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The excitation of space-wave, leaky-wave and creeping-waves in a cylindrical rod with uniaxial anisotropic permittivity and permeability is thoroughly characterized. Closed-form expressions of the space, leaky and creeping-waves in the uniaxial rod are derived for all source polarizations through the rigorous formulation, taking into account all higher-order azimuthal harmonics and TE-TM mode coupling for the space-wave and creeping-waves. Conditions for obtaining a high space-wave and leaky-wave directivity are thoroughly examined for all source polarizations with variation in the vertical and horizontal permittivity / permeability and the rod-radius. The leaky-wave phase and attenuation constants with variation in the rod parameters are investigated to identify the leaky-wave propagation regions of the uniaxial rod. Beam-scanning characteristics for the space- and leaky-waves are examined with variation in the uniaxial rod-parameters. The complex pole-loci for the azimuthally propagating creeping-waves around the uniaxial rod is also investigated to identify the leaky, creeping and the trapped-mode regions.

In this article, a unified framework of vegetation scattering using radiative transfer (RT) theory for passive and active remote sensing of vegetated land surfaces, especially those associated with moderate-to-large vegetation water contents (VWCs), e.g., forest field, is presented. The framework allows for modeling passive and active microwave signatures of the vegetated field with the same physical parameters describing the vegetation structure. RT equations are solved by a numerical iterative approach for both passive and active configurations. This approach allows including higher order scattering, which represents multiple scattering. In fields such as forests with large VWCs, associated with large scattering albedo and optical thickness, multiple scattering effects are critical. In the active iterative approach, cyclical terms are identified and backscattering enhancement is included by doubling contributions from cyclical terms. The method is applied to aspen trees in forest fields to compute the brightness temperatures and backscattering coefficients for passive and active remote sensing configurations, respectively. In the passive configuration, for forest field with VWC of 15 kg/m², the deviation between the zeroth-order brightness temperature, i.e., the tau-omega model results, and multiple scattering results around 40°observation angle, can be as large as 50 K for vertical polarization and 35 K for horizontal polarization. In the active configuration, the deviation between first-order results, which is identical to the distorted Born approximation, and the multiple scattering results around 40°incidence angle, is about 1.6 dB for VV and 0.7 dB for HH polarization. Multiple scattering is shown to be crucial for accurate forward modeling, especially over forested areas. The proposed approach is thus suitable for vegetation scattering with large VWCs. Furthermore, the proposed model is validated with the passive and active L-band sensor (PALS) acquired in SMAPVEX12 measurements in 2012, which demonstrates the applicability of this model.

Millimeter-wave (MMW) radar sensors are considered key components of autonomous vehicles. Because of the performance degeneration of cameras and lidars under inclement weather conditions, robust autonomy must rely on radar sensors to perform target detection and classification. Unlike basic target classifier methods in literature that make use of target velocity, the proposed approach is far more comprehensive and can be applied to targets with zero-Doppler. Depending on the radar type and target range, this paper presents four target classification models based on four different types of radar data: statistical RCS, distributed (time-domain) RCS, range-azimuth angle radar images and 3D radar images. The classification models are implemented by machine learning approaches artificial neural network (ANN) and convolutional neural network (CNN) with a comprehensive simulated dataset. Good classification accuracies are demonstrated, and the proposed model is validated with measured data. Different radar target classification approaches are compared, which clearly reveals the trade-off between classification performance and system complexity. The proposed radar target classification methods can be applied effectively to both static and dynamic targets, at near or far ranges, using traditional or imaging radars, resulting in improved safety for autonomous vehicles in a wide variety of complex environments.

A broadband, dual-polarized, and common-aperture $2\times 2$ cavity-backed slot antenna (CBSA) array for enabling full-duplex wireless communication is presented. The antenna consists of a thin rectangular cavity appropriately loaded with metallic septa to excite multiple resonances of similar desired field distribution to achieve consistent radiation characteristics over a wide bandwidth. Four pairs of concentric orthogonal radiating slots are cut out on one of the broad walls of the cavity all of which are fed by two concentric orthogonal slots on the opposite broad wall of the cavity. The cavity is fed by an end-launch coaxial-to-waveguide transition to excite one of the channels. The other channel is excited by a two-pronged microstrip line symmetrically crossing over the other cavity feeding slot. Due to the out-of-phase coupling from the two prongs of the microstrip line to the other port, this type of excitation is shown to provide an unprecedented level of isolation between the two channels over a wide bandwidth. To achieve a high-efficiency and fabrication accuracy of this complex architecture, the antenna is 3-D printed out of HP PA12 Nylon and is then metalized by the silver spraying technology. The fabricated antenna provides a minimum of 55 dB of isolation and 8 dB of return loss over 44% fractional bandwidth from 4.8 to 7.5 GHz. Keeping its size smaller than $1.3 \lambda _{L} \times 1.3\lambda _{L} \times 0.27\lambda L$ (where $\lambda _{L}$ is the free space wavelength at the lowest frequency), and its weight less than $70g$ , the antenna provides a minimum gain of 10 dB for both polarizations and exhibits more than 20 dB polarization discrimination within the main beam over the entire band.

Electromagnetic scattering model from 3-D snowpack with arbitrary thickness is considered. A fully coherent model is presented through the usage of the Statistical S-Matrix Wave Propagation in Spectral Domain approach (SSWaP-SD). The computer-generated snow media is constructed using 3-D spatial exponential correlation function along with Lineal-Path function to preserve the connectivity of the snow particles as well. The SSWaP-SD in conjunction with a Method of Moments (MoM) code based on the discrete-dipole approximation (DDA) is chosen to leverage both the time-efficient computations of the DDA and the full-coherency of the SSWaP-SD method simultaneously. The SSWaP-SD depends on the discretization of the medium into thin slabs. Several realizations of a thin snow slab are solved numerically to form the statistics of the scattering matrix representing such a thin snow layer. For an arbitrarily thick snow layer, thin slabs of a snowpack are analyzed and a corresponding polarimetric N-port (representing different directions of scattering) S-matrix is generated. These S-matrices are cascaded using SSWaP-SD method to calculate the total forward and backward bistatic scattered fields in a fully coherent way. The simulation results of the backscattering from an arbitrary thick snow layer are presented and validated with measurements.

In this letter, a hybrid analytical and numerical simulation method is presented for the time-efficient calculation of bistatic scattering from the buried targets. This computation is used for a fixed transmitter and target location but for a moving receiver to generate the data set for a 3-D synthetic aperture radar (SAR) simulation. In this method, initially, the current distribution on the target when illuminated by the transmitter (in the absence of the receiver) is calculated. Then, the receiver is used as a transmitter and its field is calculated everywhere in the lower half-space. The reciprocity is then used to find the open-circuit voltage across the receiver terminals due to the equivalent current distribution on the target. This approach substitutes $\boldsymbol {N}$ full-wave simulations for $\boldsymbol {N}$ sampling points required in SAR imaging with two simulations leading only to a factor of $\boldsymbol {N/2}$ reduction in the simulation time. The proposed method is validated by comparing it with the brute-force full-wave simulation method as well as by image reconstruction from the data set generated by the method.

This article reports on a novel direct frequency multiplication and phase modulation (PM) scheme for mechanical antennas operating at the very-low-frequency band (3–30 kHz) and lower. This antenna is intended as a transmitter for underwater and underground communication applications. In mechanical antennas, instead of using signal generators to drive the antenna for radiation, the radiation is induced by accelerating (rotating) electric charges or permanent magnets. One important challenge for mechanical antennas is to find an approach to modulate the information signal with sufficient bandwidth. We propose a novel PM scheme by rotating a permanent magnet which is sandwiched between two pairs of orthogonal bow-tie shape high- $\mu $ magnetic material plates. By moving the angular position of the magnetic plates, it is shown that the phase and amplitude of the EM signal can be modulated. The amplitude modulation is significant and this provides an additional important feature, which allows the antenna to generate higher harmonics of the rotation frequency, and thereby it facilitates radiation at a much higher frequency than the rotational frequency of the magnet. Hence, higher range and more radiated power can be achieved. Here, the performance of proposed mechanical antenna with its PM and frequency multiplication capability is first verified analytically and through numerical simulations using ANSYS Maxwell. Then, a prototype of the antenna is fabricated with the aid of low cost 3-D-printing technology and its performance is measured. Measurements are in good agreement with the simulation predictions.

Slope stability models developed based on the physical mechanism of landslides show the effectiveness in landslide probability assessment, while they have rarely been applied in the field of radar remote sensing. Inspired by the related work, this article proposes a new quantitative method for rainfall-induced landslide probability assessment based on safety factors (SFs) using soil moisture estimation from synthetic aperture radar (SAR) images. In order to combine slope stability models with SAR measurement, first, soil moisture that plays a vital role in slope stability models is estimated by SAR techniques from vegetated slope terrain. In this article, we propose a new SAR data processing model for potential landslide areas and a modified physical-based scattering model for short vegetation. The estimated results are qualitatively verified by the tropical rainfall measuring mission (TRMM) instrument and are quantitatively verified by the field investigation. Second, we study the water table level that plays another vital role in slope stability models and cannot be retrieved from SAR data. The analysis indicates that it can be treated as a constant in the case of unsaturated soil moisture. Combining with other geotechnical parameters that do not change with external circumstances, we simplify the slope stability model, of which effectiveness is tested by the visual interpretation. Finally, the SF maps are obtained by the simplified slope stability model using soil moisture estimated from the corresponding SAR images. The field investigation shows that all the observed landslides are located in the unstable areas, indirectly verifying the proposed method.

This article presents a full-wave numerical method that, in conjunction with Monte Carlo simulation, is used to calculate the effective propagation constant of a random volume of conducting wires of arbitrary length and curvature. Bistatic scattering from many realizations of nonoverlapping metallic wires confined within a fictitious spherical enclosure is calculated to determine the mean electric field scattered by the random medium. This mean scattered field is then used to determine the effective permittivity of the random medium using an inverse scattering scheme. These values are then compared with the values given by the Foldy approximation for many particle sizes, geometries, and number densities within a given medium. An investigation into the results is undertaken to determine an expanded macromodel for 3-D electromagnetic propagation through such a random medium, and the applicability of Foldy’s approximation is determined using this model.

One of the fundamental technical challenges of any new spaceborne vegetation remote sensing mission is the determination of what sensor(s) to place onboard and what, if any, overlapping modes of operation they will employ as each onboard sensor adds significant cost to the overall mission. In this article, the remote sensing of forest parameters using multimodal remote sensing is presented. In particular, polarimetric radar, Light Detection And Ranging (LiDAR), and near-IR passive optical sensing platforms are employed in conjunction with physics-based models. These models are used to accurately estimate forest aboveground biomass as well as canopy height in homogeneous areas. It is shown that this proposed method is capable of achieving high accuracy estimates while using minimal ancillary data in the estimation process. We present a method to combine measured data sets with our geometric and electromagnetic sensor models to develop a forest parameter estimation algorithm that fuses multimodal remote sensing technologies with a minimal amount of ground information and yields an accurate estimate of forest structure including dry biomass and canopy height with rms errors of 1.6 kg/m2 and 1.68 m respectively.

The dielectric‐coated impedance cylinder configuration is investigated thoroughly with respect to the space‐wave, leaky‐wave and creeping‐wave excitations for all source polarizations. A very high directivity is achieved for the space‐ and leaky‐wave radiation for the antenna structure compared to that from a dielectric cylindrical rod investigated earlier. It is also observed that such high directivity for the dominant‐mode leaky‐wave is accompanied by higher‐order azimuthal harmonics with low attenuation constant contributing to the leaky‐wave pole, instead of the n = 0 harmonic for the previous configuration. A large enhancement in the space‐wave directivity is also achieved, contributed by higher space‐wave harmonics compared to lower‐order azimuthal harmonics for the cylindrical dielectric rod antenna. The space‐wave and leaky‐wave directivities are examined with variation in the design parameters and azimuthal harmonics. The phase and attenuation characteristics of the leaky‐wave are used to evaluate the optimized leaky‐wave structure parameters and appropriate azimuthal harmonics that result in high directivity for the configuration. The creeping‐wave characteristics for the configuration are also investigated, with a relatively stronger creeping‐wave excitation observed for an inductive surface‐impedance.

Four‐dimensional (4D) relativistic scattering of electromagnetic waves from an arbitrary collection of uniformly translational moving lossy dielectric spheres is discussed. Two reference frames, four 4D coordinate systems and Lorentz transformation are used to obtain the scattered electromagnetic fields. The direct scattering of the spheres and their interactions are considered with a novel approach. The introduced method is straightforward and the analytical relations for the fields are achieved. To check the validity of the proposed method, different examples for both stationary and moving scatterers are investigated. The effects of key parameters such as the size, material, velocity, number, position of the spheres and also the frequency of the incident wave are discussed. The derived scattered fields are valid for low, medium and high velocities but according to practical applications low and moderate velocities are highlighted in numerical results.

A broadband (1–35 GHz) time-/frequency-domain reflection technique for measuring the complex dielectric constant of low-loss dielectric slabs is presented. The technique uses a coplanar waveguide (CPW) transmission line sample holder for measurements. In addition to the standard substrate materials, the technique is suitable for the characterization of materials, such as glass, as it does not require complicated sample machining and preparation. The procedure is based on comparing the measured effective dielectric constant and the attenuation constants for both the loaded and unloaded CPW line sample holder. The effective dielectric constant and the attenuation constant are first modeled as a function of the real and imaginary parts of the superstrate’s dielectric constant using a 2-D numerical line calculator. Samples under test are assumed to be nonmagnetic and have their material resonances at frequencies substantially above 35 GHz. The technique is validated through measurements of known Rogers RT/duroid samples. Measured complex dielectric constants of different materials are in very good agreement with data reported in materials’ datasheets.

This letter presents a new filter to suppress radio frequency interference (RFI) in microwave remote sensing instruments, particularly a wideband autocorrelation radiometer (WiBAR). RFI increases the noise floor and results in a decreased signal-to-noise ratio (SNR) of the WiBAR delay peak. The proposed filter operates like a Fabry–Perot interferometer (FPI) in optics and has the frequency response of a comb filter with many evenly spaced alternating pass and stop bands. The frequencies of these bands are varied with an electrically adjustable phase shifter. The design of a comb filter with an airy finesse value of 20 has been verified by simulation and measurement.

A novel ultrawideband directive antenna for ground-penetrating radar (GPR) applications is presented in this letter. The antenna is designed for operation at frequencies below 500 MHz to allow for high-depth-resolution imaging of the deeply buried targets such as buried pipes and other deep underground man-made structures such as tunnels. The antenna is formed by a pair of coupled antennas inspired by an electrically narrow very low profile (ENVELOP) antenna with differential feeding to provide a directive pattern normal to the antenna ground over a wide bandwidth. An antenna operating in the range of 140–510 MHz is designed and fabricated using the printed circuit board (PCB) substrates. The antenna is designed and optimized for operation in the proximity of a half-space soil medium to achieve efficient coupling of power from the air to the ground. The measured reflection coefficient of the antenna near concrete and wet soil covered by sand and grass is shown to be better than −9.5 dB within the specified band. The simulated realized gain of the antenna in the presence of dry soil is better than 4.9 dBi within the range of 140–510 MHz. The antenna is low-profile with a total height of 153 mm and a lateral dimension of 600 $\text {mm} \times 600$ mm. The performance of the antenna for GPR applications is evaluated through a field measurement of a buried object in wet soil with high losses.

A new passive microwave remote sensing technique, wideband autocorrelation radiometry (WiBAR), directly measures the microwave propagation time difference of multipath microwave emission from low-loss layered surfaces such as a dry snowpack and a freshwater lake icepack. The microwave propagation time difference through the pack yields a measure of its vertical extent. However, the presence of variable pack thicknesses within a footprint of the radiometer’s antenna will add complexity to the retrieved time delay. This issue is more severe for WiBAR on airborne and spaceborne platforms than WiBAR on ground-based platforms since the footprint for a given radiometer antenna is larger. From a simple forward model for a layer having distinct thickness values within one footprint (pixel), the system requirements for resolving these distinct thickness values are derived. A subpixel lake ice thickness distinction of 3.7 cm is demonstrated with an X-band WiBAR instrument at an incidence angle of 70°.