Ocean doppler anomaly and ocean surface current from Sentinel 1 tops mode
Summary (1 min read)
1. INTRODUCTION
- The Doppler centroid anomaly recorded over ocean with a Synthetic Aperture Radar (SAR) can be used to obtain range directed velocity which has been demonstrated to provide valuable estimates of the near surface wind speed, ocean surface current [1],[2],[3] and sea ice sea drift [4].
- The basis for their analysis is the Sentinel-1 Level 2 products from IW and EW modes collocated with Lagrangian drifters and model data.
- Secondly, the methodology for retrieving the ocean surface current (OSC) from the Doppler anomaly is outlined.
- In Section 4 some validation results are presented and discussed.
3.2. Doppler anomaly analysis
- The Doppler anomaly contains information of the ocean surface wind field, sea state and ocean surface current.
- An estimate of the ocean surface current (OSC) induced Doppler is then given as: (2) ϖ dcosc =ϖ dcphys −ϖ dccdop Eq.(2) is then converted to ground range radial surface current by the relation: (3) Ur = ϖ dc osc 2krad sinθ where krad is the radar wavenumber and θ is the local incidence angle.
- The Ur values are collocated with Lagrangian drifters and circulation models for validation.
- For the Norwegian Coastal areas, the wind vector field is provided from the AROME model with a spatial resolution of 2.5 km.
- For the Agulhas areas, the collocated Oscar current field and the GlobCurrent field are extracted using the Ocean Virtual Laboratory toolbox.
4. RESULTS
- The analysis of Sentinel 1 Doppler anomaly shows strong signatures and dynamic of surface current and wind/wave induced drifts.
- In Fig. 3 is shown an example of S1A IW ascending acquisition over the Agulhas current outside South-Africa.
- Is it expected that the Doppler anomaly is dominated by wind/wave effects, and less by coastal current.
- Altogether 32 scenes in IW and EW were analysed using the procedure described above.
- A standard deviation of 0.39 m/s and a bias of 0.38 m/s are observed between S1A and Lagrangian drifters.
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Citations
4 citations
Cites background from "Ocean doppler anomaly and ocean sur..."
...In fact, Johnsen et al. (2016) reported that the Sentinel-1 Doppler products contain artifacts in the range direction as well as biases across the entire product (noticeable over land where the Doppler anomaly should be zero) due to inaccurate attitude and antenna miss pointing calibrations....
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...It is anticipated that since the current gradient, rather than the surface current velocities, are of interest, a precise estimate of the Doppler anomaly may not be required suggesting that biases in the Doppler product (as noted by Johnsen et al. (2016)) should not prevent GSNW detection....
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...Secondly, since the gradient is evaluated along the azimuth direction, range artifacts noticed in the Sentinel1 RVL products (as described by Johnsen et al. (2016)) would not interfere with the detection algorithm....
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...Our results indicate that the GSNW can be detected in these products (regardless of the known artifacts presented by Johnsen et al. (2016)) especially in cases where the RVL captures the GS in a favorable orientation (aligned in the radar range direction)....
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4 citations
4 citations
2 citations
Cites methods from "Ocean doppler anomaly and ocean sur..."
...Analysis of the Doppler information from Sentinel 1 allows retrieving surface current velocity [6]....
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References
298 citations
197 citations
"Ocean doppler anomaly and ocean sur..." refers background or methods in this paper
...The wind/wave Doppler contribution (π dc cdop U10 ( ) ) is predicted using the CDOP [3] with input the best possible wind field, U10 extracted from numerical weather model....
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...The wind/wave Doppler contribution (ϖ dccdop U10( ) ) is predicted using the CDOP [3] with input the best possible wind field, U10 extracted from numerical weather model....
[...]
...4c) together with CDOP is used to predict and remove the wind/wave signal from the S1A Doppler anomaly....
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...The Doppler centroid anomaly recorded over ocean with a Synthetic Aperture Radar (SAR) can be used to obtain range directed velocity which has been demonstrated to provide valuable estimates of the near surface wind speed, ocean surface current [1],[2],[3] and sea ice sea drift [4]....
[...]
157 citations
23 citations
"Ocean doppler anomaly and ocean sur..." refers background in this paper
...The Doppler centroid anomaly recorded over ocean with a Synthetic Aperture Radar (SAR) can be used to obtain range directed velocity which has been demonstrated to provide valuable estimates of the near surface wind speed, ocean surface current [1],[2],[3] and sea ice sea drift [4]....
[...]
6 citations
"Ocean doppler anomaly and ocean sur..." refers methods in this paper
...A high-precision Doppler centroid anomaly estimator was developed and implemented as part of the Sentinel-1 Level 2 ocean processor [5],[6]....
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Frequently Asked Questions (10)
Q2. What is the main contribution to the standard deviation and bias?
The main contribution to the standard deviation and bias is expected to be the uncertainty in attitude as well as in the wind field used to remove the contribution to the Doppler anomaly from wind and waves.
Q3. What is the use of data from Agulhas?
Data from Agulhas (South-Africa) and Norwegian Coast are used in combination with numerical models, higher-order satellite products, and Lagrangian drifters.
Q4. What is the purpose of this paper?
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.
Q5. What is the name of the ocean processor?
A high-precision Doppler centroid anomaly estimator was developed and implemented as part of the Sentinel-1 Level 2 ocean processor [5],[6].
Q6. What is the purpose of this work?
This work is performed under the S1 Mission Performance Centre activities and the SEOM S1-4SCI Ocean Study, funded by ESA/ESRIN, and CIRFA – 237906/O30 funded by Norwegian Research Council.
Q7. What is the possible wind field for the EW mode?
The wind/wave Doppler contribution (ϖ dccdop U10( ) ) is predicted using the CDOP [3] with input the best possible wind field, U10 extracted from numerical weather model.
Q8. What is the meaning of the term?
Index Terms— Sentinel 1A, Doppler Anomaly, OceanSurface CurrentThe Doppler centroid anomaly recorded over ocean with a Synthetic Aperture Radar (SAR) can be used to obtain range directed velocity which has been demonstrated to provide valuable estimates of the near surface wind speed, ocean surface current [1],[2],[3] and sea ice sea drift [4].
Q9. Where is the second test area located?
A second test area is outside the North-Norwegian coast where the continental shelf and the coastal current are close to the coastline.
Q10. What is the wind vector field for the Agulhas area?
A Lagrangian drifter was within the area at the time of acquisition as indicated in Fig. 3b (red arrow), with a velocity of 1.1 m/s and direction of 228 degN.