Along-track Interferometric Synthetic Aperture Radar Research Articles

Distributed satellite system autonomous orbital control with recursive filtering

In this article, we propose a recursive orbital elements filter for autonomous control of Distributed Satellite Systems (DSS) that significantly reduces the variance of relative orbital elements between the observed and the desired satellite orbits. Leveraging satellite kinematics and control inputs data, combined with a model of relative dynamics, the filter provides smooth and continuous orbital control, while minimizing propellant consumption. In conjunction with Precise Point Positioning (PPP) navigation, the proposed filter enables onboard continuous low-thrust control compatible with high-performance electric propulsion. We also propose a restricted transverse/normal control law that simplifies the thruster's configurations and/or attitude manoeuvres required for propulsion pointing. The applicability and validity of our proposed techniques are verified by numerical simulations with two case studies: a constellation for Differential Interferometric Synthetic Aperture Radar (DInSAR) for global infrastructure monitoring; and a maritime domain awareness mission based on along-track interferometric synthetic aperture radar which requires single-pass interferometry for responsive ship traffic surveillance, and the coverage of a very large maritime zone with high revisit rates.

A Distributed Satellite System for Multibaseline AT-InSAR: Constellation of Formations for Maritime Domain Awareness Using Autonomous Orbit Control

Space-based Earth Observation (EO) systems have undergone a continuous evolution in the twenty-first century. With the help of space-based Maritime Domain Awareness (MDA), specially Automatic Identification Systems (AIS), their applicability across the world’s waterways, among others, has grown substantially. This research work explores the potential applicability of Synthetic Aperture Radar (SAR) and Distributed Satellite Systems (DSS) for the MDA operation. A robust multi-baseline Along-Track Interferometric Synthetic Aperture Radar (AT-InSAR) Formation Flying concept is proposed to combine several along-track baseline observations effectively for single-pass interferometry. Simulation results are presented to support the feasibility of implementing this acquisition mode with autonomous orbit control, using low-thrust actuation suitable for electric propulsion. To improve repeatability, a constellation of this formation concept is also proposed to combine the benefits of the DSS. An MDA application is considered as a hypothetical mission to be solved by this combined approach.

Influence of Radar Parameters and Sea State on Wind Wave-Induced Velocity in C-Band ATI SAR Ocean Surface Currents

Wind wave-induced artifact surface velocity (WASV) is an important component of the sea surface motions detected by synthetic aperture radar (SAR) systems. Understanding the characteristics of the interference of WASV on SAR current velocity estimates is necessary to improve the accuracy of retrievals. In this study, we assessed and analyzed the sensitivity of WASV in C-band along-track interferometric (ATI) SAR to radar configuration, wind field, swell field, and a wave spectrum model. Results showed that the influence of wind speed on WASV increased with the current velocity. The swell also affected WASV, especially at higher wind speeds; WASV was more strongly influenced by swell amplitude than by swell wavelength. In terms of radar configurations, results showed that VV polarization was more suitable than HH polarization in the estimation of WASV. The interference of WASV was minimal at moderate incidence angles (around 40°), and an appropriate ATI baseline selection was also given. The WASV was more strongly influenced by sea states than by the wave spectrum model or by a spreading function. The findings of this study improve our understanding of WASV and provide a reference for the design of future ATI SAR current measurement instruments and projects.

Velocity Estimation of Ocean Surface Currents in along-Track InSAR System Based on Conditional Generative Adversarial Networks

Velocity estimation of ocean surface currents is of great significance in the fields of the fishery, shipping, sewage discharge, and military affairs. Over the last decade, along-track interferometric synthetic aperture radar (along-track InSAR) has been demonstrated to be one of the important instruments for large-area and high-resolution ocean surface current velocity estimation. The calculation method of the traditional ocean surface current velocity, as influenced by the large-scale wave orbital velocity and the Bragg wave phase velocity, cannot easily separate the current velocity, characterized by large error and low efficiency. In this paper, a novel velocity estimation method of ocean surface currents is proposed based on Conditional Generative Adversarial Networks (CGANs). The main processing steps are as follows: firstly, the known ocean surface current field diagrams and their corresponding interferometric phase diagrams are constructed as the training dataset; secondly, the estimation model of the ocean surface current field is constructed based on the pix2pix algorithm and trained by the training dataset; finally, the interferometric phase diagrams in the test dataset are input into the trained model. In the simulation experiment, processing results of the proposed method are compared with those of traditional ocean surface current velocity estimation methods, which demonstrate the efficiency and effectiveness of the novel method.

Correction: Yang, J., et al. Phase Imbalance Analysis of GF-3 Along-Track InSAR Data and Ocean Current Measurements. Remote Sens. 2021, 13, 269

The authors would like to make the following correction of [...]

Application of MA-ATI SAR for Estimating the Direction of Moving Water Surface Currents in Pi-SAR2 Images

A multiaperture along-track interferometric synthetic aperture radar (MA-ATI SAR) theory was proposed to obtain velocity vectors of moving objects. This theory utilizes two antennas to estimate two velocity components (aft- and fore-looking directions) by multi/sublook processing. The moving direction can then be calculated by the two velocity components. Combining it with the conventional range velocity estimation, the velocity vector can be estimated. In order to validate its methodology, the present study applies MA-ATI SAR to airborne polarimetric interfereometric SAR-2 (Pi-SAR2) for estimating the directions of moving water surface currents of rivers. The results were in good agreement with the true moving directions. The estimated accuracy showed the direction error of approximately 20° for the three cases out of four. One case showed the higher error, since there may be complicated flow patterns. The findings of this study have implications for the applicability of the MA-ATI method to an airborne SAR system.

Observing Sea Surface Current by Gaofen-3 Satellite Along-Track Interferometric SAR Experimental Mode

Sea surface current is a research hotspot in oceanography. Space-borne along-track interferometric synthetic aperture radar (along-track InSAR, ATI) is a promising sensor for measuring high-resolution sea surface current field, and there is no operational system in orbit yet. To support future space-borne ATI systems, based on the ATI experimental mode of the Gaofen-3 (GF-3) satellite, the first sea surface current observing experiment was conducted in the Jiaozhou Gulf in China, in 2019. Meanwhile, SAR observations and in situ instrument measurements of the current are obtained in the experiments. The data is first preprocessed by a processor specially developed for the GF-3 ATI data. Then, the current is extracted based on the M4S mode. The retrieved current of the Jiaozhou Gulf is compared with ground-based high frequency surface wave radar data. The results show that the root mean square error of the surface current observed by the GF-3 satellite is less than 0.2 m/s.

Ship Velocity Estimation in Airborne Along-Track Interferometric SAR Imagery Based on the Fractional Fourier Transform

Synthetic aperture radar (SAR) was originally exploited to image stationary scenes. However, it is important to derive target information of velocity for many applications. The fractional Fourier transform (FrFT) is a generalization of the classical Fourier transform and is well-known as a useful tool to estimate the chirp rate of linear frequency-modulated (LFM) signals. Motion compensation is critical to moving target imaging. It is difficult for us to obtain the actual motion parameters in real scenarios. Based on the moving target echo model in airborne along-track interferometric SAR (ATI-SAR) and expression of the ATI phase, a method is proposed to estimate the ship velocity by combining the ATI phase with FrFT. First, we use the FrFT to evaluate the chirp rate of the moving target echo. Then, we construct an equation to estimate the ship velocity using the chirp rate estimation, peak response time, and ATI phase. Finally, the simulation experiments are used to validate the effectiveness of the proposed method.

Improved Method to Suppress Azimuth Ambiguity for Current Velocity Measurement in Coastal Waters Based on ATI-SAR Systems

Measurements of ocean surface currents in coastal waters are crucial for improving our understanding of tidal atlases, as well as for ecosystem and water pollution monitoring. This paper proposes an improved method for estimating the baseline-to-platform speed ratio (BPSR) for improving the current line-of-sight (LOS) velocity measurement accuracy in coastal waters with along-track interferometric synthetic aperture radar (ATI-SAR) based on eigenvalue spectrum entropy (EVSE) analysis. The estimation of BPSR utilizes the spaceborne along-track interferometry and considers the effects of a satellite orbit and an inaccurate baseline responsible for azimuth ambiguity in coastal waters. Unlike the existing methods, which often assume idealized rather than actual operating environments, the proposed approach considers the accuracy of BPSR, which is its key advantage applicable to many, even poorly designed, ATI-SAR systems. This is achieved through an alternate algorithm for the suppression of azimuth ambiguity and BPSR estimation based on an improved analysis of the eigenvalue spectrum entropy, which is an important parameter representing the mixability of unambiguous and ambiguous signals. The improvements include the consideration of a measurement of the heterogeneity of the scene, the corrections of coherence-inferred phase fluctuation (CPF), and the interferogram-derived phase variability (IPV); the last two variables are closely related to the determination of the EVSE threshold. Besides, the BPSR estimation also represents an improvement that has not been achieved in previous work of EVSE analysis. When the improved method is used on the simulated ocean-surface current LOS velocity data obtained from a coastal area, the root-mean-square error is less than 0.05 m/s. The other strengths of the proposed algorithm are adaptability, robustness, and a limited user input requirement. Most importantly, the method can be adopted for practical applications.

Along-Track Interferometric SAR Systems for Ground-Moving Target Indication: Achievements, Potentials, and Outlook

Along-track interferometric (ATI) synthetic aperture radar (SAR) has proven a powerful tool for ground-moving target indication (GMTI). ATI-SAR systems use more than one antenna (at least two), mounted on a moving platform and displaced along the moving direction. Each SAR antenna provides a complex image of the same ground scene acquired with a very small time delay, depending on the separation (baseline) between the antennas. Starting from these interferometric images, ground-moving targets can be detected and their radial velocity estimated.

Validation of MA-ATI SAR Theory Using Numerical Simulation for Estimating the Direction of Moving Targets and Ocean Currents

A theory of multiaperture along-track interferometric (MA-ATI) synthetic aperture radar (SAR) was previously proposed to estimate the velocity vector of moving targets by utilizing conventional ATI SAR data. In the MA-ATI SAR method, multi/sublook processing is applied to both the raw data acquired by the fore-and-aft antennas, yielding two sets of forward-looking images and two sets of backward-looking images. From the interferograms of corresponding directions, two velocity components can be estimated, and hence the moving directions of a target are also obtained. The velocity vector is calculated by combining the estimated moving direction with the velocity in the range direction measured by the conventional ATI method. In order to show initial validation of this theory, a numerical SAR simulator, which has a function to produce a time series SAR raw data, was further developed to simulate MA-ATI SAR by taking into account the multilook processing. In the present validation study, this simulator (in time-domain processing) was applied to estimate the moving directions of a moving point target and ocean currents in the case of an airborne radar system. In particular, the effect on the subaperture azimuth beam angle was discussed since it is a key factor for the estimation. The preliminary results demonstrated that the estimation accuracy exhibited a direction error of 5°-10° for both the moving target and ocean currents, respectively, when half of the subaperture azimuth beam angle was larger than 1.25°.

Surface Deformation Related to the 2019 Mw 7.1 and 6.4 Ridgecrest Earthquakes in California from GPS, SAR Interferometry, and SAR Pixel Offsets

Abstract We analyzed Synthetic Aperture Radar (SAR) images from Copernicus Sentinel-1A and 1B satellites operated by the European Space Agency and the Advanced Land Observation Satellite-2 (ALOS-2) satellite operated by the Japan Aerospace Exploration Agency and Global Navigation Satellite System (GNSS) data from the Network of the Americas for the 4 July 2019 Mw 6.4 and 5 July (local; 6 July UTC) Mw 7.1 Ridgecrest earthquakes. We integrated geodetic measurements for the 3D vector field of coseismic surface deformation for the two events, using SAR data from Sentinel-1 and ALOS-2 satellites. We combined less precise large-scale displacements from SAR images by pixel offset tracking or matching, including the along-track component, with the more precise SAR interferometry (Interferometric Synthetic Aperture Radar [InSAR]) measurements in the radar line of sight (LoS) direction and intermediate-precision along-track InSAR to estimate all three components of the surface displacement for the two events together. We also estimated the coseismic deformation for the two earthquakes from time-series processing of continuous Global Navigation Satellite System data stations in the area. InSAR coherence and coherence change maps the surface disruptions due to fault ruptures reaching the surface. Large slip in the Mw 6.4 earthquake was on a NE-striking fault that intersects with the NW-striking fault that was the main rupture in the Mw 7.1 earthquake. The main fault bifurcates towards the southeast ending 3 km from the Garlock Fault. The Garlock fault had triggered slip of about 20 mm in the radar LoS along a short section directly south of the main rupture. About 3 km northwest of the Mw 7.1 epicenter, the surface fault separates into two strands that form a pull-apart with about 1 m of down-drop. Further northwest is a wide zone of complex deformation.

Accuracy and Error Analysis of Vector Measurement of Ocean Surface Current by Multi-Aperture Along-Track Interferometric SAR

The ocean surface current vector contains velocity and direction information, and its acquisition method has important research value in marine applications. Multi-aperture along-track interferometric (MA-ATI) SAR is a novel method of current vector detection. However, due to the lack of real experimental data validation, complex marine environment and diverse system parameters have an uncertain impact on the current measurement ability of MA-ATI SAR. Therefore it is necessary to analyze the measurement accuracy and error of this method in theory. Based on the current measurement principle of MA-ATI SAR, this article deduces and establishes the accuracy and error simulation model, and analyzes the influence of radar parameters and marine environment on the velocity and direction results. In the simulation results, the airborne MIMO-SAR radar parameters is more suitable to use MA-ATI method for accurate current measurement, and the accuracy of current measurement is higher for the current with high velocity and direction close to azimuth. It is found that the variation of wind direction will lead to obvious errors in MA-ATI current results, and it is pointed out that accurate current results can be obtained by iteratively correcting the velocity error of sub-apertures using M4S model.

Measurements of Surface River Doppler Velocities With Along-Track InSAR Using a Single Antenna

Nowadays, a worldwide database containing the historical and reliable data concerning the water surface speed of rivers is not available and would be highly desirable. In order to meet this requirement, the present work is aimed at the design of an estimation procedure for water flow velocity by means of synthetic aperture radar (SAR) data. The main technical aspect of the proposed procedure is that an along-track geometry is synthesized using a single antenna and a single image. This is achieved by exploiting a multichromatic analysis in the Doppler domain. The application of this approach allows us to obtain along-track interferometry equivalent virtual baselines much lower than the equivalent baseline corresponding to the decorrelation time of raw data preserving data coherence. The performance analysis, conducted on live airborne full-polarimetric SAR data, highlights the effectiveness of the proposed approach in providing reliable river surface velocity estimates without the need of multiple passes on the observed scene.

A Theory of Multiaperture Along-Track Interferometric Synthetic Aperture Radar

In this letter, a novel theory is proposed to measure the velocity vector of moving targets such as ocean currents by multiaperture along-track interferometric synthetic aperture radar (MA-ATI SAR), utilizing the conventional dual-antenna ATI SAR data. In this system, the range component of the velocity vector is first computed with the conventional ATI SAR algorithm by correlating the two sets of complex images produced through the fore and aft antennas. For estimating the direction of the velocity vector, multilook/sublook processing is applied to two sets of range-compressed azimuth raw data. Now, there are four sets of complex subimages, two of which are forward-looking and the others are backward-looking. Then, from these subimage pairs, the two sublook interferograms of the corresponding directions are produced. The direction of the moving targets can be estimated either from the forward- and backward-looking azimuth velocity components, or directly from the interferometric phases. With this direction and the range velocity component, the velocity vectors of ocean currents and also moving hard targets on land and sea can be estimated.

Study on the simulation of simultaneous full AT‐POLINSAR

With the maturity of the instantaneous full-polarisation measurement, the instantaneous full-polarimetric system is becoming an important development direction of the polarimetric synthetic aperture radar (POLSAR); however, as an important research branch of the synthetic aperture radar (SAR), the along-track interferometry (ATI) technology for the ground moving target indication (GMTI) is still largely confined to be single polarimetric. In order to facilitate the study of GMTI using a full-polarimetric along-track interferometric SAR (AT-POLINSAR), this study first models the transmitting and receiving signals of AT-POLINSAR and points out its realisability in the existing technical condition of POLSAR. Then, the 6D vector interferometric model for the full-polarimetric echoes is established, where the differences of data processing with the single-polarimetric ATI are analysed. Finally, two simulation methods of the AT-POLINSAR images are given for the generation of signal-level and image-level data, where the results have shown that both the data quality of the interferometric system and its detection performance for moving targets can be improved by the introduction of full polarisation.

A New Azimuth Ambiguity Suppression Algorithm for Surface Current Measurement in Coastal Waters and Rivers With Along-track InSAR

We present a new algorithm for suppressing azimuth ambiguities when measuring surface currents with an along-track interferometric (ATI) synthetic aperture radar in heterogeneous scenes, such as coastal waters or rivers. The key of the proposed algorithm involves a careful analysis of a parameter called the eigenvalue spectrum entropy (EVSE), which is defined as the entropy of the eigenvalue spectrum of the ATI covariance matrix computed in the Doppler domain. The physical meaning of EVSE is that it serves as a descriptor for the degree to which an unambiguous signal component and an ambiguous one are mixed. With the help of EVSE, azimuth ambiguities can be suppressed. Simulation results demonstrate that as compared with the conventional ATI method without azimuth ambiguity suppression, the proposed algorithm allows for a pronounced improvement in the current measuring accuracy. Other advantages of the proposed algorithm lie in the fact that it is not only adaptive, due to its ability to automatically capture the useful Doppler band whose width and position may both vary for different radar and scene parameters, but it also needs a minimal number of user inputs, making it a quite attractive algorithm for routine implementation.

Slow and small target CFAR detection of polarimetric along-track interferometric SAR using coherence optimization

Slow and small target CFAR detection of polarimetric along-track interferometric SAR using coherence optimization

Ship Detection Based on Oceanic Displaced Phase Center Antenna Technique in Along-Track Interferometric SAR

This paper proposes a new, yet powerful constant false alarm rate (CFAR) method for ship detection in along-track interferometric (ATI) synthetic aperture radar (SAR) images. First, based on the property that sea surface is not stationary, a detecting metric, named oceanic displaced phase center antenna (ODPCA), is defined, which is an extended version of the conventional displaced phase center antenna (DPCA) technique to the case of oceanic target detection. In order to implement the automatic and adaptive detection in varying sea backgrounds in practical applications, the models characterizing the statistics of the ODPCA of sea clutter are then analytically derived and developed. Next, the completed processing chain of CFAR detection based on the ODPCA metric is given. Preliminary experiments performed on the measured L- and C-band VV-polarization ATI-SAR data by NASA/JPL's AIRSAR validate that ODPCA has better capability of clutter suppression in oceanic situation and near computation efficiency than the conventional DPCA. Moreover, the generalized gamma distribution is suitable for characterizing the ODPCA statistics in spite of sea surface roughness and velocity. Therefore, the proposed CFAR method based on the ODPCA provides a promising technique for the detection of ships using ATI-SAR data.

A detection method for near-ship wakes based on interferometric magnitude, phase and physical shape in ATI-SAR systems

ABSTRACTNear-ship wakes approximately form a triangle region on the sea surface, and they differ from the background sea in a wave pattern, wave propagation direction and velocity, and fluid particle orbital motion. The differences are observed by along-track interferometric synthetic aperture radar (ATI-SAR) systems, and can be properly measured by the ATI techniques. In this paper, a detection method for near-ship wakes based on interferometric magnitude, phase and physical shape in ATI-SAR systems is proposed. The complex multi-look interferometric information for near-ship wakes is first utilized to detect potential pixels. Then, a cluster algorithm is designed based on the interferometric phase and spatial position characteristics of near-ship wakes. After clustering the detected pixels and performing image close operation, the near-ship wakes, which is the triangular region with a much larger physical area than those of common ships or other false alarms, can be identified. Finally, simulation results validate the effectiveness of the proposed method.