Measuring snow thickness over Antarctic sea ice with a helicopter-borne 2

243 Pages·2012·6.08 MB·English

by  Galin, N

Measuring snow thickness over Antarctic sea ice
with a helicopter-borne 2 - 8 GHz FMCW radar.
Natalia Galin
B.E. Electrical Engineering (Hons.),
New South Wales University, 2006
Submitted in fullment of the requirements for the
Degree of Doctor of Philosophy
at the University of Tasmania.
Institute forMarineandAntarctic Studies,theUniversity ofTasmania
January 2012

Abstract
Antarctic sea ice and its snow cover are integral components of the global climate system, yet
many aspects of their vertical dimensions are poorly understood, making their representation in
global climate models poor. Remote sensing is the key to monitoring the dynamic nature of sea
ice and its snow cover. Reliable and accurate snow thickness data from an airborne platform is
currently a highly sought after data product. Remotely sensed snow thickness measurements can
provideanindicationofprecipitationlevels. Thesearepredictedtoincreasewitheffectsofclimate
change, and are difcult to measure as snow fall is frequently lost to wind-blown redistribution,
sublimation and snow-ice formation. Additionally, accurate regional scale snow thickness data
willincreasetheaccuracyofseaicethicknessretrievalfromsatellitealtimeterfreeboardestimates.
Airborne snow-depth investigation techniques are one method for providing regional estimation
of these parameters. The airborne datasets are better suited to validating satellite algorithms, and
are themselves easier to validate with in-situ measurement. The development and practicality
of measuring snow thickness over sea ice in Antarctica using a helicopter-borne radar forms the
subject of this thesis. The radar design, a 2 - 8 GHz Frequency Modulated Continuous Wave
Radar, is a product of collaboration and the expertise at the Centre for Remote Sensing of Ice
Sheets,KansasUniversity.
This thesis presents a review of the theoretical basis of the interactions of electromagnetic waves
withthesnowandseaice. Thedominantgeneralphysicalparameterspertinenttoelectromagnetic
sensingarepresented,andthenecessaryconditionsforunambiguousidenticationoftheair/snow
and snow/ice interfaces by the radar are derived. Itis found that the roughnesses of the snow and
ice surfaces are dominant determinants in the effectiveness of layer identication in this radar.
Motivated bythese results, theminimumsensitivity requirements fortheradar arepresented.
Experiments withtheradarmountedonasledconrmthattheradariscapable ofunambiguously
detecting snow thickness. Helicopter-borne experiments conducted during two voyages into the
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East Antarctic sea-ice zone show however, that the airborne data are highly affected by sweep
frequencynon-linearities,makingidenticationofsnowthicknessdifcult. Amodelforthesource
ofthesenon-linearities intheradarisdeveloped andveried,motivatingthederivation ofanerror
correcting algorithm. Applicationofthealgorithmtotheairbornedatademonstrates thattheradar
isindeed receiving reections fromtheair/snow andsnow/ice interfaces.
Consequently, this thesis presents the rst in-situ validated snow thickness estimates over sea
ice in Antarctica derived from a Frequency Modulated Continuous Wave radar on a helicopter-
borne platform. Additionally, the ability of the radar to independently identify the air/snow and
snow/ice interfaces allows for a relative estimate of roughness of the sea ice to be derived. This
parameter is a critical component necessary for assessing the integrity of satellite snow-depth
retrieval algorithms such as those using the data product provided by the Advanced Microwave
Scanning Radiometer-EarthObserving Systemsensor onboardNASA’’sAquasatellite.
This thesis provides a description, solution or mitigation of the many difculties of operating a
radar from a helicopter-borne platform, as well as tackling the difculties presented in the study
of heterogeneous media such as sea ice and its snow cover. In the future the accuracy of the
snow-depth retrieval results can be increased as technical difculties are overcome, and at the
same time the radar architecture simplied. However, further validation studies are suggested to
better understand theeffect oftheheterogeneous nature ofseaiceand itssnowcoverontheradar
signature.
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Contents
StatementofDeclaration i
Abstract iii
Contents v
ListofFigures xi
ListofTables xxiii
ListofAcronyms xxv
Acknowledgements xxvii
1 Introduction 2
1.1 WorldClimateandthePolarRegions . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 SeaIceandClimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 SeaIceObservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 Satellite RemoteSensingofSeaIceThickness . . . . . . . . . . . . . . 10
1.2.2 SnowThickness onSeaIceRemoteSensing . . . . . . . . . . . . . . . 14
1.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3.1 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
v
2 IntroductiontoRadarPrinciplesandSummaryofOperatingParameters 22
2.1 OverviewofFMCWRadar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Operating DesignParameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3 Performance Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 Frequency Resolution: δf . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 RangeResolution: δR . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.3 Unambiguous Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.4 Operating Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.5 Antenna Gainand3dBBeamwidth . . . . . . . . . . . . . . . . . . . . 32
2.3.6 ActiveArea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.7 ThermalNoisePowerDensity: N . . . . . . . . . . . . . . . . . . . . 34
T
2.3.8 MinimumDetectable Signal: MDS . . . . . . . . . . . . . . . . . . . . 34
2.3.9 SamplingFrequency: f . . . . . . . . . . . . . . . . . . . . . . . . . . 35
s
2.4 FMCWDistributed Target RadarRangeEquation . . . . . . . . . . . . . . . . . 36
2.4.1 SignaltoNoiseRatio: SNR . . . . . . . . . . . . . . . . . . . . . . . . 38
2.4.2 Theoretical MinimumBackscattering Coefcient: σ0 . . . . . . . . . . . 39
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3 Electromagnetic WaveInteraction withSnowandSeaIceusingRAASTI 43
3.1 WideBandwidth andLayeredMedia . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1 Qualitative Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.1.2 Quantitative Considerations . . . . . . . . . . . . . . . . . . . . . . . . 46
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3.2 Conditions forUnambiguous Interface Identication . . . . . . . . . . . . . . . 49
3.2.1 PowerSplitting attheBoundary ofTwoMedia . . . . . . . . . . . . . . 49
3.2.2 Modelling theCondition forSnowLayerIdentication . . . . . . . . . . 52
3.2.3 EffectofSnowWetness . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3 Sensitivity ofSnowThickness EstimatestoErrorsinRefractive Index . . . . . . 56
3.4 Backscattering Coefcient Estimation . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.1 Surface Scatter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4.2 VolumeScatter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4 Sled-BasedRadarTests-Fastice,DavisStation,Antarctica 66
4.1 RadarOperating Conditions andAssumptions . . . . . . . . . . . . . . . . . . . 66
4.1.1 Experimental Description . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.2 ThePhysical Conditions ofSnowonFastIce,DavisStation . . . . . . . . . . . . 70
4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.3.1 TypeI:SnowPitAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.3.2 TypeII:Blizzard TailStudy . . . . . . . . . . . . . . . . . . . . . . . . 79
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5 DevelopmentofaNon-linearity Correcting Algorithm 85
5.1 ProblemDescription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.3 Non-linearities asaFunction ofTarget Range . . . . . . . . . . . . . . . . . . . 89
5.3.1 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.3.2 Non-linearity Characterisation . . . . . . . . . . . . . . . . . . . . . . . 92
5.3.3 Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.4 Non-linearity Correction Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 94
5.4.1 Performance Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
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6 Helicopter-borneRadarExperiments,EastAntarctica 102
6.1 RadarHelicopter Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.2 SeaIcePhysicsandEcosystemeXperiment, SIPEX’’07 . . . . . . . . . . . . . . 106
6.2.1 CraneExperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.3 Voyage 1,2008 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.3.1 Experiment Description . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.3.2 In-situ Gathered DataSummary . . . . . . . . . . . . . . . . . . . . . . 115
6.3.3 Helicopter Flights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.3.4 RadarOperating Conditions andAssumptions . . . . . . . . . . . . . . . 122
6.3.5 PeakDetection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
6.3.6 Converting peakseparation tosnowthickness . . . . . . . . . . . . . . . 129
6.3.7 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.3.8 SystemErrorEstimate . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7 ConclusionsandRecommendations 138
7.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7.1.1 Suggested Technical Improvements . . . . . . . . . . . . . . . . . . . . 141
7.2 RelatedWorkandOutlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
A RAdarforAntarcticSnowThicknessInvestigation (RAASTI) 145
A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
A.2 SIPEX’’07,RadarVersion1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
viii
A.2.1 Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
A.2.2 YIG(cid:239)YttriumIron GarnetCurrentControlled Oscillator . . . . . . . . . 151
A.2.3 YIGMainCoilDriver . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
A.2.4 Laboratory Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 155
A.3 V1’’08,RadarVersion2.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
A.3.1 YIGMainCoilDriver . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
A.3.2 YIGHeaterTests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
A.3.3 Internal DelayLines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
A.3.4 Antenna Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
A.3.5 Sled-based RadarOperation . . . . . . . . . . . . . . . . . . . . . . . . 164
A.3.6 Helicopter BootVibration Tests . . . . . . . . . . . . . . . . . . . . . . 165
A.4 National Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
A.4.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
A.4.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
A.4.3 RAASTIFileFormat . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
B SIPEX’’07RAASTIFlightSummary 174
B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
B.2 FlightDetails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
B.2.1 Alpha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
B.2.2 Bravo/Charlie/Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
B.2.3 Golf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
B.2.4 Hotel/India/Juliet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
B.2.5 Mike/November/Oscar . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
B.2.6 Papa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
B.2.7 Sierra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
B.2.8 Uniform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
B.2.9 VictorandCraneTest. . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
B.2.10 Yankee/Zulu/Zulu Two . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
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C HardDiskDriveAnti-VibrationTests 188
C.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
C.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
D Characterisation ofthe8-ElementTEMHornAntennas 195
D.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
D.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
D.3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
D.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
D.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
E TheEffectofConstantAntennaGainonUWBRadarSystems. 203
E.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
E.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
E.3 ProblemSetup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
E.4 Quantitative Analysis andSimulation . . . . . . . . . . . . . . . . . . . . . . . 205
E.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
E.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
References 209
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