AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Tomographic processing of AlpTomoSAR airborne data for observing the internal structure of Alpine glaciers: algorithm
description, challenges, and future perspectives
Stefano Tebaldini, Thomas Nagler, Helmut Rott, Achim Heilig , Adriano Meta, Alex Coccia, Davide Giudici, Dirk Schüttemeyer, Malcolm Davidson
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Introduction
Track 1
Reference Track
(Master)
Track n
height
θ
π/2
TomoSAR imaging access to the 3D structure of the illuminated media
Tomography is a fundamental tool to:
o Investigate the phenomenology of Radar scattering
o Guide physical modeling through direct observation
Mariotti d'Alessandro, M.; Tebaldini, S., GRSL, 2012
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
from SAOCOM CS Science Report, ESA, 2015
Introduction
Spaceborne TomoSAR is intensively being studied for forest application
Tandem-L, SAOCOM-CS
o Currently under evaluation o Possible launch in 2018 for SAOCOM-CS o L-Band o Simultaneous InSAR pairs
BIOMASS
o Selected as the next ESA Earth Explorer Core Mission o Expected launch in 2020 o P-Band o Repeat pass
Ho Tong et al., TGRS, 2012
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Introduction
What about ice ?
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Introduction
What about ice ?
AlpTomoSAR: First campaign entirely focused on the study of the internal structure of glacier ice using TomoSAR
o Analogy/complementarity/synergy w.r.t. Nadir looking systems
TomoSAR
o Spatial Coverage
o Sensitivity to rough internal surfaces
Sounder/GPR:
o Accurate vertical layering on transects
o Sensitivity to specular scattering (interfaces)
o Added value w.r.t. single baseline inversion
InSAR & PolInSAR single baseline surveys yield sensitivity to scattering depth, not 3D structure
o Assessment of the added value of spaceborne TomoSAR surveys over glaciers
Key questions:
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Test site
Test site: Mittelbergferner, Austrian Alps
o temperate glacier at the main ridge of the Alps in Tyrol
o main test area is a flat plateau in the upper part of the glacier between 3000 and 3200 m
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Field Works
Field activities
o Setting up Corner Reflectors
o Stratigraphy of winter snow pack
Density / Hardness, ice layers, grain size
o Transects of snow depth
o GPR Measurements
GPR Equipment: o IDS dual-frequency 200/600 MHz o Total length of GPR profile: 18 km
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
SAR Acquisitions
North-South
South-North
SAR Equipment: o FMCW SAR by MetaSensing o Transmitted bandwidth: 150 MHz o Central frequency: 1275 MHz o Fully polarimetric o Spatial resolution ≤ 2 x 2 m (ground
range, azimuth)
Flights: o Two flight directions o 20+20 passes
Aircraft o CASA C-212 operated by BLOM-CGR
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Flight Trajectories
Strong trajectory perturbation caused by wind/
turbulence
o Max along track variation > 50 m
o No auto-piloting system
o Proximity to mountain peaks
-2000
-1000
0
1000
2000
-50
0
50
1004450
4500
4550
4600
azimuth [m]ground range [m]
heig
ht
[m]
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
2D Focusing
2D Focusing via Time Domain
Back Projection on a reference
DEM
o Optimal motion compensation
o Common target wavenumbers in
all passes
o Automatic coregistration at the
reference elevation
HH – dir 1
HH – dir 2
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
TomoSAR provides resolution in elevation by jointly focusing data from multiple baselines
Phase screens result in signal defocusing
o Spaceborne: tropospheric and ionospheric phase screens
o Airborne: errors of the navigational system on the order of a fraction of a wavelength
Phase calibration through Phase Center Double Localization
joint estimation of target and aircraft positions
ground range
hei
ght
Data covariance matrix (NxN)
p
ref
p
n
p
n rr ˆ4
ˆ
ground range
hei
ght
p
refr
p
nr̂
p
mr̂
(y,z)
(Y,Z )
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
InSAR analysis of 15 x 15 interferometric pairs
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration Estimated trajectory errors
-1500 -1000 -500 0 500 1000 1500 -1
-0.5
0
0.5
1
azimuth [m]
Trajectory error - ground range [m]
-1500 -1000 -500 0 500 1000 1500 -1
-0.5
0
0.5
1
azimuth [m]
Trajectory error - height [m]
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Phase Calibration
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
3D Focusing
“Extreme” acquisition geometry:
o Large baselines Large range offset variations
o Large yaw angle variations Doppler plane
variations
Range and azimuth coregistration strongly depends
on initial focusing height
z1
z2
Focusin
g
heig
ht
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
3D Focusing
3D Focusing via Time Domain Back Projection
o Optimal treatment of large height-dependent range and azimuth coregistration offsets
o Preservation of common horizontal wavenumbers
o Automatic 3D geocoding
o The computational burden of this step was greatly reduced by adapting the sampling frequency of
defocused data to the effective amount of azimuth displacement induced by yaw angle variations
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
3D Focusing
Lidar DTM
Lidar DTM
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Ice Velocity Correction
Wave propagation through ice
o Wave refraction
o Changing delay-to-distance conversion law
Targets are not focused at their geometrical
position
True target position
Apparent target position
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Ice Velocity Correction
Wave propagation through ice
o Wave refraction
o Changing delay-to-distance conversion law
Targets are not focused at their geometrical
position
True target position
Apparent target position
TomoSAR correction
1. Mapping between true and apparent position
based on Fermat's minimum time principle
2. Subsequent Tomogram interpolation
minimum time travel path
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Physical Analysis - CRs gr
ou
nd
ran
ge
azimuth
CRs appear higher than surface scattering Surface scattering from ice/snow interface
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Physical Analysis – Lower Horizon
o Clear signal from the ice/snow interface in co-polarized channels
o Clear signal from down to 60 m beneath in all polarizations
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Physical Analysis – Lower Horizon
o Lower horizon retrieval through surface-picking
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Comparison to 600 MHz GPR Transects
o TomoSAR vs GPR comparison by sampling TomoSAR cubes
along GPR transects
o 600 MHz GPR transects processed down to 25 m
o TomoSAR transects processed down to 60 m
600 MHz GPR - 140227 AH
distance
depth
[m
]
300 400 500 600 700 800 900
0
5
10
15
20
25
TomoSAR - Direction 1 - HV
distance
he
igh
t w
.r.t
.lid
ar
[m]
300 400 500 600 700 800 900 -60
-50
-40
-30
-20
-10
0
TomoSAR - Direction 2 - HV
distance heig
ht
w.r
.t.lid
ar
[m]
300 400 500 600 700 800 900 -60
-50
-40
-30
-20
-10
0
Bedrock
Direction 1 Direction 2
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
600 MHz GPR - 140227 AC
distance
depth
[m
]
600 700 800
0
5
10
15
20
25
TomoSAR - Direction 2 - HH
distance
heig
ht w
.r.t.lid
ar
[m]
600 700 800 -60
-50
-40
-30
-20
-10
0
TomoSAR - Direction 2 - HV
distance heig
ht w
.r.t.lid
ar
[m]
600 700 800 -60
-50
-40
-30
-20
-10
0
Comparison to 600 MHz GPR Transects
Firn
transitions from ice to
soaked firn (???)
Direction 2 o TomoSAR vs GPR comparison by sampling TomoSAR cubes
along GPR transects
o 600 MHz GPR transects processed down to 25 m
o TomoSAR transects processed down to 60 m
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Comparison to 200 MHz GPR Transects
200 MHz GPR - 140227 AF
distance
depth
[m
]
0 500 1000 1500 2000 2500
0
20
40
TomoSAR - Direction 1 - HV
distance
heig
ht w
.r.t.lid
ar
[m]
0 500 1000 1500 2000 2500 -60
-40
-20
0
TomoSAR - Direction 2 - HV
distance
heig
ht w
.r.t.lid
ar
[m]
0 500 1000 1500 2000 2500 -60
-40
-20
0
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Comparison to 200 MHz GPR Transects
200 MHz GPR - 140227 AF
distance
depth
[m
]
0 500 1000 1500 2000 2500
0
20
40
TomoSAR - Direction 1 - HV
distance
heig
ht w
.r.t.lid
ar
[m]
0 500 1000 1500 2000 2500 -60
-40
-20
0
TomoSAR - Direction 2 - HV
distance
heig
ht w
.r.t.lid
ar
[m]
0 500 1000 1500 2000 2500 -60
-40
-20
0
Firn area Crevasses Firn area
Bedrock/ground reflection
AlpTomoSAR: observing the internal structure of Alpine glaciers Fringes 2015 - Frascati
Conclusions
o Analogy/complementarity/synergy w.r.t. Nadir looking systems
Different subsurface features could be detected by both GPR and TomoSAR
Wave penetration down the bedrock ( ≈ 60 m)
o Added value w.r.t. single baseline inversion
3D imaging allowed observation of the complexity of ice internal structure (over depth-only
information)
o Assessment of the added value of spaceborne TomoSAR surveys over glaciers
Need to fully assess spaceborne performance in all environmental conditions
Potentially unprecedented mapping of ice features worldwide using the next generation of
spaceborne TomoSAR systems
Key questions:
AlpTomoSAR: First campaign entirely focused on the study of the internal structure of glacier ice using TomoSAR