Table of Contents

Introduction

Chapter 1. The Physical Basis of Synthetic Aperture Radar Imagery

1.1. Electromagnetic propagation

1.2. Matter-radiation interaction

1.3. Polarization

Chapter 2. The Principles of Synthetic Aperture Radar

2.1. The principles of radar

2.2. The SAR equations

2.3. Acquisition geometry of SAR images

Chapter 3. Existing Satellite SAR Systems

3.1. Elements of orbitography

3.2. Polar orbiting SAR satellites

3.3. Satellites in non-polar orbit

3.4. Other systems

3.5. Airborne SARs

Chapter 4. Synthetic Aperture Radar Images

4.1. Image data

4.2. Radiometric calibration

4.3. Localization precision

Chapter 5. Speckle Models

5.1. Introduction to speckle

5.2. Complex circular Gaussian model of single-look and scalar speckle

5.3. Complex circular multi-variate Gaussian model of vectorial or multi-look speckle

5.4. Non-Gaussian speckle models

5.5. Polarimetric radar speckle

Chapter 6. Reflectivity Estimation and SAR Image Filtering

6.1. Introduction

6.2. Estimations of reflectivity R

6.3. Single-channel filters with a priori knowledge of the scene

6.4. Multi-channel filters

6.5. Polarimetric data filtering

6.6. Estimation of filter parameters

6.7. Filter specificities

6.8. Conclusion

Chapter 7. Classification of SAR Images

7.1. Notations

7.2. Bayesian methods applied to scalar images

7.3. Application of the Bayesian methods to ERS-1 time series

7.4. Classification of polarimetric images

Chapter 8. Detection of Points, Contours and Lines

8.1. Target detectors

8.2. Contour detectors

8.3. Line detectors

8.4. Line and contour connection

8.5. Conclusion

Chapter 9. Geometry and Relief

9.1. Radar image localization

9.2. Geometric corrections

Chapter 10. Radargrammetry

10.1. Stereovision principles: photogrammetry

10.2. Principles of radargrammetry

10.3. Results of radargrammetry

10.4. Conclusion

Chapter 11. Radarclinometry

11.1. Radarclinometry equation

11.2. Resolution of the radarclinometry equation

11.3. Determination of unknown parameters

11.4. Results

11.5. Radarpolarimetry

11.6. Conclusion

Chapter 12. Interferometry

12.1. Interferometry principle

12.2. Interferogram modeling

12.3. Geometric analysis of data

12.4. Applications of interferometry

12.5. Limitations of interferometry imaging

Chapter 13. Phase Unwrapping

13.1. Introduction

13.2. Preprocessing of InSAR data

13.3. Phase unwrapping methods

Chapter 14. Radar Oceanography

14.1. Introduction to radar oceanography

14.2. Sea surface description

14.3. Image of a sea surface by a real aperture radar

14.4. Sea surface motions

14.5. SAR image of the sea surface

14.6. Inversion of the SAR imaging mechanism

Bibliography

List of Authors

Index

First published in France in 2001 by Hermes Science Publications entitled “Traitement des images de RSO”

First published in Great Britain and the United States in 2008 by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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© ISTE Ltd, 2008

© Hermes Science, 2001

The rights of Henri Maître to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

 [Traitement des images de RSO. English] Processing of synthetic aperture radar images / edited by Henri Maître.

 p. cm.

 Includes bibliographical references and index.

 ISBN 978-1-84821-024-0

1. Synthetic aperture radar. I. Maître, Henri.

 TK6592.S95T73 2008

 621.3848--dc22

2007022559

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN: 978-1-84821-024-0

Introduction

Synthetic aperture radar imagery was born as a result of an exciting process extending over more than half a century and involving parallel advances in physics, electronics, signal processing and finally image processing. As radars first appeared at the eve of World War II, their prime task was surveillance, i.e. detection. They gradually acquired reconnaissance capabilities: very low resolution images were produced by space scans, while a persistent display made it possible to distinguish echoes from different reflectors. To move on to an actual image, all that was needed was to accelerate the scan and organize systematically collected echoes along two directions. But above all, major improvements had to be achieved on two key parameters: resolution, which, due to the particular wavelengths that were used, was rather poor at a useful monitoring range, and on the other hand, discriminating power, i.e., receiver sensitivity to major relevant dynamics. Both parameters were improved in the wake of manifold technical progress, but also thanks to some decisive choices, including side vision that helped remove the dominant echo of orthogonal reflection and synthetic aperture that paved the way to virtually unlimited resolution capabilities. As uncomplicated as these ideas may appear, they could not have materialized without proper technological backing. It thus took a continuous movement back and forth between methodological, conceptual strides and progress in areas such as sensors and emitters, electronic components and processing algorithms for radar imaging to eventually emerge on a par with optical imaging as a basic remote sensing tool.

By the 1960s, the essentials of radar imaging that make it so attractive nowadays had been investigated and recorded. Its foundations ranging from the capacity of discriminating among different materials to that of penetrating through various covers and vegetation layers, from geometrical effects to depolarization properties, from stereoscopic, interferometric and clinometric capacities to differential wavelength properties, had all been laid down. This progress, however, was not widely publicized. Born on the spur of anti-aircraft defense needs, radar imaging was still closely connected with military applications. As a result, even its most outstanding advances, which were often regarded as strategic, were very slow to seep into other areas of industry or research. By its very complexity, particularly its hi-tech requirements, radar imaging was out of bounds for many industrial applications, and academics would not get into it without solid support from some powerful constructors. Even having a look at images from synthetic aperture radars was a lot of trouble. This was not only due to obvious property restrictions, but also to the complex way in which they were obtained. These images, which often were the product of experimental sensors, were very hard to use. The basic acquisition parameters that will be detailed further in this work were subject to endless adjustments. Intermediate processing was also constantly improving, involving transient changes that were not always fully documented. To users, a significant leap forward was made with the advent of civilian satellite sensors such as SEASAT, SIR-A and -B, and especially the ERS family. These systems made it possible to establish a number of reference products that became accessible to all laboratories and helped expand the application range to a considerable extent. Whole areas, from natural disaster prevention to geological and mining surveys, from cartography to polar route monitoring and from forestry management to sea surveys, thus opened up to the use of radar imaging.

Radar imaging has numerous advantages over optical imaging. From among them, we have to underline its capacity of working in any weather, which is particularly useful in frequently overcast countries such as those located in the equatorial belt. In addition, its coherent imaging properties (i.e., its capacity of collecting amplitude and phase signals) are used to attain remarkable resolutions in the synthetic aperture version, while interferometry uses them to measure extremely fine altitudes and control some even finer displacements (accounting for bare fractions of the operating wavelength). The penetration capacity of radar waves is also linked to microwave frequency. It helps them get across light foliage and detect underground structures provided they are shallowly buried in very dry environments. Finally, radar waves are for the most part polarized, and the extent to which they are depolarized by different media that backscatter them is a great source of information for agriculture, geology and land management.

Nevertheless, radar imaging is less attractive for its edge over optical imaging than for the way it complements the latter. The formation of radar images, for instance, is governed by time-of-flight laws rather than the projection imaging we are familiar with. Moreover, radar imaging is especially sensitive to the geometric properties of targets, whether microscopic (e.g., roughness, surface effects) or macroscopic (e.g., orientation, multiple reflections). On the other hand, optical imaging is more sensitive to the physicochemical properties (e.g., emissivity, albedo, color) of targets. Radars are sensitive to properties such as the nature of materials (metallic targets, for example) and their condition (such as soil humidity or vegetation dryness) that optics is frequently unable to perceive. Finally, optical imaging depends on a source of light, which is usually the Sun, while radar imaging has nothing to do with this. As a result, radar images, as compared to optical images, have higher daytime and seasonal stability, but depend to a greater extent on the position of the sensor when it takes a shot.

For all these reasons, many satellites have been equipped with imaging radars. While some of them were merely experimental, others lived on through their descendants, such as the Lacrosse family, which are US military satellites, and the ERS, which are European civilian satellites. These satellites are permanent sources of information on our planet. Such information is mostly processed by photograph interpreters, but automatic techniques are gaining ground, driven by an increased amount of images that need to be processed and the growing demand for reliable and quantitative measurements. This work is designed to contribute to the development of such automatic methods.

It therefore covers the three basic types of tools that are required to digitally process images supplied by synthetic aperture radars, namely:

– physical concepts that help account for the main propagation phenomena and substance-radiation interactions and provide notions on how radars and their supporting platforms operate;

– mathematical models that statistically describe the very peculiar characteristics of radar issued signals and the properties we may expect of them; and

– image processing methods to suit specific applications: detection, reconnaissance, classification or interpretation.

The careful, simultaneous consideration of these three types of properties has helped devise effective automatic methods for extracting information from radar images. For many years, users only adapted to radar imaging a number of algorithms that had worked successfully in optical remote sensing. These commercially available programs were well known to photo interpreters and had been amply tested on images from Landsat, SPOT and Meteosat. Applied to radar images, they yielded very poor results, strengthening the belief that these were definitely unmanageable by automatic tools and open to nothing but qualitative interpretation. It is the goal of this work to disprove this belief and provide the necessary elements for turning the asset of radar imagery to good account.

Chapter 1

The Physical Basis of Synthetic Aperture Radar Imagery ¹

1.1. Electromagnetic propagation

The physics behind radar image formation is complex and involves several different topics. Some deal with electronic components devoted to transmission and reception of the wave, but they will not be discussed here. Other aspects, namely wave propagation and the interaction between microwave frequency waves and materials, are more important for our purposes. These two topics are the subject of this chapter. Electromagnetism obviously underlies both these phenomena and we begin with a review of useful results in this area.

1.1.1. The laws of propagation in homogenous media

1.1.1.1. Basic equations

An electromagnetic wave such as that emitted by radars is characterized at any point in space and at every moment by four vector values: (electric field), (electric displacement), (magnetic induction) and (magnetic field).

These quantities verify Maxwell’s equations, which in the absence of free charges and current densities are written as [JAC 75]:

In the linear stationary case, the fields, the electric displacement and the magnetic induction are ruled by the following relations:

where images is the permittivity and µ is the permeability. We will consider them as scalar values in this book (they are tensors in the general case of anisotropic dielectrics).

The electric field and magnetic field vectors are sufficient to characterize this electromagnetic wave for an unbounded, homogenous, isotropic medium which is free of charges and currents. We will use Maxwell’s equations to show that every component of these fields verifies the wave equation:

[1.1]images

We thus observe the electromagnetic energy transmission; v is the propagation velocity of the electromagnetic wave.

By denoting images₀ the vacuum permittivity and μ₀ the vacuum permeability, we deduce c, i.e. the speed of light, as being:

In the general case and in the absence of any charge or current, the relative permittivity and relative permeability of the propagation medium are normally used, which makes it possible to express the propagation velocity according to c:

The refractive index n for a propagation medium is defined as:

Note that in non-magnetic media we will deal with µr = 1, which leads to

Since the medium is unbounded, and are perpendicular to each other at any and both are perpendicular to the propagation direction that represents the energy path, which is also called a ray.

If a preferred direction can be specified by convention in the plan (, ), we will then be able to characterize (and therefore ) in terms of its polarization, i.e., its orientation with respect to the defined direction.

1.1.1.2. Propagation equation solution

In the presence of an isotropic radiation source located at the solution of propagation equation [1.1] at any point in space is written:

[1.2]images

The wave then propagates from the source (homogenous medium) in such a way that the wavefront, i.e., the normal ray surface everywhere in space, is a sphere centered on the source: the propagation between the source and any observer is carried out in a straight line.

In the specific case of satellite systems, the objects impinged by the electromagnetic wave are far enough from the antenna so that the wave can be seen as locally plane around the study zone (Fraunhoffer zone). Moreover, the only waves that are generally taken into account are quasi-monochromatic waves that have a frequency (harmonic case) and are defined by their wavelength and wave vector

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