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Proceedings ArticleDOI

Ocean doppler anomaly and ocean surface current from Sentinel 1 tops mode

10 Jul 2016-pp 3993-3996

TL;DR: Results show strong Doppler signal and dynamics from coastal areas caused by a mixture of surface current and wind/wave induced drifts at a spatial resolution of around 2 km2 in IW mode and 4km2 in EW mode.
Abstract: Processing and analysis of Doppler information from Sentinel 1A Interferometric Wide (IW) and Extra Wide (EW) modes are performed for assessing the capabilities of mapping ocean surface current field. Data from Agulhas (South-Africa) and Norwegian Coast are used in combination with numerical models, higher-order satellite products, and Lagrangian drifters. Results show strong Doppler signal and dynamics from coastal areas caused by a mixture of surface current and wind/wave induced drifts at a spatial resolution of around 2 km2 in IW mode and 4km2 in EW mode. Doppler values of up to 70 Hz are observed, corresponding to a surface drift velocity of 3.5 m/s. The Sentinel 1 retrieved surface current component is in reasonable agreement with the circulation models and drifter measurements. Surface current values up to 1.5 m/s are observed in the central Agulhas current, with a standard deviation of around 0.39 m/s with respect to Lagrangian drifters.
Topics: Drifter (59%), Ocean surface topography (55%), Doppler effect (51%), Sea surface temperature (50%)

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Geoscience and Remote Sensing Symposium (IGARSS),
2016 IEEE International. 10-15 July 2016
Pages 3993-3996
http://dx.doi.org/10.1109/IGARSS.2016.7730038
http://archimer.ifremer.fr/doc/00356/46713/
© IEEE 2016
Achimer
http://archimer.ifremer.fr
Ocean doppler anomaly and ocean surface current from
Sentinel 1 tops mode
Johnsen Harald
1, *
, Nilsen Vegard
1
, Engen Geir
1
, Mouche Alexis
2
, Collard Fabrice
3
1
Northern Research Institute, Box 6434, N-9294 Tromsoe, Norway
2
Laboratoire d'Ocanographie Spatiale - Ifremer, 29280 Plouzane, France
3
OceanDataLab, 29280 Locmaria Plouzane, France
* Corresponding author : Harald Johnsen, email address : harald.johnsen@norut.no
Abstract :
Processing and analysis of Doppler information from Sentinel 1A Interferometric Wide (IW) and Extra
Wide (EW) modes are performed for assessing the capabilities of mapping ocean surface current field.
Data from Agulhas (South-Africa) and Norwegian Coast are used in combination with numerical models,
higher-order satellite products, and Lagrangian drifters. Results show strong Doppler signal and
dynamics from coastal areas caused by a mixture of surface current and wind/wave induced drifts at a
spatial resolution of around 2 km2 in IW mode and 4km2 in EW mode. Doppler values of up to 70 Hz
are observed, corresponding to a surface drift velocity of 3.5 m/s. The Sentinel 1 retrieved surface
current component is in reasonable agreement with the circulation models and drifter measurements.
Surface current values up to 1.5 m/s are observed in the central Agulhas current, with a standard
deviation of around 0.39 m/s with respect to Lagrangian drifters.
Keywords : Doppler effect, Sea surface, Sea measurements, Ocean temperature, Surface treatment,
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Figure 3: Sentinel 1A IW Doppler anomaly (a) and the ground
range velocity component (b) from ascending mode acquisition
over the Agulhas area outside South-Africa. The Lagrangian
drifter position and direction is indicated with a red arrow in (b).
The velocity from the drifter was 1.1 m/s.
a)
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Citations
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Journal ArticleDOI
TL;DR: A two-scale model (KaDOP) is proposed to fit the observed DC, based on the radar modulation transfer function (MTF) previously developed for the same data set, and the wave-induced DC contribution is expressed as a function of MTF, significant wave height, and wave peak frequency.
Abstract: Multi-year field measurements of sea surface Ka-band dual-co-polarized (vertical transmit–receive polarization (VV) and horizontal transmit–receive polarization (HH)) radar Doppler characteristics from an oceanographic platform in the Black Sea are presented. The Doppler centroid (DC) estimated using the first moment of 5 min averaged spectrum, corrected for measured sea surface current, ranges between 0 and ≈1 m/s for incidence angles increasing from 0 to 70 ∘ . Besides the known wind-to-radar azimuth dependence, the DC can also depend on wind-to-dominant wave direction. For co-aligned wind and waves, a negative crosswind DC residual is found, ≈−0.1 m/s, at ≈20 ∘ incidence angle, becoming negligible at ≈ 60 ∘ , and raising to, ≈+0.5 m/s, at 70 ∘ . For our observations, with a rather constant dominant wave length, the DC is almost wind independent. Yet, results confirm that, besides surface currents, the DC encodes an expected wave-induced contribution. To help the interpretation, a two-scale model (KaDOP) is proposed to fit the observed DC, based on the radar modulation transfer function (MTF) previously developed for the same data set. Assuming universal spectral shape of energy containing sea surface waves, the wave-induced DC contribution is then expressed as a function of MTF, significant wave height, and wave peak frequency. The resulting KaDOP agrees well with independent DC data, except for swell-dominated cases. The swell impact is estimated using the KaDOP with a modified empirical MTF.

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11 citations


Cites background or methods from "Ocean doppler anomaly and ocean sur..."

  • ...researchers [4], [11], [14], [56] and, second, on the following arguments specific to this work....

    [...]

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Journal ArticleDOI
Abstract: We explore the potential of computing coastal ocean surface currents from Moderate-Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) satellite imagery using the maximum cross-correlation (MCC) method. To improve on past versions of this method, we evaluate combining MODIS and VIIRS thermal infrared (IR) and ocean color (OC) imagery to map the coastal surface currents and discuss the benefits of this combination of sensors and optical channels. By combining these two sensors, the total number of vectors increases by 58.3 % . In addition, we also make use of the different surface patterns of IR and OC imagery to improve the tracking performance of the MCC method. By merging the MCC velocity fields inferred from IR and OC products, the spatial coverage of each individual MCC field is increased by 65.8 % relative to the vectors derived from OC images. The root mean square (RMS) error of the merged currents is 18 cm · s − 1 compared with coincident HF radar surface currents. A 5-year long time serious of merged MCC computed currents was used to investigate the current structure of the California Current (CC). Weekly, seasonal, and 5-year mean flows provide a unique space-time picture of the oceanographic variability of the CC.

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Cites methods from "Ocean doppler anomaly and ocean sur..."

  • ...The ocean surface currents also can be retrieved using Doppler anomaly from Sentinel-1 [5]....

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TL;DR: Though the agreement is not perfect, the experiment demonstrates that the Doppler technique is capable of measuring a signal from the ice if the ice is fast moving, however, for typical ice speeds, the uncertainties quickly grow beyond the speeds the authors are trying to measure.
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References
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Abstract: The median Doppler shift of radar echoes is analyzed in measurements by ENVISAT's Advanced Synthetic Aperture Radar (ASAR) over the ocean. This Doppler centroid differs from a predicted signal based on the predicted motion of the satellite and Earth. This anomaly, converted to a surface Doppler velocity U D , appears to be of geophysical origin. Two wide-swath images over the Gulf Stream around Cape Hatteras suggest that U D contains high-resolution information on surface currents, while on a global scale, U D is found to vary with the wind speed in the range direction. A simple quantitative forward model is proposed, based on a practical two-scale decomposition of the surface geometry and kinematics. The model represents the effect of the wind through the wave spectrum, and gives U D ≈ γU 10 ∥ + U c ∥ , with U 10 ∥ and U c ∥ as the 10 m wind speed and quasi-Eulerian current in the line of sight of the radar projected on the sea surface, respectively, and γ as a coefficient function of the wind speed, wave development, and radar geometry. It is found that for an incidence angle of 23°, γ ≈ 0.3 for moderate winds and fully developed seas. This model is validated with a global data set of ASAR Wave Mode observations, with colocated model winds, acquired over the global ocean during the years 2003 and 2004. The Doppler signal therefore provides the signed parameter U D that can be used to reduce the wind direction ambiguity in the inversion of high-resolution wind fields from SAR imagery. A qualitative validation of current effects is shown for the English Channel where tidal currents dominate. Thus it should be possible to combine this previously ignored geophysical Doppler signal with traditional information on sea surface roughness, in order to provide very high resolution wind and current fields.

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Abstract: Previous analysis of Advanced Synthetic Aperture Radar (ASAR) signals collected by ESA's Envisat has demonstrated a very valuable source of high-resolution information, namely, the line-of-sight velocity of the moving ocean surface. This velocity is estimated from a Doppler frequency shift, consistently extracted within the ASAR scenes. The Doppler shift results from the combined action of near surface wind on shorter waves, longer wave motion, wave breaking and surface current. Both kinematic and dynamic properties of the moving ocean surface roughness can therefore be derived from the ASAR observations. The observations are compared to simulations using a radar imaging model extended to include a Doppler shift module. The results are promising. Comparisons to coincident altimetry data suggest that regular account of this combined information would advance the use of SAR in quantitative studies of ocean currents.

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TL;DR: High-resolution Doppler frequency measurements are used to estimate the subsecond line-of-sight motion of drifting sea ice in Fram Strait and are compared with buoy measurements and a recent cross-correlation algorithm for tracking ice between pairs of images.
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5 citations


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