93 research outputs found
Towards wide-swath high-resolution mapping of total ocean surface current vectors from space: Airborne proof-of-concept and validation
No full textTwo-dimensional high-resolution maps of total surface current vectors obtained for the first time with an airborne demonstrator of the innovative Wavemill instrument concept are validated against HF radar data and compared with output from the POLCOMS high-resolution coastal ocean circulation model. Wavemill is a squinted along-track interferometric SAR system optimized for ocean surface current vector retrieval that operates at moderate incidence angles (∼30°) and is compatible with spaceborne implementation. This paper represents the first comprehensive validation of the current retrieval capabilities of squinted along-track SAR interferometry in support of its development as a future European Space Agency Earth Explorer mission.
Wavemill airborne data were acquired in October 2011 in Liverpool Bay off the west coast of Great Britain in light southerly wind (5.5 m/s) and maximum tidal ebbing flow (0.7 m/s) conditions. Contributions to the measured SAR interferometric phase by surface gravity waves, known as the Wind-wave induced Artefact Surface Velocity (WASV), were removed using our best estimate of wind conditions and the (Mouche et al., 2012) empirical correction derived from Envisat ASAR. Validation of the 1.5 km resolution Wavemill current vectors against independent current measurements from HF radar gives very encouraging results, with Wavemill biases and precisions typically better than 0.05 m/s and 0.1 m/s for surface current speed, and better than 10° and 7° for current direction.
The sensitivity of the current retrieval to the wind vector used to compute the WASV is estimated. A ± 1 m/s error (bias) in wind speed has minimal impact on the quality of the retrieved currents. In contrast, the choice of wind direction is critical: a bias of ± 15° in the direction of the wind vector degrades the accuracy of the airborne current speed against the HF radar by about ± 0.2 m/s. This highlights the need for future instruments to provide calibrated SAR Normalised Radar Cross Section data to support retrieval of wind and current vectors simultaneously.
Comparisons of POLCOMS surface currents with HF radar data indicate that the model reproduces well the overall temporal evolution of the tidal current (correlation of spatial fields against HF radar over two tidal cycles of 0.9) but that the model features a systematic 1-h delay in the timing of the maximum ebbing flow in eastern parts of the domain near the Mersey Bar Light buoy. At the maximum ebb flow, the model underestimates the current speed (bias of −0.2m/s) with respect to the HF radar and Wavemill data at the time of the flights. Both the HF radar and Wavemill data reflect much greater snapshot spatial variability of the ocean surface current field than is present in the model, resulting in poor correlation of instantaneous spatial fields (< 0.5) between POLCOMS and the HF radar data. The Wavemill data reveal high spatial variability of ocean surface currents at fine scales, which are not visible in the 4km resolution HF radar data. Wavemill detects several strong (1–1.5m/s) localized current jets associated with deeper bathymetry channels in shallow waters (< 10 m) that are too narrow or too close to land to be observed by the HF radar. The study confirms the value of synoptic wide-swath maps of high-resolution ocean surface current vectors for coastal applications and to validate and develop high-resolution ocean circulation models
Simultaneous ocean surface current and wind vectors retrieval with squinted SAR interferometry: Geophysical inversion and performance assessment
No full textSimultaneous measurements of ocean surface current and wind vectors at the ocean submesoscale (O [1–10 km]) are needed to improve our understanding of upper ocean mixing, air-sea interactions, ocean biophysical processes and large-scale oceanic transports. A new satellite mission concept called SEASTAR aims to do just that. The concept is a Ku-band along-track interferometric Synthetic Aperture Radar (SAR) system with two squinted beams pointing ±45° from broadside and incidence angles around 30°. The paper presents an inversion strategy to retrieve simultaneously ocean surface current and wind vectors and reports on the performance obtained with different wind/current conditions and instrument configurations. Results are based on numerical simulations using a Bayesian approach and existing geophysical model functions (GMFs) of the microwave Normalized Radar Cross Section (NRCS) and Doppler shift.Using the baseline two-look instrument configuration and realistic instrument noise figures (radiometric resolution: kp = 5 and 12%; Δdf = 2 and 5 Hz), the root-mean square errors (RMSE) of the retrieved current and wind vectors are typically better than [0.1 m/s, 10°] for current and [0.5 m/s, 5°] for wind. This inversion setup yields four ambiguous solutions within a current range of ∼1 m/s. The addition of dual polarization (VV, HH) capability helps to discriminate these ambiguities. The retrieval performance depends weakly on geophysical parameters such as wind speed, current velocity or current direction, but is sensitive to wind direction because of its strong effect on current retrieval through the wind-wave induced artifact surface velocity (WASV). Larger retrieval errors are obtained when the wind is aligned with one of the antenna line-of-sight (LoS) directions, although errors remain typically below [0.2 m/s, 25°] for current and [0.5 m/s, 15°] for wind. Improving the retrieval performance regardless of wind direction could be achieved either with lower noise figures on σ0, or with higher incidence angles, or by including an additional third-look direction in azimuth (e.g. to achieve a configuration similar to Metop/ASCAT scatterometers) as per the SEASTAR mission concept submitted to EE10
Wind-Wave induced velocity in ATI SAR Ocean Surface Currents: First experimental evidence from an airborne campaign
No full textConventional and along-track interferometric (ATI) Synthetic Aperture Radar (SAR) sense the motion of the ocean surface by measuring the Doppler shift of reflected signals. Measurements are affected by a Wind-wave induced Artefact Surface Velocity (WASV) which was modelled theoretically in past studies and has been estimated empirically only once before with Envisat ASAR by Mouche et al., (2012). An airborne campaign in the tidally dominated Irish Sea served to evaluate this effect and the current retrieval capabilities of a dual-beam SAR interferometer known as Wavemill. A comprehensive collection of Wavemill airborne data acquired in a star pattern over a well-instrumented validation site made it possible for the first time to estimate the magnitude of the WASV, and its dependence on azimuth and incidence angle from data alone. In light wind (5.5 m/s) and moderate current (0.7 m/s) conditions, the wind-wave induced contribution to the measured ocean surface motion reaches up to 1.6 m/s upwind, with a well-defined 2nd order harmonic dependence on direction to the wind. The magnitude of the WASV is found to be larger at lower incidence angles. The airborne WASV results show excellent consistency with the empirical WASV estimated from Envisat ASAR. These results confirm that SAR and ATI surface velocity estimates are strongly affected by WASV and that the WASV can be well characterized with knowledge of the wind knowledge and of the geometry. These airborne results provide the first independent validation of Mouche et al., 2012, and confirm that the empirical model they propose provides the means to correct airborne and spaceborne SAR and ATI SAR data for WASV to obtain accurate ocean surface current measurements. After removing the WASV, the airborne Wavemill retrieved currents show very good agreement against ADCP measurements with a root mean square error (RMSE) typically around 0.1 m/s in velocity and 10° in direction
New Map of Mexico, Texas and parts of the bordering states.
No full textScale approximately 1:5,000,000.1 map : outline color ; 94 x 65 cmBelow author: Revue et augmentee par C.H. Picquet, geographe du roi et du Duc d'Orleans.Below place of publication: Publi en 1834; revue en 1837, 1839, 1840, 1845
Simultaneous ocean surface current and wind vectors retrieval with squinted SAR interferometry: Geophysical inversion and performance assessment
No full textSimultaneous measurements of ocean surface current and wind vectors at the ocean submesoscale (O [1–10 km]) are needed to improve our understanding of upper ocean mixing, air-sea interactions, ocean biophysical processes and large-scale oceanic transports. A new satellite mission concept called SEASTAR aims to do just that. The concept is a Ku-band along-track interferometric Synthetic Aperture Radar (SAR) system with two squinted beams pointing ±45° from broadside and incidence angles around 30°. The paper presents an inversion strategy to retrieve simultaneously ocean surface current and wind vectors and reports on the performance obtained with different wind/current conditions and instrument configurations. Results are based on numerical simulations using a Bayesian approach and existing geophysical model functions (GMFs) of the microwave Normalized Radar Cross Section (NRCS) and Doppler shift.
Using the baseline two-look instrument configuration and realistic instrument noise figures (radiometric resolution: kp = 5 and 12%; Δdf = 2 and 5 Hz), the root-mean square errors (RMSE) of the retrieved current and wind vectors are typically better than [0.1 m/s, 10°] for current and [0.5 m/s, 5°] for wind. This inversion setup yields four ambiguous solutions within a current range of ∼1 m/s. The addition of dual polarization (VV, HH) capability helps to discriminate these ambiguities. The retrieval performance depends weakly on geophysical parameters such as wind speed, current velocity or current direction, but is sensitive to wind direction because of its strong effect on current retrieval through the wind-wave induced artifact surface velocity (WASV). Larger retrieval errors are obtained when the wind is aligned with one of the antenna line-of-sight (LoS) directions, although errors remain typically below [0.2 m/s, 25°] for current and [0.5 m/s, 15°] for wind. Improving the retrieval performance regardless of wind direction could be achieved either with lower noise figures on σ0, or with higher incidence angles, or by including an additional third-look direction in azimuth (e.g. to achieve a configuration similar to Metop/ASCAT scatterometers) as per the SEASTAR mission concept submitted to EE10
First multi-year assessment of Sentinel-1 radial velocity products using HF radar currents in a coastal environment
No full textDirect sensing of total ocean surface currents with microwave Doppler signals is a growing topic of interest for oceanography, with relevance to several new ocean mission concepts proposed in recent years. Since 2014, the spaceborne C-band SAR instruments of the Copernicus Sentinel-1 (S1) mission routinely acquire microwave Doppler data, distributed to users through operational S1 Level-2 ocean radial velocity (L2 OCN RVL) products. S1 L2 RVL data could produce high-resolution maps of ocean surface currents that would benefit ocean observing and modelling, particularly in coastal regions. However, uncorrected platform effects and instrument anomalies continue to impact S1 RVL data and prevent direct exploitation.
In this paper, a simple empirical method is proposed to calibrate and correct operational S1 L2 RVL products and retrieve two-dimensional maps of surface currents in the radar line-of-sight. The study focuses on the German Bight where wind, wave and current data from marine stations and an HF radar instrumented site provide comprehensive means to evaluate S1 retrieved currents. Analyses are deliberately limited to Sentinel-1A (S1A) ascending passes to focus on one single instrument and fixed SAR viewing geometry. The final dataset comprises 78 separate S1A acquisitions over 2.5 years, of which 56 are matched with collocated HF radar data. The empirical corrections bring significant improvements to S1A RVL data, producing higher quality estimates and much better agreement with HF radar radial currents.
Comparative evaluation of S1A against HF radar currents for different WASV corrections reveal that best results are obtained in this region when computing the WASV with sea state rather than wind vector input. Accounting for sea state produces S1 radial currents with a precision (std of the difference) around 0.3 m/s at ∼1 km resolution. Precision improves to ∼0.24 m/s when averaging over 21 × 27 km2, with correlations with HF radar data reaching up to 0.93. Evidence of wind-current interactions when tides and wind align and short fetch conditions call for further research with more satellite data and other sites to better understand and correct the WASV in coastal regions.
Finally, 1 km resolution maps of climatological S1A radial currents obtained over 2.5 years reveal strong coastal jets and fine scale details of the coastal circulation that closely match the known bathymetry and deep-water coastal channels in this region. The wealth of oceanographic information in corrected S1 RVL data is encouraging for Doppler oceanography from space and its application to observing small scale ocean dynamics, atmosphere and ocean vertical exchanges and marine ecosystem response to environmental change
An Early Warning System for Tipping Points in the Greenland Ice Sheet and the North Atlantic Subpolar Gyre: Exploring the Edge of the Possible with AEROSTATS.
No full textSatellite and In Situ Sampling Mismatches: Consequences for the Estimation of Satellite Sea Surface Salinity Uncertainties
No full textalidation of satellite sea surface salinity (SSS) products is typically based on comparisons with in-situ measurements at a few meters’ depth, which are mostly done at a single location and time. The difference in term of spatio-temporal resolution between the in-situ near-surface salinity and the two-dimensional satellite SSS results in a sampling mismatch uncertainty. The Climate Change Initiative (CCI) project has merged SSS from three satellite missions. Using an optimal interpolation, weekly and monthly SSS and their uncertainties are estimated at a 50 km spatial resolution over the global ocean. Over the 2016–2018 period, the mean uncertainty on weekly CCI SSS is 0.13, whereas the standard deviation of weekly CCI minus in-situ Argo salinities is 0.24. Using SSS from a high-resolution model reanalysis, we estimate the expected uncertainty due to the CCI versus Argo sampling mismatch. Most of the largest spatial variability of the satellite minus Argo salinity is observed in regions with large estimated sampling mismatch. A quantitative validation is performed by considering the statistical distribution of the CCI minus Argo salinity normalized by the sampling and retrieval uncertainties. This quantity should follow a Gaussian distribution with a standard deviation of 1, if all uncertainty contributions are properly taken into account. We find that (1) the observed differences between Argo and CCI data in dynamical regions (river plumes, fronts) are mainly due to the sampling mismatch; (2) overall, the uncertainties are well estimated in CCI version 3, much improved compared to CCI version 2. There are a few dynamical regions where discrepancies remain and where the satellite SSS, their associated uncertainties and the sampling mismatch estimates should be further validated
The OSCAR Instrument: SAR Processing and Calibration
No full text15th European Conference on Synthetic Aperture Radar, 23-26 April 2024, Munich, Germany.-- 6 pages, 11 figures, 1 tableThe OSCAR instrument is a gimbal-based multi-channel interferometric Ku-band SAR system recently developed and built within the framework of a European Space Agency funded project Ocean Surface Currents Airborne Radar demonstrator. The OSCAR system is tailored to the observations of the ocean surface motion and retrieval of wind. This paper presents the development background of the OSCAR instrument. It also presents the methodology and techniques used to process and calibrate the OSCAR data up to co-registered intra-channel phase interferometric complex SAR images. Calibration over land shows that velocity accuracy of 5cm/s is achieved. Results from the OSCAR functional campaign and from the first OSCAR operational campaign, the SeaSTARex, are presentedWith the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe
SEASTARex: Technical Assistance to Earth Explorer 11 SEASTAR Phase 0 campaign. Final Report.
No full textSEASTARex is an airborne and ground truth scientific campaign to support the SeaSTAR phase 0 candidature for ESA’s Earth Explorer 11. Deploying the Ocean Surface Current Airborne Radar (OSCAR) airborne SAR instrument, this scientific campaign represents the first of a SAR instrument
of its configuration and the first simultaneous retrieval of total surface current vectors (TSCV) and ocean surface vector winds (OSVW) from interferometric SAR data.
A first campaign was conducted in the Iroise Sea with a wide range of ground truth measurements deployed (HF radar, mooring, X-band marine radar, stereo camera), tasked SAR images, in addition to high-resolution and mature numerical models to simulate ocean waves and current. This campaign
consists of three sites: a site over the Ouessant island characterized by strong tidal current gradients, a homogeneous site South of the island where the mooring was deployed and a third area offshore in the Bay of Biscay parallel to an ASCAT pass. The campaign acquired data over four days (17, 22, 25, 26 May 2022). Comparison between OSCAR derived Total Surface Current Vectors and ground
truth measurements give correlation and RMSE of 0.08m/s and 8.5° in velocity and direction respectively. These first results have been added to the EE11 SeaSTAR Report for Assessement (ESA, 2023) and a paper was submitted (McCann et al., 2023).
Based on this success and on the unique opportunity to fly simultaneously with NASA/CNES SWOT (Surface Water and Ocean Topography) satellite mission during its daily CalVal repeat phase and a wide range of in situ measurements (e.g. BioSWOT-Med) all relevant to OSCAR and SeaSTAR, a
second OSCAR airborne campaign was then planned. This campaign was executed in May 2023 and consisted of three acquisition days (5, 7, 8 May 2023) with a rose pattern centered over the left SWOT sub-swath at (41.09°N, 4.29°W) generating a disk of about 40km diameter. Acquired data during this Med Sea campaign are not analyzed in the frame of this project, but the analysis was supported through a CNES contract and preliminary results are integrated in the EE11 SeaSTAR Assessment Report (ESA, 2023
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