Remote Sensing with Imaging Radar

Remote Sensing with Imaging Radar

This book is concerned with remote sensing based on the technology of imaging radar. It assumes no prior knowledge of radar on the part of the reader, commencing with a treatment of the essential concepts of microwave imaging and progressing through to the development of multipolarisation and interferometric radar, modes which underpin contemporary applications of the technology.

The use of radar for imaging the earth’s surface and its resources is not recent. Aircraft-based microwave systems were operating in the 1960s, ahead of optical systems that image in the visible and infrared regions of the spectrum. Optical remote sensing was given a strong impetus with the launch of the first of the Landsat series of satellites in the mid 1970s. Although the Seasat satellite launched in the same era (1978) carried an imaging radar, it operated only for about 12 months and there were not nearly so many microwave systems as optical platforms in service during the 1980s. As a result, the remote sensing community globally tended to develop strongly around optical imaging until Shuttle missions in the early to mid 1980s and free-flying imaging radar satellites in the early to mid 1990s became available, along with several sophisticated aircraft platforms. Since then, and particularly with the unique capabilities and flexibility of imaging radar, there has been an enormous surge of interest in microwave imaging technology.

Unlike optical imaging, understanding the theoretical underpinnings of imaging radar can be challenging, particularly when new to the field. The technology is relatively complicated, and understanding the interaction of the incident microwave energy with the landscape to form an image has a degree of complexity well beyond that normally encountered in optical imaging. A comprehensive understanding of both aspects requires a background in electromagnetic wave propagation and vector calculus. Yet many remote sensing practitioners come from an earth sciences background within which it is unlikely they will have acquired that material. So that they can benefit from radar technology it is important that a treatment be available that is rigorous but avoids a heavy dependence on theoretical electromagnetism. That is the purpose of this book. It develops the technology of radar imaging, and an understanding of scattering concepts, in a manner suited to the background of most earth scientists, supported by appendices that summarise important mathematical concepts. That enables the treatment to move quickly to the practical aspects of imaging, since it does not require early chapters that focus on electromagnetic theory rather than radar itself.

In addition to being a resource book for the user this treatment is also intended to be used as a teaching text, at senior undergraduate or graduate level.

After providing a framework for the book in Chapter 1 including a commentary on the knowledge that is assumed on the part of the reader, Chapters 2 and 3 cover the fundamentals of radar and how images are formed. The material is set in the context of multipolarisation radar which is the hallmark of modern microwave imaging technology. Chapter 4 covers errors in radar data and how they can be corrected, while Chapter 5 is devoted to the landscape and how it responds to incident microwave energy; that chapter is central to using radar in remote sensing.

An important application of radar is interferometry, which allows the derivation of detailed topographic information about the landscape from a collection of radar images, and means by which landscape changes with time can be detected. This covered in Chapter 6.

An emerging technology in the remote sensing context is bistatic radar in which the source of radiant energy and the receiver are not necessarily collocated, as has been the case for most remote sensing imaging radars to date. An introduction to the technology of bistatic radar is the topic of Chapter 7.

Interpretation of imagery is a logical end point in most remote sensing studies. Chapter 8 covers the range of approaches to radar image interpretation in common use, including statistical and target decomposition methods for thematic mapping.

The book concludes with a brief coverage of passive microwave imaging in Chapter 9. There is sufficient natural microwave energy emanating from the landscape that it can be used to produce coarse resolution images that find particular value in soil moisture studies and sea surface assessment.

Appendices are included that introduce the reader to the concept of complex numbers, summarise essential results in matrices and vectors, demonstrate how images are formed from the recorded radar signal data, and provide other supplementary material.

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