| 1. |
- Dammann, Dyre Oliver, 1985, et al.
(författare)
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Instantaneous sea ice drift speed from TanDEM-X interferometry
- 2019
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Ingår i: Cryosphere. - : Copernicus GmbH. - 1994-0424 .- 1994-0416. ; 13:4, s. 1395-1408
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Tidskriftsartikel (refereegranskat)abstract
- The drift of sea ice is an important geophysical process with widespread implications for the ocean energy budget and ecosystems. Drifting sea ice can also threaten marine operations and present a hazard for ocean vessels and installations. Here, we evaluate single-pass along-track synthetic aperture radar (SAR) interferometry (S-ATI) as a tool to assess ice drift while discussing possible applications and inherent limitations. Initial validation shows that TanDEM-X phase-derived drift speed corresponds well with drift products from a ground-based radar at Utqiagvik, Alaska. Joint analysis of TanDEM-X and Sentinel-1 data covering the Fram Strait demonstrates that S-ATI can help quantify the opening/closing rate of leads with possible applications for navigation. S-ATI enables an instantaneous assessment of ice drift and dynamic processes that are otherwise difficult to observe. For instance, by evaluating sea ice drift through the Vilkitsky Strait, Russia, we identified short-lived transient convergence patterns. We conclude that S-ATI enables the identification and analysis of potentially important dynamic processes (e.g., drift, rafting, and ridging). However, current limitations of S-ATI are significant (e.g., data availability and they presently only provide the cross-track vector component of the ice drift field) but may be significantly reduced with future SAR systems.
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| 2. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Correlation between the Backscatter and Doppler Shift Modulations in SAR Images and Its Effect on the Doppler Centroid Estimation
- 2023
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2023-July, s. 3962-3965
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Konferensbidrag (refereegranskat)abstract
- A known and challenging effect in the retrieval of ocean surface currents from synthetic aperture radar (SAR), is the so called wave-induced velocity bias. This effect is due to the correlation between the normalized radar cross section (NRCS) and Doppler shift modulations, which are generated by the long modulating waves. This paper analyses two collocated NRCS and Doppler frequency shift images acquired by the along-track interferometric SAR TanDEM-X at two polarizations VV and HH. These images capture a swell traveling nearly in the range direction. First, it is shown that the NRCS and Doppler shift modulations, due to the modulating swell wave, are highly correlated. Second, it is shown that this correlation affects the mean Doppler shift. Consequently, the ocean current estimation can be biased by approximately 0.2 m/s, which can be significant at low current magnitudes and when high-accuracy current retrievals are required.
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| 3. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Empirical Relationship Between the Doppler Centroid Derived From X-Band Spaceborne InSAR Data and Wind Vectors
- 2022
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 60
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Tidskriftsartikel (refereegranskat)abstract
- One of the challenges in ocean surface current retrieval from synthetic aperture radar (SAR) data is the estimation and removal of the wave-induced Doppler centroid (DC). This article demonstrates empirically the relationship between the dc derived from spaceborne X-band InSAR data and the ocean surface wind and waves. In this study, we analyzed over 300 TanDEM-X image pairs. It is found that the general characteristics of the estimated dc follow the theoretically expected variation with incidence angle, wind speed, and wind direction. An empirical geophysical model function (GMF) is fit to the estimated dc and compared to existing models and previous experiments. Our GMF is in good agreement (within 0.2 m/s) with other models and data sets. It is found that the wind-induced Doppler velocity contributes to the total Doppler velocity with about 15% of the radial wind speed. This is much larger than the sum of the contributions from the Bragg waves (~0.2 m/s) and the wind-induced drift current (~3% of wind speed). This indicates a significant (dominant) contribution of the long wind waves to the SAR dc. Moreover, analysis of dual-polarized data shows that the backscatter polarization ratio (PR=σ⁰VV/σ⁰HH) and the dc polarization difference (PD=|dcVV|-|dcHH|) are systematically larger than 1 and smaller than 0 Hz, respectively, and both increase in magnitude with incidence angle. The estimated PR and PD are compared to other theoretical and empirical models. The Bragg scattering theory-based (pure Bragg and composite surface) models overestimate both PR and PD, suggesting that other scattering mechanisms, e.g., wave breaking, are involved. In general, it is found that empirical models are more consistent with both backscatter and Doppler data than theory-based models. This motivates a further improvement of SAR dc GMFs.
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| 4. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Measurements of Sea Surface Currents in the Baltic Sea Region Using Spaceborne Along-Track InSAR
- 2019
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 57:11, s. 8584-8599
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Tidskriftsartikel (refereegranskat)abstract
- The main challenging problems in ocean current retrieval from along-track interferometric (ATI)-synthetic aperture radar (SAR) are phase calibration and wave bias removal. In this paper, a method based on differential InSAR (DInSAR) technique for correcting the phase offset and its variation is proposed. The wave bias removal is assessed using two different Doppler models and two different wind sources. In addition to the wind provided by an atmospheric model, the wind speed used for wave correction in this work is extracted from the calibrated SAR backscatter. This demonstrates that current retrieval from ATI-SAR can be completed independently of atmospheric models. The retrieved currents, from four TanDEM-X (TDX) acquisitions over the Öresund channel in the Baltic Sea, are compared to a regional ocean circulation model. It is shown that by applying the proposed phase correction and wave bias removal, a good agreement in spatial variation and current direction is achieved. The residual bias, between the ocean model and the current retrievals, varies between 0.013 and 0.3 m/s depending on the Doppler model and wind source used for wave correction. This paper shows that using SAR as a source of wind speed reduces the bias and root-mean-squared-error (RMSE) of the retrieved currents by 20% and 15%, respectively. Finally, the sensitivity of the sea current retrieval to Doppler model and wind errors are discussed.
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| 5. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Phase calibration of TanDEM-X ATI-SAR data for sea surface velocity measurements
- 2017
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2017-July, s. 922-925
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Konferensbidrag (refereegranskat)abstract
- It has been demonstrated that Along Track Interferometric (ATI) SAR is a useful tool to retrieve ocean surface currents. The ATI SAR provides an interferometric phase (hereafter called phase) which is directly related to the Line-Of-Sight (LOS) component of the surface velocity. The accuracy of ocean currents retrieval is highly dependent on the phase processing. For instance, a properly processed phase must equal zero over static targets. The measured TanDEM-X phase rarely (if ever) satisfies this condition which indicates a phase offset independent of the surface properties. The offset can be either due to a phase synchronization issue or to using inaccurate orbital and attitude information in the processing. The objective of phase calibration is the estimation and the removal of the offset and possible trends from the measured phase. In this paper, the topographic phase is simulated using a Digital Elevation Model (DEM) and baseline information. The calibration is carried out by estimating the average of the phase over land, after topography correction, and subtracting the estimated value from the measured phase. Finally, the residual phase trend is removed using a second order polynomial fitting.
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| 6. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Wind direction ambiguity removal using along-track InSAR: A case study
- 2018
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2018-July, s. 3262-3265
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Konferensbidrag (refereegranskat)abstract
- The main problem in wind retrieval using SAR imagery is the lack of the wind direction information. A few methods have been proposed to extract the wind direction from SAR images. The main limitation of these methods is the 180 degrees ambiguity in the direction. Usually, an external source of wind direction is used to remove this ambiguity. This study exploits the Along-track Interferometric SAR (ATI-SAR) phase to demonstrate its usefulness to tackle this problem. A method is proposed to remove the wind direction ambiguity using the ATI-SAR phase information. This is based on the fact that the interferometric phase is related to the sea surface motion direction. Depending on the sign convention, the phase is positive/negative for advancing/receding target respectively. This effect is used to assist the wind extraction algorithm to select the most plausible direction. The results show a very good agreement with atmospheric model and visual investigation.
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| 7. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Wind-wave effect on ATI-SAR measurements of ocean surface currents in the Baltic Sea
- 2016
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Ingår i: 36th IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2016; Beijing; China; 10-15 July 2016. ; 2016-November, s. 3982-3985
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Konferensbidrag (refereegranskat)abstract
- Along-Track Interferometric (ATI) SAR has demonstrated through several studies a capability to detect ocean surface currents. One of the most challenging problems in ocean surface current retrieval using SAR is the removal of the wind-wave contribution. The phase difference provided by ATI-SAR technique is directly related to the radial velocity of the moving ocean surface. In order to infer the current-only velocity from the total phase the wind-wave contribution need to be removed. This is achieved by simulation of SAR Doppler spectra from given wind fields. This paper investigates the effect of the local wind on ATI-SAR phase. A study case, where the backscatter modulation is dominated by the wind variation, is illustrated using TanDEM-X data over the Baltic Sea. It is shown that retrieving high resolution winds from SAR data using an empirical wind model and using the retrieved winds as input to the SAR imaging model improves the simulated SAR signatures.
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| 8. |
- Gommenginger, Christine, et al.
(författare)
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SEASTAR: A mission to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas
- 2019
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 6:JUL
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere e.g. freshwater, pollutants. As numerical models continue to evolve towards finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed towards spaceborne implementation within Europe and beyond.
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