Assessment and modeling of angular backscattering variation in ALOS ScanSAR images over ...

116 Pages·2008·4.79 MB·English

Assessment and modeling of angular
backscattering variation in ALOS ScanSAR
images over tropical forest areas

Juan Pablo Ardila Lopez
February, 2008

Assessment and modeling of angular backscattering
variation in ALOS ScanSAR images over tropical forest
areas

by

Juan Pablo Ardila Lopez

Thesis submitted to the International Institute for Geo-information Science and Earth Observation in
partial fulfilment of the requirements for the degree of Master of Science in Geo-information Science
and Earth Observation, Specialisation: Geoinformatics.

Thesis Assessment Board

Chairman: Prof.Dr. A. Stein
External examiner: Dr. Y. A. Hussin
First supervisor: Dr. V.A. Tolpekin
Second supervisor: Ms. Dr. Ir. W. Bijker

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION
ENSCHEDE, THE NETHERLANDS
Disclaimer

This document describes work undertaken as part of a programme of study at the International
Institute for Geo-information Science and Earth Observation. All views and opinions expressed
therein remain the sole responsibility of the author, and do not necessarily represent those of the
institute.
Abstract
The use of radar images provides the most effective and timely alternative to monitor forest changes
and map land cover types in tropical forest areas. Radar images, captured in the microwave range of
the spectrum are weather and sun illumination independent, two factors which usually inhibit the use
of optical satellite data. Likewise, radar acquisition in the low frequencies can be effectively used to
study the physical structure of the forest, which is highly relevant for forestry applications.
Nevertheless, some radar issues should still be addressed for a successful implementation of radar in
those areas. One of these fundamental issues is the backscattering variation in terms of the incident
angle. Although this has been investigated before, it has been done to a great extent in simple land
covers and in specific conditions. The incident angle variation is a rather complicated phenomenon
when studied for tropical forests of complex physical structure and with seasonal floods. Besides, if
these areas are investigated under large L band penetration microwaves it is even more difficult to
assess.
This research focuses on analyzing radar brightness dependency of one of the most promising SAR
products for tropical monitoring systems: L band ScanSAR images. In particular, the radar brightness
dependency on incident angle is studied under different viewing geometry observations. ScanSAR
products, due to wide swath length extension, offer a high variation in range of observed incident
angles. Three ScanSAR images of Colombian tropical forest with incident angle variations of 25
degrees are analyzed. The radar backscattering behaviour in terms of incident angle is studied with the
support of spatial information such as digital elevation model, optical satellite data, ecosystems map
and existing land cover classifications. The study of angular backscattering variation is investigated by
image and for different land cover types. The presented results confirm an effect of incident angle in
radar backscattering for the studied images. This effect depends on land cover type, moisture content
and the physical structure of the reflecting targets.
After incident angle effect is measured two approaches are followed for the consideration of the
angular backscattering variation. The first one normalizes and corrects radiometrically the
backscattering values by means of a cosine correction to the power index n. A second approach seeks
to understand and explain backscattering variation by means of a semi empirical backscattering model.
The model applied is an extended version of the water cloud model considering second order
backscattering interactions. A successful implementation of the model explains fairly well the
observed backscattering of flooded forest areas with R2 larger than 0.78 while it does not provide good
predictions for very dense forest class.
Keywords: rain forest, incident angle, incidence angle, ALOS PalSAR, backscattering modelling,
water cloud model, rain forest.
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Acknowledgements
The research presented in this document was carried out at the International Institute for Geo-
Information Science and Earth Observation (ITC) in the Netherlands, with the support of the
Fellowship Programme of the Nuffic Organization. To these institutions and all their staff I am deeply
indebted for making this possible. I also want to extend my gratitude to the SIMCI-UNODC Project in
Colombia, The Humboldt Institute of Colombia, Alaska ALOS-ASF node, and M. Sousa Jr. from
INPE- Brazil for all the information they kindly provided for this research and for their rapid support.
I want to express my thanks to the assessment board committee of this thesis for taking the time to
read this document and making sense of the ideas and statements proposed here.
This research is in good part the result of the efforts and discussions with my two supervisors: Dr.
Tolpekin and Dr. Bijker. I express my most sincere appreciation for all the time they dedicated to me
during the last six months and for all their advice and orientation when I was doing research. Although
some popular beliefs about the Europeans being cold they proved this wrong. I was always
overwhelmed by their encouragement and the confidence they granted me.
I also thank Dr. Yousif Hussin for his constant enthusiasm and motivation during the Advanced RS
module and the first stages of this research. Special thanks to Marcela Quinones through the distance,
for all her attention and consideration with me. For all her expert contributions in the radar field and
for inspiring me with her love for our ever sacred tropical forests in Colombia and for her very good
use of technology to assist me from Wageningen. I hope she recovers her health soon.
I extend my most sincere thanks to Dr. Rodolfo Llinas, head of the SIMCI-UNODC project who trust
me and encouraged me when I just started applying my knowledge at the beginning of my career. All
that I learnt during my time at SIMCI has contributed enormously to this research. Likewise I have to
thank all the SIMCI project staff, Orlando for being my second father, Sandra for her messages,
Martha Paredes and JP Latorre for their advice, friendship and for encouraging me to do the MSc,
Maria Ximena for making me feel home through the telephone, Oscar for his time and friendship, and
Leonardo for all his deep thoughts. Silva Lauffer, thanks for being my family in Europe.
Looking back in time I am equally grateful to all my colleagues and former colleagues at ITC. For
letting me know a little bit about their lives and their countries as well. I met many valuable persons
here in ITC that make me belief we are not that different at all. Many thanks to my dear friends: Juan
Francisco, Sally, Jorge, Andrei, Du Ye, Arik, Punya, Shango, Chin and Quiuju. Mila, special thanks
must go to you for your valuable help with the prepositions and the correct use of the verbs in this
report. I use a good time of your PhD project indeed.
Without Gonyito my MSc time would have not been so great. Thank you for letting me know you and
being with me all this time. Roomy thank you for trying the bicycle and for Candy. I am astonished for
your energy to come with me travelling around Europe. I lost the count of how many countries we
visited the last year. Thanks also for letting me know how delicious can be the food in your country.
Last but not least I want to thank my family. Mother, sorry for all this time you have been alone, and
for always understanding and supporting my ideas. I ought you everything. To my grandparents, my
sister and all my family go special thanks. Nubia and Arturo: thank you for always being there.
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Table of contents
1. Introduction ...................................................................................................................................... 1
1.1. Motivation and problem statement. .......................................................................................... 1
1.2. Research identification ............................................................................................................. 2
1.2.1. Research objectives .......................................................................................................... 2
1.2.2. Research questions ........................................................................................................... 2
1.2.3. Intended innovation .......................................................................................................... 2
1.3. Scientific context ...................................................................................................................... 3
1.4. Thesis structure ......................................................................................................................... 4
2. Radar remote sensing and related work............................................................................................ 5
2.1. Incident angle ........................................................................................................................... 7
2.2. Relief distortion ........................................................................................................................ 8
2.3. Terrain reflectivity .................................................................................................................... 9
2.4. Radar backscatter dependency ............................................................................................... 11
2.4.1. Wavelength and frequency ............................................................................................. 11
2.4.2. Polarization ..................................................................................................................... 12
2.4.3. Surface roughness ........................................................................................................... 12
2.4.4. Dielectric constant .......................................................................................................... 13
2.4.5. Incident Angle ................................................................................................................ 13
2.5. Speckle ................................................................................................................................... 15
2.5.1. Multilooking ................................................................................................................... 16
2.5.2. Radar calibration ............................................................................................................ 16
2.6. The ALOS system .................................................................................................................. 16
2.6.1. PalSAR processing levels ............................................................................................... 18
2.6.2. PalSAR calibration ......................................................................................................... 18
2.6.3. PalSAR antenna pattern .................................................................................................. 18
2.7. Related work .......................................................................................................................... 20
2.7.1. On incident angle dependency........................................................................................ 20
2.7.2. On correction methods ................................................................................................... 22
2.7.3. On backscatter modelling ............................................................................................... 24
2.7.3.1. Water cloud model ................................................................................................. 24
2.7.3.2. Extended water cloud model .................................................................................. 26
3. Study area, materials and methods ................................................................................................. 29
3.1. Study area ............................................................................................................................... 29
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3.1.1. Land cover types ............................................................................................................ 30
3.2. Materials ................................................................................................................................ 32
3.2.1. Radar images ................................................................................................................. 32
3.2.2. Satellite images .............................................................................................................. 34
3.2.3. Digital elevation model ................................................................................................. 35
3.2.4. Ecosystem map .............................................................................................................. 36
3.3. Methods ................................................................................................................................. 37
4. Data processing ............................................................................................................................. 41
4.1. Radar data .............................................................................................................................. 41
4.2. Satellite images ...................................................................................................................... 43
4.3. Digital elevation model ......................................................................................................... 44
4.4. Ecosystem map ...................................................................................................................... 45
5. Radar backscattering analysis and modelling ................................................................................ 47
5.1. Radar backscattering statistics ............................................................................................... 47
5.2. Calculation of incident angle ................................................................................................. 50
5.3. Antenna pattern effect ........................................................................................................... 51
5.4. Radar backscattering analysis at image level ........................................................................ 52
5.5. Radar backscattering analysis by reflecting targets ............................................................... 54
5.5.1. Class 1 - Dense wet forest ............................................................................................. 55
5.5.2. Class 2 - Dense flooded forest ....................................................................................... 57
5.5.3. Class 3 - Tall and dense wet forest ................................................................................ 60
5.5.4. Class 4 - Short forest ..................................................................................................... 62
5.5.5. Class 5 – Savannah ........................................................................................................ 64
5.6. Correction model for incident angle dependency .................................................................. 66
5.6.1. Correction model ........................................................................................................... 66
5.6.1.1. Normalization class 2 (flooded forest) .................................................................. 67
5.6.1.2. Normalization class 5 (Savannah) ......................................................................... 68
5.7. Backscattering modelling ...................................................................................................... 70
5.7.1. Initial parameter values for model fitting ...................................................................... 70
5.7.2. Backscattering modelling of dense wet forest (class 1) ................................................. 71
5.7.3. Backscattering modelling of flooded forest (class 2) .................................................... 73
5.7.4. Model assessment .......................................................................................................... 75
6. Discussion ..................................................................................................................................... 77
6.1. On backscattering analysis by reflecting targets ................................................................... 77
6.2. On simplified correction model ............................................................................................. 81
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6.3. On backscattering modelling .................................................................................................. 81
6.4. On used materials ................................................................................................................... 84
6.5. On applied methods ................................................................................................................ 84
6.6. On applications of this study .................................................................................................. 85
7. Conclusions and recomendations ................................................................................................... 87
7.1. Conclusions ............................................................................................................................ 87
7.2. Recommendations .................................................................................................................. 88
Appendix I. Acquisition modes of ALOS system .................................................................................. 89
Appendix II. Radar backscattering of a tree trunk modeled as a dihedral corner reflector .................... 90
Appendix III. Land cover photographs in study area ............................................................................. 91
Appendix IV. Distribution of land cover types in ALOS image 1 ......................................................... 94
Appendix V. Metadata of PalSAR images ............................................................................................. 95
Appendix VI. Radar brightness expressions for ALOS PalSAR images ............................................... 96
Appendix VII. Rain statistics for image acquisition dates ..................................................................... 97
Appendix VIII. Observations of Leaf Area Index in tropical forest ecosystems ................................... 98
References .............................................................................................................................................. 99

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List of figures
Figure 2-1 Radar pulse interactions with objects on the ground ............................................................. 5
Figure 2-2 Radar observation geometry .................................................................................................. 6
Figure 2-3 Range resolution and azimuth resolution .............................................................................. 7
Figure 2-4 Diagram of incident angle and local incident angle ............................................................... 7
Figure 2-5 Increment of incident angle as function of radar range ........................................................ 8
Figure 2-6 Relationship between incident, look and depression angle .................................................... 8
Figure 2-7 Schematic diagram of radar foreshortening, radar layover and radar shadow ....................... 9
Figure 2-8 Brightness expressions for radar backscattering .................................................................. 10
Figure 2-9 Roughness surface reflector ................................................................................................. 13
Figure 2-10 Schematic radar backscattering variation with incident angle ........................................... 14
Figure 2-11 Radar backscattering variation in terms of surface roughness and incident angle ............ 15
Figure 2-12 Number of looks of a SAR system .................................................................................... 16
Figure 2-13 ScanSAR observation mode .............................................................................................. 18
Figure 2-14 Uncalibrated and calibrated ScanSAR images in Amazon forest. ..................................... 19
Figure 2-15 L band HH polarization backscattering statistics ............................................................... 20
Figure 2-16 Modelled L-HH backscatter from the floodplain forest stand ........................................... 21
Figure 2-17 Flooded forest backscattering in L band ............................................................................ 21
Figure 3-1 Study area ............................................................................................................................ 29
Figure 3-2 Distribution of considered land cover classes ...................................................................... 32
Figure 3-3 Layout of radar images investigated in the research ............................................................ 33
Figure 3-4 Layout of the satellite images in the study area ................................................................... 34
Figure 3-5 Digital elevation model for study area ................................................................................. 35
Figure 3-6 Ecosystem map for study area ............................................................................................. 36
Figure 3-7 Workflow methodology applied in this research ................................................................. 37
Figure 3-8 Methods implemented in research ....................................................................................... 38
Figure 3-9 Segmentation output implemented in SegSAR .................................................................... 38
Figure 4-1 Workflow for radar data processing .................................................................................... 41
Figure 4-2 Original image and terrain corrected image ......................................................................... 42
Figure 4-3 Georreferenced image and geocoded image ....................................................................... 42
Figure 4-4 Workflow for optical satellite image processing ................................................................. 44
Figure 4-5 Workflow for DEM processing ........................................................................................... 45
Figure 4-6 Workflow for processing of ecosystem map ....................................................................... 46
Figure 5-1 Forest window for PDF test ................................................................................................. 47
Figure 5-2 Comparison of intensity theoretical distribution with observation for test region ............... 49
Figure 5-3 Comparison of amplitude theoretical distribution with observation for test region ............ 49
Figure 5-4 Schematic diagram of incident angle values for the three ALOS radar images .................. 50
Figure 5-5 Calculation of incident angle image for ALOS image 2 ...................................................... 51
Figure 5-6 Radiometric azimuth bins product of antenna pattern correction. Image 1-3 ...................... 52
Figure 5-7 Backscattering values along the range. Image 1. Image 3 ................................................... 53
Figure 5-8 Land cover sampling strategy for analysis of radar backscattering ..................................... 54
Figure 5-9 Workflow sampling class 1 .................................................................................................. 55
Figure 5-10 Sampling areas class 1. Image 1. Image 2 ........................................................................ 55
Figure 5-11 Sigma values Image 1 and 3 for class 1 along the range ................................................... 56
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