Polarisation: Applications in Remote Sensing

Polarisation: Applications in Remote Sensing , S. R. Cloude

Oxford U. Press, New York, 2010. $120.00 (453 pp.). ISBN 978-0-19-956973-1

Remote sensing is a hot topic these days. Climatologists and NASA scientists depend on it, as do the US Air Force, the US Navy, and the CIA. A key technique widely used to decode that remotely acquired data is polarization analysis, which is why Shane Cloude’s book Polarisation: Applications in Remote Sensing is both timely and relevant.

Cloude has been working on polarimetric radar since the 1980s and has seen polarimetric analysis grow from a largely theoretical exercise mostly useful for entertaining a few electromagneticians to a capability now incorporated into almost every imaging radar system designed to acquire scientific data. In Polarisation, he has assembled a nice compendium of the tools and techniques used for polarimetric analysis of remotely sensed data. The book emphasizes his specialty, the construction and use of images from radar data, but it is also useful as a reference to a wider variety of electromagnetic sensors.

The author has tried and largely succeeded at compiling results from many works in the primary literature and presenting them with a consistent notation. The many symbols used by the large and somewhat disparate remote-sensing community confuse scientists new to the field, so Cloude’s attempt at standardization will likely be one of the book’s longer-lived legacies. For example, he demystifies the difference between the FSA (forward-scattering alignment) and BSA (backscattering alignment) coordinate conventions for describing the polarization of transmitted and received waves: Getting all the signs right in the scattering vectors and matrices is important if a data modeler wants to use measurements for a real application.

Inclusiveness is the book’s strength—the text covers many relevant approaches to polarimetric analysis and discusses how each is related to similar but differently emphasized algorithms. As examples, the first four chapters include excellent coverage of depolarization and decomposition, Cloude’s primary work for the past 25 years. That portion of the book will be much appreciated by scientists interested in the automatic classification of targets and terrain from polarimetric radar measurements.

However, Polarisation is less satisfying from a scholarly perspective. It scarcely contains historical context or derivations but focuses instead on presenting various results and discussing how they are related. Major topics, such as interferometric synthetic aperture radar (InSAR), are adequately summarized but presented with little insight into their essential physics. That is a missed opportunity for InSAR in particular, given that the applications of polarimetric InSAR get their own chapter. The later chapters of the book tend to cover applications of polarimetry, but without the rigor of the earlier analytic-technique presentations. And although the application section contains some discussion of modeling, much of its space is devoted to idealized models of volume scattering, especially from vegetated surfaces. That rather specialized application set seems to me to be nice theoretically but lacks proven utility. In general, the chapters covering applications read a bit too much like a list of ongoing areas of research rather than solid demonstrations of why a particular technique is chosen to study the terrain under investigation.

Polarisation is promising as a reference for quantitative specialists outside the radar community who want to understand what polarimetry has to offer their particular field. It is also a fine reference for students who need to learn and keep the notation straight for coding and experimental design. Researchers engaged in a detailed study of polarimetric scattering analysis will want to directly access the primary literature. So will those who find, for example, that the limitations for a given technique are often driven more by its inherent restrictions than by those imposed by a chosen analytical approach—as is the case for polarimetric InSAR, whose data cannot be decorrelated. Nonetheless, it is a worthwhile book to keep on your shelf if you ever want to relate a particular polarization-based, remotely sensed finding to your own research.