AGU Fall Meeting 2012
OS21A-1677
Extracting Ocean Surface Currents from SAR: MCC and Doppler Centroid
1Aerospace
1William
Engineering Sciences, University of Colorado, Boulder, CO, USA
2Nansen Environmental and Remote Sensing Center, Norway
A. Qazi*,
J. Emery
2Morten W. Hansen
*[email protected]
Envisat ASAR IMG GEC VV-pol
25 m - Calibrated - Speckle filtered
20080417 - 1804 GMT
SAR MCC
The primary ocean backscattering mechanism for SAR is Bragg scattering through
small-scale surface capillary waves. Ocean surface slicks cause damping of surface
waves through “Marangoni damping”, which in turn damps the backscatter in SAR
intensity images. C-band SAR instruments aboard Envisat (ASAR) and ERS2 (AMI-SAR)
offer a unique possibility of observing temporal changes in sub-mesoscale to
mesoscale ocean surface features due to 30-minute time-lag in their orbits during
certain portions of their missions coincidence. The Maximum Cross-Correlation (MCC)
method (Emery et al., 1986) is applied here to Envisat-ERS2 30-minute lag image pairs
to generate ocean surface currents at 3 km resolution.
The MCC_SAR currents are compared separately with CODAR current fields of 6 km
and 2 km resolution (Kim et al., 2008). Some sample plots of re-gridded vectors are
shown below, along with the histograms of residuals (u, v, and direction). As a
preliminary statistical analysis, the residual histograms for both the 2 km and 6 km
CODAR data are tending towards Gaussian, which is an indication that differences
between CODAR and MCC_SAR currents are due to random errors. The means of the u
and v residuals, however, are not zero and show a mean +ve value; this suggests that
the MCC_SAR vectors show an overall larger magnitude than the CODAR vectors.
ERS2 AMI-SAR IMG GEC VV-pol
25 m – Calibrated – Speckle filtered
20080417 - 1834 GMT
Change Detection Difference Image
(Envisat – ERS2)
MCC_SAR Currents – Raw
20080417: 1804-1834 GMT
MCC_SAR Currents – Filtered
20080417: 1804-1834 GMT
MCC_SAR Currents – Geocorrected
20080417: 1804-1834 GMT
30 min lag
42
30
σ⁰ (dB)
σ⁰ (dB)
27
σ⁰ (dB)
1Waqas
-58
-73
-44
MCC_SAR Currents - 20090817: 1758-1830 GMT
CODAR Currents - 20090817: 1800
GMT
MCC_SAR Currents - 20080809: 1820-1850 GMT
CODAR Currents - 20080809: 1800-1900 GMT
MCC_SAR Currents - 20090408: 1815-1846 GMT
CODAR Currents - 20080809: 1800-1900 GMT
MCC_SAR
CODAR 2 km
MCC_SAR
CODAR 2 km
MCC_SAR
CODAR 2 km
MCC_SAR
CODAR 6 km
MCC_SAR
CODAR 6 km
MCC_SAR
CODAR 6 km
Histograms of Residuals: MCC_SAR – CODAR 2 km
Histogram of u Residuals
Histograms of Residuals: MCC_SAR – CODAR 6 km
Histogram of θ Residuals
Mean: 8.24 cm/s
Std: 17.77 cm/s
Histogram of u Residuals
Histogram of v Residuals
Mean: 5.37 cm/s
Std: 16.9 cm/s
N
Histogram of θ Residuals
Histogram of v Residuals
N
Mean: 7.1 cm/s
Std: 15.85 cm/s
Mean: 7.96 cm/s
Std: 20.87 cm/s
Mean: 19.95⁰
Std:
103.76⁰
Mean: 10.13⁰
Std:
99.19⁰
E
E
Doppler Centroid
Envisat Doppler Centroid Radial Currents
20090513: 055933 GMT
CODAR 6 km Radial Currents
20090513: 060000 GMT
MCC_SAR Radial Currents
20080806: 1843-1913 GMT
Envisat Doppler Centroid Radial Currents
20080806: 060347 GMT
+ve radial direction
Radial Velocity (cm/s)
+ve radial direction
Radial Velocity (cm/s)
+ve radial direction
Radial Velocity (cm/s)
+ve radial direction
Radial Velocity (cm/s)
The Doppler centroid tracking method is a SAR single-antenna
Doppler shift method which enables estimates of across-track
(XT) radial currents. The method utilizes the residual Doppler
shifts in the single-antenna return due to relative motion of the
target with respect to the antenna. For satellite platforms, the
centroid of the measured Doppler spectrum is dominated by the
antenna velocity relative to the rotating Earth. This major
contribution to the Doppler centroid, along with satellite attitude
and antenna pointing contributions, can be modeled and
removed from the measured Doppler spectrum. The resultant
“Doppler centroid anomaly” depicts the Doppler shift due to
geophysical processes, which over the ocean are winds, surface
currents, and waves.
Envisat Wide Swath Mode (WSM) intensity images are
provided with a low-resolution (around 10 km) resolution
Doppler centroid grid, which is processed to generate radial
currents (Hansen et al., 2011). Two separate cases of Doppler
Centroid derived radial current fields are lotted here, respectively,
with 6-km resolution CODAR and SAR_MCC derived currents,
both rotated on their axis to bring the u-component along the
SAR look direction. Visual comparison with CODAR shows the
same general features. The SAR_MCC currents are not coincident
in time with the Doppler Centroid currents (~12 hours lag);
however the 12 hour difference eliminates the impact of 6-hour
and 12-hour tides. Still, the SAR_MCC coverage is too sparse to
make any proper inspection. Further overlapping datasets are
being processed for statistical comparison.
References:
Acknowledgements:
Emery, W. J., Thomas, A. C., Collins, M. J., Crawford, W. R., & Mackas, D. L. (1986). An objective method for computing advective surface velocities from sequential infrared satellite images.
J. Geophys. Res., 91, C11, p. 12865-12878.
Kim, S. Y., Terrill, E. J, & Cornuelle, B. D. (2008). Mapping surface currents from HF radar radial velocity measurements using optimal interpolation. J. Geophys. Res., 113, C10023.
Hansen, M. W., Collard, F., Dagestad, K.-F., Johannessen, J., Fabry, P., & Chapron, B. (2011). Retrieval of Sea Surface Range Velocities From Envisat ASAR Doppler Centroid Measurements.
IEEE Trans. Geosci. Rem. Sens., 49 (10), p. 3582-3592.
Envisat and ERS2 SAR images provided by ESA under Cat-1 project 10009. CODAR currents over the US West Coast provided by Sung Yong Kim
(Scripps Institution of Oceanography, CA, USA). Partial funding by NASA Physical Oceanography Program. Waqas Qazi is partially supported by
the U.S. State Dept.’s Fulbright program.