Microwaves and Radar InstituteMicrowaves and Radar Institute

Correction of Ionospheric Distortions in Low Correction of Ionospheric Distortions in Low

Frequency Interferometric SAR DataFrequency Interferometric SAR Data

Jun Su Kim, Pau Prats & Konstantinos P. Papathanassiou

Microwave and Radar Institute German Aerospace Center

Microwaves and Radar InstituteMicrowaves and Radar Institute

SAR imaging through the Earth’s ionosphereSAR imaging through the Earth’s ionosphere

Displacement (due to group delay)

Defocusing (dispersion)

Small

Proportional to TEC (TEC) itself

Azimuth distortionsRange distortions

Displacement (due to phase adv.)

Defocusing (ionosphere variation)

Large and visible

Proportional to derivative of dTEC/dx (dTEC/dx) for displacement

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Azimuth shift decorrelationAzimuth shift decorrelation

Determination of azimuth shift

TECPRF

20

piercing Δdx

d

cfD

vΔa

f

Δa

piercingv

PRF

fD

x

: Azimuth shift [pixel]

: Velocity of piercing point

: Pulse repetition frequency

: Doppler rate

: azimuth distance

TEC change rate of 5 TECU/100 km

Satellite orbit

Varying ionosphere

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Azimuth shift: result (Azimuth shift: result ( comparison) comparison)

Before After

ALOS-PalSAR: Collville, Alaska

-histogram

Shift FRContour: Impovement larger than 0.2

Best case (low temp, weak TEC var.)

Pauli

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Interferometric phase: First order effectInterferometric phase: First order effect

0

TEC4

cf

Δφ

Determination of interferometric phase

Sensitivity

L-band: 13.30 rad./TECU= 2.12 cycle/TECU

1 cycle ~ 0.47 TECU

P-band:38.82 rad./TECU= 6.18 cycle/TECU

1 cycle ~ 0.16 TECU

Interferograms under ionospheric phases

Collville Beaver

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Interferometric phase: correction and resultsInterferometric phase: correction and results

- =

Original FR Corrected phase

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Combined estimation: principleCombined estimation: principle

Satellite orbit

Mean -> FR

Slant -> shift

Varying ionosphere

CET111

N

Ω

CET1111

6

CET211

12TEC2

NL

NNLx

Divide Ionosphereinto N segments

1TEC 2TEC 3TEC NTEC

20

ˆ

cmf

κBe

Coherent length = L

TNTECTECTECCET 21

Measure of FR Measure of Azimuth shift

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Combined estimation: estimation (phase and coherence)Combined estimation: estimation (phase and coherence)

CET

Δa

Ω

aΔ

Ω

G0

0G

Equation to be solved

Solution:

FR only Combined

Interferometric phase correction

Co

her

ence

co

mp

aris

on

:

a b

y 3-

way

s

obs1D

121DCET dT

DT

CGWGCG

Microwaves and Radar InstituteMicrowaves and Radar Institute

Correction using estimated Correction using estimated TECTEC

Adoption of air-borne motion compensation

12

14

,

2

v

fr

rf

aiono

ionoadecomp

Flow chart

SLC

Decompress

iono

Compress

SLC’

riono=0 km riono=100 km riono=200 km riono=300 km

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Full band vs. Sub-band az. shift estimationFull band vs. Sub-band az. shift estimation

Low

Middle

HighFull

Low Middle High

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Sub-band Correction schemeSub-band Correction scheme

6 sublooks

3 sublooks

Full band

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Differential az. sub-band interferogramDifferential az. sub-band interferogram

GROUND

ORBIT

Ionospheric intensity

S1 S2 S3

2,1x

ΔTEC

1ΔTEC

2ΔTEC

H

h

D

Wv

NΔx

r

asat

1

H

h

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Differential az. sub-band interferogramDifferential az. sub-band interferogram

N=2

The more sub-bands (N) we have, (when coherence is good)

1. the finer structure we get,

2. we can avoid phase unwrapping,

3. the higher resistance to the decorrelation it has.

N=9

a estimation

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Simulated 6MHz P-band dataSimulated 6MHz P-band data

Without Ionospheric disturbances

Coherence-range

HH channel SLC

Coherence with undisturbed master

(N=81)

Under Ionospheric disturbances

(kp=3, Ckl=50)

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Interferometric phase retrieval Interferometric phase retrieval (kp=3, Ckl=50)(kp=3, Ckl=50)

Ionospheric phase screen

Coherence as a function of range

Nr of Looks =3

A = 3,000 m

Nr of Looks =33

A = 300 m

Nr of Looks =10

A = 1,000 m

Nr of Looks =100

A = 100 m

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Interferometric phase retrieval (Far Range)Interferometric phase retrieval (Far Range)

Ionospheric phase screen

Nr of Looks =3

A = 3,000 m

Nr of Looks =33

A = 300 m

Nr of Looks =10

A = 1,000 m

Nr of Looks =100

A = 100 m

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ConclusionConclusion

Azimuth full band approach

Azimuth shift can be corrected for slow ionospheric variations.

Combination of Az shift and FR gives better TEC estimation.

Air-borne motion compensation method can be applied.

Azimuth sub-band approachSub-band approach always give finer structure of the ionosphere

Azimuth shift is compensated also in sub-bands.

Sub-band inter-interferogram is well coincidence with direct shift estimation

Performance increases with increasing number of looks (i.e. degreasing azimuth apperture) on the cost of spatial resolution.

Microwaves and Radar InstituteMicrowaves and Radar Institute

Correction of Ionospheric Distortions in Low Correction of Ionospheric Distortions in Low

Frequency Interferometric SAR DataFrequency Interferometric SAR Data

Jun Su Kim, Pau Prats & Konstantinos P. Papathanassiou

Microwave and Radar Institute German Aerospace Center