Ground-based Synthetic Aperture Radar System Research Articles

Analysis of Geometric Characteristics and Coverage for Moon-Based/Spaceborne Bistatic SAR Earth Observation

With the rapid development of Earth system science, a new understanding of the complete Earth system has highlighted the crucial importance of integrated observations, especially in research involving large-scale geoscience phenomena. As an active sensor with all-time and all-weather capabilities, synthetic aperture radar (SAR) has been widely used in recent decades for Earth observation. However, the existing spaceborne, airborne, and ground-based SAR systems have difficulty providing temporally consistent and spatially continuous Earth observation data on a global scale. As Earth’s only natural satellite, the Moon is a very promising Earth observation platform. By deploying a transmitter on the Moon and a receiver on the high-orbit satellite, a Moon-based/spaceborne bistatic synthetic aperture radar (MS-BiSAR) can be formed. In this paper, the MS-BiSAR geometric model of Earth observation was established using ephemeris and orbit propagators with reference system transformations, and three different MS-BiSAR configurations were used to calculate and analyze their geometric characteristics and Earth observation coverage. The results show that with the advantage of wide swaths, continuous observation capabilities, and large coverage, such an MS-BiSAR could significantly contribute to monitoring and understanding large-scale geoscience phenomena.

Structural displacement monitoring using ground-based synthetic aperture radar

This study develops a ground-based synthetic aperture radar (GBSAR) imaging system and interferometric processing framework for structural health monitoring (SHM). Radar remote sensing for displacement monitoring plays a vital role in SHM. Radar sensors and, more specifically, GBSAR systems have gained more attention in recent studies due to their capability to overcome the limitations of other contact or non-contact SHM sensors. This paper proposes a three-dimensional SAR imaging system and advanced signal processing scheme to extract continuous (fast) displacement components, such as structural vibrations, and discontinuous (slow) displacements, such as subsidence or uplift, using SAR time-series datasets. Using controlled experiments, this study shows that the developed GBSAR can monitor sub-millimeter displacements with high spatial resolution and 0.02 mm precision in the LOS direction. Moreover, several simulations are carried out to evaluate the proposed displacement monitoring algorithms. Accordingly, the proposed continuous monitoring framework can measure fast LOS displacements with better than 0.03 mm precision by reducing the effects of radar movement, clutters, and disturbances. Also, evaluations on a simulated monitoring scene at 20 m from the GBSAR’s LOS show that the proposed discontinuous displacement monitoring algorithm can estimate 3D displacement vectors with sub-millimeter precision. The results show that the developed GBSAR can be used for repeat-pass LOS displacement monitoring and the proposed algorithms demonstrate high potential for measuring continuous sub-second LOS displacements and long-term 3D displacement vectors.

A Novel GB-SAR System Based on TD-MIMO for High-Precision Bridge Vibration Monitoring

Ground-based synthetic aperture radar (GB-SAR) is a highly effective technique that is widely used in landslide and bridge deformation monitoring. GB-SAR based on multiple input multiple output (MIMO) technology can achieve high accuracy and real-time detection performance. In this paper, a novel method is proposed to design transmitting and receiving array elements, which increases the minimum spacing of the antenna by sacrificing several equivalent phase centers. In MIMO arrays, the minimum antenna spacing in the azimuth direction is doubled, which increases the variety of antenna options for this design. To improve the accuracy of the system, a new method is proposed to estimate channel phase errors, amplitude errors, and position errors. The position error is decomposed into three directions with one compensated by the phase error and two estimated by the strong point. Finally, we validate the accuracy of the system and our error estimation method through simulations and experiments. The results prove that the GB-SAR system performs well in bridge deformation and vibration monitoring with the proposed method.

A Ground-Based Interferometric Synthetic Aperture Radar Design and Experimental Study for Surface Deformation Monitoring

A new ground-based interferometric synthetic aperture radar has been developed by iRadar Sdn Bhd, in collaboration with the Centre for Remote Sensing and Surveillance Technologies, Multimedia University, Malaysia. This ground-based radar operates at Ku-band, single VV-polarization, with range resolution up to 0.175 m and azimuth resolution of 5.81 mrad at 1 km. Indoor test and outdoor field measurements have been conducted to verify the performance and specifications of the developed ground-based synthetic aperture radar (GBSAR). This system can be deployed for surface deformation monitoring especially at the high-risk slope for landslide monitoring and detection. This article presents the basic theory of the GBSAR system, the hardware description, as well as some indoor and outdoor field measurement results of the developed system.

Evaluating the deformation monitoring capability of a ground based SAR system with MIMO antenna

Evaluating the deformation monitoring capability of a ground based SAR system with MIMO antenna

Performance of SAR Polarimetric Calibration Using Hybrid Corner Reflectors: Numerical Simulations and Experimental Measurements

In this article, we devise a polarimetric calibration method utilizing only three types of calibration targets, i.e., trihedral, dihedral, and a 22.5°-rotated dihedral corner reflectors and requiring no assumptions on the scene statistics. The influence of the roll angle in a 22.5°-rotated dihedral corner reflector is assessed by a full-wave numerical simulation. The effectiveness and accuracy of the method are verified with a ground-based synthetic aperture radar system and an airborne L-band Pi-SAR data. We demonstrate that an amplitude error as low as 0.5 dB and a phase error within 3° can be achieved, showing an excellent agreement between the theoretical and calibrated polarimetric signatures for the reference targets. We further exploit the calibration on the Pauli decomposition, Yamaguchi four-component decomposition, and the estimation of the polarimetric parameters derived from target scattering vector method decomposition to show the superior performance of the devised calibration method.

Deformation Monitoring of Slopes With a Shipborne InSAR System: A Case Study of the Lancang River Gorge

Differential synthetic aperture radar interferometry (DInSAR) is a remote sensing technique to monitor ground deformation using repeat-pass data and suitable processing methods. Different platforms have been used to acquire DInSAR data such as space-borne, air-borne, car-mounted, and ground-based SAR systems. Space-borne SAR systems are relatively mature, but air-borne and other systems have attracted increasing interest of the remote sensing community due to their operational flexibility and observation capabilities. In this paper, a new InSAR platform is proposed for the deformation monitoring of reservoir slopes. The shipborne InSAR system uses frequency modulated continuous wave (FMCW) technology with one-transmit-dual-receive configuration. First, we describe the data acquisition process at the Lancang River Bank in October 2019. Subsequently, we present the imaging and interferometric capabilities of the new shipborne InSAR system. Moreover, we analyze the acquired radar data and present an assessment of the quality of the single look complex (SLC) SAR imagery and interferometric products. We focus on the influence of platform stability and baseline bias. Finally, we compare the calculated deformation with the measured deformation obtained from corner reflectors deployed in the monitored area. A line-of-sight error within 2cm is obtained. This paper proposes a novel solution for deformation monitoring of reservoir slopes and a new approach for InSAR applications.

Analysis of a Bistatic Ground-Based Synthetic Aperture Radar System and Indoor Experiments

Recent advancement of satellite synthetic aperture radar (SAR) techniques require more sophisticated system configurations such as the use of bistatic antennas or multi-frequencies. A ground-based experiment is a cost-effective and efficient way to evaluate those new configurations especially in the early stage of the system development. In this paper, a ground-based synthetic aperture radar (GB-SAR) system was constructed and operated in a bistatic mode at Ku-band where a receiving antenna (Rx) follows a transmitting antenna (Tx) separated by a baseline B. A new bistatic GB-SAR focusing algorithm was developed by modifying a conventional range-Doppler algorithm (RDA), and its performance has been evaluated by comparing the results with those from a back-projection algorithm (BPA). The results showed good performance of RDA at far range approaching nominal resolutions of 9.4 cm in range and 4.5 cm in azimuth, but limited quality at near range due to the approximation used in RDA. Signals from three trihedral corner reflectors (CR) reduced with increasing B, showing a typical bidirectional scattering behavior of CR. This GB-SAR system will be a testbed for new SAR imaging configurations with variations in antenna positions and target properties.

GROUND-BASED & SPACEBORNE SAR INTERFEROMETRIC TECHNIQUES SUPPORTING THE MANAGEMENT OF EMERGENCIES IN THE HEIMDALL PROJECT

Abstract. This paper describes the application of spaceborne and Ground Based radar interferometry as a tool for assisting the management of emergencies related to land movements. The support of the two techniques is carried out integrating some products, using tools available from a software and hardware architecture specifically developed within HEIMDALL, a H2020 project, devoted to providing a multi-hazard Cooperative Management, for Data Exchange, Response Planning and Scenario Building. Deformation maps obtained processing Sentinel-1 SAR images, updated every six days, can provide information over a large area, to be used during the preparedness and recovery phases. Data acquired through a Ground Based SAR system installed in-situ, provide a continuous and discontinuous (periodical) monitoring, aiming at supporting response and recovery phases. The products, in both cases, consist of deformation maps and temporal series, velocity of displacement, obtained through the application of Persistent Scatterer Interferometry (PSI).

An approach to wide-field imaging of linear rail ground-based SAR in high squint multi-angle mode

Ground-based synthetic aperture radar (GB-SAR) has been successfully applied to the ground deformation monitoring. However, due to the short length of the GB-SAR platform, the scope of observation is largely limited. The practical applications drive us to make improvements on the conventional linear rail GB-SAR system in order to achieve larger field imaging. First, a turntable is utilized to support the rotational movement of the radar. Next, a series of high-squint scanning is performed with multiple squint angles. Further, the high squint modulation phase of the echo data is eliminated. Then, a new multi-angle imaging method is performed in the wave number domain to expand the field of view. Simulation and real experiments verify the effectiveness of this method.

A Monostatic/Bistatic Ground-Based Synthetic Aperture Radar System for Target Imaging and 2-D Displacement Estimation

A monostatic/bistatic ground-based synthetic aperture radar (GB-SAR) system using an optical electric field sensor (OEFS) as the bistatic receiving unit has been designed for target imaging and 2-D displacement estimation purposes. The designed system has the capability of acquiring the monostatic and bistatic SAR images simultaneously from different looking angles, enabling the displacement vector estimation of the illuminated area. The spatial baseline between the monostatic and bistatic receivers is selected according to the estimation precision analysis. Experimental results are presented to evaluate the performance of the designed system and the proposed method. The displacement estimation accuracies in x- and y-directions can reach up to millimeter level.

Three-Dimensional Ground-Based SAR Imaging Algorithm Based on Keystone Formatting and Subblock

Three-dimensional (3D) ground-based synthetic aperture radar (GB-SAR) draws attention because of the ability to obtain high accuracy 3D information of the monitoring terrain. Besides, the 3D GB-SAR system is welcome due to the flexibility and large coverage in angle and range. However, existing 3D imaging algorithms for GB-SAR data focusing have limits of high computational complexity or narrow applicable scope. To realize 3D displacement monitoring with high spatial resolution and short revisit time, in this paper, a novel 3D imaging algorithm is proposed. Based on characteristics of the model of echo data from the large range and wide-view angle scenario, the proposed method uses keystone formatting to complete range migration correction and subblocks dechirping to realize horizontal focus. With the method, the reflectivity of the monitoring scenario is obtained by only one time of linear interpolation and several times of fast Fourier transforms. The main advantages of this algorithm are high imaging precision and low computational cost, and in addition, it is applicable for large illuminating coverage including the near-field and the far-field of the radar aperture. The imaging results are sampled on pseudo-spherical grid, aiming to simplify the formulation. Finally, this method is extensively validated with simulations.

An Analysis of Metaheuristic Algorithms Used for the Recovery of a Failed Antenna Element in an Antenna Array

A ground-based synthetic aperture radar system (GB-SAR) comprising of 12 horn antennas in its transmitter section and 12 Vivaldi antennas in its receiver section is used for the monitoring of the Tessina landslide in Italy and the sinking of the Costa Concordia ship in Italy. Alternate 2×6 geometry of 12 coaxial cavity horn antennas have been proven to yield a higher half-power beamwidth and low peak sidelobe ratio resulting in a better quality of the image captured. This 2×6 planar array, when fed with power according to the Dolph Chebyshev weighting polynomial, removes side lobes entirely and results in an increase half-power beamwidth (HPBW) value. A failure of any single element in the 2×6 array could lead to the alteration of the radiation pattern thereby jeopardizing the monitoring activity. This paper explores the usage of metaheuristic algorithms in order to recover the weight of the principal antenna element of the 2×6 coaxial cavity horn antenna array. The algorithms explored are Gradient Descent, Simulated Annealing, Firefly algorithm, Bat algorithm, Flower Pollination algorithm, Cuckoo search algorithm, and Harmony Search algorithm. Among these ones, the Firefly algorithm performs the best in finding the weight of the failed antenna element of the 2×6 planar array.

The Integrated Simulation and Processing Tool for Ground Based Synthetic Aperture Radar (GBSAR)

Ground-based Synthetic Aperture Radar (GBSAR) is a tremendous example of the extended applications of Synthetic Aperture Radar (SAR). GBSAR is extremely useful in human-made structure observations, terrain mapping, landslide monitoring and many more. However, the process of designing and developing the GBSAR system is rather costly and time-consuming. It would be of a great advantage for system designers to have a realistic simulation and designing tool to anticipate the results before the implementation of the final design. In this paper, we are going to present the integrated simulation and designing tool that we have developed for a generic GBSAR system. We named it iSIM v2.0

The Integrated Simulation and Processing Tool for Ground Based Synthetic Aperture Radar (GBSAR)

Ground-based Synthetic Aperture Radar (GBSAR) is a tremendous example of the extended applications of Synthetic Aperture Radar (SAR). GBSAR is extremely useful in human-made structure observations, terrain mapping, landslide monitoring and many more. However, the process of designing and developing the GBSAR system is rather costly and time-consuming. It would be of a great advantage for system designers to have a realistic simulation and designing tool to anticipate the results before the implementation of the final design. In this paper, we are going to present the integrated simulation and designing tool that we have developed for a generic GBSAR system. We named it iSIM v2.0.

Novel MIMO‐SAR system applied for high‐speed and high‐accuracy deformation measurement

As a novel system, multiple-input multiple-output synthetic aperture radar (MIMO-SAR) has achieved rapid development in the past years. Based on the MIMO technique, multiple transmitting and receiving antennas could be utilised to compose a specific geometrical structure and acquire a large aperture. The MIMO-SAR system can be utilised to take deformation measurement to scenes with high deformation rate and can avoid the problems of phase errors caused by the antennas' vibration on the rail. This study introduces the basic principle of the MIMO technique and the key problems in the system design. Experimental datasets are acquired with the MIMO-SAR and GB-SAR (ground-based SAR) systems simultaneously. Comparisons of the imaging results indicate that the MIMO-SAR system has a better repeat-track interferometry performance than the GB-SAR system. In addition, deformation measurement results validate the high-accuracy measurement ability of the MIMO-SAR system.

Design and preliminary verification of L‐band ground‐based SAR system for the vegetated area

Ground-based synthetic aperture radar (GB-SAR) is a novel system applied deformation monitoring of buildings, mountains, and mining slopes with the advantages of high-resolution, large-scale monitoring, and fast image acquisition image interval. Most of current GB-SAR systems work in the high-frequency band, without the ability of vegetation penetration. So here, a GB-SAR system used for vegetated area is designed on the carrier frequency and other important parameters under the requirements for vegetation penetration, high-resolution and large-scale imaging. At last, the imaging ability and vegetation penetration ability of the designed GB-SAR system are assessed and verified through simulations and experiments.

Measurement of Pier Deformation Patterns by Ground-Based SAR Interferometry: Application to a Bollard Pull Trial

In this paper, we describe a new methodology for the nondestructive measurement of absolute displacements of a pier during a bollard pull trial by ground-based synthetic aperture radar (GBSAR) interferometry. This technique measures displacement patterns with a submillimeter precision in any weather conditions, operating at a distance up to $\text{4 km}$ from the target area. Bollard pull trials are performed to study the deformation response of a pier when a static pull is applied by a tug to a bollard on the pier edge. The precise measurement of the pulling force and the corresponding displacement pattern of the pier around the bollard is a useful piece of information for the back-analysis studies during the assessment phases of recently built piers. An experiment is carried out to measure pier's displacements at 12 co-located corner reflectors (CRs) and surveying prisms, by SAR interferometry and topographic techniques during a bollard pull trial. The GBSAR results have been validated at the CR locations using the displacement measurements provided by topographic survey. The pulling force applied to the bollard is measured by a load cell specifically customized to precisely measure the pulling force during the trial. Results demonstrate that GBSAR systems can provide a useful tool for the assessment of harbor infrastructures, such as piers, measuring absolute displacements with near-real time capabilities.

Impact of Wind-Induced Scatterers Motion on GB-SAR Imaging

Ground-based synthetic aperture radar (GB-SAR) sensors represent a cost-effective solution for change detection and ground displacement assessment of small-scale areas in real-time early warning applications. GB-SAR systems based on stepped linear frequency modulated continuous wave signals have led to several improvements such as a significant reduction of the acquisition time. Nevertheless, the acquisition time is still long enough to force a degradation of the quality of the reconstructed images because of possible short-term variable reflectivity of the scenario. This reduction of the quality may degrade the differential interferometric detection process. In scenarios where interesting targets are surrounded by vegetation, this is normally related to atmospheric conditions, in particular, the wind. The present paper characterizes the effect of the short-term variable reflectivity in the GB-SAR image reconstruction and evaluates its equivalent blurring effect, the decorrelation introduced in the SAR images, and the degradation of the extracted parameters. In order to validate the results, the study assesses different GB-SAR images obtained with the RISKSAR-X sensor, which has been developed by the Universitat Politecnica de Catalunya.

Near Range Radar Imaging Based on Block Sparsity and Cross-Correlation Fusion Algorithm

In this paper, we propose a novel processing model for compressive sensing (CS)-based stepped-frequency continuous-wave (SFCW) radar near range imaging, which takes the azimuth dependence of the reflection coefficients of targets into consideration. Based on the block sparse property of the received signal in the defined dictionary, 2-D images of the targets can be obtained at each spatial sampling point. A cross-correlation method is then employed to fuse these 2-D images to obtain the final result. Random undersamplings of frequencies and spatial sampling points are conducted to reduce the data acquisition time, the data size, and the computational complexity. Experimental results of an SFCW-based MIMO radar and a ground-based SAR system show that, compared with the conventional matched filtering-based methods, the proposed method can provide artifacts-reduced higher resolution images by using reduced frequencies and spatial sampling points. We also demonstrate that, compared to the conventional CS-based methods, due to the more suitable established observation model, the proposed method can achieve better imaging results with fewer artifacts for near range targets.