High Resolution Radar System Research Articles

The ExoMars 2028 WISDOM antenna assembly: Description and characterization

While ground penetrating radars have been extensively researched on Earth, the high-resolution exploration and imaging of the shallow subsurface of celestial bodies in our solar system is still in its early stages, with only a handful of systems capable of the task.Designing high-resolution radar systems can be a complex task due to the large frequency bandwidth required by the antennas to achieve high vertical resolution. The WISDOM GPR, as part of the 2028 ExoMars mission, is a highly capable and challenging instrument in this context, given its fully-polarimetric setup and mission constraints on the operating environment, robustness, as well as mass and size budget.This paper outlines the development and characterization process of the WISDOM antenna assembly, which can serve as a model for future radar systems. Furthermore, it presents the results of the antenna characterization as the foundation for instrument calibration and optimal radar sounding outcomes.

Estimation of Moving Direction and Size of Vehicle in High-Resolution Automotive Radar System

Estimation of Moving Direction and Size of Vehicle in High-Resolution Automotive Radar System

Spaceborne HRWS-SAR-GMTI System Design Method with Optimal Configuration

The spaceborne high-resolution and wide-swath synthetic aperture radar (HRWS-SAR) system combined with the ground moving target indication (GMTI) mode provides a promising prospect in the realization of wide-area target surveying and high-resolution target imaging. In this paper, a system design method is proposed for an HRWS-SAR-GMTI system with ideal reconstruction configuration. In the proposed method, the whole azimuth receiving channels are uniformly divided into multiple groups, where HRWS-SAR imaging is implemented in each sub-group and then GMTI processing is performed based on the reconstructed SAR images. Then, an optimal candidate PRF is properly selected with respect to the optimal reconstruction configuration. After that, the digital beam forming scanning on receive (DBF-SCORE) technique is applied to further enlarge the range swath and improve the noise equivalent scattering coefficient (NESZ). Based on the predesigned system, HRWS-SAR image-based GMTI processing can finally be accomplished. The effectiveness of the proposed method is validated by simulated experiments.

Implementation of MIMO Radar-Based Point Cloud Images for Environmental Recognition of Unmanned Vehicles and Its Application

High-performance radar systems are becoming increasingly popular for accurately detecting obstacles in front of unmanned vehicles in fog, snow, rain, night and other scenarios. The use of these systems is gradually expanding, such as indicating empty space and environment detection rather than just detecting and tracking the moving targets. In this paper, based on our high-resolution radar system, a three-dimensional point cloud image algorithm is developed and implemented. An axis translation and compensation algorithm is applied to minimize the point spreading caused by the different mounting positions and the alignment error of the Global Navigation Satellite System (GNSS) and radar. After applying the algorithm, a point cloud image for a corner reflector target and a parked vehicle is created to directly compare the improved results. A recently developed radar system is mounted on the vehicle and it collects data through actual road driving. Based on this, a three-dimensional point cloud image including an axis translation and compensation algorithm is created. As a results, not only the curbstones of the road but also street trees and walls are well represented. In addition, this point cloud image is made to overlap and align with an open source web browser (QtWeb)-based navigation map image to implement the imaging algorithm and thus determine the location of the vehicle. This application algorithm can be very useful for positioning unmanned vehicles in urban area where GNSS signals cannot be received due to a large number of buildings. Furthermore, sensor fusion, in which a three-dimensional point cloud radar image appears on the camera image, is also implemented. The position alignment of the sensors is realized through intrinsic and extrinsic parameter optimization. This high-performance radar application algorithm is expected to work well for unmanned ground or aerial vehicle route planning and avoidance maneuvers in emergencies regardless of weather conditions, as it can obtain detailed information on space and obstacles not only in the front but also around them.

Efficient Target Classification Based on Vehicle Volume Estimation in High-Resolution Radar Systems

In this paper, we propose a method for efficient target classification based on the spatial features of the point cloud generated by using a high-resolution radar sensor. The frequency-modulated continuous wave radar sensor can estimate the distance and velocity of a target. In addition, the azimuth and elevation angle of the target can be estimated by using a multiple-input and multiple-output antenna system. Using the estimated distance, velocity, and angle, the 3D point cloud of target can be generated. From the generated point cloud, we extract the point cloud for each individual target using the density-based spatial clustering of application with noise method and a camera mounted on the radar sensor. Then, we define the convex hull boundaries that enclose these point clouds in both 3D and 2D spaces obtained by orthogonally projecting onto the xy, yz, and zx planes. Using the vertices of convex hull, we calculate the volume of the targets and the areas in 2D spaces. Several feature points, including the calculated spatial information, are numerized and configured into feature vectors. We design an uncomplicated deep neural network classifier based on minimal input information to achieve fast and efficient classification performance. As a result, the proposed method achieved an average accuracy of 97.1%, and the time required for training was reduced compared to the method using only point cloud data and the convolutional neural network-based method.

The High-Resolution Synthetic Aperture Radar System and Signal Processing Techniques: Current progress and future prospects

The High-Resolution Synthetic Aperture Radar System and Signal Processing Techniques: Current progress and future prospects

Mapping Floods during Cloudy Weather Using Radar Satellite Images

Monitoring floods using optical satellite images in winter is challenging. Because of the thickness of clouds, the electromagnetic bands used by satellites are too narrow and the resulting signals are reflected into space. The purpose of this study is to assess the potential of implementing a high-resolution synthetic aperture radar system (SAR) for the purpose of mapping floods in Jordan while the weather is cloudy. TerraSAR-X satellite was utilized to acquire images of high resolution for flood monitoring. This research employed TerraSAR-X thresholding images to identify flood-prone areas and track floods over time using time-series images. It was possible to determine the places of the landscape that have been impacted by the flood and to establish the degree of damage by conducting an analysis of the changes that have occurred in the landscape. The accuracy of the study's findings was confirmed using backscatter texture analysis on the images, which revealed a small difference between thresholding and texture analysis approaches of less than 3% in the mapped test region's flood area. While monitoring floods, it is recommended to use TerraSAR-X images, particularly in conjunction with other data sources, such as weather predictions and hydrological models. KEYWORDS: Flood mapping, Polarimetric SAR, TerraSAR-X, Texture analysis, Thresholding, Interferometric SAR

On-Chip Photonic Generation of Tunable Wideband Phase-Coded Linearly-Chirped Microwave Waveforms

On-chip photonic generation of tunable wideband phase-coded linearly-chirped microwave waveforms (PC-LCMWs) is proposed and experimentally demonstrated. The on-chip generation system consists of two main parts. One part is a hybrid Fourier-domain mode-locked opto-electronic oscillator (FDML-OEO) for a wideband linearly-chirped microwave waveform (LCMW) generation; the other part is a high-speed microwave photonic phase shifter (MPPS) to perform the phase-coding. By synchronizing the two parts, a PC-LCMW is finally generated. In the proposed chip, a thermally-tunable ultrahigh-Q micro-ring resonator (MRR) is a key component which is incorporated in the FDML-OEO loop to select the oscillation frequency, and an electrically-tunable micro-disk resonator (MDR), as another key component, is used to code the phase of the generated microwave signal while maintaining the amplitude by controlling the free carrier concentration in the lateral PN junction of the disk. With the use of the two key component chips, an experiment is performed and different PC-LCMWs are generated with a tunable center frequency from 12 to 16 GHz, a bandwidth from 2 to 6 GHz, and phase code of 7-bit and 13-bit Barker code. In particular, a PC-LCMW with a maximum bandwidth as wide as 6 GHz and an ultra-large time-bandwidth product (TBWP) as large as 1.96×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> is experimentally demonstrated. The proposed integrated PC-LCMW generation system holds great advantages including ultracompact configuration and fast tunability, which holds a high potential for applications in modern high-resolution multi-functional radar system.

Radar Target Classification Receiver Using Sparse Regression and Target Tailored Matched Filters

Waveform design is a commonly used approach to enhance target classification in high resolution radar systems. In the monostatic case of detecting some extended target, inherent to many of these system models is an assumption of a known target impulse or frequency response. A practical issue with this method is that for nonsimple target cases, such a response can change drastically for varying aspect or viewing angles. In this paper, we first develop a sparse regression method for classifying target class and aspect angle. Next, we derive matched filters tailored to dictionaries of target response profiles representing these variations in aspect angle. The desirable goal of real-time classification is met by suitably reducing the computational cost and not relying on a series of measurement and adaption cycles to achieve classification. The result is a series of filters matched to the target dictionary data and can be used in a hypothesis testing approach to classification. Results are shown for radar cross-sectional data generated from computer-aided design models of different targets.

Ground Moving Target Detection With Nonuniform Subpulse Coding in SAR System

For the high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) system, the increasing imaging width results in a serious range ambiguity problem, which affects the performance of ground moving target indication (GMTI). In this article, a novel nonuniform subpulse coding (NSPC) scheme is proposed. It is characterized by resorting to range-frequency band resources and detailed coding design for each subpulse, enabling the beam auto-scanning in elevation. Also, the bandpass filtering and digital beamforming (DBF) technology with improved data reconstruction are utilized to realize the separation of subpulses and suppress range ambiguity. The NSPC technique exchanges the signal bandwidth for increasing swath without range ambiguity, and the coded subpulses can be directed to the prescribed regions, while skipping the invalid areas where the echoes are blocked. After that, through the robust principal component analysis (RPCA) method, the moving target detection is performed for each separated region without residual range-ambiguous interference. The proposed approach has been theoretically deduced in detail and the simulation experiments demonstrate its effectiveness.

Detection of range-spread targets based on order statistics

High resolution radars (HRRs) transmit a wideband signal to achieve a high range resolution. This paper presents a range-spread target detector based on order statistic in Gaussian white noise. Considering that the target spreads in multiple range cells in high resolution radar systems, we sort the range cells by energy in the detection window, and then the energy is sequentially accumulated and detected until a decision is made. An algorithm for computing the test thresholds of the detector is proposed. In addition, the detector is compared with other detectors under a given probability of false alarm by Monte Carlo simulations, which proves the proposed detector is more powerful with full.

Synchronization for mini‐UAV‐based high‐resolution BiSAR systems: Lost direct signal estimation based on Hampel filtering

In mini-unmanned aerial vehicle-based high-resolution bistatic synthetic aperture radar systems, the rotation of the unmanned aerial vehicle platforms will cause direct signal loss, resulting in the reduction of synchronization error compensation accuracy. Here, a synchronization algorithm is proposed to estimate the lost direct signal. First, bistatic time and phase synchronization signal models are established. Then, Hampel filtering is used to estimate the time delay of the lost direct signal, and the time and phase synchronization errors are estimated based on third-order polynomial fitting. A high-resolution synthetic aperture radar image is used to demonstrate the validity of the proposed algorithm in mini-unmanned aerial vehicle-based high-resolution bistatic synthetic aperture radar systems.

Three-Dimensional Coordinate Extraction Based on Radargrammetry for Single-Channel Curvilinear SAR System

With the rapid development of high-resolution synthetic aperture radar (SAR) systems, the technique that utilizes multiple two-dimensional (2-D) SAR images with different view angles to extract three-dimensional (3-D) coordinates of targets has gained wide concern in recent years. Unlike the traditional multi-channel SAR utilized for 3-D coordinate extraction, the single-channel curvilinear SAR (CLSAR) has the advantages of large variation of view angle, requiring fewer acquisition data, and lower device cost. However, due to the complex aerodynamic configuration and flight characteristics, important issues should be considered, including the mathematical model establishment, imaging geometry analysis, and high-precision extraction model design. In this paper, to address these challenges, a 3-D vector model of CLSAR was presented and the imaging geometries under different view angles were analyzed. Then, a novel 3-D coordinate extraction approach based on radargrammetry was proposed, in which the unique property of the SAR system, called cylindrical symmetry, was utilized to establish a novel extraction model. Compared with the conventional approach, the proposed one has fewer constraints on the trajectory of radar platform, requires fewer model parameters, and can obtain higher extraction accuracy without the assistance of extra ground control points (GCPs). Numerical results using simulated data demonstrated the effectiveness of the proposed approach.

A Novel Space-Borne High-Resolution SAR System with the Non-Uniform Hybrid Sampling Technology for Space Targets Imaging

In order to reduce the complexity of the receiving system for wideband signals, a novel space-borne high-resolution synthetic aperture radar (SAR) system with the non-uniform hybrid sampling technology is proposed in this paper. The non-uniform hybrid sampling technology is firstly applied in the SAR imaging system for the detection of space targets. The non-uniform hybrid sampling technology is able to optimize the transmitted and receiving timing of SAR signals, reducing the requirement of the sampling rate of the analog-to-digital converter (ADC) for the reception of the wideband echoes from space targets. Meanwhile, according to the oversampling requirement of SAR imaging in the azimuth direction, a theoretical model of non-uniform hybrid sampling parameters and relative velocity between the SAR system and the space targets is established. A series of simulation experiments with different targets and different non-uniform hybrid parameters are performed, and an X-band SAR imaging experimental system is constructed to verify the effectiveness of the proposed non-uniform hybrid sampling technology. The experimental results show that the imaging resolution is better than 8 cm. When the non-uniform hybrid sampling interval is 15 us, the imaging quality is consistent with imaging results of the Nyquist real-time sampling, and it is easier to implement in the high-resolution imaging for space targets.

A Two-Step Wide-Scene Polar Format Algorithm for High-Resolution Highly-Squinted SAR

Traditional polar format algorithm (PFA) is widely used in high-resolution and highly-squinted (HRHS) synthetic aperture radar (SAR) systems, because of its unique decoupling ability for any squint angle. However, the image size limit in traditional PFA, affected by the wavefront curvature, is too small to meet the modern wide-scene requirements. To break the limit, a new two-step PFA is proposed in this letter. It uses PFA twice. The first PFA is to obtain a coarse focused big image, from which a number of subimages can be filtered out. Each subimage could be recovered to the original phase history domain and reprocessed by another modified PFA, i.e., second PFA, to get a precisely focused subimage. The final full-image can be obtained easily by mosaicking subimages together. This two-step approach is derived in detail and could obviously enlarge the valid PFA scene. It is validated by simulated and real data.

A Novel Channel Errors Calibration Algorithm for Multichannel High-Resolution and Wide-Swath SAR Imaging

For a spaceborne high-resolution and wide-swath synthetic aperture radar (HRWS-SAR) system, it usually uses the digital beamforming technology. However, in practice, because of the influences of temperature, antenna pattern, receiving antenna, and other error factors, there may exist the range synchronization time errors, amplitude errors, and phase errors among different spatial channels. These nonideal factors will significantly degrade the multichannel data reconstruction performance, resulting in a smeared SAR image. To address this issue, in this article we propose a novel channel error correction algorithm based on the orthogonal projection theory. First, the optimal weight of each Doppler ambiguity component is calculated by the orthogonal projection. Then, the cost function is constructed based on the power maximization criterion, from which the channel phase errors can be obtained. Finally, the HRWS-SAR imaging can be finely realized after performing the channel balancing. Compared with the conventional phase error estimation method, the proposed algorithm does not require to perform the matrix eigenvalue decomposition, avoiding the signal leakage phenomenon under low SNR case. The effectiveness of the proposed algorithm is validated by both airborne and space-borne real SAR data.

Radiometric Degradation Associated With Terrain Height Variations and Pulse Duration in Scan-On-Receive SAR Images

Scan-on-receive (SCORE) is a key digital beamforming (DBF) technique for future high-resolution wide-swath spaceborne synthetic aperture radar (SAR) systems. Compared to a conventional approach, it allows improving the signal-to-noise ratio and the range ambiguity suppression. Nevertheless, it also exposes the system to new errors, associated with terrain height variations and pulse duration. This work investigates the mutual effect of these error sources on the SCORE SAR image. A novel, closed, a mathematical expression is derived, respectively, for the impulse response function of the image formation process and for the radiometric loss affecting the image pixels. This makes it possible to predict and quantify the effect of terrain height variations and pulse duration as a function of the system, processing, and geometric parameters. The numerical results, based on the end-to-end simulation of the SAR image formation process in different operational scenarios, highlight the relevance of this effect and the derived analytical description, especially in view of the demanding radiometric quality requirements imposed on future SAR images.

An Unambiguous Imaging Method of Moving Target for Maritime Scenes With Spaceborne High-Resolution and Wide-Swath SAR

In azimuth multichannel high-resolution and wide-swath (AMC-HRWS) synthetic aperture radar (SAR) system, undersampling of azimuth signal results in the failure of the single-channel echo imaging, so the echoes of all the channels should be reconstructed to obtain the unambiguous SAR image. Many methods have been proposed to reconstruct the stationary scene echoes. However, moving targets (MTs) will cause false targets in the reconstructed SAR image, which has a great influence on SAR image interpretation and moving target detection, especially in maritime scenes. Hence, we propose an unambiguous imaging method of the moving target for maritime scenes with AMC-HRWS SAR. First, azimuth deramp processing is introduced to obtain coarse-focused moving targets, and then their echoes can be extracted from the sea clutter. Based on the extracted data, a radial velocity estimation method is proposed, which is high efficiency and does not require redundant channels. In addition, based on the estimated radial velocity and the characteristics of the coarse-focused signal, a novel signal reconstruction method of moving target is presented. The reconstruction performance of this method is not affected by channel imbalance, and false targets can be removed even if there is radial velocity error. Besides, the effectiveness of this scheme is verified by the simulation data and GaoFen-3 SAR real data. The estimated radial velocities of ships are verified by automatic identification system (AIS) information. The imaging results show that false targets are effectively suppressed, and moving targets are located in correct positions.

An Effective Clutter Suppression Approach Based on Null-Space Technique for the Space-Borne Multichannel in Azimuth High-Resolution and Wide-Swath SAR System

In this article, an effective clutter suppression algorithm is presented for the space-borne azimuth multiantenna high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) system, which is based on the null-space technique. First, the echo of bistatic geosynchronous-low earth orbit (GEO-LEO) azimuth multiantenna HRWS SAR-ground moving target indication (GMTI) system is utilized to deduce the coarse-focused image of moving targets and clutter, where the Chirp Fourier transform (CFT) in azimuth is involved. Then, the matrix form can be utilized to describe the coarse-focused image of the multiantenna SAR system and the corresponding covariance matrix can be estimated. After that, the null-space is constructed using the covariance matrix corresponding to clutter, where at least a redundant channel freedom is required. Since the null-space vector is orthogonal to signal-space vector, it can be used to suppress the clutter. As an equivalent phase is brought by the slant velocity, the moving targets’ echo can be preserved during clutter suppression. Then, the optimization and suboptimization vectors for clutter suppression are introduced. It is worth noting that the proposed clutter suppression algorithm is robust for the antenna mismatch, that is, the antenna mismatch in phase and the corresponding position error. Finally, the theoretical investigations are validated using some simulation experiments, where the experiments for bistatic GEO-LEO and single-platform azimuth multiantenna HRWS SAR-GMTI system are both included. In addition, the real measured single-platform azimuth multiantenna HRWS SAR data experiments are also performed.

Photonics-assisted joint high-speed communication and high-resolution radar detection system.

A photonics-aided dual-functional system that can simultaneously implement high-speed communication and high-resolution radar detection is proposed. In the proposed system, high-speed orthogonal frequency division multiplexing signals and broadband linear frequency modulated signals are simultaneously realized based on photonic heterodyning. Based on this system, a net rate of 46.55 Gbit/s and radar with a 15 GHz bandwidth in the W-band were achieved.