SATELLITE OCEANOGRAPHY An Introduction for Oceanographers

and Remote-sensing Scientists

I. S. Robinson Lecturer in Physical Oceanography

Department of Oceanography University of Southampton

JOHN WILEY & SONS Chichester • New York • Brisbane • Toronto • Singapore

Published in association with

PRAXIS PUBLISHING Chichester

Table of Contents

Foreword 11

Preface 13

Chapter 1 Oceanography from space? 1.1 A new field of marine science? 17 1.2 A quarter of a century of ocean observation from space 18 1.3 The value of satellite data to oceanography 19 1.4 The scope of the book 22

Section A FUNDAMENTALS OF REMOTE SENSING IN OCEANOGRAPHY

Chapter 2 The possibilities in space — space hardware and data transmission

2.1 Space platforms and their orbits 26 2.1.1 Satellite vehicles 26 2.1.2 Satellite orbital dynamics 31 2.1.3 Spatial and temporal sampling characteristics of orbits . . .36

2.2 Sensors on satellites 41 2.2.1 Sensors in relation to the electromagnetic spectrum 41 2.2.2 Sensor measurements 45 2.2.3 Spatial sampling capabilities of sensors 47 2.2.4 Recent and planned satellite sensors which have

oceanographic applications 51 2.3 Data retrieval 51

2.3.1 Satellite-to-earth data transmission 54 2.3.2 Data dissemination 56

Chapter 3 The possibilities for oceanography 3.1 The oceanographic capabilities of satellite sensors 60 3.2 Ocean science in the 1980s .* 64 3.3 Dynamical oceanographic processes 65

3.3.1 Ocean circulation 66 3.3.2 Tides 71

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3.3.3 Fronts 72 3.3.4 Upwelling 74 3.3.5 Mixed-layer dynamics 75 3.3.6 Coastal and estuarine dynamical and topographic

phenomena 76 3.4 Ocean waves 77 3.5 Space and time scales of oceanographic phenomena, in relation to

satellite sampling 84

Chapter 4 Principles of remote sensing of the sea 4.1 Introduction 89 4.2 Sensor calibration 90 4.3 Atmospheric correction 92 4.4 Positional registration 96 4.5 Geophysical calibration 99 4.6 Oceanographic sampling for'sea truth' 100

Chapter 5 Principles of image processing 5.1 Image processing and oceanography 106 5.2 Digital image data from satellites 107 5.3 Image-processing hardware 112 5.4 Geometric processing 114 5.5 Masks and overlays 119 5.6 Smoothing, filtering, and noise reduction 121 5.7 Measurement of spatial variability 124 5.8 Enhancement of single-band radiometric information 124 5.9 Presentation of multichannel image data 129 5.10 Atmospheric correction and geophysical calibration 130

Section В OCEANOGRAPHIC APPLICA TIONS OF SA TELLITE REMOTE SENSING

Chapter 6 Visible wavelength 'ocean-colour' sensors 6.1 Introduction 134

6.1.1 Scope 134 6.1.2 Colour sensors on Landsat 135 6.1.3 The Coastal Zone Colour Scanner on Nimbus 7 139

6.2 Aspects of optical theory relevant to ocean-colour viewing from space 141

6.2.1 Definition of some optical quantities 141 6.2.2 Measurements made by a satellite sensor 144 6.2.3 Optical pathways in the atmosphere 148 6.2.4 Practical strategies for the atmospheric correction of

visible wavelength data 152 6.2.5 Scattering and absorption of light in sea-water 155 6.2.6 Oceanographic interpretation of ocean colour 157 6.2.7 Light-scattering at the sea surface 164

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6.3 Calibration and application of CZCS data 166 6.3.1 Sensor calibration 166 6.3.2 Atmospheric corrections 168 6.3.3 CZCS calibration algorithms 173 6.3.4 Applications in biological oceanography 175 6.3.5 Applications in dynamical oceanography 180

6.4 Oceanographic uses of Landsat MSS data 184 6.4.1 Data processing 184 6.4.2 Calibration of Landsat data for sediment and

chlorophyll 185 6.4.3 Sediment movement studied with Landsat data 188 6.4.4 Other oceanographic processes viewed by Landsat 190 6.4.5 Bathymetry using Landsat 191 6.4.6 Coastal geomorphology 192

6.5 Oceanographic use of other visible-wavelength satellite data 192

Chapter 7 Sea-surface temperature from infrared scanning radiometers 7.1 Introduction 194 7.2 The physics of infrared radiation 199

7.2.1 Thermal emission 199 7.2.2 Atmospheric absorption 201 7.2.3 Infrared sensors 204 7.2.4 Brightness temperature calibration of AVHRR d a t a . . . . 205

7.3 The near-surface thermal structure in the ocean 207 7.3.1 The skin temperature 207 7.3.2 The thermocline structure of the ocean 208 7.3.3 The sea-surface skin-temperature deviation 210 7.3.4 Surface slicks 214

7.4 Atmospheric correction and SST calibration techniques 215 7.4.1 Atmospheric correction strategies. 215 7.4.2 Multispectral and multilook techniques 216 7.4.3 Cloud-removal techniques 219 7.4.4 SST operational products 221

7.5 The potential uses of SST data from satellites 224 7.5.1 Climatology 224 7.5.2 Global changes in SST 225 7.5.3 Atmospheric response to SST anomalies 225 7.5.4 Weather prediction (1 to 10 days) 225 7.5.5 Gas exchange across the air—sea interface 226 7.5.6 Surface heat flux as it affects the ocean heat budget. . . . 226 7.5.7 Deep convection and water-mass formation 227 7.5.8 Dynamical oceanography 227 7.5.9 Pollution studies 228

7.6 Examples of the application of satellite SST data 232 7.6.1 Global SST, and SST-anomaly, maps 232 7.6.2 Ocean eddies 233

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7.6.3 Upwelling 237 7.6.4 Ocean fronts 237 7.6.5 The Mediterranean Sea 240 7.6.6 Shallow-sea fronts 240 7.6.7 Coastal-fringe fronts 244 7.6.8 Commerical fisheries 246

Chapter 8 Passive microwave radiometers 8.1 The physical principles of passive microwave radiometry 248

8.1.1 Overview 248 8.1.2 Thermal emission in the microwave 249 8.1.3 The radiation received by the satellite antenna 250 8.1.4 Atmospheric and solar radiation effects 252 8.1.5 Sea-surface emissivity. 254 8.1.6 Skin depth of sea-surface emission 258

8.2 Microwave radiometer design 260 8.2.1 Radiometer design constraints 260 8.2.2 The Seasat and Nimbus 7 SMMR 261 8.2.3 Other satellite microwave radiometers 263

8.3 Oceanographic interpretation of passive microwave data from space 264

8.3.1 Potential applications 264 8.3.2 SST and surface-wind retrieval algorithms for SMMR . . . 266

8.4 Comparison between infrared and microwave radiometers for SST measurement 270

8.5 Applications of SMMR data 272

Chapter 9 Satellite altimetry of sea-surface topography 9.1 Introduction 278

9.1.1 An exciting new oceanographic instrument 278 9.1.2 Principles of satellite altimetry 279

9.2 Distance measurement with a radar altimeter 281 9.2.1 The instrument design 281 9.2.2 Corrections for atmospheric transmission 284 9.2.3 Surface roughness errors 285 9.2.4 Altimeter specifications 286

9.3 Establishing a datum 287 9.3.1 Orbit determination 287 9.3.2 Summary of height errors relative to the reference

ellipsoid 289 9.3.3 The geoid 289

9.4 Application of altimetry to the study of ocean currents 290 9.4.1 Geostrophic balance 290 9.4.2 The possibilites for ocean-circulation determination. . . .292 9.4.3 Examples of ocean-circulation phenomena observed

with Seasat altimeter data 293

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9.5 Tidal observations with the altimeter 300 9.5.1 The value of satellite altimetry to tidal studies 300 9.5.2 Extraction of tidal information from altimeter data . . . . 301 9.5.3 Examples of altimeter applications to ocean-tidal

science 303 9.6 Ocean bathymetry from satellite altimeters 306 9.7 The selection of satellite orbits for altimeters 306

Chapter 10 Active microwave sensing of sea-surface roughness 10.1 Introduction to the remote sensing of sea-surface roughness 310 10.2 Radar reflection from the sea surface 311

10.2.1 Backscatter cross-section 311 10.2.2 Specular reflection and scattering 311 10.2.3 Resonant Bragg scattering 313 10.2.4 Polarisation 315

10.3 Wind-generated surface-wave roughness 315 10.4 Surface films and slicks 317 10.5 Dynamical causes of sea-surface roughness patterns 319 10.6 Artificial causes of sea-surface roughness patterns 321

Chapter 11 The altimeter as a surface-roughness sensor 11.1 A nadir-viewing radar 324 11.2 Physical principles of operation 324

11.2.1 The effect of waves on the return-pulse shape 324 11.2.2 The effect of surface roughness on the return-pulse

strength 328 11.3 Performance of wave-height algorithms 329 11.4 Performance of wind-speed algorithms 331 11.5 Oceanographic applications of satellite-altimeter-derived

wave height 332

Chapter 12 Synthetic aperture radar 12.1 High-resolution imaging radars 344 12.2 Principles of SAR operation 345

12.2.1 SAR geometry 345 12.2.2 Range resolution 347 12.2.3 Azimuth resolution using the synthetic aperture

principle 351 12.2.4 SAR processing 356 12.2.5 SAR image errors due to moving targets 358 12.2.6 Image degradation due to speckle 362

12.3 SAR imaging of ocean waves 363 12.3.1 A controversial problem 363 12.3.2 Hydrodynamic modulation 365 12.3.3 Electromagnetic modulation 366 12.3.4 Motion effects 368

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12.3.5 High sea states 371 12.3.6 Directional wave spectra from SAR images 371

12.4 Observations of ocean waves with Seasat SAR 372 12.4.1 A description of the Seasat SAR 372 12.4.2 Wave imagery 375 12.4.3 Wave spectra from the Seasat SAR 380

12.5 Internal waves imaged by SAR 384 12.5.1 Observations of internal waves in Seasat data 384 12.5.2 Internal-wave theory 387 12.5.3 Surface-roughness imaging of internal waves 390 12.5.4 Oceanographic information content of SAR images of

internal waves 391 12.6 Other oceanographic phenomena imaged by SAR 392

Chapter 13 Microwave scatterometers 13.1 Introduction 395 13.2 Wind scatterometry 396

13.2.1 Surface-wind stress and the near-surface wind-velocity profile 396

13.2.2 The development of wind-speed and wind-stress algorithms for interpreting radar backscatter measurements 398

13.3 Experience with the Seasat scatterometer 401 13.3.1 The SASS design and operation 401 13.3.2 SASS data processing 405 13.3.3 The SASS wind-vector algorithm 406 13.3.4 Removing the direction alias 410 13.3.5 Validation of the SASS algorithms 413

13.4 Applications of wind scatterometry 415 13.5 Wave scatterometry 417

Chapter 14 The way forward 14.1 Satellite sensors for oceanography in the future 419

14.1.1 Introduction 419 14.1.2 The ERS-1 payload 420 14.1.3 TOPEX 422

14.2 Developments in data analysis 423 14.3 Trends in ocean science 425 14.4 Conclusion 426

References 428

Index 444