Experimental Methods in the Physical Sciences

Volume 47

Optical Radiometryfor Ocean ClimateMeasurements

Edited by

Giuseppe ZibordiInstitute for Environment and Sustainability

Joint Research Centre

Ispra, Italy

Craig J. DonlonEuropean Space Agency/ESTEC

Noordwijk

The Netherlands

Albert C. ParrSpace Dynamics Laboratory, Utah State

University, Logan, UT, USA

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Contents

List of Contributors xvVolumes in Series xviiForeword xxiPreface xxiii

1. Introduction to Optical Radiometry and OceanClimate Measurements from Space

James A. Yoder and B. Carol Johnson

1.1. Ocean Climate and Satellite Optical Radiometry

James A. Yoder, Kenneth S. Casey and Mark D. Dowell

1. Introduction 31.1 Characteristics of a Climate-Observing System 4

2. Global Climate Observing System Requirements for ECVsand CDRs 62.1 Ocean Color Radiometery 72.2 Sea Surface Temperature 8

3. From Essential Climate Variables to Climate Data Records 104. Conclusion 11

References 12

1.2. Principles of Optical Radiometry and MeasurementUncertainty

B. Carol Johnson, Howard Yoon, Joseph P. Rice and Albert C. Parr

1. Basics of Radiometry 141.1 Introduction 141.2 Radiance 171.3 Irradiance 211.4 Reflectance 231.5 Distance and Aperture Areas in Radiometry 28

2. Radiometric Standards and Scale Realizations 302.1 Sources 302.2 Radiometers 38

v

3. The Measurement Equation 423.1 Background and a Review of the Concepts 423.2 Measurement Equation Examples 463.3 Uncertainty in Ocean Color Measurements 57

4. Summary 61Acknowledgments 62References 62

2. Satellite Radiometry

Charles R. McClain and Peter J. Minnett

2.1. Satellite Ocean Color Sensor Design Conceptsand Performance Requirements

Charles R. McClain, Gerhard Meister and Bryan Monosmith

1. Introduction 742. Ocean Color Measurement Fundamentals and Related

Science Objectives 753. Evolution of Science Objectives and Sensor Requirements 804. Performance Parameters and Specifications 84

4.1 Spectral Coverage and Dynamic Range 844.2 Coverage and Spatial Resolution 864.3 Radiometric Uncertainty 874.4 SNR and Quantization 894.5 Polarization 904.6 Additional Characterization Requirements 914.7 On-Board Calibration Systems 92

5. Sensor Engineering 935.1 Basic Sensor Designs: Whiskbroom and Pushbroom 955.2 Design Fundamentals and Radiometric Equations 965.3 Performance Considerations 995.4 Sensor Implementation 104

6. Summary 107Acronyms 108Symbols and Dimensions 109

7. Appendix. Historical Sensors 1097.1 CZCS and OCTS 1107.2 SeaWiFS 1117.3 MODIS 1137.4 MERIS 115References 116

vi Contents

2.2. On Orbit Calibration of Ocean Color ReflectiveSolar Bands

Robert E. Eplee, Jr and Sean W. Bailey

1. Introduction 1212. Solar Calibration 124

2.1 SD Degradation 1252.2 SD Radiometric Response Trends 1262.3 SNR on Orbit 1282.4 Uncertainties in the Solar Calibration Data 128

3. Lunar Calibrations 1283.1 ROLO Photometric Model of the Moon 1293.2 Lunar Radiometric Response Trending 1303.3 Uncertainties in Lunar Calibration 1313.4 Lunar Calibration Intercomparisons 133

4. Spectral Calibration of Grating Instruments 1355. Vicarious Calibration 137

5.1 NIR/SWIR Band Calibration 1395.2 Visible Band Calibration 1405.3 Alternative Approaches 142

6. On-orbit Calibration Uncertainties 1426.1 Accuracy 1436.2 Long-term Stability of the TOA Radiances 1436.3 Precision of the TOA Radiances 1446.4 Combined Uncertainty Assessment 144

7. Comparison of Uncertainties Across Instruments 1458. Summary of On-orbit Calibration 149

References 150

2.3. Thermal Infrared Satellite Radiometers: Designand Prelaunch Characterization

David L. Smith

1. Introduction 1542. Radiometer Design Principles 155

2.1 Performance Model 1592.2 Signal to Noise 160

3. Remote Sensing Systems 1613.1 Along Track Scanning Radiometers (ATSR) 1613.2 Sea and Land Surface Temperature Radiometer (SLSTR) 1643.3 Advanced Very High Resolution Radiometer (AVHRR) 1653.4 MOderate Resolution Imaging Spectroradiometer

(MODIS) 1663.5 Visible Infrared Imaging Suite (VIIRS) 1673.6 Spinning Enhanced Visible and Infrared Imager (SEVIRI) 171

Contents vii

4. Calibration Model 1724.1 Radiometric Noise 1744.2 Nonlinearity 1744.3 Offset Variations 176

5. On-Board Calibration 1765.1 Calibration Sources 178

6. Pre-launch Characterization and Calibration 1826.1 Blackbody Calibration 1826.2 Instrument Radiometric Calibration 184

7. Conclusions 197References 198

2.4. Postlaunch Calibration and Stability: ThermalInfrared Satellite Radiometers

Peter J. Minnett and David L. Smith

1. Introduction 2012. On-Board Calibration 203

2.1 (A)ATSR Radiometric Calibration 2042.2 AVHRR Calibration 2092.3 MODIS and VIIRS Radiometric Calibration 2132.4 MODIS Spectroradiometric Calibration Assembly for

On-Orbit Stability 2142.5 MODIS Mirror Response versus Scan Angle 216

3. Comparisons with Reference Satellite Sensors 2183.1 Spatial Comparisons 2193.2 Temporal Comparisons 2203.3 Simultaneous Nadir Overpasses 2223.4 Instruments on the Same Satellite 223

4. Validating Geophysical Retrievals 2254.1 Cloud Screening 2294.2 Atmospheric Correction Algorithm 2304.3 Geophysical Validation 2324.4 Ship-Board Radiometers 236

5. Discussion 2376. Conclusions 239

References 239

3. In Situ Optical Radiometry

Craig J. Donlon and Giuseppe Zibordi

3.1. In situ Optical Radiometry in the Visible and NearInfrared

Giuseppe Zibordi and Kenneth J. Voss

1. Introduction and History 2482. Field Radiometer Systems 249

viii Contents

2.1 General Classification: Multispectral and Hyperspectral 2492.2 Irradiance Sensors 2502.3 Basic Radiance Sensors 252

3. System Calibration 2543.1 Linearity Response 2553.2 Temperature Response 2553.3 Polarization Sensitivity 2563.4 Stray Light Perturbations 2573.5 Spectral Response 2573.6 Angular Response of Irradiance Sensors 2583.7 Rolloff of Imaging Systems 2603.8 Immersion Effects 2603.9 Absolute Response 263

4. Measurement Methods 2644.1 In Water Systems 2654.2 Above Water Systems 2674.3 Radiometric Data Products 268

5. Errors and Uncertainty Estimates 2735.1 Calibration Specific Sources of Uncertainties 2745.2 Instrument Specific Sources of Uncertainties 2765.3 Methods and Field Specific Sources of Uncertainties 2775.4 Examples of Uncertainty Budget for Radiometric

Products 2826. Applications 285

6.1 Sky and Sea Radiance Distribution 2856.2 In-water Light Field Polarization 2876.3 Bio-Optical Models 2896.4 Validation of Satellite Radiometric Products 2916.5 In situ Data and System Vicarious Calibration 293

7. Summary and Outlook 294References 295

3.2. Ship-Borne Thermal Infrared Radiometer Systems

Craig J. Donlon, Peter J. Minnett, Andrew Jessup, Ian Barton,William Emery, Simon Hook, Werenfrid Wimmer,Timothy J. Nightingale, and Christopher Zappa

1. Introduction and Background 3062. TIR Measurement Theory 311

2.1 General Considerations 3112.2 SSTskin Ship-Borne Radiometer Measurement Challenges 3172.3 Practical Measurement of SSTskin from a Ship-Borne

Radiometer 3203. TIR Field Radiometer Design 321

3.1 TIR Detectors 3283.2 TIR Radiometer Spectral Definition 3363.3 Beam Shaping and Steering 3413.4 Thermal Control System 350

Contents ix

3.5 An Environmental System to Protect and ThermallyStabilize the Radiometer 351

3.6 Instrument Control and Data Acquisition 3533.7 A Calibration System 3543.8 Summary 3613.9 Additional Comments 363

4. Examples of FRM Ship-Borne TIR Radiometer Designand Deployments 3634.1 The DAR-011 Filter Radiometer 3634.2 The SISTeR Filter Radiometer 3644.3 NASA JPL NNR 3684.4 The Calibrated Infrared In situ Measurement System 3714.5 ISARdQuasi Operational Ocean Field Radiometers 3754.6 Use of Unmanned Airborne Vehicles BESST Radiometer 3804.7 Spectroradiometers 3824.8 Derivation of Air Temperature Using a Spectroradiometer 3874.9 TIR Cameras 389

5. Future Directions 3936. Conclusions 395

Acknowledgments 395References 395

4. Theoretical Investigations

Barbara Bulgarelli, Menghua Wang and Christopher J. Merchant

4.1. Simulation of In Situ Visible RadiometricMeasurements

Barbara Bulgarelli and Davide D’Alimonte

1. Overview 4072. The RTE and Its Solution Methods 408

2.1 The Radiative Transfer Equation 4082.2 Deterministic Solutions of the RTE 4102.3 Monte Carlo Solutions of the RTE 410

3. Simulations of In Situ Radiometric MeasurementPerturbations 4133.1 Overstructure Perturbations 4143.2 Perturbations Induced by Sea-Surface Waves 429

4. Summary and Remarks 441References 442

4.2. Simulation of Satellite Visible, Near-Infrared,and Shortwave-Infrared Measurements

Menghua Wang

1. Introduction 4522. OceaneAtmospheric System 4553. Simulations 457

x Contents

3.1 Ocean Radiance Contributions 4573.2 The TOA Atmospheric Path Radiance Contributions 4643.3 Atmospheric Diffuse Transmittance 4703.4 Simulated and Satellite-Measured TOA Radiances 471

4. Summary 478Disclaimer 479References 479

4.3. Simulation and Inversion of Satellite ThermalMeasurements

Christopher J. Merchant and Owen Embury

1. Introduction 4892. Radiative Transfer Simulation for Thermal Remote Sensing 4903. Propagation of Thermal Radiation through Clear Sky 4934. Simulation of Interaction with Aerosol and Cloud 5005. Simulation of Surface Emission and Reflection 5026. Use of Simulations in Thermal Image Classification

(Cloud Detection) 5047. Use of Simulations in Geophysical Inversion (Retrieval) 5098. Use of Simulations in Uncertainty Estimation 5169. Conclusion 521

References 523

5. In Situ Measurement Strategies

Giuseppe Zibordi and Craig J. Donlon

5.1. Requirements and Strategies for In situ Radiometry inSupport of Satellite Ocean Color

Giuseppe Zibordi and Kenneth J. Voss

1. Introduction 5322. Overview of Past and Current Field-Related Radiometric

Activities 5332.1 Field Measurements 5332.2 Intercomparisons 5382.3 Data Repositories 542

3. Requirements and Strategies for Future Satellite Ocean-ColorMissions 5433.1 Field Measurements for System Vicarious Calibration 5443.2 Field Measurements for the Validation of Satellite Data

Products 5463.3 Field Measurements for Bio-Optical Modeling 5473.4 Protocols Revision and Consolidation 5473.5 Calibration and Characterization of Field Radiometers 5473.6 Data Reduction, Quality Control, and (re)Processing 548

Contents xi

3.7 Accuracy Tailored to Applications 5493.8 Archival and Access 5493.9 Intercomparisons to Secure Accuracy and Best

Practice 5493.10 Standardization and Networking 5503.11 Development and Implementation 551

4. Summary and Way Forward 551References 552

5.2. Strategies for the Laboratory and Field Deploymentof Ship-Borne Fiducial Reference Thermal InfraredRadiometers in Support of Satellite-Derived SeaSurface Temperature Climate Data Records

Craig J. Donlon, Peter J. Minnett, Nigel Fox, and Werenfrid Wimmer

1. Introduction 5582. Fiducial Reference Measurements for SST CDRs

and Uncertainty Budgets 5592.1 FRM TIR Ship-Borne Radiometer Network 5622.2 The Importance of Uncertainty Budgets 563

3. Laboratory Intercalibration Experiments for FRMShip-Borne Radiometers 585

4. Ship-Borne Radiometer Field Intercomparison Exercises 5905. Protocols to Maintain the SI Traceability of FRM Ship-Borne

TIR Radiometers for Satellite SST Validation 5955.1 Definition of Measurement Methodology 5955.2 Definition of Laboratory Calibration and Verification

Methodology and Procedures 5955.3 Predeployment Calibration Verification 5965.4 Postdeployment Calibration Verification 5965.5 Uncertainty Budgets 5965.6 Improving Traceability of Calibration and Verification

Measurements 5965.7 Accessibility to Documentation 5975.8 Archiving of Data 5975.9 Periodic Consolidation and Update of Calibration

and Verification Procedures 5986. Summary and Future Perspectives 598

Acknowledgments 598References 599

6. Assessment of Satellite Products for ClimateApplications

Frederic Melin and Gary K. Corlett

xii Contents

6.1. Assessment of Satellite Ocean Colour Radiometryand Derived Geophysical Products

Frederic Melin and Bryan A. Franz

1. Introduction 6092. Validation of Satellite Products 610

2.1 Validation Protocol 6102.2 Validation Metrics 6122.3 Analysis of Validation Results 6142.4 Model-Based Approaches to Uncertainty Analysis

and Error Propagation 6183. Comparison of Cross-Mission Data Products 621

3.1 Band Shift Correction 6223.2 Point-by-Point Comparison 6243.3 Analysis of Time Series 6263.4 Climate Signal Analysis 628

4. Conclusions 631Acknowledgments 632References 632

6.2. Assessment of Long-Term Satellite Derived SeaSurface Temperature Records

Gary K. Corlett, Christopher J. Merchant, Peter J. Minnettand Craig J. Donlon

1. Introduction 6392. Background 640

2.1 Assessment of Top of Atmosphere BrightnessTemperatures 641

2.2 Validation Uncertainty Budget 6432.3 Reference Data Sources 647

3. Assessment of Long-Term SST Datasets 6493.1 Example 1: Long-Term SST Data Record Assessment 6523.2 Example 2: Long-Term Component Assessment 6543.3 Quantitative Metrics 6573.4 Demonstrating Traceability to SI 6593.5 Stability 6633.6 Validation of Uncertainties 669

4. Summary and Recommendations 673References 674

Index 679

Contents xiii

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List of Contributors

Sean W. Bailey, Ocean Biology Processing Group, NASA Goddard Space FlightCenter, Greenbelt, MD, USA; FutureTech Corporation, Greenbelt, MD, USA

Ian Barton, CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia

Barbara Bulgarelli, European Commission, Joint Research Centre, Ispra, Italy

Kenneth S. Casey, NOAA Oceanographic Data Center, Silver Spring, MD, USA

Gary K. Corlett, Department of Physics and Astronomy, University of Leicester,Leicester, UK

Davide D’Alimonte, Centre for Marine and Environmental Research, University ofAlgarve, Faro, Portugal

Craig J. Donlon, European Space Agency/ESTEC, Noordwijk, The Netherlands

Mark D. Dowell, European Commission, Joint Research Centre, Ispra, Varese, Italy

Owen Embury, Department of Meteorology, University of Reading, Reading, UK

William Emery, Aerospace Engineering Sciences Department, University of Colorado,Boulder, CO, USA

Robert E. Eplee, Jr, Ocean Biology Processing Group, NASA Goddard Space FlightCenter, Greenbelt, MD, USA; Science Applications International Corporation,Beltsville, MD, USA

Nigel Fox, National Physical Laboratory (NPL), Teddington, Middlesex, UK

Bryan A. Franz, NASA, Goddard Space Flight Center, Greenbelt, MD, USA

Simon Hook, NASA Jet Propulsion Laboratory, California Institute of Technology,Pasadena, CA, USA

Andrew Jessup, Applied Physics Laboratory, University of Washington, Seattle, WA,USA

B. Carol Johnson, Sensor Science Division, National Institute of Standards andTechnology, Gaithersburg, MD, USA

Charles R. McClain, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Gerhard Meister, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Frederic Melin, European Commission, Joint Research Centre, Ispra, Italy

Christopher J. Merchant, Department of Meteorology, University of Reading,Reading, UK

Peter J. Minnett,Meteorology & Physical Oceanography, Rosenstiel School of Marineand Atmospheric Science, University of Miami, Miami, FL, USA

xv

Bryan Monosmith, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Timothy J. Nightingale, RAL Space STFC Rutherford Appleton Laboratory, Harwell,Oxford, Didcot, UK

Albert C. Parr, Sensor Science Division, National Institute of Standards andTechnology, Gaithersburg, MD, USA; Space Dynamics Laboratory, Utah StateUniversity, Logan, UT, USA

Joseph P. Rice, Sensor Science Division, National Institute of Standards andTechnology, Gaithersburg, MD, USA

David L. Smith, RAL Space, Science and Technologies Facilities Council, HarwellOxford, Oxford, UK

Kenneth J. Voss, Physics Department, University of Miami, Coral Gables, FL, USA

Menghua Wang, NOAA Center for Satellite Applications and Research, College Park,Maryland, USA

Werenfrid Wimmer, Ocean and Earth Science, University of Southampton, EuropeanWay, Southampton, UK

James A. Yoder, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

Howard Yoon, Sensor Science Division, National Institute of Standards andTechnology, Gaithersburg, MD, USA

Christopher Zappa, Ocean and Climate Physics Division, Lamont-Doherty EarthObservatory of Columbia University, Palisades, NY, USA

Giuseppe Zibordi, European Commission, Joint Research Centre, Ispra, Italy

xvi List of Contributors

Foreword

The view of the Earth from space has become an icon of our time. First seenthrough the spectacular photographs taken by the Apollo astronauts, it showedus the Earth, which had seemed limitless to our ancestors, to be small andfragile, a vulnerable oasis for life in the vast vacuum of space. If no otherbenefit had ever come from the space age, those pictures alone would havejustified the effort to leave the Earth, for they changed our view of the planetforever.

But those photographs, it turned out, were just the beginning of what canbe learned by looking down on the Earth from space. Only from the vantagepoint in orbit above the planet can we really get the whole picturedseeing far enough to give a truly global view, but also with sufficient detail toget down to the local scale. Since the time of the early satellites, the numberand sophistication of remote sensing measurements has grown hugely, so thatwe now have a nearly continuous view of the Earth from space that is highlyresolved in area, time, and wavelength. Terabytes of data now flood down fromour satellites, documenting the view of Earth from space in unprecedenteddetail. If only we can make sense of it all, it offers the chance to understandour home planet as never before, allowing us to see how every locality fits intothe whole picture. For the oceans in particular, this is a transformative view,because over large areas they are only rarely visited by people or instrumentsto make in situ observations. Much of our uncertainty over prediction ofseasonal and longer term changes originates in this ignorance of the oceans,which are the main storage for heat in the climate system and the site of halfthe world’s biological productivity.

This book describes the latest knowledge and techniques in visible andinfrared radiometry from satellites. These regions of the electromagneticspectrum can be used to give important information about several aspects ofthe oceans: the infrared observations can be used to measure sea surfacetemperature, which is a fundamental variable needed for climate and weatherprediction studies. Visible measurements characterize ocean color, from whichwe can derive estimates of chlorophyll and other pigments to enable charac-terization of the plankton community. The plankton are in turn the base of theocean food chain and play important roles in the Earth’s carbon cycle, both inthe rapid changes occurring today as a result of human activitiesdclimatechange and ocean acidificationdand over the longer term for in maintaining ahabitable planet.

xxi

As the contributions here illustrate, making sense of the flood of data fromsatellites is no easy task: it requires meticulous attention to detail. The sensorsmust be continually calibrated and the data validated, so that long-termrecords, constructed over time from successive instruments, can be relied on tobe free from drift. This is of critical importance for studies of climate change,where any long-term change in temperature must be carefully separated frominstrumental effects. To achieve this kind of reliability requires continuous andextended free exchange and cooperation between all those involveddfrom thedesigners and engineers who build the sensors, those interpreting the data,and researchers making in situ observations who provide the ground truth.However, there is a rich return on this effort for our civilization as a whole, forfrom it we can understand our home planet as never before.

Andrew WatsonUniversity of Exeter 27th July 2014

xxii Foreword

Preface

Climate change science relies on the combined use of models and measure-ments to advance understanding of climate fluctuations and trends, and ulti-mately to formulate predictions. Gathering measurements for climate changeinvestigations requires well-characterized observing systems and the imple-mentation of strategies to detect decadal variations that are much smaller thanthose occurring at daily or interannual scales. This requirement imposes thecollection of uninterrupted time series of highly accurate measurementstraceable to accepted international standards that collectively constitute theevidence baseline for climate research.

Satellite systems provide a quasi-synoptic global sampling dimension ofclimate data measured using a variety of instruments operated over the Earth’ssurface. Like any observing system devoted to the generation of climate-quality data records, space-based instruments supporting climate change in-vestigations need to deliver continuous highly accurate measurements withdefined uncertainties. This imposes lifetime calibration and validationprocesses for each component of the end-to-end observing system and for thederived data products.

During the last few decades, several space missions have been designed tosupport ocean climate studies through measurement of physical, biological,and chemical variables. Among the various remote sensing technologies,optical sensors operating in the visible, near-infrared, and thermal infraredspectrum are well suited to measure variables such as sea surface temperatureand water leaving radiance at timescales varying from hours to days andgeographical-scales from tens of meters to kilometers. While the sea surfacetemperature has relevance for the heat, gas, and momentum coupling betweenthe atmosphere and ocean, reconstruction of patterns associated withdynamical processes such as surface currents, eddies, and upwelling, thewater-leaving radiance in the visible spectral region is fundamental for thequantification of optically significant seawater constituents, including phyto-plankton biomass, that play a major role in the Earth’s carbon cycle.

Optical remote sensing technologies used to generate climate-quality datarecords share the need for thorough prelaunch characterization and absolutecalibration of the satellite radiometer. These activities are then followed by thepostlaunch monitoring of the radiometer stability over the mission lifetime, thecontinuous assessment of data product quality, and finally, successive

xxiii

reanalysis and reprocessing of all data in conjunction with better under-standing of error sources. The postlaunch activities largely rely on in situreference measurements for the development and assessment of the algorithmsand methods applied to determine each climate variable, and successively forthe continuous validation of derived satellite products. Furthermore, referencemeasurements are required to homogenize climate data records obtained frommultiple or successive satellite instruments. Because of this, advances inremote sensing optical technology demand progress to deliver in situ referenceinstrumentation, measurement methods, and field strategies. Such progressembraces the design of increasingly precise and stable field optical radiome-ters, the improvement of laboratory techniques for their characterization andabsolute calibration, the assessment of measurement methods and fieldintercomparison strategies, and finally, advances in the creation and handlingof data repositories.

This book, through a number of contributions from various authors, pre-sents the state of the art for optical remote sensing and shows how it can beapplied for the generation of marine climate-quality data products. The variouschapters are grouped into six thematic parts each introduced by a brief over-view. The different parts include: (1) requirements for the generation ofclimate data records from satellite ocean measurements and the basic radi-ometry principles addressing terminology, standards, measurement equation,and uncertainties; (2) satellite visible and thermal infrared radiometryembracing instrument design, characterization, and pre- and postlaunchcalibration; (3) in situ visible and thermal infrared reference radiometryincluding overviews on basic principles, technology, and measurementmethods required to support satellite missions devoted to climate change in-vestigations; (4) computer model simulations as fundamental tools to supportinterpretation and analysis of both in situ and satellite radiometric measure-ments; (5) strategies for in situ reference radiometry to satisfy missionrequirements for the generation of climate data records; and finally, (6)methods for the assessment of satellite data products.

The expectation of the editors is that this book will become a workingtool, as either a reference text or as background literature for discussions, forstudents and scientists interested in ocean climate studies and satelliteradiometry.

Giuseppe ZibordiCraig J. DonlonAlbert C. Parr

xxiv Preface