Estimation of radial sea currents in the northern
Adriatic Sea close to the Venice Lagoon inlets #ID2141
ASI CSK Workshop
Stefano Zecchetto
ISAC-CNR, Uos Padova
[email protected]
Gianfranco Fornaro
IREA-CNR, Napoli
[email protected]
ASI CSK Workshop, Roma 27-29 March 2012
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Estimation of radial sea currents in the northern Adriatic
Sea close to the Venice Lagoon inlets #ID2141
Summary
1. Project description
2. Tools
3. Cases selection and geophysical
characterization
4. Image processing
5. Results
6. Critical points and future activities
ASI CSK Workshop, Roma 27-29 March 2012
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Project description
This project is aimed to assess if and with what accuracy the
radial sea surface currents can be evaluated using the CosmoSkymed SAR images. The radial sea surface current will be
computed from the map of the Doppler shift obtained from level 0
Stripmap Himage Cosmo-SkyMED images, and compared with the
radial sea surface current obtained from a 3-D oceanographic
model (SHYFEM)
The test area includes part of the northern Adriatic Sea and of the
Venice Lagoon (45° 10' – 45° 40' N, 12° 00' – 13° 00' E); the main
focus is on the three lagoon inlets because here the sea currents
reach the highest values of the Mediterranean Sea.
ASI CSK Workshop, Roma 27-29 March 2012
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Project description
Motivations
This area has been chosen
because the currents at the Venice lagoon inlets
reach
the
maximum
velocities
of
the
Mediterranean currents (up to 1.5 m/s)
for the availability of in-situ data set (current,
tide, wind)
for the availability of hydrodynamic model
routinely running
ASI CSK Workshop, Roma 27-29 March 2012
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Project description
Institutes involved
Istituto Scienze dell'Atmosfera e del Clima (ISAC), CNR, Uos. Padova
Istituto di Scienze Marine (ISMAR), CNR, Uos. Venezia
Istituto per il Rilevamento Elettromagnetico dell'Ambiente (IREA), CNR,
Uos. Napoli
Istituzione Centro Previsioni e Segnalazioni Maree (ICPSM),
Comune di Venezia, Venezia
ASI CSK Workshop, Roma 27-29 March 2012
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Project Tools
Shallow water HYdrodynamic Finite Element Model 3-D version
(SHYFEM, http://sites.google.com/site/shyfem/home)
ASI CSK Workshop, Roma 27-29 March 2012
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Project Tools
Shallow water HYdrodynamic Finite Element Model 3-D version (SHYFEM)
ASI CSK Workshop, Roma 27-29 March 2012
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Project Tools: Cosmo-SkyMed Stripmap Himage images
Area imaged ~50 km x 50 km
Grid size: 2.7 m in azimuth, 2.3 m in range
Polarization: HH (archived images)
Compromise between the area size and the spatial resolution
ASI CSK Workshop, Roma 27-29 March 2012
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Two examples of selected images
1 April 2009 17.01 GMT
Ws=8 m/s
Wdir=50°
Current: outflow
27 July 2009 17.01 GMT
Ws=6.6 m/s
Wdir=118°
Current: outflow
ASI CSK Workshop, Roma 27-29 March 2012
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Geophysical characterization: sea current field
1 April 2009 17.01 GMT
Ws=8 m/s
Wdir=50°
Current: outflow
27 July 2009 17.01 GMT
Ws=6.6 m/s
Wdir=118°
Current: outflow
ASI CSK Workshop, Roma 27-29 March 2012
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Processing of the selected images
• Development of the algorithm for DC estimation
• Analysis of the images characteristics
• Calibration of DC variations
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Development of the algorithm for DC estimation
DC estimation carried out locally after azimuth focusing to mitigate the problems
associated with the range migration
Raw data
Range
Compression
Azimuth
Focusing
Madsen DC
Estimation
1
f DC =
arg [ R ( 1 ) ]
2πT
R(n) azimuth self-correlation, T=1/PRF
-540 Hz
-440 Hz
DC estimation on block of 512x512 pixels (~1.4 km x 1.2 km). Frequency
Resolution of each block: 6 Hz (~ 10 cm/s)
ASI CSK Workshop, Roma 27-29 March 2012
Spurious DC variations: test on a internal area
The analysis of the azimuth spectrum has shown variations of the
DC with the range. Such variations are not annotated in the data.
Calibration of DC variations
Calibrated DC
f DC ( x,r ) =f DC ( x,r )−a−br−cr
A
1 N
a = ∑ ai
Parameters are
N i =1
estimated over
1 N
b = ∑ bi
several (say N) range
N i =1
lines and averaged
1 N
c = ∑ ci
N i =1
ASI CSK Workshop, Roma 27-29 March 2012
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Calibration of DC variations
Before DC
calibration
SCS-B Provided by ASI
After DC
calibration
E' stata inoltrata ad ASI la richiesta di accedere a stime di variazione del
Centroide Doppler che, oltre al contributo di tipo geometrico, portino in conto
anche di effetti di distorsione elettronica del fascio, ovvero di accedere a dati e
misure d’interesse per la valutazione della distorsione elettronica del fascio.
ASI CSK Workshop, Roma 27-29 March 2012
Range starting window sampling
On several images it has been experienced the presence of a power
decrease at far range, probably due to a non optimal use of the range
starting sampling window associated with the range first time.
Antenna pattern in range
extracted from the image of 25
Jan. 2011
25 Jan. 2011 Uncalibrated
SSC slant range geometry
ASI CSK Workshop, Roma 27-29 March 2012
Radial sea current maps
From SHYFEM
From CSK
1 April 2009 17.01 GMT
Ws=8 m/s
Wdir=50
Current: outflow
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5.4.1.1 Radial sea current maps
From CSK
From SHYFEM
1 April 2009 17.01 GMT
Ws=8 m/s
Wdir=50°
Current: outflow
ASI CSK Workshop, Roma 27-29 March 2012
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Radial sea current maps
From SHYFEM
From CSK
27 July 2009 17.01 GMT
Ws=6.6 m/s
Wdir=118°
Current: outflow
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Critical points
the results obtained in this project do not correspond to the extensive work carried out. The main
reason is the the presence in the CSK derived Doppler frequency maps of a spurious DC component
caused by the non orthogonality between the lookup direction and the satellite speed. This component
often hinders the geophysical signal present in the image.
access to the estimates of the DC variability due, besides the geometric contribution, to the electronic
distortion of the radar beam, is essential
spatial and frequency resolution of the CSK Doppler frequency map. We have used block of size
512x512 samples thus leading to a resolution of about 1.4 km in azimuth (approximately SN) and 1.2 km
in ground range (approximately WE). This resolution may be not sufficient to resolve features of similar
spatial scale, especially at the lagoon inlets where the surface current is higher. Furthermore, the
frequency resolution estimated of 6 Hz, leads to a sea surface current resolution of about 10 cm s -1 ,
which appears for the images analysed too coarse to resolve the features of the radial currents, which
have been found to have a range of the order of 20 cm s-1
ASI CSK Workshop, Roma 27-29 March 2012
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Possible future activity
Once solved the problems of the DC variability due to the geometric contribution and to the
electronic distortion of the radar beam
Select areas of stronger sea surface currents (outside the Mediterranean Sea)
Select CSK Stripmap Himage images at VV polarization
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