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Contributors:
F.J. Meyer1), P. Rosen2), X. Pi2), K. Chotoo3), K. Papathanassiou4), J.S. Kim4), Ch. Carrano5)
1)Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 2)Jet Propulsion Laboratory, Pasadena, CA 3)User Systems Inc., Crofton, MD 4)German Aerospace Center, Oberpfaffenhofen, Germany 5)Boston College, Boston, MA
SAR AND THE IONOSPHERE: CHALLENGES AND OPPORTUNITIES
Holistic Microwave Remote Sensing
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected]
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 2
INTRODUCTION OF IONOSPHERIC EFFECTS IN SAR UNDERSTANDING AND CORRECTING IONOSPHERIC ARTIFACTS
MAPPING THE IONOSPHERE USING SAR DATA
2
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 3
Ionospheric Effects on Microwave Signals
• For linearly polarized SARs, ionospheric effects can be derived from Appelton-Hartree equation, describing two main signal effects:
1. Ionosphere-induced phase shift 𝝍𝒊𝒐𝒏𝒐 𝒇𝟎 :
ψ𝑖𝑜𝑛𝑜 𝑓0 = −2𝜋𝑓01
106 𝑁𝑖𝑜𝑛𝑜 𝑓0, ℎ
𝑐𝑑ℎ ≈ −2𝜋
𝐾
𝑐𝑓0𝑇𝐸𝐶
with TEC = 𝑁𝑖𝑜𝑛𝑜 𝑓0, ℎ 𝑑ℎ is the Total Electron Content, and
𝐾 = 12 ∙𝑒4𝜋2𝑚𝜀0 = 40.28 𝑚3 𝑠2
2. Rotation of the polarization orientation (Faraday rotation) 𝛀 𝒇𝟎 :
𝛺 𝑓0 = −2𝜋𝑓01
106 𝑁𝑖𝑜𝑛𝑜 𝑓0, ℎ
𝑐
𝑓𝐻𝑓0𝑐𝑜𝑠 𝜃 𝑑ℎ ≈
𝐾
𝑓02 𝐵 ∙ 𝑐𝑜𝑠 𝜃 ∙ 𝑇𝐸𝐶
with𝑓𝐻 = 𝐵 𝑒 2𝜋 is the electron gyro frequency and 𝜃 is angle between magnetic field and signal propagation
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 4
Observable SAR Distortions
Impacts of the Ionosphere on SAR Performance
• Ionospheric distortions depend on the spatial variability of the ionosphere:
Azimuth Defocusing
InSAR phase distortions
Range geometry distortions
Azimuth geometry distortions
Range Defocusing
Range shifts
Faraday rotation
4
Ph
ase D
isto
rtio
ns
Po
lari
zati
on
Dis
tort
ion
s
Sp
ati
ally h
ete
rog
en
eo
us
Ion
os
ph
ere
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 5
Observable SAR Distortions
Impacts of the Ionosphere on SAR Performance
• Ionospheric distortions depend on the spatial variability of the ionosphere:
Sp
ati
ally s
mo
oth
Ion
os
ph
ere
Ph
ase D
isto
rtio
ns
Po
lari
zati
on
Dis
tort
ion
s Range Defocusing
Range shifts
Faraday rotation
5
Azimuth Defocusing
InSAR phase distortions
Range geometry distortions
Azimuth geometry distortions
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 6
Ionospheric Distortions – Where and When?
• Patterned Ionosphere largely limited to: – Polar regions: Ionospheric structure possible at any time of day and year
– Equatorial regions: Structure largely after sunset until early morning
– Mid-latitudes: Ionospheric structure rare and low in magnitude
[dB] Parameters:
• KP: 4.5
• SSN: 140
• Time: 0 UTC
• Resolution: 1º×1º
Examples of Ionospheric Distortions in Low-Frequency (L-band) SAR
7
Equ
ato
rial
Reg
ion
s
Polar R
egion
s
Phase
Distortions
ALOS PALSAR L-band SAR Data
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 8
Examples of Ionospheric Distortions in Low-Frequency (L-band) SAR
Equ
ato
rial
Reg
ion
s
Polar R
egion
s
Image
Distortions
Ion
osp
heric azim
uth
disto
rtion
s [px]
ALOS PALSAR L-band SAR Data
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 9
Victoria – Wilkes Coasts, Antarctica Ionospheric Effects in Ice Velocity Measurements
Courtesy: E. Rignot, B. Scheuchl, UCI 9
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 10
INTRODUCTION OF IONOSPHERIC EFFECTS IN SAR
UNDERSTANDING AND CORRECTING IONOSPHERIC ARTIFACTS MAPPING THE IONOSPHERE USING SAR DATA
10
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 11
Concept of Ionospheric Correction of SAR Data
• Strategy: Measure SAR signal distortions Invert for ionospheric parameters using a ionospheric propagation model
Ionosphere
(TEC)
Faraday
Rotation
Azimuth
Shifts
Range
Shifts
Interferometric
phase Signal-
dispersion
20300
020000
28.40428.40428.404ffTEC
fcffTEC
fcTEC
fc
tx
Amplitude distortions
Polarimetric distortions
Phase distortions
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 12
Concept of Ionospheric Correction of SAR Data
• Strategy: Measure SAR signal distortions Invert for ionospheric parameters using a ionospheric propagation model
Ionosphere
(TEC)
Faraday
Rotation
Azimuth
Shifts
Range
Shifts
Interferometric
phase Signal-
dispersion
20300
020000
28.40428.40428.404ffTEC
fcffTEC
fcTEC
fc
tx
Amplitude distortions
Polarimetric distortions
Phase distortions
Ionospheric Correction Using Faraday Rotation (FR)
• Full-Polarimetric SAR scattering matrix under Faraday Rotation:
𝐎 = 𝐒 𝛺 = 𝐑𝐒𝐑 , where 𝐒 =𝑆𝐻𝐻 𝑆𝐻𝑉𝑆𝑉𝐻 𝑆𝑉𝑉
and 𝐑 𝛺 =cos𝛺 sin𝛺−sin𝛺 cos𝛺
• Bickel&Bates approach of estimating Faraday Rotation 𝜴 :
1. Transform into circular basis: 𝑶C =1
2
−𝑖 11 −𝑖
𝑂𝐻𝐻 𝑂𝐻𝑉𝑂𝑉𝐻 𝑂𝑉𝑉
1 𝑖𝑖 1
2. Estimate 𝜴: 𝜴 =1
4arg 𝑂12𝑂21
∗ =1
4arg 𝑒𝑖4𝛺 𝑆𝐻𝐻 + 𝑆𝑉𝑉
2
• Global phase correction accuracy 𝝈𝝋 depends on
angle with magnetic field and is worst around equator
13 Courtesy: J.S. Kim, DLR
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 14
Ionospheric Correction based on Split-Spectrum InSAR
• Interferometric phase at carrier
frequency f0:
• Split-bandwidth processing:
2
0
0
22
1
0
0
11
00
00
f
f
f
f
f
f
f
f
dispersivef
dispersivenonf
dispersivef
dispersivenonf
0f
Bb b
Lower Subband
1f
Upper Subband
2f
Subband Range Spectra
f0
0 44
000
fc
STECK
c
Rf
dispersivef
dispersivenonff
f
ff
f
ffdispersivef
21122
0
21
20
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 15
Ionospheric Correction based on Split-Spectrum InSAR
• Phase correction performance:
𝜎𝜑 =2 ∙ 𝐾
𝑐 ∙ 𝑓0𝜎∆𝑇𝐸𝐶 , 𝑤𝑖𝑡ℎ 𝜎∆𝑇𝐸𝐶 =
𝑐 ∙ 𝑓02
8𝜋 ∙ 𝑓∆ ∙ 𝑁∙1 − 𝛾2
𝛾
• Hence, high correction quality requires: – Large SAR system bandwidth large 𝑓∆ – High interferometric coherence 𝛾 – Large number of looks 𝑁 – [Error free phase unwrapping]
• Achievable performance for DESDynI mission: – Parameters: 𝑓∆ = 53𝑀𝐻𝑧; γ = 0.6; 𝑓0 = 1.27𝐺𝐻𝑧; 𝑁 = 1000
– Performance parameters for DESDynI (theoretical optimum):
𝜎∆𝑇𝐸𝐶 ≈ 0.05𝑇𝐸𝐶𝑈 and 𝜎𝜑 ≈ 0.1𝑟𝑎𝑑
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 16
Current Research: More Robust Correction through Integrated Estimator
• Integration of several estimators using least-squares: – Observation Equation:
𝑑 𝑜𝑏𝑠 =ΩΔ𝑎
Δ𝜑𝑖𝑜𝑛𝑜
=
𝐴Ω 0 00 𝐴Δ𝑎 00 0 𝐴Δ𝜑
𝑇𝐸𝐶
– Inversion:
𝑇𝐸𝐶 = 𝐴𝑇𝐶−1𝐴 −1 𝐴𝑇𝐶−1𝑑 𝑜𝑏𝑠
(Detector accuracy is included in covariance matrix 𝐶)
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 17
Development of Improved Correction Algorithms
17
• Example: InSAR pair over Alaska
Jun Su Kim; Danklmayer, A.; Papathanassiou, K., "Correction of ionospheric distortions in low frequency interferometric
SAR data," Geoscience and Remote Sensing Symposium (IGARSS), 2011 IEEE International, pp.1505-1508.
InSAR Coherence
No Ionospheric
correction applied
InSAR Coherence
a-based correction
InSAR Coherence
Joint a and W-
based correction
Am
plit
ud
e (
db
)
18
PALSAR: November 19, 2010 ALPSRP256737070
Current Research: Understanding Signatures of the Equatorial Ionosphere
24
.2 k
m
25.9 km
19
Am
plit
ude
(dB
)
24
.2 k
m
25.9 km
Am
plit
ud
e (
db
)
PALSAR: April 6, 2011 ALPSRP276867070
Current Research: Understanding Signatures of the Equatorial Ionosphere
Current Research: Understanding Signatures of the Equatorial Ionosphere
• Ionospheric Simulator: – Determine ionospheric power spectrum 𝑃𝜙 𝑘 using ionospheric model (e.g. WBMOD)
– Generate ionospheric phase screen (e.g. using IP-STATS)
– Calculate ionospheric transfer function 𝐷𝐼𝑇𝐹
– Modulate the unfocused SAR signal with 𝐷𝐼𝑇𝐹 and focus SAR Image
• Results:
Unaffected SAR image SAR image with simulated
ionospheric effects
20
Carrano, C. S., K. M. Groves, and R. G. Caton (2012), Simulating the impacts of ionospheric scintillation
on L band SAR image formation, Radio Sci., 47, RS0L20, doi:10.1029/2011RS004956.
• Comparison of threshold 𝐶𝑘𝐿 to typical ionospheric conditions:
D.P. Belcher, “Theoretical Limits on SAR Imposed by the Ionosphere,” Radar, Sonar & Navigation, IET, vol.2, issue 6, December 2008
Current Research: Understanding Signatures of the Equatorial Ionosphere
• Using simulator, estimate ionospheric turbulence level (quantified by 𝐶𝑘𝐿) at which “stripes” form in SAR data:
21
𝑪𝒌𝑳
Appearance of stripes Destruction of image L-band ( = 0.24m) 2E+33 m-2 5E+34 m-2
striping
Image destruction
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 22
INTRODUCTION OF IONOSPHERIC EFFECTS IN SAR
UNDERSTANDING AND CORRECTING IONOSPHERIC ARTIFACTS
MAPPING THE IONOSPHERE USING SAR DATA
22
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 23
Opportunities: SAR as a Source of Ionospheric Information
Pi, X., et al. (2011), Imaging ionospheric inhomogeneities using spaceborne synthetic aperture radar, Journal of Geophysical Research, 116(A4), A04303.
2-D Mapping of Auroral Structures from SAR
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 24
Opportunities: SAR as a Source of Ionospheric Information
Ionospheric variation: 2006–2011 @ 70°-90°N
24
Continental-Scale information on average ionospheric behavior
Ionospheric parameters from ~750,000 L-band images
Holistic Microwave Remote Sensing – Joint observation of Geosphere, Atmosphere, and Ionosphere
Check out our research: insar.alaska.edu Contact us at: [email protected] 25
• Ionospheric effects in SAR data well understood
• Distortions expected for low-frequency radars in latitudes ranges of < ±10° and > ±60°
• Correction methods available and successfully tested under “normal” conditions
• Current research: – Make correction technology more robust
– Understand driving mechanisms and correctability of polar cap distortions
– Understand signatures caused by equatorial scintillation
– Improve quality of ionospheric information derived from SAR
– 3D ionospheric mapping from SAR?
Summary and Future Work
25