Download - Ostst 2014 - scagliola
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.