Assessment of Sea Surface Signatures for Naval Platforms Using SAR Imagery (AssenSAR)

Lead Research Organisation: University of Bristol
Department Name: Electrical and Electronic Engineering

Abstract

In space imaging, enhanced image quality is key to the detection and characterisation of difficult and transient targets. For example, accurate evaluation of the sea surface conditions can help with the detection and characterisation of ship wakes. These provide key information for tracking (illegal) vessels and are also useful in classifying the characteristics of the wake generating vessel.

Until recently, one of the main factors hampering research into sea surface modelling was the lack of sufficient data of high enough quality, able to accurately describe the sea surface. Remote-sensing technologies have however shown remarkable progress in recent years and the availability of remotely sensed data of the Earth and sea surface is continuously growing. Several European missions (e.g., the Italian COSMO/SkyMed or the German TerraSAR-X) have developed a new generation of satellites exploiting synthetic aperture radar (SAR) to provide spatial resolutions previously unavailable from space-borne remote sensing. The UK is currently developing the first of a constellation of four satellites that will constitute the NovaSAR mission. This represents a milestone for Earth-observation capabilities but also requires the development of novel image modelling, analysis, and processing techniques, able to cope with this new generation of data and to optimally exploit them for information-extraction purposes.

Indeed, the mathematical modelling and understanding of wakes and other sea surface signatures can be greatly enhanced through image analysis and information extraction from SAR imagery. Hence, this project is concerned not only with the development and validation of new sea surface models, but also with the design of very advanced methods for enhancing SAR image quality and for subsequent information extraction.

The results of this project will be important in the detection and tracking of illegal vessels involved in smuggling goods or humans. They will also be indicative in terms of understanding and classifying the characteristics of the wake generating vessel. As a consequence, the work will directly benefit the design of stealthy vessels that can avoid such detections, reducing the risk to naval operations.

Planned Impact

AssenSAR is highly relevant to space imaging and hence to the UK's satellite industry (one of the Eight Great Technologies) where enhanced image quality can be exploited to detect and characterise difficult and transient targets.

The results of this project will be important in the detection and tracking of illegal vessels involved in smuggling goods or humans. They will also be indicative in terms of understanding and classifying the characteristics of the wake generating vessel. The work will thus inform the design of stealthy vessels that can avoid such detections, reducing the risk to naval operations.

The range of commercial and societal impact objectives are as follows:

Capability: We will help to shape UK capability across the satellite industry through better understanding of remote sensing imagery and its utility in information extraction in marine environments. Our researchers will be trained with the technical and enterprise skills needed to deliver the impact of this research across the wider community.

Commercial: Close coupling with partner organisations will facilitate pull-through into advanced products, opening new opportunities for innovation in both marine surveillance and remote sensing imaging. By liaising with the UK Satellite Applications Catapult, it is expected that the innovation potential of this project will be nurtured and eventual commercialisation of research will be facilitated. There are significant foreseeable opportunities for the creation of new starts-up to exploit the outcomes of this project.

Operational Practice: We will concentrate on the design of new denoising and super-resolution protocols for situations where automated processing is used to support human decision making. Obvious examples are in the viewing and analysis of remote sensing imagery and information extraction therefrom.

Publications

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Karakus O (2020) Ship Wake Detection in SAR Images via Sparse Regularization in IEEE Transactions on Geoscience and Remote Sensing

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Karakus O (2022) Cauchy-Rician Model for Backscattering in Urban SAR Images in IEEE Geoscience and Remote Sensing Letters

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Karakus O (2020) Detection of Line Artifacts in Lung Ultrasound Images of COVID-19 Patients Via Nonconvex Regularization. in IEEE transactions on ultrasonics, ferroelectrics, and frequency control

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Karakus O (2021) On Solving SAR Imaging Inverse Problems Using Nonconvex Regularization With a Cauchy-Based Penalty in IEEE Transactions on Geoscience and Remote Sensing

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Karakus O (2020) Correction to "Ship Wake Detection in SAR Images via Sparse Regularization" in IEEE Transactions on Geoscience and Remote Sensing

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Karakus O (2022) A Generalized Gaussian Extension to the Rician Distribution for SAR Image Modeling in IEEE Transactions on Geoscience and Remote Sensing

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Karakus O (2020) Convergence Guarantees for Non-Convex Optimisation With Cauchy-Based Penalties in IEEE Transactions on Signal Processing

 
Description 1) In order to detect ship wakes, one important first step is to detect the wake-generating vessel. That provides the starting point from which the search for sea wakes can begin. In this context, the AssenSAR team have developed a novel, superpixel-based topology for the detection of ships at sea in synthetic aperture radar (SAR) remote sensing imagery. The method is based on the so-called Constant False Alarm Rate (CFAR), which are detectors that utilise various statistical models for sea clutter in conjunction with thresholding schemes. Superpixels on the other hand are groups of connected pixels with similar colors or gray levels. The architecture based on the combination of superpixels and CFAR can be utilized with most prior sea clutter statistical models or thresholding schemes. The performance of this modified SP-CFAR algorithm was demonstrated on TerraSAR-X and SENTINEL-1 images, with superior results in comparison to classical CFAR for various background distributions.
2) Following accurate detection of ship targets in SAR images, the team have also developed a method for the detection of corresponding ship wakes. Since ship wakes mostly appear as lines in SAR images, we have used line detection methods for their identification. Unlike classical approaches that use direct line detection methods, for example based on the Radon transform, our proposed method uses an inverse problem formulation that relies on very advanced mathematical concepts related to non-convex optimisation. We have conducted extensive experiments involving a high number of ship wakes visible in SAR images acquired using different satellite platforms. The performance of our proposed method was demonstrated to be superior to that of previously proposed techniques with a significantly higher detection performance.
3) An important theoretical development during the life of the project has been the formalisation of a novel penalty function, based on the Cauchy distribution that can be used to regularise the solution to various inverse problems. Convergence guarantees have been proven and published and the resulting algorithm has been employed to solve a number of computational SAR imaging problems, including despeckling, autofocusing, or super-resolution. Notably, the same penalty function has been shown to be applicable in a completely different domain, i.e. it was used for detection of markers of COVID-19 in lung ultrasound images.
4) The visibility of sea wakes in SAR images is enabled by the interaction of the electromagnetic waves with the water surface. This is called Bragg resonance scattering and relates to both characteristics of the sea state (amplitude of waves and their direction) and SAR signal (wavelength, incidence angle of signal, etc). We investigated five different sea wave spectra and, besides hydrodynamical effects (wave elevation models), we also simulated corresponding synthetic aperture radar images. We considered different SAR parameters, including frequencies (X, C, and L-band), incidence angles, signal polarizations (VV, HH), image resolutions (2.5, 5 and 10 m), four different platform configurations (two airborne and two spaceborne), and velocity bunching effects (azimuthal cut-off, shifting and smearing). We also determined the boundary conditions for ship wake detectability in SAR images. Most importantly, the MATLAB implementation of this extremely versatile simulator is available as open source and can become a valuable resource for anyone interested in oceanography, or marine and maritime engineering.
Exploitation Route Applications of our ship and ship wake detection algorithms can be related to a diverse set of problems such as navigation and equipment positioning; radar from manned or unmanned platforms and satellites for object detection, classification, tracking, or avoidance; remote sensing of oceanographic phenomena such as fronts and eddies; law enforcement and monitoring for possible drug trafficking and illegal immigration.
One interesting potential application that emerged following our interactions with the Satellite Applications Catapult (SAC) is in illegal fishery identification. In particular, we have started talking to OceanMind, a spin-out from SAC, who provide insights and intelligence into fishing compliance to government authorities and seafood buyers, in order to enable enforcement and compliance to protect the world's fisheries. Our methods could form the basis of machine learning algorithms that would classify detected ships not associated to automatic identification systems and could hence potentially perform illegal fishing activities.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Environment,Healthcare,Government, Democracy and Justice,Security and Diplomacy,Transport

URL https://assensar.blogs.bristol.ac.uk/
 
Title AssenSAR Image Simulator 
Description This software implements a simulation framework for SAR imagery of the sea surface, which includes the superposition of sea waves and ship wakes. The methodology is based on well-proven concepts: the linear theory of sea surface modelling, Michell thin-ship theory for Kelvin wake modelling, and ocean SAR imaging theory. The modelling of the sea surface is based on the sea wave spectrum. A two-scale model is used to simulate intensity SAR images of disturbed sea surface with Kelvin wakes, including tilt and hydrodynamic modulations, while the velocity bunching model is also employed. SAR image simulator includes different parameters, such as the wind speed and direction, fetch length (sea state factors), ship parameters, platform parameters (airborne and satellite SAR platforms), and SAR scanning parameters, all of which contribute to the visualization of sea waves and ship wakes in SAR images. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://data.bris.ac.uk/data/dataset/el0p94vgxjhi2224bx78actb4/
 
Title Cauchy Proximal Splitting (CPS) 
Description These Matlab functions implement a proximal splitting method involving a non-convex penalty function based on the heavy-tailed Cauchy distribution. A function for calculating the proximal operator of the Cauchy prior is provided, and two examples are included to illustrate how to perform cost function optimisation with a forward-backward (FB) -based algorithm that implements the corresponding Cauchy proximal splitting (CPS) method. The two signal processing examples include 1D signal denoising in the frequency domain and image de-blurring. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://data.bris.ac.uk/data/dataset/15y437loa26cr2nx8gnn3l4hzi/
 
Title CauchySAR_v1 
Description This software includes MATLAB source code for solving SAR imaging inverse problems using the Cauchy proximal splitting (CPS) algorithm. CPS is a method for the optimization of a cost function that includes a non-convex Cauchy-based penalty. The convergence of the overall cost function optimization is ensured through careful selection of model parameters within a forward-backward (FB) algorithm. The package includes solutions to three standard SAR imaging inverse problems, i.e. super-resolution, image formation, and despeckling, as well as ship wake detection for maritime applications. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
Impact Nothing yet but it is part of a portfolio of tools that have been developed as part of this project and could serve the purpose of performing image analytics in SAR data. 
URL https://data.bris.ac.uk/data/dataset/2ld1on1vrlzba2o05jd5jhdhd0/
 
Title GG-Rician SAR Image Modelling 
Description This package includes the MATLAB source code for the modeling of SAR images with the Generalized-Gaussian Rician (GG-Rician) distribution. The GG-Rician distribution is a novel statistical model specifically developed for the characterization of synthetic aperture radar (SAR) images. It is based on the Rician distribution to model the amplitude of the complex SAR signal, the in-phase and quadrature components of which are assumed to be generalized-Gaussian distributed. The extension to modeling SAR images in intensity format is also included. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
Impact Nothing yet, but the code can be part of any future algorithms that rely on the statistical characterisation of SAR images to perform data analytics. 
URL https://data.bris.ac.uk/data/dataset/3nqhdd4qvorwx28hjh8bh6g3r8/
 
Title QuantLUS - CPS v1.0 
Description This source code package includes the MATLAB source codes for implementing a novel method for line artefacts quantification in lung ultrasound (LUS) images of COVID-19 patients. The method is formulated as a non-convex regularisation problem involving a sparsity-enforcing, Cauchy-based penalty function, and the inverse Radon transform. Moreover, a simple local maxima detection technique in the Radon transform domain, associated with known clinical definitions of line artefacts, is employed. The method accurately identifies both horizontal and vertical line artefacts in LUS images. In order to reduce the number of false and missed detection, the method includes a two-stage validation mechanism, which is performed in both Radon and image domains. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://data.bris.ac.uk/data/dataset/z47pfkwqivfj2d0qhyq7v3u1i/
 
Title Ship Wake Detection in SAR Images via GMC Regularization 
Description This code implements a method for detecting ship wakes in synthetic aperture radar (SAR) images of the sea surface. The method is based on a linear model assumption for the wakes and hence the Radon transform is employed, within an inverse problem formulation, for detecting the wakes. The cost function associated with the image formation model includes a sparsity enforcing penalty, i.e., the generalized minimax concave (GMC) function. Despite being a nonconvex function, the GMC penalty allows the overall cost function to remain convex. The proposed solution is based on a Bayesian formulation, whereby the point estimates are recovered using a maximum a posteriori (MAP) estimation. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://data.bris.ac.uk/data/dataset/f2q4t5pqlix62sv5ntvq51yjy/
 
Title AssenSAR Image Simulator 
Description This software implements a simulation framework for SAR imagery of the sea surface, which includes the superposition of sea waves and ship wakes. The methodology is based on well-proven concepts: the linear theory of sea surface modelling, Michell thin-ship theory for Kelvin wake modelling, and ocean SAR imaging theory. The modelling of the sea surface is based on the sea wave spectrum. A two-scale model is used to simulate intensity SAR images of disturbed sea surface with Kelvin wakes, including tilt and hydrodynamic modulations, while the velocity bunching model is also employed. SAR image simulator includes different parameters, such as the wind speed and direction, fetch length (sea state factors), ship parameters, platform parameters (airborne and satellite SAR platforms), and SAR scanning parameters, all of which contribute to the visualization of sea waves and ship wakes in SAR images. 
Type Of Technology Software 
Year Produced 2021 
Open Source License? Yes  
Impact This constitutes the basis for further work done on automatically identifying ship targets, done in collaboration with SSTL as part of a follow-on EPSRC Impact Acceleration Award.