The estimation of speckle noise or its suppression is a crucial need to estimate geophysical parameters from radar remote sensing. This is particularly important for the ocean wave spectra retrieval from measured radar backscatter coefficients, as speckle noise can completely dominate the signal in certain conditions. Such measurements are performed by the spaceborne SWIM radar, a near-nadir looking, conical scanning instrument carried by the CFOSAT satellite dedicated to the measurement of directional spectra of ocean waves from the analysis of backscattered fluctuations. In this study, we compare empirical results on the speckle spectrum obtained from the SWIM observations using three different types of acquisition and a theoretical model. We show that a cross-spectral method is efficient for estimating the speckle energy spectrum with a look separation of the order of 13 ms, but shows some limits when this time lag is increased to about 40ms because of the limited footprint overlap. In the case of an azimuthal scanning geometry like SWIM, the speckle energy increases by several order of magnitude in a sector of about ±15° close to the along-track direction. From the theoretical model, we conclude that this is due to the decrease of the Doppler bandwidth in this look geometry and that the variance of the surface scatterer velocities limits the speckle energy increase. It also explains the sensitivity to wind speed of the speckle energy in this azimuthal sector. The almost linear decrease of speckle energy with wavenumber is explained by the spectral response of the radar impulse, slightly modified by range decimation and resampling applied in the first steps of processing. Close to the along-track direction, the variation of speckle energy with latitude is mainly explained by the variation in the integration time imposed by the SWIM on-board command. The model results also show that the impact of Earth rotation on the speckle noise is small. Finally, we show that the speckle correction has an impact on the shape of the wave spectrum derived from the SWIM inversion.

The surface wave investigation and monitoring (SWIM) instrument, onboard China and France Oceanography Satellite (CFOSAT), is a Ku-band real-aperture radar, with six beams that illuminate the ocean surface at near-nadir incidences ranging from 0° to 10°. In this study, we investigate the effect of rain on the normalized radar cross section (NRCS) measured by SWIM under both tropical cyclone (TC) and non-TC conditions. Rain primarily attenuates the radar backscatter from the ocean surface. Under non-TC conditions, when wind speeds are below 21 m/s, the NRCS reduction is small (4–5 dB) under heavy rain (>15 mm/h) conditions. The impact of rain generally decreases as wind speed increases, especially at the smallest incidence angle. At low wind speeds, the NRCS reduction is also sensitive to the incidence angle. Based on a simplified model and the differing sensitivities of NRCS to surface roughness at two near-nadir incidence angles, we find that rain also influences the surface signal at incidence angles below 10°, in addition to the dominant effect of atmospheric attenuation, which leads to a consistent NRCS reduction across all wind speeds and incidence angles. Specifically, it contributes positively to NRCS at wind speeds above 7 m/s and negatively at lower wind speeds. This behavior is attributed to increased surface roughness from splashes and ring waves at low wind speeds, and to decreased roughness due to wave damping at moderate and high wind speeds. In TC environments, the NRCS tends to saturate under light to moderate rain rates (15 mm/h) conditions, the NRCS continues to decrease as wind speed increases. In both TC and non-TC conditions, beyond atmospheric attenuation, the impact of rain on surface roughness must be considered to fully explain the observed NRCS variations with wind and rain rate.

Surface waves investigation and monitoring (SWIM) can provide global wave spectra, but under small sea conditions, the presence of parasitic peaks at low wavenumbers, surfboard effects, and residual speckle noise lead to performance degradation of SWIM wave height spectrum products. To reduce the impacts of the above factors on the SWIM wave height spectrum, in this article, a convolution neural network (CNN) method based on BU-Net is proposed for calibrating SWIM omnidirectional wave height spectra with buoy measurements under sea states (wind wave mainly/swell mainly) and sea surface conditions (wind speed from 9 to 19 m/s, and significant wave height (SWH) from 0.8 to 3.4–4.2 m). The calibration results show that the impact of the above factors on the SWIM omnidirectional wave height spectrum can be corrected. The correlation coefficients between the corrected SWIM beams 6°, 8°, and 10° and the buoy mean omnidirectional wave height spectrum are all greater than 0.90, and the relative error of the peak wavenumber is within 10%. The relative error of the integrated energy is mostly less than 20%. In addition, the performance of spectral integration parameters (effective wave height$H_{s}$, and energy wave period$T_{m-10}$) of each spectral beam of SWIM has been verified using Meteo-France WAve Model (MFWAM) reanalysis data. The validation results show that RMSE of$H_{s}$and$T_{m-10}$, for the corrected SWIM beam 6° (8°, 10°) under wind wave sea conditions are 0.31 m (0.32, 0.25 m) and 0.50 s (0.51, 0.49 s), respectively; those for swell cases are 0.16 m (0.16, 0.13 m) and 0.87 s (0.77, 0.72 s), respectively.

Since 2018, for the first time, space measurements of colocated wind vectors and wave spectral characteristics are available thanks to the French/Chinese CFOSAT mission, which carries a wind scatterometer (SCAT) and a wave scatterometer (SWIM). Four years after its launch, CFOSAT data processing has been improved to reach a high level quality, leading to a reprocessing of the whole mission dataset. This paper focuses on the CFOSAT SWIM data reprocessing, the product performance, now homogeneous over mission lifetime, the complementarity with SAR observations and some scientific contributions to oceanography.

The availability of wide swath Significant wave heights (SWH) such as those retrieved in the frame of CFOSAT and HY2B satellite missions opens important perspectives for the improvement of operational wave forecasting. The objective of this work is to analyze the impact of the combined assimilation of wide swath SWH and directional wave spectra on the integrated wave parameters in the analysis and forecast periods. The results show a significant improvement of the SWH estimate in different ocean regions. We clearly showed the persistency of the assimilation up to 3 days in the forecast period. Most striking is the ability of the combined assimilation to effectively correct swell tracking for storm events, such as Hurricane Pablo in 2019.

For the first time, co-located wind vectors and wave spectral characteristics are available thanks to the French/Chinese CFOSAT mission, which includes a wind scatterometer SCAT and a wave scatterometer SWIM. Three years after its launch, CFOSAT data is thoroughly qualified and various scientific work has been undertaken. This paper focuses on CFOSAT SWIM data, its performance and scientific contribution to oceanography, coastal and sea ice study.

Speckle noise is inherent to radar measurements. For applications which need both a high temporal and high spatial resolution, a classical method for the reduction of the speckle noise by filtering the backscattered signal may not be sufficient. In particular, when radar observations are used to estimate ocean wave spectra from relative fluctuations of the radar signal within a given footprint, a method must be implemented to correct for the speckle effect in the Fourier domain (i.e., density spectrum). A theoretical background to model the speckle density spectrum for a radar with near-nadir incidences was proposed by Jackson in 1981 but it is based on a stationary sea surface assumption and ignores the variation of the main factor in the four-frequency moment near the origin. In this article, we revisit this theoretical background to extend this model to a time-varying sea surface and alleviate some assumptions on the Fresnel phase formulation. The results from the model applied in the configuration of an airborne system indicate that not only the displacement of the radar but also the dynamic properties of the sea surfaces have a significant effect on the speckle noise spectrum in certain directions of observations. The effects depend on the radar look direction in azimuth, and on sea surface conditions (wind speed, wind direction with respect to the aircraft route, surface wave spectrum). This new model is validated against observations of the airborne near-nadir incidence scatterometer—Ku-band Radar for Observation of Surfaces (KuROS). We show in particular that the errors between the experimental estimation of the omni-directional speckle noise spectrum from KuROS and the prediction by our model are below 10%.

The surface wave investigation and monitoring (SWIM) aboard the China-France Oceanography Satellite (CFOSAT), a pioneer conically scanning wave spectrometer, was successfully launched on October 29, 2018. Its innovative configuration composed of one nadir and five rotating near-nadir beams is designed to simultaneously observe the directional wave spectrum at a global scale. In this study, we systematically implement the spectral analysis of the radar backscattering with the periodogram technique to obtain the fluctuation spectrum for each azimuth direction. The 2-D fluctuation spectrum of the three spectral beams ($\theta = 6^\circ $, 8°, and 10°) combines all the azimuth directions within one entire rotation of 360°. The case study demonstrates that the wave features (peak wavelength and direction) are roughly consistent between the estimated fluctuation spectrum and the collocated WaveWatch III wave slope spectrum. A marked up-to-downwave asymmetry of the fluctuation spectrum with larger spectral level in the upwave direction for all the three spectral beams is observed. A ratio is defined between the fluctuation spectrum within the [0°, 180°] sector relative to the [180°, 360°] sector. Statistics display that this ratio is greater than 1 when it denotes the up-to-downwave ratio and smaller than 1 for the down-to-upwave ratio. This observed spectrum asymmetry is linked to the asymmetric modulation from upwind to downwind. In addition, we employ such finding to help remove the 180° wave direction ambiguity from a practical point of view. Preliminary results of the direction ambiguity removal display a bias of 41.3°, 40.6°, and 36.7° for the beams. The 10° beam shows slightly better performance compared to the other two beams in terms of bias and standard deviation. This shall lay a strong basis for the operational implementation of such algorithm to resolve the direction ambiguity.

The comparison and verification of ocean wave spectrum by remote sensing and by in-situ measurements at the spectral level is quite rare, because the use of the traditional comparison method lead to very limited spatio-temporal matching pairs. In this paper, a new comparison method is proposed. With this method, under different sea conditions (wind wave mainly/swell mainly) and sea surface conditions (wind speed smaller than 20m/s, significant wave height from 1m to 7m), mean directional wave height spectra from SWIM (Surface Waves Investigation and Monitoring) are compared at the spectral level to the buoy counterparts, in different classes of sea-state. This includes the comparison of the omni-directional wave height spectrum and the directional function at the peak wave number. The comparison results show that under medium and high sea conditions, wave directional spectra provided by the SWIM beams at 8 ° and 10 ° incidence have a high consistency with those from buoy data. Under low sea conditions, the measurement bias of SWIM wave directional spectra mainly comes from three phenomena which are, by order of importance, an abnormal lifting of spectral energy caused by non- wave components at low wave numbers (parasitic peak), from the non-linear surfboard effect in the radar imaging mechanism and from a slight underestimation of speckle noise spectral density.

The CFOSAT (China France Oceanography Satellite) mission launched in 2018 now routinely provides at the global scale, directional spectra of ocean waves. The principle is based on the analysis of the normalized radar cross-section measured by the instrument SWIM (Surface Waves Investigation and Monitoring), a near-nadir pointing Ku-Band real-aperture scanning radar. From the ocean wave spectra derived from SWIM, the principal parameters of ocean wave spectra as significant wave height, peak wavelength, and peak direction are now available to better characterize the sea-state. However, it is known that these principal parameters are not sufficient not fully characterize the distribution of wave energy and understand or validate the physical processes impacting its evolution during growth order decay. Here we show that the parameters characterizing the shape of the wave spectra (e.g directional and frequency spread) can be estimated at the global scale from the SWIM measurements. We also show that they can provide consistent values of the Benjamin-Feir index, an index proposed to estimate the probability of extreme waves. Similarities of differences with the shape parameters of the MFWAM numerical wave model are also discussed.

The Chinese-French oceanography satellite, CFOSAT, was launched on October 2018. Two Ku-band scatterometers are on-board: SCAT for the wind observation and SWIM for the wave observation. After a first phase mainly dedicated to validation and identification of improvement possibilities, the ground processing and products generated were upgraded. This paper presents the main evolutions implemented and their positive impacts on the SWIM data quality.

This article describes the first results obtained from the Surface Waves Investigation and Monitoring (SWIM) instrument carried by the China France Oceanography Satellite (CFOSAT), which was launched on October 29, 2018. SWIM is a Ku-band radar with a near-nadir scanning beam geometry. It was designed to measure the spectral properties of surface ocean waves. First, the good behavior of the instrument is illustrated. It is then shown that the nadir products (significant wave height, normalized radar cross section, and wind speed) exhibit an accuracy similar to standard altimeter missions, thanks to a new retracking algorithm, which compensates a lower sampling rate compared to standard altimetry missions. The off-nadir beam observations are analyzed in detail. The normalized radar cross section varies with incidence and wind speed as expected from previous studies presented in the literature. We illustrate that, in order to retrieve the wave spectra from the radar backscattering fluctuations, it is crucial to apply a speckle correction derived from the observations. Directional spectra of ocean waves and their mean parameters are then compared to wave model data at the global scale and to in situ data from a selection of case studies. The good efficiency of SWIM to provide the spectral properties of ocean waves in the wavelength range [70-500 m] is illustrated. The main limitations are discussed, and the perspectives to improve the data quality are presented.

The accuracy of sea surface parameters retrieved from altimeter missions is predominantly governed by the choice of the so-called “retracking” algorithm, i.e., the model and inversion method implemented to obtain the surface parameters from the backscattered waveform. For continuity reasons, the choice of space agencies is usually to apply the same retracker from one satellite mission to the other to ensure long-time homogeneous series. In this article, taking the opportunity of a new configuration of the nadir pointing measurements onboard the recently launched China France Oceanography Satellite (CFOSAT) with the Surface Waves Investigation and Monitoring (SWIM) instrument (Hauseret al.,2020), the retracking method was upgraded, by implementing a novel algorithm, called “Adaptive” retracker. It combines the improvements brought by Poissonet al.,(2018) for the estimation of surface parameters from peaked waveforms over sea ice, improvements in the way the instrumental characteristics are considered in the model (mispointing, point target response) and a more accurate consideration of speckle statistics. In this article, we first show from simulations carried out in the instrumental configuration of SWIM that the Adaptive algorithm has better accuracy and performance than the classical MLE4 algorithm. Then, the geophysical parameters obtained with real data from SWIM are analyzed with comparisons to reference data sets (model and products from altimeters). We show that this new algorithm has several benefits with respect to the classical MLE4 method: no need of lookup tables to correct biases, significant noise reduction on all geophysical variables especially the significant wave height, and performance of inversion over a large set of echo shapes, resulting from standard oceanic scenes as well as highly specular conditions such as over bloom or sea ice.

The Chinese-French oceanography satellite, CFOSAT, was launched on October 2018. Two Ku-band scatterometers are on-board: SCAT for the wind observation and SWIM for the wave observation. This paper presents the most recent results on the SWIM data quality analysis a few months after the end of the CAL/VAL phase.

The China-France Oceangraphy SATellite (CFOSAT) is an innovative satellite mission with wind and waves measurements on oceans. This paper aims to evaluate the first results of the assimilation of the wave data provided by CFOSAT in the wave model MFWAM. Model runs are implemented during the Calibration/Validation phase of the mission. The results are compared to the wave data from altimeters and buoys. The first results are promising and indicate a significant improvement of wave heights in the different ocean basins (high latitudes, intermediate latitudes and the tropics). Azimuthal cut-off sensitivity tests for the SWIM wave spectra are also examined in this study. We also discussed the impact of combined wave spectra and the ones from the incidence angles of SWIM (6, 8 and 10°). In other respects this study also investigate the complementary use of SWIM and SAR from CFOSAT and SAR from Sentinel-1 for combined assimilation in the wave model MFWAM

This paper presents the goal, organization and first results of the calibration and validation activities devoted to the Ku-band wave scatterometer SWIM in operation since the end of October 2018 after the launch of the China-France Oceanography SATellite CFOSAT.

The increasing number of directional wave spectra from satellites is a very important opportunity for the improvement of operational wave forecast. This will lead to a better understanding of swell dissipation which is still a top task in wave modeling. This work aims to investigate how the assimilation of different sources of directional wave spectra will impact the analysis and forecast of wave parameters. Two satellites, sentinel-1A and 1B, provide the level 2 SAR wave spectra, which have been qualified for operational use regarding to the calibration/validation phase developed in the last two years. This year the launch of the satellite CFOSAT will ensure a third source directional wave spectra from the real aperture radar SWIM and a significant wave height at nadir look. Synthetic wave data of SWIM and wave spectra from S1A and S1B are assimilated conjointly in the wave model MFWAM for 1 cycle of 13 days during the southern winter season. Two sets of synthetic SWIM data with and without instrumental errors have been tested in the assimilation. The validation of the results with altimeters significant wave heights shows a significant impact in the periods of analysis and forecast. In this study we also investigated the impact of instrumental errors provided by SWIM simulator and the wavelength cut-off on the assimilation results.

This paper is on the retrieval of spectral properties of surface ocean waves from the real-aperture radar KuROS carried on an airplane. The system was designed to prepare the satellite mission CFOSAT and to contribute to its validation. In addition, KuROS provides Doppler measurements which are analyzed as Doppler velocity spectra co-located with the wave height spectra derived from intensity measurements. Data have been acquired from 2013 to 2017 in various circumstances including fetch-limited, swell, or mixed sea conditions and under moderate to high wind conditions. In this paper we present results on the directional spectra obtained and on the associated integral parameters such as significant wave height, peak direction, peak wavelength (or frequency), directional and frequency spreads. Comparisons with outputs of the wave model hindcast (MFWAM model) are presented. We also show some results on the Doppler fluctuation spectra.

The wave scatterometer, SWIM, will be on-board the Chinese French oceanographic mission, CFOSAT. It will complete the Chinese wind scatterometer, SCAT. They will provide the scientific community with joint wind and wave measurements for the very first time. The launch is now coming soon: autumn 2018. SWIM is a Ku-band real-aperture radar with 6 rotating fan-beams pointing near nadir. The main characteristics and performance of the instrument measured during the Satellite Integration Campaign, products and ground segment development are presented in this paper, as well as predicted performances estimated from simulations.

This paper provides an overview of the surface waves investigation and monitoring (SWIM) instrument which will be one of the two payload instruments carried by China France Oceanography SATellite (CFOSAT) with a planned launch date in mid-2018. SWIM is a real aperture wave scatterometer operated at near-nadir incidence angles and dedicated to the measurement of directional spectra of ocean waves. The SWIM flight model is currently being assembled and tested, its performance is being assessed and its prototype data processing algorithm is being developed. The aim of this paper is to provide a complete overview on the motivations and scientific requirements of this mission, together with a description of the design and characteristics of the SWIM instrument, and the analysis of its expected performances based on a prelaunch study. An end-to-end simulator has been developed to evaluate the quality of the data products, thus allowing the overall performance of the instrument to be assessed. Simulations run with two subsets of full orbit subsets show that the performances of the instrument and the inversion algorithms will meet the scientific requirements for the mission.

With a low cut-off for the directional wave spectra CFOSAT will impact both mixed sea and swell. The complementary use of CFOSAT and SAR wave spectra will enhance the impact of the assimilation and therefore induce a more accurate sea state. The CFOSAT mission is on one hand a big challenge for operational wave forecasting system, and on the other hand a good opportunity for wave modellers in order to improve the source and dissipation terms in the wave model.

Knowledge of the distribution of wave slopes in the sea is important for understanding a number of processes occurring at or near the air-sea interface. The sea slope probability density function (pdf) of sea waves is quasi-Gaussian, being able to be expressed as a Gram-Charlier expansion to fourth-order. However, the method is absent for remote sensing the sea slope pdf in a large scale. SWIM (Surface Waves Investigation and Monitoring), used on the CFOSAT (China-France Oceanography Satellite) mission, the first space borne wave scatterometer in the world can obtain two dimensional (2D) scattering measurements of sea surface in the incidence and azimuth direction with high resolution, which made it possible to measure 2D slope pdf with high accuracy. In the paper, the feasibility of remote sensing quasi-Gaussian sea slope by SWIM is studied by the simulation.

SWIM is part of the CFOSAT satellite mission payload. It is designed for the measurement of directional ocean wave spectra. It is a Ku-band real-aperture radar with 6 rotating fan-beams pointing near nadir. The main characteristics of the instrument, data, products are presented in this paper, as well as performances estimated from simulations.

The Chinese and French Space Agencies are jointly preparing an innovative mission, CFOSAT (China France Oceanography Satellite) devoted to the monitoring of the ocean surface and its related science and applications. This paper gives an overview of the scientific objectives, the mission and instrument characteristics, the expected data products and their performance.

It is known that the slope probability density function (pdf) of the sea surface is non-Gaussian and can be expressed as a Gram-Charlier to fouth-order expension, including seven parameters: slope variances in upwind and crosswind direction, two skewness coefficients and three peakedness coefficients. Cox and Munk[1] had gotten all the seven parameters varying with the wind speed using optical data. However, those parameters for the sea surface at optical band and microwave band are different, and until now it is not clear for microwave band what values the seven parameters are. Wave scatterometer is a kind of microwave sensor specially designed for wave spectrum remote sensing and could provide σ0in high resolution of the range and azimuthal direction so that it is possible to remote sense the slope pdf for a certain space-time. In this paper, firstly, the accuracy of Geometric Optical (GO) model is evaluated, which is used for confirming the valid range of incidence angle to invert; then, a two dimensional (2D) inversion method is developed for remote sensing all the seven parameters in non-Gaussian slope pdf by the wave scatterometer based on GO model; finally those parameters are inverted by the method using the Ku-band airborne wave scatterometer KuROS measurements.

This paper presents the under-going development of the ground segment algorithms of the SWIM instrument. SWIM is a wave scatterometer which will be embarked on the Chinese French oceanography mission, CFOSAT. SWIM aims at measuring the 2D oceanic wave spectra; it is a Ku band real aperture radar. Simulations are performed to get data along the satellite track: radar signals are obtained simulated interaction with a realistic sea surface and taking into account the radar geometry. Then, the simulated data are processed with software prototypes.

We have designed and developed a new airborne Ku-band Doppler radar, called KuROS, to prepare the CFOSAT satellite mission for measuring ocean surface wind and waves. The main characteristics of this new radar are presented, and first results obtained from a campaign held in 2013 illustrated. Both intensity and Doppler information are used to estimate the directional spectra of ocean waves. Radar cross-section and directional spectra are assessed trough comparisons with independent information.

The CFOSAT mission is an innovative spatial mission for oceanography: for the very first time, both wind and wave vectors will be measured at the global ocean surface. This paper presents the wave scatterometer, SWIM and its associated scientific performances.

A renewal of interest for the radiometric L-band Sea Surface Salinity (SSS) remote sensing appeared in the 1990s and led to the Soil Moisture and Ocean Salinity (SMOS) satellite launched in November 2009 and to the Aquarius mission (launched in June 2011). However, due to low signal to noise ratio, retrieving SSS from L-band radiometry is very challenging. In order to validate and improve L-band radiative transfer model and salinity retrieval method used in SMOS data processing, the Cooperative Airborne Radiometer for Ocean and Land Studies (CAROLS) was developed. We analyze here a coastal flight (20 May 2009), in the Gulf of Biscay, characterized by strong SSS gradients (28 to 35 pss-78). Extensive in-situ measurements were gathered along the plane track. Brightness temperature$(T_{b})$integrated over 800 ms correlates well with simulated$T_{b}$(correlation coefficients between 0.80 and 0.96; standard deviations of the difference of 0.2 K). Over the whole flight, the standard deviation of the difference between CAROLS and in-situ SSS is about 0.3 pss-78 more accurate than SSS fields derived from coastal numerical model or objective analysis. In the northern part of the flight, CAROLS and in-situ SSS agree. In the southern part, the best agreement is found when using only V-polarization measured at 30$^{\circ}$incidence angle or when using a multiparameter retrieval assuming large error on$T_{b}$(suggesting the presence of biases on H-polarization). When compared to high-resolution model SSS, the CAROLS SSS underlines the high SSS temporal variability in river plume and on continental shelf border, and the importance of using realistic river run-offs for modeling coastal SSS.

SWIM (Surface Wave Investigation and Monitoring) is a spaceborne radar pointing at nadir and small incidence angles, scanning in azimuth. It is designed for the measurement of directional ocean wave spectra and will be embarked on the CFOSAT (China France Oceanography SATellite) mission to be launched in 2014. The CFOSAT project is now in the C/D phase (manufacturing phase). Taking into account the very last definition of the instrument, we present here the expected performances for estimates of significant wave height and spectral parameters of long ocean waves. These performances have been obtained using numerical simulations taking into account instrument specifications and realistic sea surface conditions, in particular those corresponding to the Atlantic storm of November 2002, which caused the Prestige ship sinking and its terrible oil slick.

SWIM is a Ku-band radar designed for wave directional spectrum estimation. This radar operates at six incidence angles (from 0° to 10°) with a complete azimuth scanning covering a swath of 180 km. The phase B (addressing preliminary design) of SWIM is currently under finalization. In, the preliminary design and associated performance analysis have been published taking into account the first results of Phase B design. This paper is focused on the last performance assessment of this phase B for all the measurements performed by the SWIM instrument.

SWIM is a Ku-band radar designed for wave directional spectrum estimation. This radar operates at six incidence angles (from 0° to 10°) with a complete azimuth scanning. SWIM is currently in Phase B (concept and design phase). In [1, 2], the preliminary design and associated performance analysis have been published taking into account the end of Phase A design. This paper is focused on the performance assessment of the SWIM instrument based on the new developments which occur during Phase B. In addition, major reviews have been carried out on the performance analysis.

The CAROLS L-band radiometer, which is built and designed as a copy of DTU EMIRAD II instrument will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer STORM) in coordination with in situ field campaigns for futur SMOS CAL/VAL activities. A validation campaign with four flights was made over the South West of France and the Bay of Biscay (Atlantic Ocean) in September 2007. Different instrumented sites were over ocean and land surfaces were coverecd. Moreover, in order to qualify the radiometric data, different types of aircraft maneuvers were performed over ocean: circle flights, wing and nose wags. We present in this paper the first analysis of the data quality using these ocean measurements. We show a very good sensitivity of both channels.

SWIM is a Ku-band radar designed for wave directional spectrums estimation. This radar operates at six incidence angles (0deg to 10deg) with complete azimuth scanning. This paper presents the simulation tool developed for defining SWIM design and evaluating SWIM performance. The simulation tool is an end-to-end simulator, i.e. from the sea surface to the estimated wave spectrum. Some simulations are discussed, showing that the preliminary design of SWIM will fulfill the scientific requirements.

The CAROLS, L band radiometer, is built and designed as a copy of EMIRAD II radiometer of DTU team. It is a Correlation radiometer with direct sampling and fully polarimetric (i.e 4 Stockes). It will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer (STORM) and IEEC GPS system, Infrared CIMEL radiometer, one visible camera), in coordination with in situ field campaigns for SMOS CAL/VAL. The instruments are implemented on board the French research airplane ATR42. A validation campaign with four flights was made over south west of France, Hourtin Lake and Bay of Biscay (Atlantic Ocean) in September 2007. In order to qualify the radiometer data, different types of aircraft movements were realized: circle flights, wing and nose wags. Simultaneously to flights, different ground measurements were made over continental surfaces and ocean. First results show a good quality of data over ocean surfaces. For continental surfaces, important Radio-Frequency Interferences (RFI) were observed over a large part of the studied region.

Airborne radar observations of the sea surface at C- Band and small incidence angles are used to investigate some properties of the surface slope probability density function (pdf). The method is based on the analysis of the variation of the radar cross-section with incidence angle, assuming that the backscatter can be described by the Geometrical Optics theory. First, we show that roughness properties with scales larger than 12 cm can be analyzed in our configuration (C-Band, incidence 7 to 16deg). The radar data are then analyzed in terms of filtered mean square slope under the assumption of a Gaussian slope pdf. Dependence with wind speed of the upwind and total mss are analyzed and compared to various published studies. Finally an analysis of the radar data under a non-Gaussian assumption for the slope pdf is proposed, by applying the compound model suggested by [1].

We present an analysis of the azimuth anisotropy of the ocean radar backscatter from C-Band observations in VV and HH polarizations from the airborne radar STORM during the VALPARESO experiment. Comparisons with existing semi- empirical models show that none of them are in agreement with the anisotropy of the radar signal, and the trend with wind speed is too weak compared to the observations. This is attributed to a shortcoming in the description of the short surface waves anisotropy. By considering an additional mechanism for nonlinear wave interactions, we show that the short wave anisotropy is modified and can explain the anisotropy of the radar signal.

We propose here to use a combination of radar observations in HH and VV polarizations at C-Band to study the different contributions to the sea surface backscattering - polarized and non-polarized parts- and to characterize the relation between azimuth anisotropy of the signal and the surface properties.

This paper presents an analysis of measurements of the normalized radar cross-section (NRCS) in vertical and horizontal polarizations over the ocean obtained from the C-band airborne radar STORM. The dataset was collected during the experiment called "Validation with a Polarimetric Airborne Radar of ENVISAT SAR over the Ocean (VALPARESO)", which took place during the calibration/validation phase of the ENVISAT Advanced Synthetic Aperture Radar (ASAR). From this dataset, the properties of the polarization ratio are discussed and in particular its dependencies with radar geometry (incidence and azimuth angle) as well as with meteorological conditions (wind and sea state). The polarization ratio is found to be dependent on incidence and azimuth angles. Its dependence with incidence angle is found to be significantly different from empirical models previously proposed in the literature. It also exhibits some correlation with surface conditions (wind and wave) with a more important correlation with significant wave steepness. Two new analytical formulations are proposed to model the polarization ratio, one as a function of incidence angle only, the second one with additional dependence with azimuth angle. It is shown that it is necessary to consider an azimuth-dependent polarization ratio for incidence angles larger than 30/spl deg/. Comparisons with the polarization ratio from ENVISAT ASAR images are used to assess this model.

In order to improve the wave forecast, the assimilation of directional wave spectra in wave model has been developed at Meteo France. The assimilation scheme is tested for real ENVISAT ASAR wave spectra provided by the European Space Agency. The scheme takes into account the limitation in frequency related to the ASAR instrument. The preliminary results show that the assimilation of wave spectra is efficient and works correctly. The impact of the assimilation on wave parameters is significant and stays effective in the forecast period. In addition, statistical analysis has shown good skill in term of root mean square and assimilation index for the analyzed wave parameters (wave height, direction and frequency).

Studies the dependence of sea surface brightness temperatures on atmospheric stability. We used data measured by the TRMM microwave radiometer (TMI), at 10.7 and 19.35 GHz. These measurements are collocated with meteorological analyses of the ECMWF model. The stability parameter is (z/L), where z is the height at which wind speed is evaluated (10 m in this study), and L is the Monin-Obukhov length. Significant variations with stability are evidenced, depending on the SST and the surface wind range. The major result is that a significant decrease of the brightness temperatures with stability is observed, in agreement with model simulations.

The main objective of SWIMSAT is to measure the directional spectra of waves thanks to a space-borne radar. The instrumental concept involves a real-aperture radar using a multi-beam conical scanning, in order to provide measurements of the spectral properties of the wave field, and to estimate the profile of radar cross-section depending on incidence (0/spl deg/ to 10/spl deg/) and azimuth (0/spl deg/ to 360/spl deg/). The paper provides an overview of the mission and of the measurement principle, and the status on instrument design.

Observational results derived from the airborne C-band scatterometer RESSAC looking at small incidence angles (/spl les/21/spl deg/) were acquired in the vicinity of the Gulf Stream during the Surface Wave Dynamics Experiment in February and March 1991. Although the instrument was primarily designed to study the directional spectra of long waves, it is also possible to process the data to examine the behaviour of the radar cross-section /spl sigma//sup 0/ versus incidence /spl theta/ and azimuth /spl phi/. Although the classical physical or empirical models of /spl sigma//sup 0/(/spl theta/,/spl phi/) predict that the anisotropy of /spl sigma//sup 0/ versus azimuth increases with wind speed for a given incidence angle /spl theta/, it is found that under some conditions, this behaviour does not occur. By contrast, the anisotropy shows strong minima on certain occasions. Given the high-resolution upper ocean current measurements from Airborne expendable Current Profilers deployed on 5 March 1991, the regions of anisotropic minima could be identified from the three sets of observations (March 4, 5, and 7) downwind of the core of the Gulf Stream with a maximum current of 2 m s/sup -1/, and downwind of the current speed maxima of an anticyclonic-rotating warm core ring juxtaposed along the North Wall. On three other days (February 27, March 2 and 6), no clear anisotropic minima of was found, because the upper ocean current effects were obscured by variability in surface wind field.>

The RESSAC radar system, used from aircraft platforms for measuring directional spectra of ocean waves in the gravity domain (wavelengths from 30 to 400 m), is presented. The instrument consists of a C-band (5.35-GHz) FM/CW radar system. The transmitting and receiving antennas look toward the surface at a low incidence angle ( approximately=14 degrees for the center of the antenna beam) and rotate around a vertical axis. When the data are processed, the known antenna gain pattern is removed from the recorded signal, making it possible to estimate the sea surface slope variance, which in turn is used to determine the tilt modulation transfer function, without the need of any external wind measurement. Fully normalized spectra obtained from RESSAC are compared to other data sets.>