SAR AND THE IONOSPHERE: CHALLENGES AND · PDF fileSAR data," Geoscience and Remote Sensing...

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]


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



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



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



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



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



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



Victoria – Wilkes Coasts, Antarctica Ionospheric Effects in Ice Velocity Measurements

Courtesy: E. Rignot, B. Scheuchl, UCI 9



INTRODUCTION OF IONOSPHERIC EFFECTS IN SAR

UNDERSTANDING AND CORRECTING IONOSPHERIC ARTIFACTS MAPPING THE IONOSPHERE USING SAR DATA

10



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



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



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



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)



Development of Improved Correction Algorithms

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• 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



• 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


• 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



INTRODUCTION OF IONOSPHERIC EFFECTS IN SAR

UNDERSTANDING AND CORRECTING IONOSPHERIC ARTIFACTS

MAPPING THE IONOSPHERE USING SAR DATA

22



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



Opportunities: SAR as a Source of Ionospheric Information

Ionospheric variation: 2006–2011 @ 70°-90°N

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Continental-Scale information on average ionospheric behavior

Ionospheric parameters from ~750,000 L-band images



• 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

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SAR AND THE IONOSPHERE: CHALLENGES AND · PDF fileSAR data," Geoscience and Remote Sensing...

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