Speckle reduction by along-track antenna pattern compensation on stacks of single look echoes

1. 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. 2
Outline
Speckle, multilooking and range noise
Equivalent number of looks
Along-track antenna pattern compensation
Experimental results
Conclusions

3. 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) 






N N SNR g
p
h
2
1
0.8


w (l, r) s(l, r) w(l, r) sl  
   ML
N
ML l
ml
w l r N
s l r w l r
N
w l r
ML
( , ) 1 0,1/
( , ) ( , )
1
( , )
1
  
   

Single look echo:
Multilooked echo:
Range noise

4. 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
ML N
Var I
E I
ENL    
[ ]
[ ]2
50 100 150 200 250
0
25
50
75
100
125
150
175
200
225
250
Waveform sample [N]
ENL [N]
0
0.5
1
Number of averaged looks
Experimental
50 100 150 200 250
0
0.5
1
1.5
Waveform sample [N]
ENL [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. 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 samples [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 
(N 1) look 
Along-track
antenna
pattern
(n) look 
Also the speckle is scaled
by the antenna pattern.


  
NML
l
A
ML
ml G l s l r w l r
N
w l r
1
( ) ( , ) ( , )
1
( , )

6. 6
Along-track antenna pattern compensation
20 40 60 80 100 120 140
0
0.5
1
1.5
2
2.5
x 10
-13
Single look echoes [N]
Power [W]
20 40 60 80 100 120 140
0
0.5
1
1.5
2
2.5
x 10
-13
Single look echoes [N]
Power [W]
Along-track
pattern
compensation
Pitch
estimation [1]
CryoSat
along-track
antenna pattern
1
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. 7
Along-track antenna pattern compensation
20 40 60 80 100 120 140
0
0.5
1
1.5
2
2.5
x 10
-13
Single look echoes [N]
Power [W]
20 40 60 80 100 120 140
0
0.5
1
1.5
2
2.5
x 10
-13
Single look echoes [N]
Power [W]
Along-track
pattern
compensation
Pitch
estimation [1]
CryoSat
along-track
antenna pattern
1
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. 8
Experimental results
CryoSat Stacks of Single look echoes over ocean
Alongtrackantenna pattern compensationComputeENLMultilookingComputeENLComputeENLMultilookingComputeENLComparisonComparison StackL1b

9. 9
ENL for single look echoes
50 100 150 200 250
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Waveform sample [N]
ENL [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. 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.
50 100 150 200 250
0
25
50
75
100
125
150
175
200
225
250
Waveform sample [N]
ENL [N]
No antenna compensation
0
0.5
1
ENL
Smoothed ENL
Normalized waveform
50 100 150 200 250
0
25
50
75
100
125
150
175
200
225
250
Waveform sample [N]
ENL [N]
Antenna compensation
0
0.5
1
ENL
Smoothed ENL
Normalized waveform

11. 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. 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 1400
0
50
100
150
Waveforms [N]
ENL [N]
No antenna comp.
Antenna comp.
200 400 600 800 1000 1200 1400
-50
0
50
100
150
Waveforms [N]
ENL increase [%]
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. 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.