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Satellite Radar Altimetry Ever Sea Ice Satellite Radar Altimetry ever Sea Ice by Seymour William Clarke Laxon A thesis submitted to the University of London for the degree of Doctor of Philosophy Mullard Space Science Laboratory Department of Physics and Astronomy University College London November 1989 1 ProQuest Number: 10630903 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10630903 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 Frontispiece : Antarctic sea ice extent from February 1987 to February 1988 as mapped by the Geosat radar altimeter. 2 ABSTRACT The thesis concerns the analysis and interpretation of data from satellite borne radar altimeters over ice covered ocean surfaces. The applications of radar altimetry are described in detail and consider monitoring global climate change, the role that sea ice plays in the climate system, operational applications and the extension of high precision surface elevation measurements into areas of sea ice. The general nature of sea ice cover is discussed and a list of requirements for sea ice monitoring is provided and the capability of different satellite sensors to satisfy needs is examined. The operation of satellite borne altimeter over non-ocean surfaces is discussed in detail. Theories of radar backscatter over sea ice are described and are used to predict the radar altimeter response to different types of sea ice cover. Methods employed for analysis of altimeter data over sea ice are also described. Data from the Seasat altimeter is examined on a regional and global scale and compared with sea ice climatology. Data from the Geosat altimeter is compared with co-incident imagery from the Advanced Very High Resolution Radiometer and also from airborne Synthetic Aperture Radar. Correlations are observed between the altimeter data and imagery for the ice edge position, zones within the ice cover, new ice and leads, vast floes and the fast ice boundary. An analysis of data collected by the Geosat altimeter over a period of more than two years is used to derive seasonal and inter-annual variations in the total Antarctic sea ice extent. In addition the retrieval of high accuracy elevation measurements over sea ice areas is carried out. These data are used to produce improved maps of sea surface topography over ice- covered ocean and provide evidence of the ability of the altimeter to determine sea ice freeboard directly. In addition the changing freeboard of two giant Antarctic tabular icebergs, as measured by the Geosat altimeter, is presented. As a summary the achievements are reviewed and suggestions are made towards directions for further work on present data sets and for future data from the ERS-1 satellite. 3 PREFACE I would like to acknowledge the contribution of the people who have helped and guided me during my period of study towards the work described in this thesis. I am greatly indebted to my supervisor, Dr. Chris Rapley, for his constant encouragement and guidance both in matters scientific and in the wider aspects of pursuing academic research. I would also like to thank Dr. Neil McIntyre for his encouragement and help in matters glaciological, Dr. Wyn Cudlip for discussions on the operation of the altimeter and for his invaluable efforts with much of the software used at MSSL, and to Dr. Duncan Wingham, for encouraging a physical approach to remote sensing at an important early stage in my study. I would also like to thank all the past and present members of the remote sensing group at MSSL for enabling me to pursue my study in a constantly stimulating environment. I am also grateful to Len Culhane and to the department of Physics and Astronomy at UCL for allowing me to pursue my study at MSSL. I must also acknowledge the contributions of several outside institutions towards the work presented in this thesis. Thanks are due to the Norwegian Polar Institute for allowing me to participate in the Lance cruise, and to the other members of the cruise for making it a very memorable experience. I would also like to express thanks to the workers at the Navy Oceans Research and Development Activity (NORDA) in Mississipi, particularly Dr. Don Johnson, Dr. Jeff Hawkins and Dr. Florence Fetterer, for allowing me to spend time at their institution and to thank Fred Abell and Needer Chase for their efforts in data processing for a very demanding visitor. I would also like to acknowledge the contributions to this work by the Algorithm Development Facility at the Earth Observation Data Centre, Farnborough, for their efforts in providing the Polar Reference Data Set and the Geosat GDR data. The work presented in this thesis is my own, unaided work, except where otherwise acknowledged. 4 TO MY PARENTS LIST OF CONTENTS P a g e TITLE PAGE 1 FRONTISPIECE 2 A B S T R A C T 3 P R E F A C E 4 LIST OF CONTENTS 6 LIST OF FIGURES 14 LIST OF PLATES 18 LIST OF TABLES 19 CHAPTER 1 APPLICATIONS OF SATELLITE RADAR ALTIMETER OBSERVATIONS OVER SEA ICE Ui Introduction 21 1.1 Monitoring global climate change 22 1.1.1 Components of the climate system 22 1.1.2 Global climate models 23 1.1.3 Climate feedback mechanisms 23 1.1.4 The possibility of Global climate change 24 1.1.5 Monitoring Global climate change 26 1.1.6 Sea ice as an indicator of climate change 28 1.2 The roie of sea ice in regional and global meteorology 28 1.2.1 Sea ice-Ocean-Atmosphere interaction 28 1.2.2 Effects of sea ice on the atmosphere 29 1.2.3 Sea ice and global ocean circulation 30 1.2.4 Sea ice data important in determining influences on 32 clim ate 1.3 Sea ice models 32 1.4 Operational applications 34 1.4.1 Requirements for operational sea ice information 34 6 LIST OF CONTENTS (continued) Page 1.4.2 Use of satellite data for ice information and 35 forecasting 1.4.3 Operational uses of data from the Geosat altimeter 36 1.4.4 Operational sea ice products from ERS-1 36 1.4.5 Sea ice parameters for operational applications 36 Polar oceanography and Geophysics 37 1.5.1 Measurements provided by satellite radar altimeters 37 1.5.2 Altimeter measurements of sea surface height 37 1.5.3 Altimeter measurements of ocean waves 38 1.5.4 Extension of altimetry into polar regions 40 1 .6 Summary 40 CHAPTER 2 SEA ICE CHARACTERISTICS AND OBSERVATIONAL TECHNIQUES 2.0 Introduction 43 2.1 Physical characteristics of sea ice 43 2.2 The nature of global sea ice cover 46 2.2.1 Differences in Arctic and Antarctic sea ice cover 46 2.2.2 Zones observed in Arctic seasonal sea ice cover 50 2.2.2.1 Fast ice. 50 2.2.2.2 The shear zone 51 2.2.2.3 The marginal ice zone 51 2.2.3 Zones observed in Antarctic sea ice cover 53 2 Data requirements for sea ice monitoring 54 2.4 Remote sensing of sea ice 55 2.4.1 Visible/Infra-red observations of sea ice 58 2.4.2 Passive microwave observations of sea ice 60 2.4.3 Synthetic Aperture Radar observations of sea 62 ice 2.4.4 Radar altimetry over sea ice 64 2.4,4.1 Analysis of GEOS-3 data 64 7 LIST OF CONTENTS (continued) Page 2.4.4,2 Ice freeboard measurements using satellite altimetry 65 ZAAAI Satellite radar altimeter observations of swell penetration 66 ZAAA Physical interpretation of radar altimeter return waveformsover sea ice 66 2.4.4.5 Radar altimeter observations during MIZEX-84 67 2.4.4.6 Comparions of Radar altimeter and Synthetic Aperture Radar observations of sea ice 69 2.4.4.7 Comparisons of Seasat altimeter data with ice charts 70 2.4.4.8 Comparisons of airborne altimetry with visible imagery 70 2.4.4.9 Operational use of Geosat radar altimeter 71 data over sea ice 2.4.4.10Previous work on altimetry oversea ice - discussion 72 2.5 Satellite instrument capability for providing sea ice 73 param eters 2.6 Summary 74 CHAPTER 3 THE SEASAT ALTIMETER m Introduction 75 3.1 Principles of radar altimetry 75 3.1.1 Return pulse profile 75 3.1.2 Modes of operation 76 3.1.3 Waveform averaging 76 3.2 The Seasat altimeter system 77 3.2.1 Full deramp processing 77 3.2.2 Seasat return waveform sampling 78 3.3 Pulse limited altimeter geometry 79 3.3.1 Range ring geometry 79 3.3.2 Altimeter footprints 80 3.3.2.1 Pulse limited footprint 80 3.3.2.2 Range window footprint 80 3.3.2.3 Beam limited footprint 80 8 LIST OF CONTENTS (continued) Page 3^4 Seasat altimeter on-board processing 81 3.4.1 Seasat control loops 81 3.4.2 AGC loop processing 81 3.4.3 Height loop processing 82 3.5 Processing of non-ocean like waveforms 85 3.5.1 Peaked return AGC loop processing 85 3.5.2 Peaked return height loop processing 86 3.5.3 Telemetry summing 86 3.6 Geometric effects 87 3.6.1 Antenna mispointing 87 3.6.2 Topographic variations 88 3.6.3 Backscatter variations 89 3.7 The ERS-1 altimeter 89 3.8 Summary 90 CHAPTER 4 NORMAL INCIDENCE RADAR BACKSCATTER OVER SEA ICE 4.0 Introduction 92 4.1 Measurement of radar backscatter bv a satellite altimeter 93 4.2 Di-electric properties of sea ice 94 4.3 Reflection from a perfectly flat surface 96 4.4 Reflection from a collection of facets 98 4.4.1 Diffraction theory 98 4.4.2 Phase coherency between different facets 100 4.4.3 A facet model for normal incidence backscatter 101 from first-year
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