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.

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.

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 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.

The Space Geodesy Project (SGP) development by NASA is an effort to co-locate the four international geodetic techniques Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR), Very Long Baseline Interferometry (VLBI), Global Navigation Satellite System (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) into one tightly referenced campus and coordinated reference frame analysis. The SGP requirement locates these stations within a small area to maintain line-of-sight and frequent automated survey known as the vector tie system. This causes a direct conflict with the new broadband VLBI technique. Broadband means 2-14 GHz, and RFI susceptibility at -80 dBW or higher due to sensitive RF components in the front end of the radio receiver.