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Successive ASAR wide swath acquisitions (3 days apart) over Greater Agulhas and its return current. Color indicates the radial component of surface current (positive when the direction is facing the radar look meaning almost towards the West for those ascending tracks).
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Context 1
... successive ASAR wide swath acquisitions (3 days apart) over Greater Agulhas and its return current, after applying the wind correction, are presented on Fig. ...
Context 2
... dominant stream of the Agulhas current can be clearly identified and tracked on successive acquisitions, suggesting that this strong current and its return flow are well established over timescales larger than the satellite revisit time in this region. A global remaining bias high in the Doppler velocities estimate on Sept 19 frame ( Fig. 7 (a) ) is coming from unperfect Doppler reference but we shall remind here that restituted attitude files were not available, preventing from any correction for non-constant antenna misspointing. Land Doppler reference was used as a backup reference when ...
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Citations
... Magnitudes of U WD were higher at small incidence angles. Wind speed was the primary sea-state variable determining U WD [12,20,21]. Mouche et al. [22] derived an empirical geophysical model function (CDOP) that accounted for wind speed and direction. ...
The Doppler shift of microwave radar sea surface echoes serves as the foundation for sea surface current field retrieval; it includes the shift caused by satellite platform motion, ocean waves, and sea surface currents. The Doppler shift caused by ocean waves is known as the wave-induced Doppler velocity (UWD), and its removal is critical for the accurate retrieval of sea surface current fields. The low-incidence Ka-band real-aperture radar rotary scan regime has the capability of directly observing wide-swath two-dimensional current fields, but as a new regime to be further explored and validated, simulation and analysis of UWD in this regime have a significant influence on the hardware design and currently observed applications of this satellite system in its conceptual stage. In this study, we simulated and investigated the impacts of radar parameters and sea-state conditions on the UWD obtained from small-incidence-angle Ka-band rotational scanning radar data and verified the simulation results with the classical analytical solution of average specular scattering point velocity. Simulation results indicate that the change in the azimuth direction of platform observation affects UWD accuracy. Accuracy is the lowest when the antenna is in a vertical side-view. The UWD increases slowly with the incidence angle. Ocean waves are insensitive to polarization in the case of small-incidence-angle specular scattering. The increase in wind speed and the development of wind waves result in a substantial increase in UWD. We classified swell by wavelength and wave height and found that UWD increases with swell size, especially the contribution of swell height to UWD, which is more significant. The contribution of the swell to UWD is smaller than that of wind waves to UWD. Furthermore, the existence of sea surface currents changes the contribution of ocean waves to UWD, and the contribution weakens with increasing wind speed and increases with wind wave development.
... The CDOP model is an empirical geophysical model function that estimates the Doppler shift of wind-driven components. Researchers [12] [33] demonstrated that the DCA, the difference between the observed DC and the geometric Doppler shift, has a distinct geophysical signature based on the examination of the C-band ENVISAT/advanced SAR data at a worldwide scale. This signature has been shown by Alexis [34] to be correlated with the global wind field. ...
The Doppler scatterometer is an instrument used in real aperture radar (RAR) in conjunction with a circular scanning antenna and has wide swath and multi-angle characteristics. Many scholars have studied sea surface current vector field based on the use of the Doppler scatterometer. However, the existing methods directly use the phase difference of the spectrometer's echo to obtain the current vector field, and the resolution can be improved. This study proposes a high-resolution inversion method for ocean surface current fields based on circular scanning synthetic aperture radar (SAR). The current field is estimated by the proposed method based on the Doppler spectrum of the single complex image to realize high-resolution current vector field inversion. The current field inversion of circular scanning SAR is very sensitive to attitude and wind field errors. Accordingly, a model is established to study the influences of the main errors on Doppler shift estimation, and a global optimization method is proposed to realize high-precision current velocity inversion and synchronous error estimation. The effectiveness of this method is verified by using airborne circular scanning SAR data.
... Owing to the precise knowledge of satellite orbit and attitude, the 26 difference between the measured and predicted Doppler shifts represents a geophysical 27 quantity that comprises the line-of-sight velocity of scatters (wind, waves, and current) 28 caused by ocean surface movements. The most interested contribution of surface 29 current to the Doppler shift derived from SAR measurements is significantly 30 "contaminated" by orbital velocities of long surface waves [2]- [4]. The wave motion 31 contributions to the Doppler shift must therefore be properly removed to derive reliable 32 ocean surface current estimates from SAR data. ...
This paper proposes a Doppler velocity (DV) model based on dual co-polarized (co-pol) decomposition of a normalized radar cross-section of an ocean surface on polarized Bragg scattering and nonpolarized (NP) radar returns from breaking wave components. The dual co-pol decomposition provides a quantitative description of resonant and NP scattering, as well as their dependence on the incident angle, azimuth, and wind speed. Subsequently, the contributions of the facet (resonant Bragg waves and breakers) velocities, tilt, and hydrodynamic modulations due to long waves to the resulting DV can be quantified. The tilt modulation contributions to DV are estimated using the measured/empirical tilt modulation transfer function (MTF). The hydrodynamic modulations are mostly dominated by wave breaking and are estimated using a semiempirical model based on in situ measurements. In addition to the VV and HH radar data, which are required for dual co-pol decomposition and tilt MTF estimates, the surface wave spectrum is required in the DV determination for a given radar observation geometry. In this paper, qualitative and quantitative consistencies are presented between the model simulations and the empirical CDOP model. In a companion paper, DV analysis is presented to analyze the Sentinel-1 synthetic aperture radar measurements and collocated in situ measurements of surface wind and wave spectra.
... Their research also suggested that the SAR-measured velocity is mainly affected by the wind field due to the interactions between the wind-driven waves and surface current, and the accuracy of the current retrieval will be limited by the wind-driven waves. Based on previous studies, Collard et al. (2008) developed a new method of retrieving the sea surface current by constructing the C-band Doppler (CDOP) model from measured DC data using a neural network. The strategy is to calculate the wind-generated DC from the CDOP first and then subtract it from the total DC measured via spaceborne SAR to obtain the DCA caused by the radial current field. ...
Current direction retrieval is a significant part of the Doppler Centroid Technique (DCT) for remote sensing of the sea surface vector field, and it remains an unsolved problem that has prevented its wide application for more than 15 years. In this paper, we review and update the core concept of the DCT, through which the Ekman scheme for current direction retrieval is obtained. In addition, the real sea surface vector field at approximately 24°N with weak background flow during Hurricane Sam is retrieved. We also prove that the measured velocity of the DCT is just the magnitude of the sea surface current vector, and thus, the DCT is the most promising technique for remote sensing of sea surface current dynamics.
... These deviations were explained as effects of wave motions, i.e. modulation of the short Bragg waves by the current in addition to contributions of orbital motions of longer waves. Based on analysis of the Cband ENVISAT/ASAR data on the global scale, [5], [6] have shown that the Doppler centroid anomaly, i.e. difference between the measured Doppler centroid and the geometric Doppler shift, carries a clear geophysical signature. The authors showed that this signature is correlated to the global atmospheric circulation (wind field). ...
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.
... This anomaly represents the difference between the observed (from the sensor) and predicted (from the precise knowledge of the satellite motion) DC frequency shift. The DCa is associated with the nearsurface wind speed and surface current in the radar range direction [2]. When the wind-wave-induced contribution to the signal is removed the residual contains information about ocean surface current [3,4,5]. ...
Satellite-based Synthetic Aperture Radar (SAR) observations can provide global estimates of ocean surface motion. We used Doppler Centroid anomaly from Sentinel-1 Interferometric Wide (IW) mode observations collocated with Lagrangian ocean surface drifter (CARTHE) and high-resolution free-run ocean model (ROMS) to study Norwegian Coastal Current. The ocean surface current derived from the Sentinel-1 is in good agreement with the ocean surface drifter observations as well as the ocean surface velocity derived from ROMS. Although just one case is investigated, the study shows promising results of using Doppler shift observations from the Sentinel-1 Interferometric Wide (IW) mode for studying ocean surface currents in the coastal zone with high-spatial resolution.
... Recently, a novel data calibration technique based on the gyroscope telemetry from the satellites (OceanDataLab, 2019) has demonstrated promising capabilities to quantify the satellite attitude and hence provide reliable estimates of the Doppler shift. Hence, the wave-induced contribution to the Doppler shift (as established for the Envisat ASAR observations; Collard et al., 2008;Mouche et al., 2012) can now be revisited taking into account both wind waves and swell. ...
... The relationship between the geophysical Doppler shift (f rvl ) and the near-surface wind speed (u 10 ) and direction (Chapron et al., 2005;Collard et al., 2008;Collecte Localisation Satellites, 2019;Mouche & Chapron, 2015) are shown in Figure 3. As expected, f rvl increases with radial directed winds as well as increasing wind speed for both θ ¼ 23 ∘ and 36 ∘ as noticed in Figures 3a, 3b, 3d, and 3f. ...
The Doppler frequency shift acquired by Synthetic Aperture Radar (SAR) contains information about ocean surface motion induced by waves and the underlying ocean currents in the radar range direction. An accurate estimate of the wave‐induced contribution is therefore required to derive a reliable estimation of the ocean surface current. In this study, we developed an empirical model for estimating the wave‐induced Doppler shift based on Sentinel‐1B Wave Mode (WV) Level 2 Ocean products acquired from December 2017 to January 2018 collocated with wind field from ECMWF and wavefield from WAVEWATCH III. We found that the relationship between the wind field at 10 m height and the Doppler shift from Sentinel‐1 is in agreement with previous findings based on ASAR observations. Retraining of the conventional CDOP model for the Sentinel‐1 observations (CDOP‐S) yields distinct improvements. We speculate that the improvement is due to different sensor properties and hence biases in the data. Moreover, combing wave and wind information into the model yield considerable improvements especially for the Southern Ocean and the North Pacific. Given accurate wave bias correction, the ocean surface radial velocity maps based on 2 months of Sentinel‐1 acquisitions agree with ocean surface current climatology derived from multiyear drifter observations. This suggests that Sentinel‐1 Doppler shift observations can be used to study ocean surface currents with 20 km spatial resolutions at a monthly time scale.
... Several numerical models have been proposed to simulate WASV from the wind wave spectrum and radar parameters (e.g., Fois et al., 2015;Johannessen et al., 2008;Nouguier et al., 2018;Romeiser & Thompson, 2000). As a first practical step, the wind speed is taken as a proxy to characterize the wind wave effects on the surface motions detected by SAR Collard et al., 2008;Rouault et al., 2010). In addition, Mouche et al. (2012) considered both the wind speed and wind direction and proposed an empirical model for C-band SAR, using multiple azimuth and incidence angle observations. ...
Wind waves contribute a significant component to the sea surface currents detected by synthetic aperture radar (SAR) systems via the orbital velocity; this component is typically referred to as the wind wave‐induced artifact surface velocity (WASV). Accordingly, it is essential to precisely describe the WASV effect, so that the real sea surface currents can be obtained from SAR measurements. In order to enhance our understanding about WASV, we carried out a numerical analysis of the sea state dependence of WASV under various radar configurations. The simulated WASV compares reasonably well with observations reported in the literature. It is found that WASV not only depends on the wind speed but also increases considerably with the development of the wind waves, as characterized by wave age. WASV may increase by two to three times, in fully developed wind wave conditions, as compared to young waves. The wave age dependence appears to largely account for the observed uncertainty in WASV at a given wind speed; moreover, swell is also found to significantly influence WASV through hydrodynamic modulation of wind wave properties. These effects are stronger at higher wind speeds and vary significantly with the different radar configurations that are possible, in terms of polarizations and incidence angles. The variations in WASV using different shortwave spectral functions and angular spreading functions are examined in this manuscript.
... 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. Additionally, to estimate ocean surface velocity, the influence from the wind is typically removed from the Doppler estimate as it has been shown that wind and Doppler products are strongly correlated (Chapron et al. 2005;Vu La et al. 2017;Collard et al. 2008;Rouault et al. 2010). These artifacts, biases, and wind effects complicate the Doppler anomaly interpretation; regardless, it has been shown that strong ocean currents produce dominant signatures in the Doppler anomaly product (Chapron et al. 2005). ...
... It is worth mentioning that high wind speed removal (Vu La et al. 2017;Collard et al. 2008;Rouault et al. 2010) was not attempted since strong geostrophic currents (such as the Gulf Stream) or tidal currents have been shown to be highly correlated with the Doppler anomaly (Chapron et al. 2005). ...
This work presents a Gulf Stream (GS) North Wall (GSNW) detection algorithm applicable to Sentinel-1 Radial surface Velocity (RVL) products derived from Doppler centroid analysis of synthetic aperture radar (SAR) data collected off the east coast of Canada from February 2017 to August 2017. Visual comparison of the extracted location of the GSNW (obtained by evaluating the peak RVL gradient in the azimuth direction), and the estimated location of the GSNW as determined by the U.S. Naval Oceanographic Office and a GSNW search region (GSNWSR), indicates that the algorithm is capable of detecting the GSNW in ∼80% of the cases evaluated. Results are dependent on the geophysical orientation of the GS across the SAR swath, such that the GSNW detector performed most reliably when the GS was oriented across the swath resulting in maximized surface velocities in the range direction. When the GS meandered, or when GS eddies appeared in the RVL data, the GSNW was often partially detected or detected multiple times, reducing confidence in the extracted location. It is anticipated that the results obtained using the Sentinel-1 RVL products will be applicable to SAR data from other platforms.
... With a median wind speed of 7 m s −1 , this is a 70 cm s −1 error in the wind direction for C-band at 23 • of incidence, or Ka-band at 12 • . Such a high value is not acceptable for a single satellite pass, but these errors cancel out when the Doppler velocity is averaged over many satellite passes, 10 or more, as done by Collard et al. (2008) and Rouault et al. (2010). Even at the higher incidence angles of 58 • proposed by Bourassa et al. (2016), for which we expect G 7 in Ka-band, the wave bias is reduced by a factor of 4, but the RMSE on U WB is still significant at 20 cm s −1 , even if there is no error on the wind. ...
We propose a satellite mission that uses a near-nadir Ka-band
Doppler radar to measure surface currents, ice drift and ocean waves at
spatial scales of 40 km and more, with snapshots at least every day for
latitudes 75 to 82°, and every few days for other latitudes. The use of incidence angles
of 6 and 12° allows for measurement of the directional wave spectrum,
which yields accurate corrections of the wave-induced bias in the current
measurements. The instrument's design, an algorithm for current vector
retrieval and the expected mission performance
are presented here. The instrument proposed
can reveal features of tropical ocean and marginal ice zone (MIZ) dynamics that are
inaccessible to other measurement systems, and providing global monitoring of
the ocean mesoscale that surpasses the capability of today's nadir
altimeters. Measuring ocean wave properties has many applications,
including examining wave–current interactions, air–sea fluxes, the transport and
convergence of marine plastic debris and assessment of marine and coastal hazards.
