Ground moving target indication with synthetic aperture radars for maritime surveillance

The explosive growth of shipping traffic all over the World, with around three quarters of the total trade goods and crude oil transported by sea, has raised newly emerging concerns (economical, ecological, social and geopolitical). Geo-information (location and speed) of ocean-going vessels is crucial in the maritime framework, playing a key role in the related environmental monitoring, fisheries management and maritime/coastal security. In this scenario space-based synthetic aperture radar (SAR) remote sensing is a potential tool for globally monitoring the oceans and seas, providing two-dimensional high-resolution imaging capabilities in all-day and all-weather conditions. The combination of ground moving target indication (GMTI) modes with multichannel spaceborne SAR systems represents a powerful apparatus for surveillance of maritime activities. The level of readiness of such a technology for road traffic monitoring is still low, and for the marine scenario is even less mature. Some of the current space-based SAR missions include an experimental GMTI mode with reduced detection capabilities, especially for small and slow moving targets. In this framework, this doctoral dissertation focuses on the study and analysis of the GMTI limitations of current state-of-the-art SAR missions when operating over maritime scenarios and the proposal of novel and optimal multichannel SAR-GMTI architectures, providing subclutter visibility of small (reduced reflectivity) slow moving vessels. This doctoral activity carries out a transversal analysis embracing system-architecture proposal and optimization, processing strategies assessment, performance evaluation, sea/ocean clutter characterization and adequate calibration methodologies suggestion. Firstly, the scarce availability of multichannel SAR-GMTI raw data and the related restrictions to access it have raised the need to implement flexible simulation tools for SAR-GMTI performance evaluation and mission. These simulation tools allow the comparative study and evaluation of the SAR-GMTI mode operated with current SAR missions, showing the reduced ability of these missions to detect small and slow boats in subclutter visibility. Improved performance is achieved with the new multichannel architecture based on non-uniformly distributed receivers (with external deployable antennas), setting the ground for future SAR-GMTI mission development. Some experimental multichannel SAR-GMTI data sets over the sea and acquired with two instruments, airborne F-SAR and spaceborne TerraSAR-X (TSX) platforms, have been processed to evaluate their detection capabilities as well as the adequate processing strategies (including channel balancing). This doctoral activity presents also a preliminary characterization of the sea clutter returns imaged by the spaceborne TSX instrument in a three-level basis, i.e., radiometric, statistical and polarimetric descriptions using experimental polarimetric data. This study has shown that the system-dependent limitations, such as thermal noise and temporal decorrelation, play a key role in the appropriate interpretation of the data and so should be properly included in the physical backscattering models of the sea. Current and most of the upcoming SAR missions are based on active phase array antennas (APAA) technology for the operation of multiple modes of acquisitions. The related calibration is a complex procedure due to the high number of different beams to be operated. Alternative internal calibration methodologies have been proposed and analyzed in the frame of this doctoral thesis. These approaches improved the radiometric calibration performance compared to the conventional ones. The presented formulation of the system errors as well as the proposed alternative strategies set the path to extrapolate the analysis for multichannel SAR systems.L'increment continu del tràfic marítim arreu del món, amb gairebé tres quartes parts del total de mercaderies i cru transportats per mar, porta associats uns impactes canviants a nivell econòmic, ambiental, social i geopolític. La geo-informació (localització i velocitat) dels vaixells té un paper fonamental en el monitoratge ambiental, la gestió de la pesca i la seguretat marítima/costanera. Els radars d'obertura sintètica (SAR, sigles en anglès) embarcats en satèl·lits són una eina molt potent per al monitoratge global dels oceans i dels mars, gràcies a la seva capacitat de generar imatges d'alta resolució amb independència de les condicions meteorològiques i de la llum solar. La detecció de blancs mòbils terrestres (GMTI, sigles en anglès) combinada amb sistemes multicanal SAR és fonamental per a la vigilància de les activitats marítimes. El nivell de maduresa d'aquesta tecnologia per monitorar tràfic rodat és baix, però per al cas marítim encara ho és més. Algunes missions SAR orbitals inclouen el mode GMTI, però amb unes capacitats de detecció reduïdes, especialment per a blancs petits i lents. En aquest marc, la tesi doctoral es centra en l'estudi i anàlisi de les limitacions GMTI dels actuals sistemes SAR operant en entorns marítims, proposant noves configuracions SAR-GMTI multicanal optimitzades per a la detecció de vaixells petits (emmascarats pels retrons radar del mar) i que es mouen lentament. La present dissertació doctoral du a terme un estudi transversal que abasta des de la proposta i optimització de sistemes/configuracions, passant per l'avaluació de les tècniques de processat, fins a l'estudi del rendiment de la missió, caracterització del mar i la valoració de noves metodologies de calibratge. En primer terme, diverses eines de simulació flexibles s'han implementat per poder avaluar les capacitats GMTI de diferents missions tenint en compte la poca disponibilitat de dades multicanal SAR-GMTI. Aquests simuladors permeten l'estudi comparatiu de les capacitats GMTI de les missions SAR orbitals actuals, demostrant les seves reduïdes opcions per identificar vaixells emmascarats pels retorns del mar. En el marc de l'activitat de recerca s'han processat dades experimentals SAR-GMTI multicanal de sistemes aeris (F-SAR) i orbitals (TerraSAR-X), per tal d'avaluar les seves capacitats de detecció de blancs mòbils sobre entorns marítims, proposant les estratègies de processat i calibratge més adients. Com a part de l'activitat de recerca doctoral, s'ha portat a terme una caracterització preliminar dels retorns radar del mar adquirits amb el sensor orbital TerraSAR-X, amb tres nivells d'anàlisi (radiomètric, estadístic i polarimètric). Aquest estudi demostra que aspectes com el soroll tèrmic i la decorrelació temporal, dependents del propi sensor i de l'entorn dinàmic del mar, poden limitar la correcta interpretació de les dades, i per tant, s'han d'incloure en els models físics dels mecanismes de dispersió del mar. Les missions SAR tant actuals com futures es basen en l'explotació de la tecnologia de les agrupacions d'antenes de fase activa (APAA) per operar diferents modes d'adquisició. El procés de calibratge associat és molt complex atès el gran nombre de feixos que es poden utilitzar. En el marc de la tesi doctoral s'han proposat i avaluat metodologies alternatives de calibratge intern per aquests sistemes, amb un millor rendiment en comparació amb les tècniques convencionals. Aquestes estratègies de calibratge, juntament amb la corresponent formulació dels errors de sistema, estableixen les bases per a l'estudi i avaluació en sistemes multicanal SA

Performance Investigation on Scan-On-Receive and Adaptive Digital Beam-Forming for High-Resolution Wide-Swath Synthetic Aperture Radar

The work investigates the performance of the Smart Multi-Aperture Radar Technique (SMART) Synthetic Aperture Radar (SAR) system for high-resolution wide-swath imaging based on Scan-on-Receive (SCORE) algorithm for receive beam steering. SCORE algorithm works under model mismatch conditions in presence of topographic height. A study on the potentiality of an adaptive approach for receive beam steering based on spatial spectral estimation is presented. The impact of topographic height on SCORE performance in different operational scenarios is examined, with reference to a realistic SAR system. The SCORE performance is compared to that of the adaptive approach by using the Cramèr Rao lower bound analysis

Adaptive Digital Beam-Forming for High-Resolution Wide-Swath Synthetic Aperture Radar

Premi GMV en l’àmbit de la Tecnologia Espacial a la millor Tesis de Màster del Màster MINT. Projecte realitzat en col.laboració amb el centre German Aerospace Center (DLR)The thesis “Adaptive Digital Beam-Forming for High-Resolution Wide-Swath Synthetic Aperture Radar” is a project carried out in the Microwaves and Radar Institute (IHR) at the German Aerospace Center (DLR) The objective of this work is to investigate the possibility of applying direction of arrival (DOA) estimation methods to high-resolution, wide-swath Synthetic Aperture Radar (SAR) spaceborne systems, based on receive beam steering algorithm. In the conventional algorithm, called SCan-On-REceive (SCORE), the steering direction of the receive pattern corresponds to the expected DOA of the actual echo, which is computed assuming a stringent spherical Earth model. In presence of topographic height, SCORE works under model mismatch and losses of several dBs could be reached with respect to ideal operational conditions. This thesis analyses the possibility to overcome SCORE loss of performance by using a data-dependent, adaptive mechanism for receive beam steering. In particular, a new algorithm, denoted as Adaptive Digital Beam-Forming (ADBF), is proposed. This algorithm steers the beam based on the actual spatial distribution of the received signal power. The performance of the proposed algorithm is evaluated by Monte Carlo simulations and compared with that of the conventional, non-adaptive SCORE in different operational scenarios versus the main SAR system parameters. Moreover, based on Cramér-Rao Lower Bound analysis, the optimal performance achievable by the adaptive approach has been evaluated and used as a benchmark on the proposed algorithAward-winnin

Digital beamforming and moving target indication for future spaceborne synthetic aperture radar

During the last decade the interest on space-borne Synthetic Aperture Radars (SAR) for remote sensing applications has grown as testified by the number of recent and forthcoming missions as TerraSAR-X, RADARSAT-2, COSMO-kyMed, TanDEM-X and the Spanish SEOSAR/PAZ. In this sense, this thesis proposes to study and analyze the performance of the state-of-the-Art space-borne SAR systems, with modes able to provide Moving Target Indication capabilities (MTI), i.e. moving object detection and estimation. The research will focus on the MTI processing techniques as well as the architecture and/ or configuration of the SAR instrument, setting the limitations of the current systems with MTI capabilities, and proposing efficient solutions for the future missions. Two European projects, to which the Universitat Politècnica de Catalunya provides support, are an excellent framework for the research activities suggested in this thesis. NEWA project proposes a potential European space-borne radar system with MTI capabilities in order to fulfill the upcoming European security policies. This thesis will critically review the state-of-the-Art MTI processing techniques as well as the readiness and maturity level of the developed capabilities. For each one of the techniques a performance analysis will be carried out based on the available technologies, deriving a roadmap and identifying the different technological gaps. In line with this study a simulator tool will be developed in order to validate and evaluate different MTI techniques in the basis of a flexible space-borne radar configuration. The calibration of a SAR system is mandatory for the accurate formation of the SAR images and turns to be critical in the advanced operation modes as MTI. In this sense, the SEOSAR/PAZ project proposes the study and estimation of the radiometric budget. This thesis will also focus on an exhaustive analysis of the radiometric budget considering the current calibration concepts and their possible limitations. In the framework of this project a key point will be the study of the Dual Receive Antenna (DRA) mode, which provides MTI capabilities to the mission. An additional aspect under study is the applicability of the Digital Beamforming on multichannel and/or multistatic radar platforms, which conform potential solutions for the NEWA project with the aim to fully exploit its capability jointly with MTI techniques

Adaptive Digital Beam-Forming for High-Resolution Wide-Swath Synthetic Aperture Radar

Premi GMV en l’àmbit de la Tecnologia Espacial a la millor Tesis de Màster del Màster MINT. Projecte realitzat en col.laboració amb el centre German Aerospace Center (DLR)The thesis “Adaptive Digital Beam-Forming for High-Resolution Wide-Swath Synthetic Aperture Radar” is a project carried out in the Microwaves and Radar Institute (IHR) at the German Aerospace Center (DLR) The objective of this work is to investigate the possibility of applying direction of arrival (DOA) estimation methods to high-resolution, wide-swath Synthetic Aperture Radar (SAR) spaceborne systems, based on receive beam steering algorithm. In the conventional algorithm, called SCan-On-REceive (SCORE), the steering direction of the receive pattern corresponds to the expected DOA of the actual echo, which is computed assuming a stringent spherical Earth model. In presence of topographic height, SCORE works under model mismatch and losses of several dBs could be reached with respect to ideal operational conditions. This thesis analyses the possibility to overcome SCORE loss of performance by using a data-dependent, adaptive mechanism for receive beam steering. In particular, a new algorithm, denoted as Adaptive Digital Beam-Forming (ADBF), is proposed. This algorithm steers the beam based on the actual spatial distribution of the received signal power. The performance of the proposed algorithm is evaluated by Monte Carlo simulations and compared with that of the conventional, non-adaptive SCORE in different operational scenarios versus the main SAR system parameters. Moreover, based on Cramér-Rao Lower Bound analysis, the optimal performance achievable by the adaptive approach has been evaluated and used as a benchmark on the proposed algorithAward-winnin

A physical analysis of polarimetric SAR data statistical models

The random walk model is studied with the objective to obtain a physical explanation for the texture of polarimetric synthetic aperture radar (SAR) data. A simulator is designed to imitate the scattering process under different circumstances, taking into account different distributions for the scatterer number and the scatterer response, as well as mixtures of scatterers. Statistical analysis of the simulated data shows that the distribution of the scatterer response has an effect only when the number of scatterers in a resolution cell is very small, which appears in very high resolution data. Moreover, it is found that both the fluctuation of the scatterer number and the mixture of different targets can give non-Gaussian-distributed data. The mixture of point targets and distributed targets will lead to an extremely heterogeneous appearance, which may be a clue to analyze the urban areas in polarimetric SAR data.Peer Reviewe

Evaluation of state-of-the-art GMTI techniques for future spaceborne SAR systems -simulation validation-

This paper presents an evaluation of the state-of-the-art Ground Moving Target Indication (GMTI) techniques in the frame of space-borne Synthetic Aperture Radar (SAR) for vessel detection by means of a flexible SAR-GMTI simulator tool. Among the different GMTI techniques, Displaced Phase Center Antenna (DPCA), Along-Track Interferometry (ATI) and Imaging Space-Time Adaptive Processing (ISTAP) have been evaluatedPeer ReviewedPostprint (published version

Evaluation of state-of-the-art GMTI techniques for future spaceborne SAR systems -simulation validation-

This paper presents an evaluation of the state-of-the-art Ground Moving Target Indication (GMTI) techniques in the frame of space-borne Synthetic Aperture Radar (SAR) for vessel detection by means of a flexible SAR-GMTI simulator tool. Among the different GMTI techniques, Displaced Phase Center Antenna (DPCA), Along-Track Interferometry (ATI) and Imaging Space-Time Adaptive Processing (ISTAP) have been evaluatedPeer Reviewe

A physical analysis of polarimetric SAR data statistical models

The random walk model is studied with the objective to obtain a physical explanation for the texture of polarimetric synthetic aperture radar (SAR) data. A simulator is designed to imitate the scattering process under different circumstances, taking into account different distributions for the scatterer number and the scatterer response, as well as mixtures of scatterers. Statistical analysis of the simulated data shows that the distribution of the scatterer response has an effect only when the number of scatterers in a resolution cell is very small, which appears in very high resolution data. Moreover, it is found that both the fluctuation of the scatterer number and the mixture of different targets can give non-Gaussian-distributed data. The mixture of point targets and distributed targets will lead to an extremely heterogeneous appearance, which may be a clue to analyze the urban areas in polarimetric SAR data.Peer Reviewe

In vivo characterisation of cis-acting elements regulating Lect14 expression during mouse embryonic development

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