Estimation of radial sea currents in the northern Adriatic Sea … · Estimation of radial sea currents in the northern Adriatic Sea close to the Venice Lagoon inlets #ID2141 ASI

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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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Summary1. Project description

2. Tools

3. Cases selection and geophysical

characterization

4. Image processing

5. Results

6. Critical points and future activities

Estimation of radial sea currents in the northern Adriatic

Sea close to the Venice Lagoon inlets #ID2141


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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 Cosmo-Skymed 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.


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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


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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

Project description


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Project Tools

Shallow water HYdrodynamic Finite Element Model 3-D version (SHYFEM, http://sites.google.com/site/shyfem/home)


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Project Tools

Shallow water HYdrodynamic Finite Element Model 3-D version (SHYFEM)


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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


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Two examples of selected images

27 July 2009 17.01 GMT

Ws=6.6 m/s W

dir=118°

Current: outflow

1 April 2009 17.01 GMT

Ws=8 m/s W

dir=50°

Current: outflow


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Geophysical characterization: sea current field

1 April 2009 17.01 GMT

Ws=8 m/s W

dir=50°

Current: outflow

27 July 2009 17.01 GMT

Ws=6.6 m/s W

dir=118°

Current: outflow


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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

Development of the algorithm for DC estimationDC estimation carried out locally after azimuth focusing to mitigate the problems associated with the range migration

-540 Hz -440 Hz

Range

Compression

Raw data

Madsen DC

Estimation

Azimuth

Focusing

f DC=1

2πTarg [R (1 ) ]


R(n) azimuth self-correlation, T=1/PRF

DC estimation on block of 512x512 pixels (~1.4 km x 1.2 km). Frequency

Resolution of each block: 6 Hz (~ 10 cm/s)

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

∑=

=N

iiaN

a1

1

∑=

=N

iibN

b1

1

∑=

=N

iicN

c1

1

f DC A( x,r )=f DC ( x,r )−a−br−cr 2

Calibrated DC

Parameters are

estimated over

several (say N) range

lines and averaged


Calibration of DC variations

SCS-B Provided by ASI

Before DC

calibration

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.

Range starting window samplingOn 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. 201125 Jan. 2011 Uncalibrated

SSC slant range geometry


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Radial sea current maps

From CSK From SHYFEM

1 April 2009 17.01 GMT

Ws=8 m/s W

dir=50

Current: outflow


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5.4.1.1 Radial sea current maps


1 April 2009 17.01 GMT

Ws=8 m/s W

dir=50°

Current: outflow


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Radial sea current maps


27 July 2009 17.01 GMT

Ws=6.6 m/s W

dir=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


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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


Estimation of radial sea currents in the northern Adriatic Sea … · Estimation of radial sea currents in the northern Adriatic Sea close to the Venice Lagoon inlets #ID2141 ASI

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