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Synthetic Aperture Radar for Ionospheric Scintillation Studies
Shradha Mohanty CSRE, IIT Bombay
Visiting Student, ISR, Boston College3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 1
Overview:
• Ionospheric scintillations• Synthetic Aperture Radar (SAR)
• How it works ?• How ionosphere affects SAR ?• What are the effects ?• What has been done so far ?
• SAR for scintillation identification• Effect of anisotropy on scintillation observed by SAR• Conclusion• Future Work
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 2
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 3
Ionospheric effects on radio waves:
• Interaction of ionosphere with radio waves alters the wave properties
• Phase advance, group delay, Faraday rotation• Short-duration, high-intensity ionospheric
scintillations
• High priority due to its significant impact on satellite radio communication
• Also observed in low-frequency (L- and P-bands) synthetic aperture radar (SAR) signals
• Launch of several low-frequency spaceborne SAR missions, now and in future, has increased the enormity of the problem
wdc.nict.go.jp
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 4
Ionospheric Scintillations:
Cloud of Electrons
• Affects the amplitude and phase of signal
• Sources: Radio/ Beacon networks, Global Positioning System (GPS) and Synthetic Aperture Radar (SAR)
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SAR – An Introduction:
www.radartutorials.euwww.radartutorials.eu
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SAR – An Introduction:
• Scattering Matrix [S]• Complex amplitude relating the incident and
scattered field• Elements are complex and depends upon
frequency, target orientation and shape, relative orientation of the polarization planes
• In the case of monostatic radar, reciprocity assumption (SHV = SVH) holds true
Scattering Matrix
www.laser-portal.ru
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SAR – An Introduction:
• Coherency [T] or Covariance [C] Matrix• Second order statistical parameters [C] and [T] • Generated from target vectors, Ω and K, respectively
www.doncio.navy.mil
Effects on GPS and SAR:
Rufenach, 1975
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Effects on SAR:
• Defocusing ALOS2012706991ALOS2023056991
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Effects on SAR:
Fringes due to Earthquake
Fringes due to Ionospheric effects
Fringes due to Earthquake persisting
Fringes due to ionosphere removed
Curtesy: Gomba, G., Cong, X.Y., Eineder, M., 2015
• Phase Instability
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Effects on SAR:
• Azimuthal Streaking
ALPSRP256737070 ALPSRP276867070
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Ionospheric Studies using SAR:
• Interferometric SAR (Phase)• Range Split-Spectrum
• Range Group – Phase Delay
• Azimuth shift• Multiple Aperture Interferometry
H.S. Jung et al., 2013.
𝑑𝑑𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 =−𝐾𝐾.𝑇𝑇𝑇𝑇𝑇𝑇
𝑓𝑓2 𝑑𝑑𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑝𝑝 =𝐾𝐾.𝑇𝑇𝑇𝑇𝑇𝑇𝑓𝑓2
Brcic et al., 2010.
∆∅𝑖𝑖𝑔𝑔𝑖𝑖𝑔𝑔 =𝑓𝑓𝐿𝐿𝑓𝑓𝐻𝐻
𝑓𝑓𝑔𝑔 𝑓𝑓𝐻𝐻2 − 𝑓𝑓𝐿𝐿2(∆∅𝐿𝐿𝑓𝑓𝐻𝐻 − ∆∅𝐻𝐻𝑓𝑓𝐿𝐿)
Rosen et al., 2010.
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Ionospheric Studies using SAR:
• Polarimetric SAR (Intensity)• Scattering Matrix • Sub-banding• Image Contrast• Radar Cross-section Enhancement• Point Spread Function of Corner Reflector• Image Clutter Statistics
Faraday Rotation, TEC, Ionospheric altitude and drift velocity
S4, CKL, p
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S4 measurements from SAR: (Belcher and Cannon,2014)
• Image Contrast (cd)
𝑐𝑐𝑑𝑑2 ≅4𝑆𝑆42
𝑁𝑁
𝑁𝑁 =𝐿𝐿𝑆𝑆𝑆𝑆
𝛾𝛾 𝜆𝜆𝑧𝑧𝑅𝑅 sec 𝜃𝜃
where, cd = 𝑝𝑝𝑠𝑠𝑔𝑔𝑝𝑝𝑔𝑔𝑝𝑝 𝑔𝑔𝑜𝑜 𝑖𝑖𝑖𝑖𝑖𝑖𝑝𝑝𝑖𝑖𝑝𝑝𝑖𝑖𝑖𝑖𝑖𝑖 𝑝𝑝𝑖𝑖𝑑𝑑.𝑚𝑚𝑝𝑝𝑝𝑝𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑝𝑝𝑖𝑖𝑝𝑝𝑖𝑖𝑖𝑖𝑖𝑖
where, N = independent Fresnel zone samplesγ = anisotropy ratiozr secθ = reduced slant range
• RCS Enhancement
𝜇𝜇𝑖𝑖𝑔𝑔𝑖𝑖𝑔𝑔 = 𝜇𝜇 (1 + 𝑆𝑆42)
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CkL measurements from SAR:
(Belcher and Rogers, 2008Mannix et al., 2017)
• Point Spread Function (PSF) of Corner Reflector
𝑃𝑃𝑆𝑆𝑃𝑃𝜙𝜙 𝑘𝑘 = 𝑇𝑇𝑆𝑆𝐿𝐿𝑆𝑆( 𝑟𝑟𝑔𝑔2 + 𝑟𝑟2)−𝑝𝑝
𝑇𝑇𝑆𝑆𝐿𝐿𝑆𝑆 = 4𝛾𝛾𝑘𝑘𝑐𝑐1−𝑝𝑝𝐺𝐺𝑇𝑇𝑘𝑘𝐿𝐿 sec𝜃𝜃 (𝑟𝑟𝑝𝑝𝜆𝜆)2
𝜋𝜋Γ(𝑝𝑝2)
2𝜋𝜋 2Γ(𝑝𝑝 + 12 )
𝑘𝑘1𝑘𝑘𝑚𝑚𝑝𝑝+1
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CkL measurements from SAR:
(Belcher et al., 2017)• From SAR image Clutter
𝜈𝜈𝑖𝑖𝑔𝑔𝑖𝑖𝑔𝑔 = 𝜈𝜈(1 +𝜎𝜎𝑆𝑆𝐿𝐿𝑆𝑆2
𝑙𝑙𝑔𝑔)
𝜎𝜎𝑆𝑆𝐿𝐿𝑆𝑆2 = 8𝜋𝜋𝛾𝛾2−𝑝𝑝𝐺𝐺 sec𝜃𝜃 (𝑟𝑟𝑝𝑝𝜆𝜆)2𝑐𝑐𝑝𝑝
𝑝𝑝 − 110−6𝐿𝐿𝑘𝑘𝑚𝑚
𝑝𝑝−1𝑇𝑇𝑘𝑘𝐿𝐿
𝑝𝑝 𝐼𝐼 =2 𝜈𝜈𝜇𝜇Γ 𝜐𝜐
𝜈𝜈𝜇𝜇𝐼𝐼
𝜈𝜈−1
𝐾𝐾𝜈𝜈−1(2𝜈𝜈𝜇𝜇𝐼𝐼)
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 17
ALOS-2/PALSAR-2 Specifications:
Satellite/Sensor ALOS-2 / PALSAR-2
Frequency of Operation 1.27 GHz
Bandwidth 28 MHz
Revisit, No. of cycles/day 14 days, 15
Mode of acquisition Full (Quad.) Polarimetry [HH+HV+VH+VV]
Width x Length (30 -55) km x 70 km
Resolution (Range x Azimuth) 3 - 10 m
Data Level 1.1 Single Look Complex
Orbit, Look direction Sun-synchronous, Right-/Left-looking
Height, inclination 698 km, 97.9°Time of acquisition (LT) 00:00 ± 15 min (asc.) , 12:00 ± 15 min (des.)
http://www.eorc.jaxa.jp/ALOS-2/en/about/palsar2.htm
1800 km
N23 Mar 2015 31 Oct 2016
B B
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Scintillation Identification:
1800 km
N23 Mar 2015 31 Oct 2016
B B
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Scintillation Identification:
1800 km
N23 Mar 2015 31 Oct 2016
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Scintillation Identification:
B B
CALCUTTA
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Scintillation Observations: GPS
RAJKOT CALCUTTA
TIRUNELVELI
TIRUNELVELI
RAJKOT
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Scintillation Observations: GPS
RAJKOT CALCUTTA
TIRUNELVELI
TIRUNELVELI
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Tirunelveli:Avg. S4 = 0.1 Avg. CkL = 1.45 x 1034
Rajkot:Avg. S4 = 0.15Avg. CkL = 1.72 x 1033
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Avg. S4 from SAR = 0.19Avg. S4 from GPS at TIR = 0.10Avg. S4 from GPS at RAJ = 0.15
SAR for Scintillation Identification:
• Motivation• Image pairs with uniform background• Satellite’s viewing geometry details• Need for simple identification options
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• Coefficient of variance (CoV)• Local homogeneity/heterogeneity• Speckle filtering
HH
VV
HV
VH
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CoV of SAR Intensities at different polarizations
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CoV of other SAR quantities
TOTAL POWER VOLUME POWER
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Radar Cross-section Comparison:
Mag. Coord. of ROI centre6.35⁰N, 150.90⁰E
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Radar Cross-section Comparison:
Mag. Coord. of ROI centre12.84⁰N, 149.98⁰E
Boa Vista
Santarem
Alta Foresta
SAR Scene
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Effect of Anisotropy:19 November 2010 6 April 2011
Scene Centre5.94° S, 65.13° WToA ~ 0300 UT
B B
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Scintillation and Ionospheric Parameters from SAR
• Avg. S4, RCS Enhancement: 0.13• Avg. CkL : 1.68 x 1034 , log10(CkL) = 34.23 Axial
RatioOne-way S4 Log10(CkL)
10:1 0.1589 34.328320:1 0.1459 34.253230:1 0.143 34.235640:1 0.142 34.228950:1 0.1415 34.226460:1 0.1412 34.224570:1 0.141 34.222880:1 0.141 34.222890:1 0.141 34.2228
100:1 0.141 34.2228
• Avg. S4,Contrast : 0.14• Avg. CkL : 1.68 x 1034 , log10(CkL) = 34.23)
𝑇𝑇𝑑𝑑2 ≅4𝑆𝑆42
𝑁𝑁𝑁𝑁 =
𝐿𝐿𝑆𝑆𝑆𝑆𝜸𝜸 𝜆𝜆𝑧𝑧𝑅𝑅 sec 𝜃𝜃
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 35
15.129°N, 77.18°E
ToA~1900 UT
23 March 2015 31 October 2016
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Scintillation and Ionospheric Parameters from SAR
• Avg. S4, RCS Enhancement: 0.154• Avg. CkL : 1.72 x 1034 , log10(CkL) = 34.27 Axial
RatioOne-way S4 Log10(CkL)
10:1 0.2443 34.65520:1 0.1863 34.418730:1 0.1664 34.31440:1 0.1573 34.264850:1 0.1524 34.236960:1 0.1496 34.220770:1 0.1478 34.2180:1 0.1466 34.20290:1 0.1458 34.198
100:1 0.1452 34.194
• Avg. S4,Contrast : 0.15• Avg. CkL : 1.66 x 1034 , log10(CkL) = 34.22)
𝑇𝑇𝑑𝑑2 ≅4𝑆𝑆42
𝑁𝑁𝑁𝑁 =
𝐿𝐿𝑆𝑆𝑆𝑆𝛾𝛾 𝜆𝜆𝑧𝑧𝑅𝑅 sec 𝜃𝜃
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 37
17 March 2015 26 April 2016
N N
52.16N, 172.46WToA ~ 2350 UT
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A:B N S410:1 57.26 0.275110:2 45.607 0.245510:3 36.075 0.218410:4 29.34 0.196910:5 24.6 0.180310:6 21.17 0.167310:7 18.617 0.156910:8 16.655 0.148410:9 15.114 0.1413
10:10 13.88 0.1355
A:B N S410:1 57.26 0.275110:2 37.94 0.223910:3 27.29 0.189910:4 21.091 0.16710:5 17.127 0.150510:6 14.397 0.137910:7 12.412 0.128110:8 10.907 0.120110:9 9.7287 0.1134
10:10 8.782 0.1077
A:B N S410:1 57.26 0.275110:2 33.07 0.209110:3 22.97 0.174210:4 17.68 0.152910:5 14.49 0.138710:6 12.36 0.127810:7 10.876 0.1210:8 9.784 0.113710:9 8.955 0.1088
10:10 8.31 0.1048
A:B N S410:1 57.26 0.275110:2 30.534 0.210:3 21.193 0.167410:4 16.613 0.148210:5 13.968 0.135910:6 12.287 0.127410:7 11.149 0.121410:8 10.34 0.116910:9 9.75 0.1135
10:10 9.302 0.1109
A:B N S410:1 57.26 0.275110:2 29.775 0.198410:3 20.989 0.166610:4 16.859 0.149310:5 14.554 0.138710:6 13.13 0.131710:7 12.196 0.12710:8 11.548 0.123510:9 11.081 0.121
10:10 10.734 0.1191
A:B N S410:1 57.26 0.275110:2 30.5 0.200810:3 21.976 0.170410:4 18.038 0.154410:5 15.883 0.144910:6 14.577 0.138810:7 13.73 0.134710:8 13.15 0.131810:9 12.737 0.1298
10:10 12.434 0.1282
A:B N S410:1 57.26 0.275110:2 32.713 0.207910:3 24.16 0.178710:4 20.16 0.163210:5 17.978 0.154210:6 16.66 0.148410:7 15.812 0.144610:8 15.234 0.141910:9 14.823 0.14
10:10 14.522 0.1385
δ = 60° δ = 75° δ = 90° δ = 105°
δ = 120° δ = 135° δ = 150°
N
3/1/2018 ISR SEMINAR SERIES 2017-2018, BOSTON COLLEGE 39
Conclusion:
• SAR can be helpful in augmenting and complementing GPS in scintillation studies
• Demerit: SAR image pair with unchanged background• Need: simple identification techniques
• Coeff. of Variation of Intensity is not a good measure• Introduction of Total Power and Depolarized Power measures
• Varying response of SAR RCS to scintillation may be explained by many factors • Effect of anisotropy, and variation of axial ratio in particular, on the estimation
of scintillation and ionospheric parameters is discussed
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On-going and Future Work:
• Extending the effect anisotropy on SAR data to mid- and high- latitudes
• Partial focusing of SAR data at ionospheric phase screen height to calculate ionospheric parameters and their comparison with those calculated from raw GPS data