SAR Retracking in the Arctic

102 Pages·2016·12.68 MB·English

SAR Retracking in the Arctic
Development of a year round Arctic SAR retracker system
Ann-Theres Schulz
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Astrodynamics and Space Missions

SAR Retracking in the Arctic
Development of a year round Arctic SAR retracker system
by
Ann-Theres Schulz
ToobtainthedegreeofMasterofScience
inSpaceflightatDelftUniversityofTechnology,
tobedefendedpubliclyon15April2016at9.00.
Studentnumber: 4124251
Thesiscommittee: Ir. M.C.Naeije TUDelft,supervisor
Prof.dr.ir. P.N.A.M.Visser TUDelft
Dr.ir. E.J.O.Schrama TUDelft
Dr.ir. D.C.Slobbe TUDelft
Anelectronicversionofthisthesisisavailableathttp://repository.tudelft.nl/.
CoverPicture: Artist’simpressionofCryoSat-2. ImageCourtesy: EuropeanSpaceAgency.
Copyright(cid:13)c AstrodynamicsandSpaceMissions(AS)
Allrightsreserved.

Abstract
Developments in satellite altimetry over the past years lead to a high resolution altimeter: Synthetic
ApertureRadar(SAR).ThemajorimprovementofSARoverpulse-limitedaltimetersistheincreased
along-track resolution. Since CryoSat-2 is the first satellite to carry a SAR altimeter it serves as a
steppingstonetoimprovetheperformanceofSAR.Givenitspolarorbit,CryoSat-2providesfrequent
highresolutionmeasurementsoftheArcticregionoverchangingterrain(e.g. ice,waterandseaice).
Due to the high seasonality in the Arctic, it is of essence that the quality of SAR measurements is
not decreased with varying topography. Therefore, the purpose of this research is to improve SAR
waveform retracking in the Arctic region by analyzing different retrackers on their performance in
varying(Arctic)conditionsandcombinethepositivebehaviorintooneretrackersystem.
The research is separated into three parts, first the SAR waveforms are classified, secondly the re-
trackers’performanceisevaluatedandfinally,thecombinedretrackersystemisdiscussed.
The waveforms are classified according to open ocean, sea ice and leads based on their statistical
behavior and the number of major peaks. Leads are waveforms caused by fractures in the sea ice
resulting in water presence in the ridges. Throughout 2011, the number of waveforms per terrain
showsahighseasonalitywithalargeamountofseaiceinJanuarytoApril,highleadspercentagein
MaytoJuneandfinally,alargenumberofoceanobservationsinSeptembertoOctober.
Basedontheaccuracyandprecisionperformance,fourretrackersareevaluatedinordertodetermine
aretrackersystem: theempiricalprimarypeakCenterofGravity(COG),theprimarypeakthreshold
and the ESA retracker as well as the physical SAMOSA3 retracker. Empirical retrackers determine
the retracking point depending on the waveform statistics, while physical retrackers take the surface
propertiesintoaccount.
TheaccuracyisevaluatedusingcoastaltidegaugesandEnvisatcrossoversforoceanwaveformsonly,
asthisdataisnotrepresentativeforleadsorseaice. ForthemajorityofthemonthsSAMOSA3hasthe
highestaccuracyforoceanwaveformsasitsphysicalfullanalyticapproachprovidesagoodfitforthe
predictable ocean waveforms. The primary peak retrackers have the best precision performance for
irregularwaveformslikeseaiceandleads,astheyarelesssensitivetonoisesinceonlythemaximum
peakisanalyzed.
Theyearroundretrackersystemincludesthemostaccurateretrackerforoceanwaveformspermonth
and the best precision retracker for sea ice and leads. When combining different retrackers the bias
Master of Science Thesis Ann-Theres Schulz
ii
causedbydifferentalgorithmsneedstoberemoved. Ameanbiasstrategyisimplementedthatcom-
putesthemeanoffsetbetweentworetrackerspertrackoveropenoceanandtheoffsetisthenapplied
totheotherwaveformclasses. Abiasremovalapproachbasedonsignificantwaveheightdependency
is also applied using least squares to compute the offset per waveform class per track. Furthermore,
two hybrid models of these strategies are implemented. The first hybrid model uses the significant
waveheightmodelforoceanobservationsandthemeanbiasmethodforleadsandseaicetoremove
the offset. Within the second hybrid model, the ocean and leads bias is removed by the significant
waveheightapproach,whiletheoffsetintheseaiceclassismitigatedwiththemeanbiasmethod.
The retracker system with the primary peak center of gravity as a base retracker and a mean bias re-
movalapproachperformedbestasithasaprecisionimprovementof47.1%withrespecttoretracking
allwaveformclasseswiththeprimarypeakCOGwhileincludingthelargestdatasetforprecisionand
accuracydetermination.
Ann-Theres Schulz Master of Science Thesis
Table of Contents
Glossary xi
ListofAcronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
ListofSymbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii
Preface xv
1 Introduction 1
2 CryoSat-2 3
2-1 Mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2-2 AltimeterOperationalModes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Radar Altimetry 7
3-1 PrincipleofRadarAltimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3-2 GeophysicalandRangeCorrections . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3-3 SyntheticApertureRadar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3-4 DataAcquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Waveform Classification 17
4-1 Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4-1-1 ClassificationParameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4-1-2 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4-2 OpenOcean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4-3 Leads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4-4 SeaIce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4-5 Seasonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Master of Science Thesis Ann-Theres Schulz
iv Table of Contents
5 Retrackers 27
5-1 PrimaryPeakRetrackers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5-1-1 CenterofGravityRetracker . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5-1-2 ThresholdRetracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5-2 SAMOSARetracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5-3 CryoSat-2Retracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6 Retracker Performance 35
6-1 Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6-2 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6-2-1 TideGaugeData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6-2-2 EnvisatData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6-2-3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6-3 OptimalRetrackerSelection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
7 Retracker System 51
7-1 BiasRemovalStrategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
7-1-1 MeanDifferenceBias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
7-1-2 SWHDependencyBias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
7-1-3 HybridModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
7-1-4 HybridModelV.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7-2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7-3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7-3-1 RetrackerSystemProcessing . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7-3-2 Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
7-3-3 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
8 Conclusions and Recommendations 67
8-1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
8-2 Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
A Reference Ellipsoid Adjustment 73
B SAMOSA2 Performance 75
Bibliography 77
Ann-Theres Schulz Master of Science Thesis
List of Figures
2-1 CryoSat-2modemaskfortheArcticfrom16/04/2011to15/05/2011[8]. SARIN
DEG
indicatesdegradedSARInmode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3-1 Thegeometryofaltimetryobservations[9]. . . . . . . . . . . . . . . . . . . . . . . 8
3-2 The geophysical corrections that are applied to the sea surface measurement: Geoid
height,tidalheight,atmosphericpressureloadinganddynamiceffectsofgeostrophic
oceancurrents[10]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3-3 Errorbudgetforpastaltimetermissions[9]. . . . . . . . . . . . . . . . . . . . . . . 9
3-4 ThegeometryofSAR.Left: Thealong-trackbeamsemittedbythealtimeterreaching
thesurface. Right: ThefootprintofSAR[1]. . . . . . . . . . . . . . . . . . . . . . 12
3-5 Top panel: The closed burst method is shown as used in CryoSat-2. Bottom panel:
Theopenburstmethodwiththeindividualtransmittedpulsesingreyandthereceived
onesinpink[21]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3-6 SAR precision (blue) in cm shown as a function of significant wave height. As a
reference,theprecisionofpulselimitedaltimeters(black)isindicated[26]. . . . . . 15
3-7 FlowchartforthedataprocessingofESAlevel1bBaselineBmeasurements. . . . . 16
4-1 TheSARwaveformhasasteeppeakfollowedbyaslowlydecayingtrailingedge[1]. 18
4-2 Differentwaveformclassesbasedonpulselimitedaltimetry[31]. . . . . . . . . . . . 18
4-3 Oceanobservation. Left: Arcticoceannexttoanicesheet[38]. Right: Anopenocean
waveformwithitscomputedparameters. . . . . . . . . . . . . . . . . . . . . . . . . 22
4-4 Leadobservation. Left: LeadintheArcticocean[38]. Right: Aleadwaveformwith
itscomputedwaveformparameters. . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4-5 Seaiceobservation. Left: SeaiceintheArcticocean[38]. Right: Aseaicewaveform
withitscomputedparameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4-6 Sea ice concentration for the Kara Sea for 2011. Top row, left to right: January-
March,secondrow: April-June,thirdrow: July-Septemberandbottomrow: October-
December. Thedataistakenfrom[39]. . . . . . . . . . . . . . . . . . . . . . . . . 24
4-7 The percentage of waveforms per class on a monthly basis for 2011. This plot is
based on Level 1b data using the classification parameters described in this section.
Unclassifiedwaveformsarenotconsideredinthisplot. . . . . . . . . . . . . . . . . 25
Master of Science Thesis Ann-Theres Schulz
vi List of Figures
5-1 Linear interpolation scheme to determine the retracker point for the primary peak
thresholdmethod[4]. Eq.(5-9)isbasedonthegeometryshownhere. . . . . . . . . 29
5-2 GeometryofPrimaryPeakRetrackers. Thebluelineshowsthewaveform, thegreen
line indicates the extracted primary peak. The black dotted line shows the rectan-
gle that is fitted to the primary peak. The black asterisk and red diamond show the
retrackingpointoftheCOGandthresholdretracker,respectively. . . . . . . . . . . . 30
5-3 ThephysicalmeaningofthefittedCryoSat-2parametersonthewaveformasafunc-
tionoftimeτ [13]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6-1 Thedefinitionofaccuracyandprecisionthatwillbeusedtoanalyseandevaluatethe
retrackerperformance[48]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6-2 Precision performance of the primary peak center of gravity and threshold retracker,
SAMOSAandESAretrackerforfourdifferentwaveformclasses: ocean,leads,seaice
and unclassified. The precision is determined on a monthly basis for 2011 based on
itsstandarddeviation. Notethedifferentscalesoftheverticalaxis. . . . . . . . . . . 37
6-3 Precision performance of the primary peak center of gravity and threshold retracker,
SAMOSA and ESA retracker. The precision is determined on a monthly basis for
2011basedonitsstandarddeviation. . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6-4 Thetestregionandthetidegaugelocations. . . . . . . . . . . . . . . . . . . . . . . 39
6-5 Thetidegaugesystemgeometryovertime. MSL showsthelocalreferencetowhich
53
thetidegaugeobservationsaremeasured. ItisbasedontheMeanSeaLevelof1953
in this case. The tide gauge returns the height of the sea surface with respect to
the RLR, h . The mean sea surface with respect to the TOPEX/Poseidon
SSH_RLR_cor
referenceellipsoidisrepresentedbyh . Finally,ζ istheheightbetweenmean
MSS_DTU
seasurfaceDTU13andtheRLR,MSL ,usedtocomputetheseasurfaceheightina
53
globalreferenceframe,h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
SSH_re
6-6 Footprint of the two tracks (blue) and the tide gauge locations (red). Left: Track on
July1,2011. Right: TrackonNovember3,2011. . . . . . . . . . . . . . . . . . . . 42
6-7 Track analysis 01/07/2011. Left: The sea level anomaly as a function of latitude of
fourretrackersusingCryoSat-2data(PPCOG,PPThreshold,SAMOSAandESAre-
trackers)andEnvisatdata. Right: TimedifferencebetweenCryoSat-2measurements
andEnvisatcrossoverdatapoints. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6-8 Track analysis 03/11/2011. Left: The sea level anomaly as a function of latitude of
fourretrackersusingCryoSat-2data(PPCOG,PPThreshold,SAMOSAandESAre-
trackers)andEnvisatdata. Right: TimedifferencebetweenCryoSat-2measurements
andEnvisatcrossoverdatapoints. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6-9 The SLA of different waveforms classes using the four retrackers for 2011. Top left:
Oceanwaveforms. ThefourCryoSat-2retrackersareindicatedbythesolidlines. The
tide gauge data of the three locations is shown in the dotted lines. Top right: Lead
waveforms. Bottomleft: Seaicewaveforms. Bottomright: Unclassifiedwaveforms. . 45
6-10 DifferenceinSLAcomputedoveropenoceantothetidegaugedatafortheyear2011.
The blue line shows PP COG, red represents the PP threshold, the black dotted line
indicatesSAMOSAandtheESAretrackerisgiveninyellow. . . . . . . . . . . . . . 46
6-11 Meansealevelanomalyforthefourretrackers(solidlines)andthethreetidegauges
(dotted lines) on a monthly basis for 2011. The waveform included in this plot are
openocean,leadsandseaice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6-12 TheannualsignalinthetestregionbasedonDTU131◦x1◦ resolution[36]. Left: The
amplitudeoftheannualsignal. Right: Thephaseoftheannualsignal. . . . . . . . . 47
6-13 ThedifferenceinSLAbetweenSAMOSA2andSAMOSA3. Theoceanobservations
ofSAMOSA3arebluedots,leadsareredsquaresandseaiceisshownbyblacktriangles. 49
Ann-Theres Schulz Master of Science Thesis