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by radar, and Mars has been explored with orbiters and rovers. Jupiter and Saturn, as well as their satellites, have been mapped by sophisticated orbiters.

      The next decade will also see continuing advances in our use of spaceborne remote sensing techniques to understand the dynamics of our own planet and its environment. A number of international platforms continuously monitor our planet's surface and atmosphere using multispectral sensors, allowing us to observe long‐term global and regional changes. More sophisticated systems are being deployed to globally measure ocean salinity, soil moisture, gravity field changes, and surface tectonic motion. These systems will make full use of new developments in technology, information handling, and modeling.

Remote sensing is a young discipline that calls on a wide range of specialties and crosses boundaries between traditional scientific and technological disciplines. Its multidisciplinary nature requires its practitioner to have a good basic knowledge in many areas of science and requires interaction with researchers in a wide range of areas such as electromagnetic theory, spectroscopy, applied physics, geology, atmospheric sciences, agronomy, oceanography, plasma physics, electrical engineering, and optical engineering.

      The purpose of this text is to provide the basic scientific and engineering background for students and researchers interested in remote sensing and its applications. It addresses (1) the basic physics involved in wave–matter interactions, which is the fundamental element needed to fully interpret the data, (2) the techniques used to collect the data, and (3) the applications to which remote sensing is most successfully applied. This is done keeping in mind the broad educational background of interested readers. The text is self‐comprehensive and requires the reader to have the equivalent of a junior level in physics, specifically introductory electromagnetic and quantum theory.

      The text is divided into three major parts. After the introduction, Chapter 2 gives the basic properties of electromagnetic waves and their interaction with matter. Chapters 3–7 cover the use of remote sensing in solid (including ocean) surface studies. Each chapter covers one major part of the electromagnetic spectrum (visible/near infrared, thermal infrared, passive microwave, and active microwave, respectively). Chapters 8–12 cover the use of remote sensing in the study of atmospheres and ionospheres. In each chapter, the basic interaction mechanisms are covered first. This is followed by the techniques used to acquire, measure, and study the information (waves) emanating from the medium under investigation. In most cases, specific advanced sensors flown or under development are used for illustration.

      The text is generously illustrated and includes many examples of data acquired from spaceborne sensors. This book is based on an upper undergraduate and first‐year graduate course that we teach at the California Institute of Technology to a class that consists of students in electrical engineering, applied physics, geology, planetary science, astronomy, and aeronautics. It is intended for a two‐quarter course. This text is also intended to serve engineers and scientists involved in all aspects of remote sensing and its application.

      This book is a result of many years of research, teaching, and learning at Caltech and the Jet Propulsion Laboratory. Throughout these years, we have collaborated with a large number of scientists, engineers, and students who helped in developing the basis for the material in this book. We sincerely thank them for creating a most pleasant atmosphere for work and scientific “enjoyment.” To name them all would lead to a very long list; however, we would like to acknowledge the numerous researchers at JPL who were kind enough to read and provide suggestions on how to improve the text for this and earlier editions – they include M. Abrams, M. Chahine, J. Curlander, D. Diner, M. Freilich, M. Gierach, R. Greene, A. Khazendar, Y. Lou, D. McCleese, P. Rosen, D. Vane, S. Vannan, J. Waters, and H. Nair – as well as our students and Postdocs at Caltech, who hopefully became interested enough in this field to carry the banner. We want also to acknowledge, as immigrant Americans, the golden opportunities that this great country provided us and fellow immigrants to follow their dreams.

We could not have completed this book without the dedicated assistants and artists who typed the text of the First Edition, improved the grammar, and did the artwork, in particular, Clara Sneed, Susan Salas, and Sylvia Munoz. Special thanks to Priscilla McLean for helping with this edition.

      Charles Elachi and Jakob van Zyl

      Pasadena, California, August 2020

1 Introduction

      Remote sensing is defined as the acquisition of information about an object without being in physical contact with it. Information is acquired by detecting and measuring changes that the object imposes on the surrounding field, be it an electromagnetic, acoustic, or potential field. This could include an electromagnetic field emitted or reflected by the object, acoustic waves reflected or perturbed by the object, or perturbations of the surrounding gravity or magnetic potential field due to the presence of the object.

      The term “remote sensing” is most commonly used in connection with electromagnetic techniques of information acquisition. These techniques cover the whole electromagnetic spectrum from the low‐frequency radio waves through the microwave, submillimeter, far infrared, near infrared, visible, ultraviolet, x‐ray, and gamma‐ray regions of the spectrum.

      The advent of satellites is allowing the acquisition of global and synoptic detailed information about the planets (including the Earth) and their environments. Sensors on Earth‐orbiting satellites provide information about global patterns and dynamics of clouds, surface vegetation cover and its seasonal variations, surface morphologic structures, ocean surface temperature, and near‐surface wind. The rapid wide coverage capability of satellite platforms allows monitoring of rapidly changing phenomena, particularly in the atmosphere. The long duration and repetitive capability allows the observation of seasonal, annual, and longer term changes such as polar ice cover, desert expansion, solid surface motion, and subsidence and tropical deforestation. The wide‐scale synoptic coverage allows the observation and study of regional and continental scale features such as plate boundaries and mountain chains.

      Sensors on planetary probes (orbiters, flybys, surface stations, and rovers) are providing similar information about the planets and objects in the solar system. By now all the planets in the solar system have been visited by one or more spacecraft. The comparative study of the properties of the planets is providing new insight into the formation and evolution of the solar system.

1.1 Types and Classes of Remote Sensing Data

The type of remote sensing data acquired is dependent on the type of information being sought, as well as on the size and dynamics of the object or phenomena being studied. The different types of remote sensing data and their characteristics are summarized in Table 1.1. The corresponding sensors and their role in acquiring different types of information are illustrated in Figure 1.1.

Two‐dimensional images are usually required when high‐resolution spatial information is needed, such as in the case of surface cover and structural mapping (Figs. 1.2 and 1.3), or when a global synoptic view is instantaneously required, such as in the case of meteorological and weather observations (Fig. 1.4). Two‐dimensional images can be acquired over wide regions of the electromagnetic spectrum (Fig. 1.5) and with a wide selection of spectral bandwidths. Imaging sensors are available in the microwave, infrared (IR),

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