Inversion of Surface Parameters Using Polarimetric
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INVERSION OF SURFACE PARAMETERS USING POLARIMETRIC SAR Dissertation zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt dem Rat der Chemisch-Geowissenschatlichen Fakultät der Friedrich-Schiller-Universität Jena von Dipl. Geogr. Irena Hajnsek geboren am 22.04.1970 in Ljubljana (Slowenien) Reviewer 1. Prof. Dr. C. C. Schmullius, FSU Jena, Germany 2. Prof. Dr. E. Pottier, Universität Rennes I, France 3. Prof. Dr. W.-M. Boerner, University of Illionois at Chichago, USA Tag der öffentlichen Verteidigung 17.10.2001 External Reviewer 1. Dr. S. R. Cloude, AEL Consultants, Cuper, Scotland 2. Dr. J.-S. Lee, Naval Research Laboratory, Washington, USA 3. Dr. A. Moreira, DLR – HR, Germany Dedicated to the women working in the domain of science Resnica, ki jo govoriš, nima preteklosti in ne prihodnosti. Ona je, in to je vse, kar je potrebno Truth, which is spoken, has no past and no future it is and that is all what is required VI Abstract In this dissertation on the Inversion of Surface Parameters using Polarimetric SAR, the potential of implementing recent advances made in Polarimetric Synthetic Aperture Radar to surface parameter estimation is investigated. The sensitivity of microwave scattering to the dielectric properties and the geometric structure of bare soil surfaces makes radar remote sensing a challenge for a wide range of environmental issues related to the condition of natural surfaces. Especially, the potential for retrieving soil moisture with a high spatial and/or temporal resolution represents a significant contribution to hydrological and ecological modelling, as well as to economic optimisation of agricultural procedures. The classical point measurement methods for, soil moisture content and surface roughness, are described. The ground data acquisition as well as the SAR data acquisition campaign were performed with the E-SAR of the German Aerospace Center at Oberpfaffenhofen (DLR-OP- IHR) over two test sites in Germany, the Elbe-Auen and the Weiherbach region. The basic principles of SAR and radar polarimetry are introduced in order to provide the conceptual framework for the inversion of surface parameters from fully polarimetric SAR data, which includes the critical assessment of various novel polarimetric concepts. The prevalent methods for estimating the surface parameters from polarimetric SAR data are critically reviewed. The small perturbation model and two semi-empirical models are forming the basis of a comparative analysis. These two semi-empirical models were chosen in order to provide an overview of the evolution of ideas and techniques in the area of quantitative surface parameter estimation utilising partially polarimetric SAR image data takes. Further, the potential of polarimetric techniques recently developed in different application areas are investigated with regard to their suitability to improve the performance of the introduced inversion algorithms. First, the potential for applying the polarimetric decomposition techniques was addressed in order to repress the disturbing influence of secondary scattering processes, resulting from rough surface scatter. Then, the potential to estimate terrain slopes from fully polarimetric data utilising polarimetric techniques only is investigated and discussed as an attractive alternative to the standard method of using a-priori terrain information. A new model for the inversion of surface parameters from polarimetric SAR image data takes, the Extended Bragg Model, is introduced. The main advantages and disadvantages of this alternate model are discussed. Finally, future perspectives on how to extend the model by implementation of recent advances in polarimetric SAR interferometry are presented. Keywords: Surface Parameters Inversion, Soil Moisture Estimation, Surface Roughness Estimation, Synthetic Aperture Radar (SAR), SAR Polarimetry Kurzfassung In dieser Arbeit wurde das Potential vom polarimetrischen Synthetischen Apertur Radar - SAR zur Bestimmung von geophysikalischen Oberflächenparametern untersucht. Die Abhängigkeit des Rückstreusignals von den dielektrischen und geometrischen Eigenschaften einer vegetationslosen Bodenoberfläche wird in der Radarfernerkundung zur Bestimmung von Oberflächenparametern verwendet. Die Oberflächenparameter, volumetrische Bodenfeuchte und Oberflächenrauhigkeit, liefern als Eingangsparameter einen signifikanten Beitrag für hydrologische und ökologische Modelle und sind mit ihrer hohen räumlichen und zeitlichen Auflösung dienlich bei der Optimierung von landwirtschaftlichen Verfahren. Die klassischen Punktmessungen zur Bestimmung der Oberflächenparameter, volumetrische Bodenfeuchte und Oberflächenrauhigkeit, werden beschrieben. Die Flugzeugkampagne und die gleichzeitig durchgeführten Geländeaufnahmen wurden auf zwei Testgebieten in Deutschland, an der Elb-Aue und im Weiherbachtal durchgeführt. Die Flugzeugkampagne fand mit dem E- SAR des Deutschen Zentrums für Luft- und Raumfahrt (DLR-OP-HR) statt. Die Grundlage für die Invertierung von Oberflächenparametern bilden das SAR und die polarimetrische Konstellation des SAR, deren grundlegende Prinzipien und neue Konzepte vorgestellt werden. Ausgehend von der exakten Lösungsfindung der Maxwell Gleichungen bis hin zur Entwicklung von empirischen Modellen zur Bestimmung von Oberflächenparametern, wurden drei richtungsweisende Verfahren kritisch untersucht. Hierzu gehören das Small Perturbation Model und zwei semi-empirische Erweiterungen. Die ausgewählten semi-empirischen Erweiterungen stellen einen Überblick in der Entwicklung von Modellen und Techniken von teilpolarisierten SAR Daten dar. Ferner wurde das Potential neuer polarimetrischer Techniken untersucht, die auf die polarimetrischen SAR Daten angewendet wurden, um damit die Bedingungen für die anschließende Anwendung der semi-empirischen Modelle zu verbessern. Zum einen kamen polarimetrische Zerlegungstechniken von Streumechanismen zur Anwendung, die die störenden sekundären Streumechanismen reduzieren und zum anderen konnten mit Hilfe der polarimetrischen Informationen topographische Höhenunterschiede ohne a-priori Wissen extrahiert werden, um die Daten zur Bestimmung von Oberflächenparametern zu korrigieren. Ein neu entwickeltes Oberflächenmodel, Extended Bragg Model, zur Invertierung der Oberflächenparameter mit seinen Vor- und Nachteilen wurde vorgestellt. Dieses Model weist die geringsten Invertierungsungenauigkeiten auf. Schlüsselwörter: Oberflächenparameter-Bestimmung, Volumetrische Bodenfeuchte, Oberflächenrauhigkeit, Synthetisches Apertur Radar (SAR), SAR Polarimetrie VIII Preface At the initiation of writing my dissertation, a good friend told me that writing a thesis document is like writing down the story of all the experiences that have grown out of the previous three years of work. This gave me the courage required to commit to paper - the sometimes - difficult task of approaching the topic of Fully Polarimetric SAR Analysis from a geographer’s perspective. But, before I start with writing my ‘research story’, I would first like to thank all those who provided help and guidance during its formulation. The thesis describes work carried out, as part of a BMBF project sponsored by the German Ministry of Education and Research, at the German Aerospace Center (DLR), Institute of Radio Frequency Technology and Radar Systems in Oberpfaffenhofen (DLR-OP-IHR); a dissertation fellowship at the German Aerospace Center; and another dissertation fellowship contract with the European TMR Polarimetry Network carried out in parts at St. Andrews in Scotland under the guidance of Dr. Shane R. Cloude, and at Rennes in France under the supervision of Professor Dr.-Ing.-Habil. Eric Pottier. Foremost, among those I would like to thank, are my supervisor and former Group Leader at DLR, Dr. Christiane Cornelia Schmullius (now Professor with the University of Jena), for her introducing me to the field of radar remote sensing. Together with Prof. Dr. Peter Ergenzinger, the supervisor of my Diploma thesis, she was the main source of inspiration for choosing my thesis topic, and helped me to grow from being a student, to become a more mature and responsible scientist. I am also deeply indebted to my second supervisor, Prof. Dr.-Ing.-Habil. Eric Pottier from the University of Rennes I, for the support he provided particularly during the final stages of my doctoral research studies. Despite the differences in our background, his patient and clear explanations helped guiding me through the polarimetric labyrinth. His personal support from the very beginning was a rare privilege. I know, he will not like to hear it; but when he was working until three in the morning almost every day, he was appearing to me like a TIRED OLD PROFESSOR, yet the old Professor was still very young and never said NO to any inquisitive disturbances. Thanks again “beloved T.O.P”! A very special thanks is going to my third supervisor, Prof. Dr. Wolfgang-Martin Boerner from the University of Illinois at Chicago. He was not only my scientific supervisor but also like a favourite uncle to me and he always motivated me with his never-ending enthusiasm to any kind of application in polarimetry. His comments, even if sometimes very demanding - he was never 100% satisfied - were essential in improving the final version of this work. I hope his dream of a better understanding of our Earth in the frame of the Collaboratorium Terra Digitalis would come true one day. This work could not have been completed without the critical review and