Speckle reduction by along-track antenna pattern compensation on stacks

of single look echoes

Michele Scagliola (Aresys)

Marco Fornari (ESA-ESTEC),

Nicolas Tagliani (Aresys)

Andrea Di Giacinto (Aresys)

2

Outline

Speckle, multilooking and range noise

Equivalent number of looks

Along-track antenna pattern compensation

Experimental results

Conclusions

3

From speckle to range noise Speckle: radar echoes are the incoherent

summation of many randomly phased echoes from small scattering regions of the surface. This cause the speckle noise.

Multilooking: The main purpose of multi-

looking is to reduce the speckle through summing statistically independent looks at any given surface location.

Range noise: the uncertainty on the range

measure is function of the number of statistically independent looks (N)

SNRNNg

ph

21

8.0

),(),(),( rlwrlsrlwsl

ML

N

lMLml

Nrlw

rlwrlsN

rlwML

/1,01),(

),(),(1

),(

1

Single look echo:

Multilooked echo:

Range noise

4

Equivalent Number of Looks

On ocean surface, the ENL for CryoSat single look and multilooked echoes was evaluated and it resulted far from expected values

MLNIVar

IEENL

][

][ 2

50 100 150 200 2500

25

50

75

100

125

150

175

200

225

250

Waveform sample [N]

EN

L [

N]

50 100 150 200 2500

0.5

1

Number of averaged looks

Experimental

50 100 150 200 2500

0.5

1

1.5

Waveform sample [N]

EN

L [

N]

Experimental

Expected

The ENL is the estimate of the effective number of statistically independent looks and it is expected to be smaller than the number of averaged single look echoes

ENL for single look echoes ENL for 20Hz multilooked echoes

5

Acquisition of a stack of single looks

All the acquired single look echoes that are referred to a given point on Earth surface are gathered in a stack.

Range s

am

ple

s [

N]

Single look echoes [N]

20 40 60 80 100 120 140 160 180

100

200

300

400

500

600

Stack for SAR over Ocean

Each single look echo has been acquired for a different look angle, so that is scaled by the along track antenna pattern as function of the look angle.

)0(look

)1( Nlook

Along-track

antenna

pattern

)(nlook

Also the speckle is scaled by the antenna pattern.

MLN

l

AML

ml rlwrlslGN

rlw

1

),(),()(1

),(

6

Along-track antenna pattern compensation

20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5x 10

-13

Single look echoes [N]

Pow

er

[W]

20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5x 10

-13

Single look echoes [N]

Pow

er

[W]

Along-track pattern

compensation

Pitch estimation [1]

CryoSat along-track

antenna pattern

1 1.2 1.4 1.6 1.8 21

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

Stack profile

Antenna pattern

[1] “Pitch estimation for CryoSat by analysis of stacks of single look echoes”, Scagliola M., Fornari M., Tagliani N., submitted to Geoscience and Remote Sensing Letters, IEEE

7

Along-track antenna pattern compensation

20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5x 10

-13

Single look echoes [N]

Pow

er

[W]

20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5x 10

-13

Single look echoes [N]

Pow

er

[W]

Along-track pattern

compensation

Pitch estimation [1]

CryoSat along-track

antenna pattern

1 1.2 1.4 1.6 1.8 21

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

Stack profile

Antenna pattern

[1] “Pitch estimation for CryoSat by analysis of stacks of single look echoes”, Scagliola M., Fornari M., Tagliani N., submitted to Geoscience and Remote Sensing Letters, IEEE

The speckle from all the single look echoes in the stack is raised to the same power A better speckle reduction is expected by compensating the along-track antenna pattern

8

Experimental results

CryoSat Stacks of Single look echoes over ocean

Along track

antenna pattern

compensation

Compute ENL

Multilooking

Compute ENL

Compute ENL

Multilooking

Compute ENL

Comparison

Comparison

Sta

ck

L1

b

9

ENL for single look echoes

50 100 150 200 2500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Waveform sample [N]

EN

L [

N]

No antenna comp.

Antenna comp.

Averaging the ENL computed on 250 stacks over ocean, the ENLs in figure below have been obtained. The expected value of ENL=1 is obtained only in case that the along-track antenna pattern is compensated.

10

ENL for 20Hz multilooked waveforms The ENL has been evaluated on the 20Hz multilooked waveforms by considering 50 consecutive waveforms. By comparison it can be notice that the ENL on the 20Hz multilooked waveforms is higher just after the leading edge in case that alng-track antenna pattern is compensated.

50 100 150 200 2500

25

50

75

100

125

150

175

200

225

250

Waveform sample [N]

EN

L [

N]

No antenna compensation

50 100 150 200 2500

0.5

1

ENL

Smoothed ENL

Normalized waveform

50 100 150 200 2500

25

50

75

100

125

150

175

200

225

250

Waveform sample [N]

EN

L [

N]

Antenna compensation

50 100 150 200 2500

0.5

1

ENL

Smoothed ENL

Normalized waveform

11

ENL for 20Hz multilooked waveforms

In figure below the average ENL on about 1500 multilooked waveforms has been compared with the number of averaged looks in the stack, that represent the upper bound for the ENL. The ENL for antenna compensation is closer to the upper bound.

Retracking point

When the antenna pattern compensation is applied, the ENL is higher starting from range sample 40 (all the waveforms are aligned with respect to the sample 50, which is the reracking point computed by OCOG).

12

ENL increase at retracking point Several CryoSat acquisition over ocean have been processed to evaluate the ENL increase in correspondence of the retracking point.

200 400 600 800 1000 1200 14000

50

100

150

Waveforms [N]

EN

L [

N]

No antenna comp.

Antenna comp.

200 400 600 800 1000 1200 1400-50

0

50

100

150

Waveforms [N]

EN

L i

ncre

ase

[%

]

Average increase = 33.6848%

An average increase of about 30% has been evaluated on the CryoSat SAR acquisitions. An example is here on the right.

13

Conclusions ENL lower than 1 has been evaluated on CryoSat stacks of single look

echoes and this has been addressed to the along-track antenna pattern scaling of the echoes.

By compensating the along-track antenna pattern on stacks of single look echoes before multilooking

An ENL approximately equal to 1 has been measured for single look echoes

An ENL increase has been measured for the 20Hz multilooked echoes

An ENL increase of about 30% has been measured in correspondence of the retracking point

The next step is to verify that the variance in sea level anomaly is reduced by compensating the stack, as it is expected from ENL.