Airborne Radar for High Resolution Mapping of Internal Layers in Glacial Ice to Estimate Accumulation Rate

Size: px

Start display at page:

Download "Airborne Radar for High Resolution Mapping of Internal Layers in Glacial Ice to Estimate Accumulation Rate"

Gwendoline Owens
5 years ago
Views:

1 Airborne Radar for High Resolution Mapping of Internal Layers in Glacial Ice to Estimate Accumulation Rate Pannirselvam Kanagaratnam Radar Systems and Remote Sensing Lab University of Kansas 2335 Irving Hill Road, Lawrence, KS Tel: 785/ , Fax: 785/

2 MOTIVATION

3 Motivation Strong correlation between climate change and sea-level rise Sea level rose by about 15 cm over the last century IPCC projected sea-level to rise 5mm/yr. over the next 100 years Potential impact stronger storm surges in the coastal regions coastal erosion submerged islands diminished fresh water supplies loss of tourism Prediction of climate change is a critical technological challenge IEEE, 2002

4 Consequence Potential rise in sea level caused by melting of Greenland (left) and Antarctic ice sheets (right) Source: NASA s Solid Earth Science Program

5 INTRODUCTION

6 Introduction Sources of sea level rise Thermal expansion of the ocean Melting of mountain glaciers Contribution from polar ice sheets Large uncertainty in the polar ice sheets contribution Accurate determination of the mass balance of these ice sheets is required To assess their contribution To develop models To understand the causes To predict future contribution in response to climate change

7 Mass Balance Volumetric Approach Change in ice volume is measured Altimeters Satellite Aircraft Snow accumulation required for interpretation and validation Flux Approach Measure components that go into mass balance equation Ice thickness Ice velocity Topography Ablation Temperature Snow accumulation

8 Accumulation Map Bales et al., 2001 Krabill et al., 2001

9 Ice core sampling Ice cores Surface Inter-annual layers

10 Computation of Accumulation Rate A = dh ρ dt ρ layer water Determine chronology of ice as a function of depth Ice flow models Counting annual layers Matching δ 18 O record with another dated climatic record Radiocarbon dating of CO 2 Identifying horizons of known age

11 APPROACH

12 Obtained ice core records Approach Performed simple simulations Developed a surface-based radar system Determined optimum frequency Applied surface and volume scattering models to compute clutter Developed prototype airborne radar system Proved that internal layers can be mapped Developed operational system

13 DIELECTRIC PROPERTIES

14 Dielectric Properties of Glacial Ice Density changes Pressure exerted by annual accumulation Melt and subsequent refreezing in percolation and wet snow zones Depth hoar layers Conductivity changes Acidic deposits from volcanic eruptions Crystal-orientation fabrics

15 Accumulation Zones Benson, C.S., Stratigraphic studies in the snow and firn of the Greenland Ice Sheet, Research Report 70, US Army Cold Regions Research and Engineering Laboratory (CRREL), Hanover, New Hampshire, Müller, F., Zonation in the accumulation area of the glacier of axel Heiberg island, N.W.T., Canada, J. Glaciol., 4, pp , Patterson, 1998

16 Annual Layers in Snow National Geographic, Dec. 2001

17 Simple Simulation Air/firn Volcanic layer / firn Term TimeDelay Term1 TD1 Num=1 Delay=0.013 usec Z=50 Ohm ZRef=50. Ohm TLINP TL1 Z=35.0 Ohm L=60 meter K=2 F=fsc TanD=2.64e-4 TLINP TL2 Z=35.0 Ohm L=0.2 meter K=2 F=fsc TanD=6.2e-4 TLINP TL3 Z=35.0 Ohm L=5000 meter K=2 F=fsc TanD=2.64e-4 Term Term2 Num=2 Z=35 Ohm Var Eqn VAR VAR1 fsc=600e6 S-PARAMETERS S_Param SP2 Start=170 MHz Stop=2000 MHz Step=1.02e6

18 Simulation Results 0 Air/firn Volcanic layer / firn Reflected Power (dbm) Two-W ay Travel Time (usec)

19 SURFACE-BASED SYSTEM

20 Surface-Based System Frequency-Modulated Continuous Wave (FM-CW) radar Frequency Sweep Time Transmit Power Number of Coherent Integrations Antennas A/D Dynamic Range Sampling Rate MHz 125 ms 0.1 Watt 8 TEM Horn 12-bit, 72 db 1 MHz

21 Surface-Based System 50 MHz TCXO Transmit FM Sweep GHz GHz VGA 3 db 4-6 GHz YIG LPF Power Amp LPF 4 GHz PLO Op Amp 3 db RF detector 3 db 3 db Data System Clock C O M P U T E R LPF LPF HPF HPF I-Channel Q-Channel LO RF GHz GHz 4 GHz - IF; GHz +IF BPF 4 GHz - IF Receive

22 Experiment Shallow radar sounding at the North Greenland Ice core Project (NGRIP) site during July, 1998 and August Mounted the radar on a tracked vehicle and collect data over a 2-km transect in 1998 and a 10-km transect in 1999.

23 Location

24 Experiment Setup

25 Range Computation Density (kg/m 3 ) Density Permittivity 3 2 Permittivity Depth (m) ε = [( ε ε ) + ] v ε1

Katmai (1912) 30 35 Melt Layer (1889) 40 45

26 Internal Layers at NGRIP (I) NGRIP 1998 NGRIP 1999 ECM Depth (m) Katmai (1912) Melt Layer (1889) Distance (km)

27 Internal Layers at NGRIP (II) NGRIP 1998 NGRIP 1999 ECM 110 Unknown (1514) 120 Mt. St. Helens(1479) Depth (m) Distance (km)

28 Internal Layers at NGRIP (III) NGRIP 1999

29 Computed Accumulation Rate Measured Year Thickness (m) Avg. Density (g/cm 3 ) cm/yr. % Error Mean= 17.3 ±4.12% Accumulation rate computed from core=17.1 cm/yr.

30 Frequency Response of Layer Γ = ε ε r2 r2 + ε ε r1 r1 2πl 2sin λ m

31 Reflection Coefficient of Snow Layer Term TimeDelay Term1 TD1 Num=1 Delay=1.67 usec Z=50 Ohm ZRef=50. Ohm TLINP TL1 Z=50.0 Ohm L=148 meter K=2 F=fsc TanD=0 TLINP TL2 Z=35.0 Ohm L=0.2 meter K=2 F=fsc TanD=6.2e-4 TLINP TL3 Z=50.0 Ohm L=5000 meter K=2 F=fsc TanD=0 Term Term2 Num=2 Z=50 Ohm Var Eq n VAR VAR1 fsc=600e6 S-PARAMETERS S_Param SP2 Start=170 MHz Stop=2000 MHz Step=1.02e6 0 Layer Thickness=20 cm db(s(1,1)) freq, GHz

32 CLUTTER

33 Clutter Problem θ Air/Firn Interface C = P t λ 2 G 2 σ o 3 4 ( 4π) R A Firn/Internal Layer Interface S = P t λ 2 G 2 Γ 2 ( 4π) 2 ( 2R) 2

34 Modeling Results Surface Scattering 600 MHz 900 MHz

35 PROTOTYPE AIRBORNE SYSTEM

36 Prototype Airborne System Frequency MHz Sweep Time 100 µs PRF Transmit Power Number of Coherent Integrations Antennas A/D Dynamic Range 2 khz 1 Watt 100 TEM Horn 12-bit, 72 db Sampling Rate 50 MHz

37 Block Diagram 50 MHz TCXO MHz Transmit Chirp Synthesizer LPF BPF Limiter BPF Power Amp BPF 1 GHz PLO 2 Data System Clock C O M P U T E R LPF 31 db BPF 29 db BPF IF BPF 3 db LO RF BPF Receive

38 Installation

39 Flight Lines

40 Results Dry Snow Zone

Results (II) Percolation Zone Wet Snow Zone 480 480 490 490 500 510 500 Depth (m) 520 530 Depth (m) 510 520 540

855W 125655.00 0.00 125745.00 11.70 125835.00 23.40 125925.00 35.

41 Results (II) Percolation Zone Wet Snow Zone Depth (m) Depth (m) N W N W N W N W N W N W N W N W N W N W

42 Problems with Prototype 1. Transient response of the band-pass filter following the mixer was not optimized to minimize ringing. 2. Inadequate isolation between transmitter and receiver sections of the radar. 3. Insufficient receiver dynamic range. 4. Inadequate knowledge of the level of antenna feed through signal.

43 IMPROVED AIRBORNE SYSTEM

44 Improved Airborne Design 50 MHz TCXO Box 2, Partition MHz Transmit Chirp Synthesizer LPF 3 db BPF Power Amp BPF 1 GHz PLO 3 db Box 1, Partition 1 Data System Clock C O M P U T E R LPF LPF HPF IF LO 3 db RF High-Isolation Amp BPF Receive Box 1, Partition 2 Box 2, Partition 2

45 Installation

46 Improved High Pass Filter Gaussian filter design to minimize ringing

47 EEsof Simulation Transm itter Coupler`` Transceiver `` Delay Line Transceiver TRANSIENT Tran Tran1 StopTime=100 usec MaxTimeStep=0.5 nsec V_DC SRC2 Vdc=f ile{dac1, "v oltage"} R R1 R=50 Ohm Ref S3P SNP4 Attenuator ATTEN6 Loss=3. db VSWR=1.2 1 Re f S2P SNP36 Amplif ier AMP5 S21=f ile{dac2, "S[2,1]"} S11=f ile{dac2, "S[1,1]"} S22=f ile{dac2, "S[2,2]"} Attenuator S12=f ile{dac2, "S[1,2]"} ATTEN7 Loss=3. db VSWR=1.2 2 Tim edelay TD1 Delay =3.34 usec ZRef =50. Ohm 1 Re f S2P SNP37 2 IF Section Receiver Re f Term Term 1 Num=1 S2P Z=50 Ohm SNP38 Re f S2P SNP35 Re f S2P SNP34 Re f S2P SNP32 Re f S2P SNP31 Attenuator ATTEN10 Loss=3 db VSWR=1.2 Mixer Attenuator MIX2 ATTEN9 SideBand=LOWER Loss=3 db ImageRej= VSWR=1.2 LO_Rej1=30 db LO_Rej2=20 db RF_Rej=20 db Conv Gain=dbpolar(-8,0) Attenuator Amplif ier ATTEN8 AMP3 Loss=3 db S21=f ile{dac3, "S[2,1]"} VSWR=1.2 S11=f ile{dac3, "S[1,1]"} S22=f ile{dac3, "S[2,2]"} S12=f ile{dac3, "S[1,2]"} DAC DAC DAC DAC DAC DAC DataAccessComponent DAC1 DataAccessComponent DAC2 DataAccessComponent DAC3 DataAccessComponent DAC4 DataAccessComponent DAC5 DataAccessComponent DAC6

48 Improved Airborne Design 50 MHz TCXO Box 2, Partition 1 Transmit Chirp Synthesizer LPF 3 db BPF Power Amp BPF 1 GHz PLO 3 db Box 1, Partition 1 Data System Clock C O M P U T E R LPF LPF HPF IF LO 3 db RF High-Isolation Amp BPF Receive Box 1, Partition 2 Box 2, Partition 2

49 Comparison

50 Target Simulator Antenna Feedthru TimeDelay TD6 Delay=1.67e-7 sec ZRef=50. Ohm Reflection from air/snow interface Port P1 Num=1 PwrSplit2 PWR8 S21= S31= Transceiver S2P 1 SNP1 2 Ref TimeDelay TD7 Delay=3.33 usec ZRef=50. Ohm Transceiver S2P 1 SNP2 2 Ref PwrSplit2 PWR5 S21=0.707 S31=0.707 Port P2 Num=2 Attenuator ATTEN8 Loss=45 db VSWR=1. Fibre Optic Delay Line PwrSplit2 PWR7 S21=0.707 S31=0.707 PwrSplit2 PWR6 S21=0.707 S31=0.707 Short cable Delay Line Layer Reflections Receiver Transmitter Attenuator ATTEN11 Loss=3 db VSWR=1. Attenuator Amplifier ATTEN10 AMP1 Loss=3 db S21=dbpolar(5.5,0) VSWR=1. S11=polar(0,0) S22=polar(0,180) S12=0 TimeDelay TD5 Delay=3.33 nsec ZRef=50. Ohm Ice Layers

51 Results Air/firn interface Internal Layers

52 Results from Improved System Antenna Feedthrough Target

53 SUMMARY

54 Summary We developed a MHz airborne radar system to map the internal layers of the Greenland ice sheet. Will help overcome the limitations of surface based methods in determining the accumulation rate. Airborne radar system was developed based on surfacebased radar measurements made at NGRIP. We successfully mapped the internal layers over the Greenland ice sheet up to a depth of 120 m with better than 1 m resolution. We developed a target simulator that can be used to optimize radar performance.

55 RECOMMENDATIONS

56 Accumulation Map Bales et al., 2001 Krabill et al., 2001

57 Surface Property

58 Digital Beamforming Along Track Air/Snow Interface Null Null Internal Layer Receiver #-3 Receiver #-2 Receiver #-1 Receiver #0 Receiver #`1 Receiver #2 Receiver #3

59 Model-Based Signal Processing Raw Data Firn Model Noise Model System Model Estimate

Ultra-Wideband Radars for Measurements Over Land and Sea Ice

Ultra-Wideband Radars for Measurements Over Land and Sea Ice R. Hale, H. Miller, S. Gogineni, J.-B. Yan, F. Rodriguez-Morales, C. Leuschen, Z. Wang, J. Paden, D. Gomez-Garcia, T. Binder, D. Steinhage,