dependence proportional to the cosine square of the incidence angle along with significant depolarization (between -8 and -4 dB). The data for vegetation-covered surfaces show weak dependence on incidence angle and appreciable depolarization (between -12 and -6 dB). Empirical models for bare and vegetation-covered surfaces are proposed.

This paper examines recently measured polarimetric radar backscatter data of different types of bare and vegetation-covered surfaces at 222 GHz. Bare surfaces, such as asphalt, dirt, and concrete, are electrically rough with aggregate sizes comparable to the wavelength. This results in substantial volume and surface scattering. The data show strong angular dependence of the backscatter response from bare surfaces and significant depolarization between -8 dB and -4 dB. The data for vegetation-covered surfaces show weak dependence on incidence angle with appreciable depolarization between -12 dB and -6 dB.

This paper examines recently acquired polarimetric data of dry sand with a smooth air/sand interface at millimeter-wave (MMW) frequencies. The data characterize volume scattering from different types of sand as a function of the radar incidence angle, frequency, polarization, and particle size. The data show substantial volumetric response from dry sand at MMW frequencies with significant depolarization (between -12 and -7 dB). An empirical model is proposed and adapted for both smooth and 1-D periodic dry sand surfaces. Comparison between the expected volume scattering response of a dry sand medium with the surface scattering response of dry sand surfaces with rough interfaces shows that volume scattering is, in general, dominant, particularly at 95 GHz.

This paper examines recently acquired polarimetric data of dry sand with smooth air/sand interface at MMW frequencies. The data characterizes volume scattering from different types of sand as a function of the radar incidence angle, frequency, polarization, and particle size. The data shows substantial volumetric response from dry sand at MMW frequencies with significant depolarization (between -12 and -7 dB).

Radar imaging of deserts suffers from insufficient radar backscatter at low microwave frequencies due to the low permittivity of dry sand and relatively smooth sand surface roughness. Operating at millimeter-wave (MMW) frequencies, however, rectifies this deficiency as significant radar backscatter is generated by surface and volume scattering. This is due to the fact that sand surface roughness is electrically large and signal penetration into the dry sand, which is a homogeneous mixture of air and sand particles with dimensions comparable to a fraction of a wavelength, generates considerable volume scattering. This paper investigates both surface and volume scattering from dry sand surfaces, subject to the peculiar physical properties of sand surfaces found in sand dune-covered regions. An incoherent model is proposed that characterizes the angular dependence of volume scattering from dry sand in the presence of a 1-D rippled air/sand surface. A set of indoor experiments conducted on smooth and 1-D rippled sand surfaces at Ka-band confirms that significant volume scattering is present at MMW frequencies and that the proposed model correctly captures the observed angular dependence when 1-D surface ripples are present.

Fully polarimetric measurements of volume scattering contribution from dry layer of fine sand with smooth air/sand interface were performed at millimeter-wave frequencies. The measured radar response was compared to predictions made by the numerical solution of the DMRT model. Sand particles were modeled as spherical particles. The simulated response was able to predict the angular dependence of the data but was not able to predict accurately the absolute level, especially for the cross-polarized return. Furthermore, a simpler model, which was developed earlier for asphalt surfaces and is based on 1storder solution of RT for semi-infinite medium, was tested against the measured data. The model was not able to predict the angular dependence demonstrated by the measured data.

This paper gives a quick overview of inversion methods for bistatic tomographic processing. The methods are tested over simulated and real data. The real data has been acquired from a set of indoor experiments carried out in the Radiation Laboratory (RadLab), at the University of Michigan. A scale model with a house, some trees and a rough surface on the ground has been built to reproduce an urban scenario.

This paper introduces a novel approach to bistatic radar imaging. The proposed technique utilizes a synthetic transmit aperture and a stationary receiver to create a high resolution, 2-D bistatic SAR image of a given target scene. The technique can also be used to generate 3-D SAR images simply by utilizing a second, coherent receiver; hence operating in an interferometric bistatic SAR mode. The new technique is validated through both indoor and outdoor experiments.

This paper explores the nature of bistatic radar scattering from terrain by reporting the results of an investigation involving measurements of the hemispherical pattern of the field scattered by a random soil surface. The measurements were performed by a 35-GHz fully polarimetric radar system with transmitter and receiver modules mounted on separate rotatable arches. The acquired data were analyzed to determine the angular sensitivities of several attributes of the scattered field, including amplitudes and phase differences of the polarized scattering coefficients, and their copolarized and cross-polarized ratios. Generally speaking, the scattering pattern exhibits a weak dependence on the scattering angle thetass(except along the backward direction and forward specular direction), but it exhibits a strong dependence on the azimuth angle phi, particularly for the cross-polarized components. Much of the dependence is attributed to the vectorial definition of polarization in a standard frame of reference. Comparison of the measured data with calculations based on the second-order physical optics model reveals reasonable overall agreement between theory and observations (typically within 4 dB)

This paper introduces a technique to detect the presence of foliage-obscured targets using millimeter-wave radars operating at near-grazing incidence. Radar return from a scene where a target is concealed behind foliage consists of both foliage backscatter return and a polarimetrically distorted and reduced target return. A data acquisition and signal processing technique that takes advantage of temporal decorrelations in foliage is proposed to enhance target detection and resolve it from foliage. Furthermore, an inversion technique is proposed to remove the polarimetric distortions in the target response. The techniques are verified through a series of indoor and outdoor experiments.

Polarimetric measurements of the bistatic scattered fields from a rough, dry, soil surfaces, were performed recently at 35 GHz over the entire upper hemisphere, at both low and high incidence and scattering angles, and at near forward and backward directions. These measurements are discussed in this paper and the angular dependence of the bistatic scattering coefficients, /spl sigma//sup 0/, and the correlation between various polarization combinations are presented as well.

In this paper, we report on a series of foliage penetration experiments aimed at demonstrating the potential application of nadir looking, millimeter-wave radars in detecting targets under foliage cover. An algorithm is developed that classifies the radar return and isolates the radar return from hard targets.

The millimeter-wave (MMW) radar backscatter response of a rough, lossy, dielectric cylinder is examined both analytically and experimentally. Models for both the coherent and incoherent components of the scattered field are developed based on the geometric-optics approximation. The accuracy of the analytical models and their applicability in predicting the backscatter response of tree trunks are determined experimentally by measuring the radar backscatter at 95 GHz from a rough conducting cylinder and from a section of a tree trunk. In both cases, very good agreement was achieved between the model predictions and the measurements.

With the advent of high-frequency radio frequency (RF) circuits and components technology, millimeter-wave (MMW) radars are being proposed for a large number of military and civilian applications. Accurate and high-resolution characterization of the polarimetric radar backscatter responses of both clutter and man-made targets at MMW frequencies is essential for the development of radar systems and optimal detection and tracking algorithms. Toward this end, a new design is developed for ultrafast, wide-band, polarimetric, instrumentation radars that operate at 35 and 95 GHz. With this new design, the complete scattering matrix of a target (magnitude and phase) can be measured over a bandwidth of 500 MHz in less than 2 /spl mu/s. In this paper, the design concepts and procedures for the construction and calibration of these radars are described. In addition, the signal processing algorithm and data-acquisition procedure used with the new radars are presented. To demonstrate the accuracy and applicability of the new radars, backscatter measurements of certain points and distributed targets are compared with their analytical radar cross section (RCS) and previously measured /spl sigma//spl deg/ values, respectively, and good agreements are shown. These systems, which can be mounted on a precision gimbal assembly that facilitates their application as high-resolution imaging radar systems, are used to determine the MMW two-way propagation loss of a corn field for different plant moisture conditions.

The presence of trees in a given scene can hamper detection of nearby targets by millimeter-wave (MMW) radars especially at near grazing incidence. Proper characterization of scattering and attenuation in tree canopies is important for optimal detection algorithms. In this paper, a new technique for determining the extinction and volume backscattering coefficients in tree canopies using the measured radar backscatter response is proposed and verified experimentally. The technique, which can be applied to already available wideband radar backscatter data, is used to compute the extinction and volume backscattering coefficients of different tree canopies under various physical conditions. The dynamic range of these coefficients are presented and results at 35 GHz are compared with results at 95 GHz.

The application of the radar backscatter frequency correlation for classification and inversion of physical parameters of terrestrial targets is investigated. Traditionally, in radar remote sensing, the backscattering coefficients and the backscatter phase difference statistics of a distributed target are considered for estimating the biophysical parameters of interest. Because of the complex nature of random media scattering problems, however, target classification and parameter inversion algorithms are very convoluted. One obvious way of enhancing the success and accuracy of an inversion algorithm is to expand the dimension of the input vector space. Depending on the radar parameters, such as footprint (pixel) size, incidence angle, and the target attributes (physical parameters), the backscatter signal decorrelates as function of frequency. In this paper, analytical and experimental procedures are developed to establish a relationship between the complex frequency correlation function (FCF) of the backscatter and the radar and target attributes. Specifically, two classes of distributed targets are considered: 1) rough surfaces and 2) random media. Analytical expressions for the frequency correlation function are derived and it is shown that the effect of radar parameters can be expressed explicitly and thus removed from the measured correlation functions. The University of Michigan wideband polarimetric scatterometer systems are used to verify the theoretical models and inversion algorithms developed in this study.

The millimeter-wave (MMW) backscatter response of bare-soil was examined by conducting experimental measurements at 35 and 94 GHz using a truck-mounted polarimetric scatterometer and by developing appropriate models to relate the backscattering coefficient to the soil's surface and volume properties. The experimental measurements were conducted for three soil surfaces with different roughnesses under both dry and wet conditions. The experimental measurements indicate that in general the backscattering coefficient is comprised of a surface scattering component /spl sigma//sup s/ and a volume scattering component /spl sigma//sup v/. For wet soil conditions, the backscatter is dominated by surface scattering, while for dry conditions both surface and volume scattering are significant, particularly at 94 GHz. Because theoretical surface scattering models were found incapable of predicting the measured backscatter, a semiempirical surface scattering model was developed that relates the surface scattering component of the total backscatter to the roughness parameter ks, where k=2/spl pi///spl lambda/ and s is the rms height, and the dielectric constant of the soil surface. Volume scattering was modeled using radiative transfer theory with the packed soil particles acting as the host material and the air voids as the scattering particles. The combined contribution of surface and volume scattering was found to provide good agreement between the model calculations and the experimental observations.

Radar backscatter experiments were conducted at 35 and 95 GHz to measure the response of snow-covered ground to snow depth, liquid water content, and ice crystal size. The measurements included observations over a wide angular range extending between normal incidence and 60/spl deg/ for all linear polarization combinations. A numerical radiative transfer model was developed and adapted to fit the experimental observations. Next, the radiative transfer model was exercised over a wide range of conditions and the generated data were used to develop relatively simple semi-empirical expressions that relate the backscattering coefficient (for each linear polarization) to incidence angle, snow depth, crystal size, and liquid water content.

A three-step process is presented for estimation of forest biophysical properties from orbital polarimetric SAR data. Simple direct retrieval of total aboveground biomass is shown to be ill-posed unless the effects of forest structure are explicitly taken into account. The process first involves classification by (1) using SAR data to classify terrain on the basis of structural categories or (2) a priori classification of vegetation type on some other basis. Next, polarimetric SAR data at L- and C-bands are used to estimate basal area, height and dry crown biomass for forested areas. The estimation algorithms are empirically determined and are specific to each structural class. The last step uses a simple biophysical model to combine the estimates of basal area and height with ancillary information on trunk taper factor and wood density to estimate trunk biomass. Total biomass is estimated as the sum of crown and trunk biomass. The methodology is tested using SIR-C data obtained from the Raco Supersite in Northern Michigan on Apr. 15, 1994. This site is located at the ecotone between the boreal forest and northern temperate forests, and includes forest communities common to both. The results show that for the forest communities examined, biophysical attributes can be estimated with relatively small rms errors: (1) height (0-23 m) with rms error of 2.4 m, (2) basal area (0-72 m/sup 2//ha) with rms error of 3.5 m/sup 2//ha, (3) dry trunk biomass (0-19 kg/m/sup 2/) with rms error of 1.1 kg/m/sup 2/, (4) dry crown biomass (0-6 kg/m/sup 2/) with rms error of 0.5 kg/m/sup 2/, and (5) total aboveground biomass (0-25 kg/m/sup 2/) with rms error of 1.4 kg/m/sup 2/. The addition of X-SAR data to SIR-C was found to yield substantial further improvement in estimates of crown biomass in particular. However, due to a small sample size resulting from antenna misalignment between SIR-C and X-SAR, the statistical significance of this improvement cannot be reliably established until further data are analyzed. Finally, the results reported are for a small subset of the data acquired by SIR-C/X-SAR.>