Polarimetric Radar System Research Articles

Realizing Orthogonal Modes in Compact Cavity‐Backed Dual‐Polarized Antenna Through Simple Feeding Structures for Millimeter‐Wave AiP Applications

AbstractThis paper presents a dual‐polarized cavity‐backed antenna designed for mm‐wave applications, featuring simple feeding structures with a high‐isolation for 60 GHz compact AiP applications. The dual‐polarization design relies on two separate feed ports that excite two orthogonal modes within the same resonant cavity, achieving very high port isolation of up to 40 dB over the entire band. We conducted a detail analysis of the antenna, including its working principles and parametric studies. For verification, we fabricated an antenna test kit using standard printed process on substrates and measured the kit from the back‐side of a GSG probing platform. The proposed antenna demonstrates a wide impedance bandwidth, stable radiation patterns, very low cross‐polarization levels, and a high radiation efficiency. The co‐located cavity‐backed design ensures the compactness and facilitates easy integration with ICs in a very small AiP module. These features make the proposed antenna highly suitable for 60 GHz AiP applications, such as high‐data‐rate wireless communication and mmW polarimetric radar systems.

Strategy for vertical deformation of railway bridge monitoring using polarimetric ground-based real aperture radar system

The health monitoring of infrastructure is vital for ensuring the safety and structural integrity of bridges. Recently, ground-based real aperture radar (GB-RAR) systems have been successfully utilized in the dynamic and static monitoring of bridges. In this study, a comprehensive and innovative approach is presented to monitor the vertical deformation of a long-span metallic railway bridge and a reinforced concrete Shinkansen bridge in Japan using a polarimetric GB-RAR system. Distinct from conventional signal processing procedures, the proposed method omits the coherent scatterer selection step. Instead, polarization analysis is employed to evaluate the properties of scatterers and identify those corresponding to bridge sections requiring monitoring, while considering the structural characteristics of the bridge. Simultaneously, the signal-to-noise ratio for monitoring is enhanced by combining co-polarization responses from scatterers. Furthermore, the radar look angle is determined by accounting for the spatial configuration of the survey and the polarization orientation angle. Lastly, vertical deformation is assessed by projecting line of sight deformation in the vertical direction. The findings reveal the dynamic responses of the two bridges under diverse loading conditions, which include the transit of a low-speed train and a high-speed Shinkansen bullet train. The results demonstrate that the polarimetric GB-RAR interferometry technique, coupled with the developed algorithms, can be effectively applied to monitor any type of bridge with unparalleled spatial and temporal resolutions.

Channel Imbalance Calibration Based on the Zero Helix of Bragg-like Targets

The calibration of channel imbalances is currently the main concern in polarimetric calibration (POLCAL) since the crosstalk of recent polarimetric synthetic aperture radar (Pol-SAR) systems is lower than −20 dB. The existing channel imbalance calibration method without corner reflectors utilizes both volume-dominated and Bragg-like targets. However, there are two limitations to using volume-dominated targets. One is that the inaccurate selection of volume-dominated areas in the uncalibrated Pol-SAR images has a negative influence on the estimation of cross-polarization (x-pol) channel imbalance, which subsequently impacts the estimation of copolarization (copol) channel imbalance. The other is that there are minimal volume-dominated areas in some special applications of Pol-SAR, such as planetary exploration. Thus, only selecting Bragg-like targets to estimate the values of both transmitting and receiving channel imbalances, which is proposed in this paper, can avoid the uncertainty brought about by selecting other distributed targets in an uncalibrated imaginary. In addition, the reciprocity assumption and characteristics corresponding to H/α¯ decomposition are introduced to eliminate the phase ambiguity for the first time. Compared with previous methods, our method had an obvious advantage in terms of universality, since Bragg-like targets are common in the most illuminating areas. The novel method was applied to both the simulated data from the L-band Advanced Land Observing Satellite (ALOS) and C-band GaoFen-3 (GF-3), and to real data with corner reflectors on site. The results from the simulated data showed that the errors of the amplitude and phase estimation were less than 0.5 dB and 5.0° in most topographical features. Meanwhile, the VV/HH terms from all trihedral corner reflectors were less than 0.3 dB for amplitude, and 5.5° for phase after calibration by using the estimated channel imbalances.

A novel bivariate distribution and its divergence-based hypothesis inference: an application to the deforestation focus detection

ABSTRACT The polarimetric synthetic aperture radar (PolSAR) system is one of the most successful tools for solving remote-sensing issues. However, the images produced by this system – which uses coherent illumination – are corrupted by the multidimensional speckle noise that gives PolSAR data a multiplicative character. Therefore, tailored processing of PolSAR images is required, e.g. improved hypothesis testing and change detectors. In this work, we propose a novel bivariate distribution – called McKay bivariate (MB) - to describe a joint distribution arising from two components of the total scattering power image (SPAN). We derive closed-form expressions for the Kullback–Leibler and Rényi divergences for the MB law. We provide new two-sample divergence-based hypothesis tests and evaluate their performance using Monte Carlo experiments. Finally, we apply the new tests to real PolSAR images to evaluate the changes caused by urbanization processes in the Los Angeles and California regions. Results show that our proposals to detect changes in PolSAR images outperform the one based on the likelihood ratio.

A Fully Polarimetric Radar System for Non-Destructive Testing of Fiber Glass Layers

A Fully Polarimetric Radar System for Non-Destructive Testing of Fiber Glass Layers

Joint Design Methods of Unimodular Sequences and Receiving Filters With Good Correlation Properties and Doppler Tolerance

Sequence set design with good correlation properties and Doppler tolerance has been a classic and important problem in many multichannel systems, including but not limited to the multiple-input multiple-output (MIMO) and simultaneous polarimetric radar systems. In this article, we first consider the problem of jointly designing Doppler resilient unimodular sequences and receiving filters to minimize the metric weighted integrated sidelobe level (WISL), which can be used to construct sequence sets with “thumbtack” co-channel and zero cross-channel ambiguity functions (AFs). To control the signal-to-noise ratio (SNR) loss caused by the mismatched filter, a peak constraint function is added to the objective function based on the penalty function method. An algorithm based on the alternatively iterative scheme and general majorization-minimization (MM) method is developed to tackle the constrained joint design problem. Moreover, the proposed algorithm is then extended to optimize the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${l_{p}}$ </tex-math></inline-formula> -norm of sidelobes of AFs, which gives a way to optimize the weighted peak sidelobe level (WPSL) metric. Due to the use of the fast Fourier transform (FFT) algorithm and a general acceleration scheme, the proposed algorithm can be realized efficiently. A number of simulations are provided to demonstrate the excellent performance of the proposed sequence set synthesis algorithms. Besides, an application of using the designed Doppler resilient sequence set on the simultaneous polarimetric radar to detect multiple moving targets in a strong clutter scene is given via simulation, which further verifies the practicability of the proposed algorithm.

PolSAR Models with Multimodal Intensities

Polarimetric synthetic aperture radar (PolSAR) systems are an important remote sensing tool. Such systems can provide high spacial resolution images, but they are contaminated by an interference pattern called multidimensional speckle. This fact requires that PolSAR images receive specialised treatment; particularly, tailored models which are close to PolSAR physical formation are sought. In this paper, we propose two new matrix models which arise from applying the stochastic summation approach to PolSAR, called compound truncated Poisson complex Wishart (CTPCW) and compound geometric complex Wishart (CGCW) distributions. These models offer the unique ability to express multimodal data. Some of their mathematical properties are derived and discussed—characteristic function and Mellin-kind log-cumulants (MLCs). Moreover, maximum likelihood (ML) estimation procedures via expectation maximisation algorithm for CTPCW and CGCW parameters are furnished as well as MLC-based goodness-of-fit graphical tools. Monte Carlo experiment results indicate ML estimates perform at what is asymptotically expected (small bias and mean square error) even for small sample sizes. Finally, our proposals are employed to describe actual PolSAR images, presenting evidence that they can outperform other well-known distributions, such as WmC, Gm0, and Km.

Spatial and polarimetric information fusion using residual network for polarimetric synthetic aperture radar image classification

A polarimetric synthetic aperture radar (POLSAR) system provides an image that can be considered as a data cube containing spatial information in two spatial dimensions and polarimetric information in the scattering dimension. A spatial and polarimetric residual network (SPRN) is proposed for POLSAR image classification. At first, polarimetric features are extracted from the scattering dimension through two designed polarimetric residual blocks. Then, the processed POLSAR cube is fed to two consecutive spatial residual blocks for contextual feature extraction. Three dimensional convolutional layers are used as basic layers for simultaneous extraction and fusion of polarimetric information and correlation among neighbouring pixels in local regions. The shortcut connections are utilised to overcome the degradation problem due to increasing network depth. In addition, batch normalisation is applied to regularise the learning process. The experimental results on four real POLSAR images show the superior performance of SPRN compared to several state-of-the-art classifiers in terms of various assessment measures.

Polarimetric SAR image classification using binary coding‐based polarimetric‐morphological features

Polarimetric synthetic aperture radar (POLSAR) systems provide high resolution images containing polarimetric information. So, they have high capability in land cover classification. In this work, a binary coding-based polarimetric-morphological (BCPM) feature extraction is proposed for POLSAR image classification. At first, a set of polarimetric features is proposed. Then, a new morphological framework is introduced for contextual feature extraction from the POLSAR cube. The coherence matrix is composed from diagonal and non-diagonal elements with different information. These elements are analysed separately in the proposed method. Moreover, the amplitude and phase components of the non-diagonal elements are individually analysed using morphological filters by reconstruction. Finally, a binary coding-based polarimetric-spatial feature reduction, which uses the first order statistics, is proposed for feature transformation. The experiments on three real POLSAR images and a synthetic dataset show the superior performance of BCPM compared to several classification methods.

Realization of Low Cross-Polarization Level for GF-3 SAR Antenna

In the polarimetric synthetic aperture radar (PolSAR) system, crosstalk error is the key parameter for polarimetric imaging. The source of crosstalk is mostly from the cross-polarization level of the SAR antenna subsystem. To meet the requirements of PolSAR observing modes, a low cross-polarization level under −35 dB is realized for the SAR antenna of the GF-3 satellite. In this paper, we will describe the principles and methods for implementation of low cross-polarization level. The two polarized RF chains are totally independent to ensure high isolation, and a dual-polarized waveguide slot antenna subarray is designed with a low cross-polarization level. Besides, the low taper efficiency of the cross-polarization pattern contributes to reduce the cross-polarization, showing the advantage of the phased array antenna in the PolSAR system. The paper also displays the measured results of cross-polarization for the GF-3 SAR antenna. The study gains a better understanding of the cross-polarization pattern for phased array antenna. The methods utilized here can be extended to other polarimetric SAR systems.

Polarimetric Calibration Technique for a Fully Polarimetric Entomological Radar Based on Antenna Rotation

For entomological radar, the polarization information of the target is usually used to estimate the biological parameters, such as orientation, body length, and mass, of the insect. Thus, the accuracy of polarization measurement directly affects the performance of the biological parameters’ estimation. The polarization measurement error is mainly caused by the imbalance of amplitude and phase between two polarization channels and the crosstalk of the dual-polarization antenna. In order to obtain the correct polarization information of the target, the polarimetric calibration of the entomological radar is required. This paper proposes a new polarimetric calibration technique based on antenna rotation, which does not require the calibrator to have a specific polarization scattering matrix (PSM). Compared with the currently existing calibration techniques, no prior knowledge of the calibrator PSM is required (in fact, any fixed-point target can be used as a calibrator); thus, the errors introduced by the mechanical process can be avoided. Simulations and data measured by radar verify the effectiveness of the method. This method has the potential to be extended to other fully polarimetric radar systems in the future, such as fully polarimetric weather radar, fully polarimetric synthetic aperture radar (SAR), and so on.

Polarimetric Airborne Radar Sounding as an Approach to Characterizing Subglacial Röthlisberger Channels

Conventional airborne radar sounding techniques are well suited to the detection and characterization of flat-lying, specular subglacial water bodies. However, topographically positive, diffusively scattering Rthlisberger (R-)channels are more difficult to image, while also exerting substantial control on basal friction and ice dynamics. As subglacial R-channels share geometrical similarities with targets of interest in polarimetric ground-penetrating radar studies (i.e., cylindrical pipes), in this study we investigate whether similar concepts can be adapted to detect and characterize R-channels. While closed form analytical solutions exist for the scattering widths of perfect electrically conducting (PEC) and dielectric circular cylinders, the insight they provide for the polarimetric response of half-cylinder R-channels is limited. As such, a series of modeling experiments have been performed to characterize the scattering widths of half cylinders. Our results demonstrate that scattering from subglacial R-channels depends on numerous factors including the polarization and frequency of the incident radar wave, the size of the R-channel, and the relative orientation of the R-channel to a pair of orthogonally oriented, linearly polarized radar antennas. The results imply that patterns in the like-polarized echo powers across the signal bandwidth may be useful in inferring R-channel existence and possibly estimate R-channel size. However, as differences in scattering width are small and decrease with increasing misalignment between the acquisition system antennas and R-channel strike, achieving a high degree of radiometric resolution and stability should be a driving factor in polarimetric radar system design.

Detection of early-stage rebar corrosion using a polarimetric ground penetrating radar system

As a non-destructive testing method, ground penetrating radar (GPR) has been applied to monitor the corrosion of reinforced bar (rebar) in concrete. However, a traditional GPR system employs a pair of antennas, and can only measure single-polarization radar signals. In this paper, a hybrid-polarization GPR system is proposed for detection and evaluation of rebar corrosion. It can record rebar reflections in two orthogonal polarimetric channels, from which a full-polarimetric scattering matrix can be estimated through polarimetric decomposition and calibration. An accelerated corrosion experiment was conducted and the results prove that radar signal in the polarization channel perpendicular to the rebar orientation is more sensitive to the corrosion, compared with the commonly-used parallel polarization channel. Analysis of the scattering mechanism through H-Alpha decomposition shows that the corroded rebar shifts from the low-entropy dipole scattering to the low-entropy surface scattering. By filling the migrated GPR image with the color of the H-Alpha scattering classification, it is able to visually evaluate the early-stage corrosion of rebar in concrete.

Water Ice Detection Research in Utopia Planitia Based on Simulation of Mars Rover Full-Polarimetric Subsurface Penetrating Radar

The probe of China’s first Mars exploration mission, Tianwen-1, has been successfully launched. It will carry out scientific exploration on the topography, soil characteristics, water ice, climate, ionosphere, and physical fields of Mars. Different from other rovers landing on the moon and Mars, the Zhurong rover is equipped with a full polarimetric subsurface penetrating radar (FP-SPR) system for the first time. The radar’s mission is to depict the shallow subsurface structure of Mars and search for possible water ice. Therefore, in this paper, a 3D realistic structure model is established and numerically simulated based on the possible subsurface structure of Utopia Planitia (the landing area). Influencing factors such as topographical fluctuations, rocks, water ice, and the variation of dielectric constant of different layers are added to the model. The analysis of the acquired FP-SPR data set shows that the two-dimensional principal component analysis (2D-PCA) method can extract effective reflected signals from the radar data with noise interference and improve the data quality. These clearly imaged targets may be water ice blocks, so the application of 2D-PCA to FP-SPR data increases the imaging quality of suspected water ice targets. The results of this paper are the basis for future processing of the measured FP-SPR data on Mars, which will help to identify more details of subsurface structures.

Identification of snowfall microphysical processes from Eulerian vertical gradients of polarimetric radar variables

Abstract. Polarimetric radar systems are commonly used to study the microphysics of precipitation. While they offer continuous measurements with a large spatial coverage, retrieving information about the microphysical processes that govern the evolution of snowfall from the polarimetric signal is challenging. The present study develops a new method, called process identification based on vertical gradient signs (PIVSs), to spatially identify the occurrence of the main microphysical processes (aggregation and riming, crystal growth by vapor deposition and sublimation) in snowfall from dual-polarization Doppler radar scans. We first derive an analytical framework to assess in which meteorological conditions the local vertical gradients of radar variables reliably inform about microphysical processes. In such conditions, we then identify regions dominated by (i) vapor deposition, (ii) aggregation and riming and (iii) snowflake sublimation and possibly snowflake breakup, based on the sign of the local vertical gradients of the reflectivity ZH and the differential reflectivity ZDR. The method is then applied to data from two frontal snowfall events, namely one in coastal Adélie Land, Antarctica, and one in the Taebaek Mountains in South Korea. The validity of the method is assessed by comparing its outcome with snowflake observations, using a multi-angle snowflake camera, and with the output of a hydrometeor classification, based on polarimetric radar signal. The application of the method further makes it possible to better characterize and understand how snowfall forms, grows and decays in two different geographical and meteorological contexts. In particular, we are able to automatically derive and discuss the altitude and thickness of the layers where each process prevails for both case studies. We infer some microphysical characteristics in terms of radar variables from statistical analysis of the method output (e.g., ZH and ZDR distribution for each process). We, finally, highlight the potential for extensive application to cold precipitation events in different meteorological contexts.

Quasi-Orthogonal Z-Complementary Pairs and Their Applications in Fully Polarimetric Radar Systems

One objective of this paper is to propose a novel class of sequence pairs, called “quasi-orthogonal Z-complementary pairs (QOZCPs)”, each depicting Z-complementary property for their aperiodic auto-correlation sums and also having a low correlation zone when their aperiodic cross-correlation is considered. Construction of QOZCPs based on Successively Distributed Algorithms under Majorization Minimization (SDAMM) is presented. Another objective of this paper is to apply the proposed QOZCPs in fully polarimetric radar systems and analyse the corresponding ambiguity functions. It turns out that QOZCP waveforms are much more Doppler resilient than the known Golay complementary waveforms.

Polarimetric calibration of spaceborne and airborne multifrequency SAR data for scattering-based characterization of manmade and natural features

The Polarimetric Synthetic Aperture Radar (PolSAR) systems use electromagnetic radiations of different polarizations in the microwave frequency to collect the scattering information from targets on the Earth. Nevertheless, as with any other electronic device, the PolSAR systems are also not ideal and subjected to distortions. The most important of these distortions are the polarimetric distortions caused due to the channel imbalance, phase bias, and crosstalk between the different polarization channels. For the spaceborne PolSAR systems, the Earth’s ionosphere also contributes to an additional polarimetric distortion known as the Faraday rotation. An effort was made in this study to perform the polarimetric calibration of the Quad-pol and Compact-pol PolSAR datasets acquired using different airborne and spaceborne PolSAR systems to estimate and minimize these polarimetric distortions. The investigation was also done to analyze the impact of these polarimetric distortions on the scattering mechanisms from ground targets and on its dependency on the radar wavelength. The study was done using the UAVSAR L-band Quad-pol dataset, RADARSAT-2 Quad-pol dataset, ALOS-2 PALSAR-2, ISRO's L&S- Band Airborne SAR (LS-ASAR) Quad-pol and Compact-pol datasets, and the RISAT-1 Compact-pol dataset. Calibration of the airborne PolSAR data was carried to understand the level of polarimetric distortions in the LS-ASAR product that is a precursor mission to the spaceborne Dual-Frequency L&S Band NASA-ISRO Synthetic Aperture Radar (NISAR) mission. It is understood that the crosstalk is the dominant polarimetric distortion, which severely affects the PolSAR datasets compared to the other polarimetric distortions, and it is more for the higher wavelength PolSAR systems. The Quegan, Improved Quegan, and Ainsworth algorithms for crosstalk estimation and minimization was performed for the different Quad-pol datasets and it was found that the Improved Quegan algorithm is suitable for removing crosstalk from datasets having high crosstalk and the Ainsworth algorithm is suitable for removing crosstalk from datasets having low crosstalk. The Freeman method of the polarimetric calibration was implemented for the compact-pol datasets and it was able to considerably minimize the polarimetric distortions. The coherency matrix, scattering matrix, model-based decomposition, polarimetric signatures, and roll invariant parameter-based analysis revealed that all the datasets after polarimetric calibration were showing the correct scattering responses expected from the ground targets.

Numerical Simulation of Subsurface Penetrating Radar in Isidis Planitia on Mars for China’s First Mission to Mars

The water ice exploration mission is an important scientific target of China’s first Mission to Mars. The high frequency full polarimetric subsurface penetrating radar (SPR) system onboard the landing rover can carry out high-resolution imaging of the near-surface area of Mars, which is an important tool for detecting the presence of shallow water ice and soil structure. At present, most of the radar detection simulations are based on low-frequency antennas to analyze the geological structure of Mars down to several kilometers, with low resolution, which is difficult to detect the subsurface structure in the near-surface accurately. Considering topographic relief, interface roughness, subsurface rock, water ice, and permittivity changes at different layers, a three-dimensional near-surface model in the Isidis Planitia on Mars is established. The interpretation of the forward simulation results is of reference value for future Mars exploration missions.

Design of a Polarization Rotating SIW-Based Reflector for Polarimetric Radar Application

Polarimetric radar systems are beneficial to identifying and classifying targets but require multiple transmit or receive channels with different polarizations. This leads to a high hardware effort and thus higher costs. To use a single, linear polarized radar sensor as a polarimetric system, a frequency-dependent, polarization rotating reflector which can be placed in front of the radar antenna is presented. The reflector is based on a frequency selective surface (FSS) consisting of slot-excited substrate integrated waveguide (SIW) resonators. Resonator modes are analyzed and an equivalent circuit diagram to describe the filter functionality is developed. For using the described FSS as reflective structure, the design focuses on 45° oblique incidence. Different field vectors for normal and oblique incident angles are considered and different cavity modes for these cases are analyzed. An undesired mode is suppressed by an additional plated through via hole and slot impedances are matched. Reflector designs for normal and oblique incident angles are presented for $15 GHz$ ( $Ku$ -band) and afterward adapted to $35 GHz$ ( $Ka$ -band). The frequency band of operation with ${12\%}$ fractional bandwidth is divided into two ${4.5\%}$ subbands which allows a frequency-dependent polarization rotation of a linear polarized electromagnetic wave. Investigations on the fabrication accuracies are presented and reflectors for both bands are manufactured. Measurements are performed with a vector network analyzer and results fit well to the simulated curves. In the band of polarization rotation reflection, the matching is better than $- 13 dB$ and dielectric losses of less than $1 dB$ are achieved.

Distortions Imposed by Ionospheric Faraday Rotation Dispersion in Low-Frequency Full-Polarimetric SAR Images

Spaceborne polarimetric synthetic aperture radar (Pol-SAR) systems operating at low frequencies, such as P-band, are significantly influenced by ionospheric Faraday rotation (FR). A novel theoretical model of the effect of FR dispersion on Pol-SAR images is proposed. Three components of the range compression response are derived with the main function accompanied by two accessories. Simulation results indicate that two accessory functions in existence of FR dispersion can be comparable with the main for P-band ultrawideband systems. It can not only lead to range imaging deteriorations but also bring about extra polarimetric distortions. Finally, an airborne P-band Pol-SAR real scene is used for validation.