PREFACE

In the past few decades, the principles and techniques of digital signal processing (DSP) have been used in applications such as data and wireless communication, voice and speech analysis and synthesis, and video and image compression and expansion. Radar image processing is considered the primary application of the remote sensing field and is a new and emerging area for DSP applications. Although the primary application of satellite-based radar imaging is military surveillance, the low cost and real-time processing capability of radar imaging, together with its capability to operate under any environmental conditions (e.g., night or day, rain or snow, fog or clear sky) have opened up many commercial applications. Sea ice monitoring and disaster monitoring of events such as forest fires, floods, volcano eruptions, earthquakes, and oil spills are examples of satellite-based radar imaging applications. Airborne-based radar systems also have made radar imaging more affordable and popular. Furthermore, exploration of underground natural resources is an example of a new application.

The processing of radar images, in general, consists of three major fields: DSP principles and communication theory, knowledge of antenna and radar operation, and algorithms used to process the radar images. The purpose of this book is to include the material in these fields in one publication, to provide the reader with a thorough understanding of how radar images are processed. To further familiarize the reader with the theories and techniques used in processing radar images, MATLAB*-based programs are utilized extensively in this book in both the synthesis and analysis of the radar image. In this way, the signal waveforms are therefore made visible at various stages during computer simulation, and the capability of three-dimensional (3D) graphical displays makes many abstract results easier to understand. This book is aimed at engineers or students who have some knowledge of DSP theory and limited knowledge of communication theory and/or antenna theory, but are interested in advanced DSP applications, especially in the remote sensing field.

This book consists of three major groups of chapters. Chapters 1 and 2 provide an overview of DSP principles, reviewing signal characteristics in both analog and digital domains and describing some DSP techniques that serve as key tools in radar images processing. Chapters 3–5 discuss the basics of antenna theory, radar operation principles, modulation/demodulation, and radar target detection techniques. Chapters 6–9 discuss the properties and formation of radar images and then try to model the processing of radar images. The principles of radar image data synthesis are presented and demonstrated with computer-simulated examples. Both the range–Doppler and the Stolt interpolation algorithms are described and applied to the simulated image data and satellite radar-based image data. The results are analyzed and compared. MATLAB* programs are used extensively during the generation of various waveforms of signal processing, radar detection, and synthesis/simulation of radar image processing.

The first two chapters briefly review the DSP principles. Chapter 1 describes the characteristics of signals, followed by Fourier series representation of periodic signals. Fourier transform is then introduced to represent a signal, whether in periodic or nonperiodic form. Sampling theory and interpolation filter are derived, and some advanced sampling and interpolation techniques are reviewed. Resampling from unevenly spaced data to obtain evenly spaced data is briefly discussed at the end of the chapter. Chapter 2 addresses the discrete signal transformation in both time and frequency domains. Discrete Fourier transform (DFT), together with some of its characteristics, are reviewed. Windowing functions and the well-known fast Fourier transform (FFT) technique are covered. The discrete cosine transform (DCT), which is the byproduct of DFT, is introduced. A graphical representation of DFT provides an overview of the relationship between a continuous signal and a discrete signal. It also provides signal variations in both time and frequency domains. The chapter ends with an example of resampling with fractional interpolation based on DFT technique.

Chapters 3–5 provide a background review on antenna theory and radar operation principles. Chapter 3 starts the review of the electromagnetic field with the Maxwell equation, followed by the electromagnetic (EM) fields generated from the infinitesimal dipole. Finite-length dipole- and half-wavelength dipole-based linear antenna arrays are described. Some commonly used antennas, including the microstrip antenna, are also covered. Chapter 4 deals with the basic theory of radar signal processing. The radar range equations and other related parameters are reviewed. The Doppler frequency due to relative movement between radar and target is briefly discussed with respect to the wavefront. Some target range and motion direction detection techniques are also revealed at the end of chapter. Chapter 5 provides broad coverage of modulation/demodulation and target detection techniques used by radar systems. Amplitude modulation (AM)-based pulse Doppler frequency radar is first reviewed, followed by discussion of target detection techniques. Frequency modulation (FM)-based radars, which include pulsed linear FM (LFM), continuous-wave LFM and stepped LFM signals, are then briefly discussed. Also covered in this chapter are in-phase–quadrature-phase (I–Q) demodulator and pulse compression (or matched filtering), which serve as important tools in radar signal processing

Chapters 6–9 discuss the main topic of this book: radar image formation and processing. Chapter 6 starts with a survey of some popular imaging radars and possible applications, followed by the description of the geometry of stripmap synthetic aperture radar (SAR), which consists of broadside SAR and squint SAR. The role of Doppler frequency in radar image formation is analyzed. Also covered are the range migraion, geometric distortion, and resolution of image radar. Chapter 7 discusses the ideal system model of radar imaging. The reconstruction of 2D target function is modeled by two independent 1D functions. The model of 1D range imaging is first described, followed by discussion of the 1D cross-range imaging. Data acquisition and the frequency spectrum of radar image are also reviewed. Chapter 8 discusses the principles of radar image generation and how to synthesize the radar image. Examples of synthesizing radar image data for broadside SAR and squint SAR are presented, which include single and multiple targets. The range–Doppler algorithm on processing radar images is then reviewed and applied to the synthesized data. Chapter 9 reviews some radar image processing techniques in the wavenumber domain. The Stolt interpolation technique on radar image processing is briefly reviewed and applied to some simulated image data. The real satellite radar signal is then processed by both range–Doppler and Stolt interpolation algorithms. A comparison on these two algorithms is also provided.

Some of the material in this book was presented to graduate students in Su-Zhou University in China, and the feedback from the students was incorporated into this book. It is my hope that this book can provide enough knowledge for readers to become familiar with radar image processing. Although I have made every effort to make this a thorough and accurate book, errors and mistakes are inevitable. Any comments or feedback from readers will be welcomed and appreciated.

Acknowledgment

I would like to thank Dr. Russell Hsing of Telcordia for his support and inspiration throughout the process of writing this book. His advice made the publication of this book possible and is greatly appreciated.

Finally, I owe a lot to my family for their patience and understanding as I worked on this book. My wife, Rhoda, my children, Anna and David, my son-in-law, Scott Chong, and granddaughter, Jocelyn, all helped make this book possible in numerous ways, and I am grateful to them.

BU-CHIN WANG, PHD