Maître H. (ed.) Processing of Synthetic Aperture Radar Images

ISTE/John Wiley, 2008. — 411.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.The Physical Basis of Synthetic Aperture Radar Imagery
The Principles of Synthetic Aperture Radar
Existing Satellite SAR Systems
Synthetic Aperture Radar Images
Speckle Models
Reflectivity Estimation and SAR Image Filtering
Classification of SAR Images
Detection of Points, Contours and Lines
Geometry and Relief
Radargrammetry
Radarclinometry
Interferometry
Phase Unwrapping
Radar Oceanography