Digital Processing of Synthetic Aperture Radar Data

Transcription

1 Digital Processing of Synthetic Aperture Radar Data Algorithms and Implementation Ian G. Cumming Frank H. Wong ARTECH HOUSE BOSTON LONDON artechhouse.com

2 Contents Foreword Preface Acknowledgments xix xxiii xxv I Fundamentals of Synthetic Aperture Radar 1 1 Introduction Brief Background of SAR Radar in Remote Sensing SAR Fundamentals Spaceborne SAR Sensors Outline of the Book Example of a Spaceborne SAR Image 16 References 19 2 Signal Processing Fundamentals Introduction Linear Convolution Continuous-Time Convolution Discrete-Time Convolution Fourier Transforms Continuous-Time Fourier Transform Discrete Fourier Transform 28 vn

3 viii Contents Fourier Transform Properties Examples of Fourier Transforms Convolution Using DFTs Sampling of Signals Spectrum of Sampled Signals Signal Types Nyquist Sampling Rate and Aliasing Smoothing Windows Interpolation Sine Interpolation Spectra of Interpolation Kernels Nonbaseband and Complex Interpolation Point Target Analysis Summary Magellan Image of Venus Crater 65 References 66 3 Pulse Compression of Linear FM Signals Introduction Linear FM Signals Time Domain Representation Spectrum of the Linear FM Pulse Sampling of FM Signals Frequency and Time Discontinuities Pulse Compression Principles of Pulse Compression Time Domain Compression of Linear FM Signals Frequency Domain Matched Filter The Effect of Windows Oversampling Ratio Revisited Matched Filter Implementation Target Registration and Matched Filter Throwaway FM Rate Mismatch 98

4 Contents ix Effect of Mismatch in a Baseband Signal Effect of Mismatch in a Nonbaseband Signal Filter Mismatch and Time Bandwidth Product Summary A Wide Swath ENVISAT/ASAR Image 104 References 107 Appendix ЗА: Derivation of the Matched Filter Output 108 3A.1 Signal and Matched Filter of Equal Length 108 3A.2 Signal and Matched Filter of Unequal Length 110 Appendix 3B: Derivation of the Phase Error Due to Mismatch... Ill 4 Synthetic Aperture Concepts Introduction SAR Geometry Definition of Terms Satellite Ground Range Geometry Satellite Orbit Geometry The Range Equation Hyperbolic Form of the Range Equation Relationships Between Velocities and Angles SAR Signal in the Range Direction Transmitted Pulse Data Acquisition SAR Signal in the Azimuth Direction What Is Doppler Frequency in the SAR Context? Coherent Pulses Choice of PRF Azimuth Signal Strength and Doppler History Azimuth Parameters The Two-Dimensional Signal Data Arrangement in Signal Memory Demodulated Baseband Signal The SAR Impulse Response 146

5 x Contents Typical Radar Parameter Values SAR Resolution and Synthetic Aperture Resolution Derived from Bandwidth Synthetic Aperture Summary ScanSAR Narrow Image of Vancouver Island 155 References 156 Appendix 4A: Derivation of the Approximate Radar Velocity Appendix 4B: Quadrature Demodulation 159 4B.1 Theory of Quadrature Demodulation 160 4B.2 Errors and Corrections 161 Appendix 4C: Concept of Synthetic Aperture SAR Signal Properties Introduction Signal Spectrum in the Low-Squint Case Spectrum in the Range Doppler Domain Two-Dimensional Spectrum Signal Spectrum in the General Case Range Fourier Transform Azimuth Fourier Transform Range Inverse Fourier Transform Azimuth Aliasing and the Doppler Centroid Origin of Azimuth Aliasing and Ambiguities The Doppler Centroid The Doppler Ambiguity Doppler Centroid Variation with Range Range Cell Migration RCM Components Multiple Targets at the Same Range Target Trajectory Wraparound Point Target Examples Simulation Parameters 200

6 Contents xi 5.7 Prelude to SAR Processing Algorithms Time Domain Matched Filtering Real-Time Processed Airborne Radar Image Unfocused SAR Moving on to Better Processing Algorithms Summary 211 References 214 Appendix 5A: Range/Azimuth Coupling 215 Appendix 5B: A Note on the Azimuth FM Rate 219 5B.1 Geometric Interpretation from Slopes 220 5B.2 Approximate Signal Representation 221 II SAR Processing Algorithms The Range Doppler Algorithm Introduction Algorithm Overview RDA in the Low Squint Case Raw Radar Signal Data (Raw Data) Range Compression Azimuth Fourier Transform Range Cell Migration Correction Broadening Due to Residual RCM Azimuth Compression Example of a Low Squint RADARSAT-1 Image The High Squint Case Modifications to Handle Squint Implementation of SRC SRC Options Needed for Satellite and Airborne Cases SRC Simulation Experiments An L-Band Airborne Radar Image Example Multilook Processing Frequency and Time Relationship of Looks 265

7 xii Contents Look Extraction, Detection, and Summation Equivalent Number of Looks An Example of Multilook Processing FM Rate Error Multilook Processed Image Summary 280 References The Chirp Scaling Algorithm Introduction Overview of the Chirp Scaling Algorithm The Chirp Scaling Concept Applying Chirp Scaling to RCMC Bulk and Differential RCMC A More Accurate Expression for RCM Derivation of the Scaling Function Examples of the Size of Differential RCM CSA Processing Details Range Processing Azimuth Processing Processing Examples Simulations with Point Targets Processing of SRTM/X-SAR Data Summary 316 References The Omega-K Algorithm Introduction Overview of the Omega-K Algorithm Reference Function Multiply Stolt Interpolation The Change of Variables Interpretations of the Stolt Mapping Components of the Stolt Mapping 335

8 Contents xiii Interpretation Via Fourier Transform Properties Interpretation Via the Region of Support Interpretation Via Imaging Geometry Error Analysis Approximate Version of the Omega-K Algorithm Approximation Components Relation to the RDA and the CSA Discussion of Errors in the Approximate LJKA Processing Examples Simulation of the Full LJKA Algorithm Approximate Version of the LJKA An X-Band Airborne Radar Spotlight Image Example Summary 358 References 359 Appendix 8A: Stolt Mapping in the Wavenumber Domain The SPECAN Algorithm Introduction Overview of the SPECAN Algorithm Derivation of the SPECAN Algorithm From Convolution to SPECAN Geometric Interpretation Aliasing and FFT Length Output Sample Spacing Number of Good FFT Output Points Placement of Subsequent FFTs Stitching the FFT Outputs Together Multilook Processing Processing Efficiency Range Cell Migration Correction Time Domain Linear RCMC Skewing and Deskewing Phase Compensation 396

9 9.7 Image Quality Issues Frequency Discontinuities at the Stitching Points Azimuth FM Rate Error Radiometric Scalloping Processing Examples Simulated Point Targets ERS Image Processed by SPECAN Summary 420 References Processing ScanSAR Data Introduction ScanSAR Data Acquisition Compression of a Single-Burst Target Full-Aperture Processing The SPECAN Algorithm The Modified SPECAN Algorithm Outline of the Algorithm SRTM Processing Example The Short IFFT Algorithm The Extended Chirp Scaling Algorithm Stitching Processed Bursts Together Summary RADARSAT-1 ScanSAR Image 454 References Comparison of Algorithms Introduction Recap of the Precision Processing Algorithms Range Doppler Algorithm Chirp Scaling Algorithm Omega-K Algorithm Comparison of Processing Functions Form of Range Equation 465

10 Contents xv Implementation of the Azimuth Matched Filter Implementation of Range Cell Migration Correction Implementation of Secondary Range Compression Summary of Processing Errors QPE in the Azimuth Matched Filter QPE in Secondary Range Compression Residual RCM Examples of the Size of the Processing Errors Computation Load Basic Algorithm Operations Range Doppler Algorithm Chirp Scaling Algorithm Omega-K Algorithm Pros and Cons of Each Algorithm Pros and Cons of the RDA Pros and Cons of the CSA Pros and Cons of the ШКА Summary ASAR Image of the Strait of Messina 480 III Doppler Parameter Estimation Doppler Centroid Estimation Introduction Doppler Centroid Frequency Satellite SAR System Geometry Outline of This Chapter Doppler Centroid Accuracy Requirements Baseband Centroid Accuracy Requirements Doppler Ambiguity Accuracy Requirements Calculating Doppler Centroid from Geometry Examples of Doppler Centroid Calculations Yaw and Pitch Steering 503

11 12.4 Estimation of the Baseband Centroid from Received Data Magnitude-Based Estimation Approach Phase-Based Estimation Method Estimation of the Doppler Ambiguity from Received Data A Magnitude-Based DAR Estimation Method Phase-Based DAR Estimation Methods The Wavelength Diversity Algorithm Multilook Cross Correlation Algorithm Multilook Beat Frequency Algorithm PRF Diversity Method Comparison of DAR Algorithms Principles of the Global Estimation Procedure Spatial Diversity Estimator Quality Measures Surface Fitting Approaches Global Fit Using a Polynomial Surface Global Fit Using a Geometry Model Automatic Fitting Procedure Summary 549 References 550 Appendix 12A: Details of the Doppler Calculation 553 Appendix 12B: The Offset Frequency in DAR Algorithms B.1 Model of Signal from Each Look B.2 Multilook Cross Correlation Algorithm B.3 Multilook Beat Frequency Algorithm Azimuth FM Rate Estimation Introduction Azimuth FM Rate Accuracy Requirements Geometry Models to Compute the Azimuth FM Rate Estimating the Azimuth FM Rate from Data 574

12 Contents xvii Autofocus Via Contrast Maximization Autofocus Via Look Misregistration Phase-Based Autofocus Methods Summary A Miniature SAR System - MiSAR 584 References 586 Appendix A: RADARSAT Data CD 587 List of Acronyms 589 List of Symbols 593 Selected Bibliography 597 About the Authors 617 Index 619