WIDE-ANGLE INFRARED CLOUD IMAGING FOR CLOUD COVER STATISTICS by Paul Winston Nugent A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering MONTANA STATE UNIVERSITY Bozeman, Montana January, 2008 © COPYRIGHT by Paul Winston Nugent 2007 All Rights Reserved - ii - APPROVAL of a thesis submitted by Paul Winston Nugent This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citation, bibliographic style, and consistency, and is ready for submission to the Division of Graduate Education. Dr. Joseph A. Shaw Approved for the Department of Electrical and Computer Engineering Dr. Robert C. Maher Approved for the Division of Graduate Education Dr. Carl A. Fox - iii - STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. If I have indicated my intention to copyright this thesis by including a copyright notice page, copying is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Requests for permission for extended quotation from or reproduction of this thesis in whole or in parts may be granted only by the copyright holder. Paul Winston Nugent January 2008 - iv - ACKNOWLEDGMENTS The completion of this thesis has been made possible by the contributions of many groups. First, I’d like to acknowledge those who contributed to the funding of this project. The NASA Stennis Space Center which provided a GSRP fellowship allowing me to pursue the research described in this thesis, and the NASA Jet Propulsion Laboratory (JPL) for the contribution of equipment and monetary funding. Also I’d like to acknowledge the Montana State University Space Science Engineering Laboratory (SSEL) and the Department of Civil Engineering, both who graciously allowed for the use of their environmental chambers. I’d also like to acknowledge those who contributed to this project in other ways, no less important than funding to the completion of this project. Many of these people are the fellow members of the Montana State University Optical Remote Sensor Laboratory (ORSL) who have helped contribute to this project and my thesis in numerous ways. Either by helping run experiments, often late at night, or by putting up with the messes I often made in the lab. I’d like to acknowledge my advisor Dr. Joseph Shaw whose guidance helped me understand many of the difficult problems addressed in this thesis. I’d also like to acknowledge my parents, Terry and Jody Nugent, and many of my close friends, too many to name here. These people helped encourage me through the processes which eventually lead to the completion of this project. Their contribution cannot be adequately described with words. - v - TABLE OF CONTENTS 1. INTRODUCTION .........................................................................................................1 Motivations for Cloud Cover Studies............................................................................2 Climate and Atmospheric Research............................................................................3 Earth-space Optical Communication..........................................................................4 Requirements of an Instrument to Fit These Applications.........................................6 Methods of Cloud Detection..........................................................................................7 Passive Detection Systems..........................................................................................7 Active Detection Systems...........................................................................................9 Ground-Based Thermal Imaging for Cloud Detection.................................................10 The Infrared Cloud Imager.......................................................................................12 The Second-Generation Infrared Cloud Imager.......................................................15 2. EXTENSION OF THE ICI METHOD FOR CLOUD DETECTION TO WIDE FIELD-OF-VIEW CAMERAS...................................................................................19 Atmospheric Effects in the Long-Wave Infrared.........................................................19 Atmospheric MODTRAN Simulations....................................................................20 Need for a Radiometric Calibration and Water Vapor Measurements.....................23 The Cloud Data Processing Routine.........................................................................25 Water Vapor Radiance Correction............................................................................26 Methods of Precipitable Water Vapor Measurements..............................................28 Need for Camera-Specific Water Vapor Corrections...............................................30 The Photon Core Camera.............................................................................................33 Bandwidth of the 50° fov Photon Camera................................................................34 Bandwidth of the 100° fov Photon Camera..............................................................35 Mapping Each Camera's Per-Pixel Field of View....................................................36 Camera-Specific Data Processing and Cloud Detection..............................................39 Extended Water Vapor Correction...........................................................................40 Angle-Dependent Water Vapor Correction..............................................................42 Threshold-Based Cloud Detection with Simulated Data..........................................45 3. PHOTON CORE CAMERA CALIBRATION............................................................49 Introduction to Radiometric Calibration......................................................................49 The Camera Radiance to Digital Number Function.................................................52 Calibration for the First-Generation ICI Systems.....................................................53 Calibration for the Second-Generation ICI Systems................................................55 FPA Temperature-Dependent Data Drift.....................................................................56 Temperature-Dependent Camera Function...............................................................57 Data Error Related to Camera FPA Temperature.....................................................58 - vi - TABLE OF CONTENTS - CONTINUED Initial Corrections of the FPA Temperature-Dependent Drift..................................60 Extending the Correction to Multiple Scene Temperatures......................................66 Testing the Correction..............................................................................................73 FPA-Temperature Correction for the 100° fov Camera...........................................76 Stability of the 100° fov Camera..........................................................................76 FPA-Temperature Correction for the 100° fov Camera.......................................78 Testing the 100° fov Camera FPA Temperature Correction................................80 Stability of the FPA Correction over Time...............................................................82 Laboratory-Based Gain Calibrations...........................................................................83 Internal-shutter-based Calibrations..............................................................................87 Completed Calibration of the Camera..........................................................................93 Radiometric Accuracy of the Calibration.................................................................94 4. EXPERIMENTS USING THE PROTOTYPE SYSTEM...........................................98 Summer 2006 Experiments in Bozeman MT...............................................................98 Blackbody-Based Calibration...................................................................................99 Internal Shutter-Based Calibration.........................................................................103 Deployment at Table Mountain.................................................................................107 Fall 2007 Deployment at Bozeman MT.....................................................................109 Cloud Type Detection.............................................................................................112 Second-Generation ICI Camera Comparisons........................................................114 Bandwidth Difference Problems.........................................................................115 Comparisons of the Second-Generation ICI Systems.........................................116 Clear-Sky Comparisons with Theory..................................................................118 Loss of Calibration on the 50° fov System.........................................................119 Data Processing and Results...................................................................................121 Spatial Cloud Detection Comparisons................................................................122 Temporal Cloud Amount Comparisons..............................................................126 Radiative Forcing Measurements.......................................................................129 Applications for these Data.....................................................................................130 5. CONCLUSIONS........................................................................................................132 Contributions of This Project.....................................................................................133 Climate-Related Cloud Detection...........................................................................134 Earth-Space Optical Communication.....................................................................135 Calibration of Infrared Imaging Systems................................................................136 Quality of the Calibration..........................................................................................136 Types of Clouds that can be Observed...................................................................137 Proposed Future Work...............................................................................................138 - vii - TABLE OF CONTENTS - CONTINUED FOV-dependent Atmosphere and Cloud Emission.................................................138 Characterization of the Enclosure Window............................................................139 Sun and Moon Error Removal................................................................................140 Extended Deployment in a Harsh Environment.....................................................140 Pending Deployment at TMF.................................................................................141 REFERENCES CITED...................................................................................................142 - viii - LIST OF TABLES Table Page 2-1: Integrated Radiance for Clear and Cloudy Atmospheres....................................23 3-1: Digital Number for a Constant Source and Variable Camera Temperature.........59 3-2: Digital Number for a Constant Source and Constant Camera Temperature.........60 3-3: Variation in Digital Number Before and After Correction...................................66 3-4: Slope of the Linear Fit for Different Scene Temperatures...................................67 3-4: Variation in Digital Number Before and After Correction...................................73 3-5: Digital Number for a Constant Source and Constant Camera Temperature.........77 3-6: 100° fov Variation in Digital Number Before and After Correction....................79 3-7: Changes in the 50° fov camera 2006 and 2007 FPA temperature corrections.............................................................................................................83 3-8: Calibration Coefficients for the summer 2006 experiments.................................84 3-9: Constant Calibration versus Constant Gain: RMS Calibration Errors..................85 4-1 : Camera-A simulated cloud radiance...................................................................113 4-2: Camera-B simulated cloud radiance...................................................................113 4-3: Camera A Multi-level thresholds used for cloud type detection........................114 4-4: Camera B Multi-level thresholds used for cloud type detection.........................114 4-5: Mean cloudiness on 10-04-2007.........................................................................127 - ix - LIST OF FIGURES Table Page 1-1. The original ICI during a deployment at Poker Flat Research Range, AK. The ICI camera and calibration equipment are inside the white housing in the foreground. The instruments in the background are other instruments at the site operated by other groups. (Photo by J. Shaw)......................................13 2-1. MODTRAN-simulated downwelling atmospheric emission spectrum for three different cloud free atmospheres: (bottom) zero water vapor, (middle) 1976 U.S. Standard profile, and (top) twice the standard water vapor profile..........................................................................................................20 2-2. MODTRAN-simulated transmittance over the 7 µm to 15 µm band, using the 1976 model atmosphere with a zenith path to space.......................................21 2-3. MODTRAN-simulated downwelling emission over the 7 µm to 15 µm band, for the 1976 U.S. Standard Atmosphere model. The plot contains a clear sky (bottom) and three cloud types: cirrus at 9km (red), alto-stratus at 2.4km (green), and cumulus at 0.066km (teal).................................................22 2-4. Flow diagram of the processing algorithm used for cloud detection in the original ICI. This processing method only allowed for the detection of cloud presence, and not cloud type.......................................................................25 2-5. For an upward-viewing thermal camera the increasing atmospheric path length with increasing zenith angle causes an increase in perceived radiance.................................................................................................................31 2-6. The downwelling radiance increases with zenith angle. The majority of the change occurs in the 8 µm to 13 µm region, showing the increase in perceived water vapor with increasing angle........................................................31 2-7. Band-integrated radiance plotted versus zenith angle..........................................32 2-8. The Photon Core thermal cameras built by Indigo Systems: (left) the 50° fov camera, and (right) the 100° fov camera........................................................33 2-9. Optical bandwidth of a typical Photon Camera. The major response lies within the atmospheric water vapor window, over which water in the atmosphere is the primary emission source..........................................................34