Hindcast model results indicate that there have been statistically significant changes in global ocean wave climate over the last 40-years. In particular, the Southern Ocean shows increases in wind speeds and wave heights which are associated with a strengthening of the Southern Ocean westerlies and a migration of these systems south. Global altimeter data now spans the period back to 1985 and hence provides a valuable validation source for such model results. As the altimeter dataset spans up to 10 separate missions, it is critically important that this multi-mission altimeter dataset is calibrated in a consistent manner. Two such long-term calibrated altimeter missions have been used to estimate global ocean wave height trends (Young and Ribal, 2019; Timmermans et al., 2020). Both show many similarities in terms of the global distribution of trends, but there are also significant differences in the magnitudes. Young and Ribal (2019) calibrate each separate altimeter against buoy data. In contrast, the Timmermans et al. (2020) dataset is a combination of calibrations against buoys and calibrations of altimeters against previous missions.

Although calibrations against buoys may seem a robust approach, it does raise concerns about changes in buoy hull types and processing methods during the extended observation period. Such changes may introduce non-homogeneity and impact long-term trend estimates. The proposed paper re-calibrates the full multi-mission global altimeter record firstly against buoys. In a second calibration a single altimeter is calibrated against buoys and then earlier and later altimeter missions are calibrated against overlapping satellite altimeters. The differences between these approaches is examined in detail and potential errors assessed. The analysis will consider the impacts of geographical distribution of calibration data (buoy and altimeter-altimeter matchups), as well as sampling density of altimeters.

Based on the results, the limitations of the altimeter datasets will be explored. In addition, the capabilities of accurately determining trends in both mean and upper percentile values of significant wave height will be determined.

Young, I.R. and A. Ribal, 2019, Multi-platform evaluation of global trends in wind speed and wave height, Science, 364, 548-552.

Timmermans, B.W., C. P. Gommenginger , G. Dodet and J.-R. Bidlot, 2020, Global Wave Height Trends and Variability from New Multimission Satellite Altimeter Products, Reanalyses, and Wave Buoys, Geophys. Res. Lett., 47.

Accurate knowledge and understanding of the sea state and its variability is crucial to numerous oceanic and coastal engineering applications, but also to climate change and related impacts including coastal inundation from extreme weather and ice-shelf break-up. An increasing duration of multi-decadal altimeter observations of the sea state motivates a range of global analyses, including the examination of changes in ocean climate. For ocean surface waves in particular, the recent development and release of products providing observations of altimeter-derived significant wave height make long term analyses fairly straightforward. In addition, advances in imaging SAR processing for some missions have made available multivariate observations of sea state including wave period and sea state partition information such as swell wave height. Records containing multivariate information from both Envisat and Sentinel-1 are included in the version 3 release of the European Space Agency Climate Change Initiative (CCI) for Sea State data product.

In this study, long term trends and variability in significant wave height spanning the continuous satellite record are intercompared across high-quality global datasets using a consistent methodology. In particular, we make use of products presented by Ribal et al. (2019), and the recently released products developed through the ESA Sea State CCI. Regional differences in mean climatology are identified and linked to low and high sea states, while temporal trends from the altimetry products, and two reanalysis and hindcast datasets, show general similarity in spatial variation and magnitude but with differences in equatorial regions and the Indian Ocean. Discrepancies between altimetry products likely arise from differences in calibration and quality control. However, multidecadal observations at buoy stations also highlight issues with wave buoy data, raising questions about their unqualified use, and more fundamentally about uncertainty in all sea state products.

In addition to wave height, global climatologies for wave period are also intercompared between the recent Sea State CCI product, ERA 5 reanalysis and in situ observations. Results reveal good performance of the CCI products but also raise questions over methodological approach to multivariate sea state analysis. For example, differences in computational approach to the derivation of higher order summaries of wave period, such as the zero-crossing period, lead to apparent discrepancy between satellite products and reanalysis and modelled data. It is clear that the broadening diversity of reliable sea state observations from satellite, such as provided by the Sea State CCI project, motivates new intercomparisons and analyses, and in turn elucidates inconsistencies that have been previously overlooked.

We discuss these results in the context of both the current state of knowledge of the changing wave climate, and the on-going development of Sea State CCI altimetry and imaging SAR products.

Sentinel-6 Michael-Freilich (S6-MF) is the new Copernicus mission having for objectives to provide high-precision ocean altimetry measurements (sea-surface height, wave height and wind). To achieve this objective, the S6-MF satellite carries a radar altimeter of new generation, Poseidon-4 (supported by a new highly precise microwave radiometer, AMR-C). The Poseidon-4 altimeter evolves significantly from its predecessors (Cryosat-2 and Sentinel-3) featuring higher performance than this previous SAR altimeter generation. In particular it embeds a new operating mode, currently termed interleaved, which allows to make use of a higher number and continuous stream of pulse echoes within a coherent processing interval, maximizing the fully focused SAR (FF-SAR) processing capabilities [Egido & Smith, 2017]. It is expected substantial improvements in terms of noise reduction, but also in the focusing process (an important step forward compared to the Cryosat-2 and Sentinel-3 missions which are currently impacted by sidelobes in the along-track point target response (PTR) caused by the lacunar sampling of the closed-burst operation mode [Egido & Smith, 2017]). This gain in resolution and noise reduction would allow to obtain much more details of the ocean surface structures, at smaller scales than what have already been achieved (first recovering the signals filtered around the 200 m band stop by the close-burst mode and possibly restituting much lower wavelength signals - down to few tens of meters - as long as the signal to noise ratio is sufficiently low and that the orbital wave velocities effect does not degrade too much the azimuthal resolution).
The objective of this study is to provide a comprehensive and in-depth assessment of the S6-MF FF-SAR performances over open ocean and see the extent to which it may optimize geophysical parameters and possibly recover valuable information (Hs and swell period retrievals) as it was tentatively made by Rieu et al. [2020] with Sentinel-3 data. A first objective of the study will be the selection of an optimal configuration of the FF-SAR processing chain in case of acquisitions over the sea surface to generate waveforms that allow to assess the achievable performance of the S6-MF FF-SAR over open ocean. Special attention will be also paid at mesoscales which still remain not well observed and understood, in order to evaluate the capability of this new high-resolution and high posting rate technique to improve the observability of SSH signals within this wavelength range. However, uncertainty remains regarding the orbital wave velocity effects that shall degrade the theoretical azimuth resolution of this approach and may limit the access to finer scales. This study seeks to answer the question to know whether the ocean could immediately benefit from the FF-SAR processing, and open up new and concrete perspectives for oceanography from space.

References
Egido, A., Smith, W.H.F., 2017. Fully focused SAR altimetry: theory and applications. IEEE Trans. on Geosci. Remote Sens. 55 (1), 392–406. https://doi.org/10.1109/TGRS.2016.2607122.
Rieu, P., Moreau, T., Cadier, E., Raynal, M., Clerc, S., Donlon, C., Borde, F., Boy, F., Maraldi, C., 2020. Exploiting the Sentinel-3 tandem phase dataset and azimuth oversampling to better characterize the sensitivity of SAR altimeter sea surface height to long ocean waves. Adv. Space Res. https://doi.org/10.1016/j.asr.2020.09.037, ISSN 0273-1177.

1. INTRODUCTION
Since October, 29th 2018, the new space-borne system CFOSAT (China France Oceanography Satellite) [1] has been deployed for measuring ocean surface parameters. This mission, developed under the responsibilities of the French and Chinese Space agencies (CNES and CNSA) was designed to monitor at the global scale, ocean surface winds and waves. It is composed of two radar sensors both scanning in azimuth: SCAT, a fan-beam wind scatterometer [2], and SWIM designed for wave measurements [3]. With its collocated measurements of ocean surface wind and waves, CFOSAT aims at better understanding processes at the ocean surface and ocean/atmosphere interactions and at improving atmospheric and oceanographic models and predictions by feeding forecast systems through assimilation. This paper focuses on the SWIM measurements. SWIM is an innovative Ku-band real-aperture wave scatterometer, with 6 low-incidence rotating beams [2].
This new instrument allows for the first time the systematic production of directional spectra of ocean waves with a real-aperture radar system. This usefully complements the existing missions based on SAR systems which also provide spectral information on surface ocean waves but with more limitations [4]. After an important task of CALibration and VALidation (CAL/VAL) on the instrument and products at the beginning of the mission, the expected performances have been demonstrated, as it is recalled in section 2. In section 3, we present the studies performed by the science teams, showing the potential of CFOSAT mission for several Oceanography applications and even more.
2. Products performances
SWIM provides several types of information: directional wave spectra and their dominant parameters, significant wave height (SWH), wind speed and backscattering profiles.

From its nadir beam, the SWH and wind speed are provided in addition to 0. To retrieve these parameters from the radar nadir echoes, the “Adaptive algorithm” was implemented in the SWIM ground segment [6].
This ensures the same level of performance over ocean as conventional altimetry missions, in spite of the SWIM instrument lower measurement rate (4.5Hz vs 20Hz). It also improves the relevance of the retrieved parameters on specific areas such as near sea-ice, or bloom or rain impacted surfaces.

The normalized radar cross-section 0 profiles are provided at level 2 as averaged values per bins of 0.5° in incidence and 15° in azimuth. They are referenced in the geometry of the wave cells, which are boxes of about 70 x 90 km². The mean trend of these profiles is globally consistent with results provided by GPM datasets [7]. The dependence of 0 with wind speed is very consistent (less than 1 dB difference) with the GPM data mean trend. The smallest sensitivity to wind speed is observed for the 10° and 8° beams (1dB to 1.5 dB difference between 5 and 20 m/s), making them the most valuable incidences for the wave inversion as the dominant effect in the 0 fluctuations within the footprint will be the tilt of the long waves.


Wave slope spectra are determined based on 6°, 8° and 10° beam measurements. Directional wave spectra are processed at level 2 in the wave cell geometry (boxes of about 70 x 90 km). Impact of speckle noise is mitigated in the processing using an empirical model of the density spectrum of speckle noise. It was shown in [5] that its impact is the most important when then antenna look direction is aligned with the satellite track (at ±15°). In this direction it also varies with latitude and sea state conditions. The modulation transfer function which is used to convert the directional modulation into wave slope spectra is currently estimated by using the nadir SWH as reference. This processing configuration leads to the current wave spectra and wave parameter performances [8]: waves are identified for wavelength from 50 to 500m and the main wave parameters (significant wave height, dominant wavelength and dominant direction) extracted from spectra are consistent with models and buoy observations. Comparisons with in-situ data, models and Sentinel-1 will be shown during the conference.

3. Perspectives of CFOSAT SWIM data for coastal and ocean applications
The use of the SWIM data is now extended beyond the open ocean (coastal regions, sea ice) and to new applications (Stokes current, extreme wave identification).

For coastal regions, the wave directionality is a key driver for sediment transport and overtopping parameter in case of severe storms. In this frame, wave spectra from SWIM has a high potential to correct initial conditions of operational coastal wave models, which ensure a reliable wave submersion warnings. Moreover better sampled SWH, estimated at high resolution (5Hz) from the nadir beam, captures accurately surface changes induced wave/currents interactions observed close to coast. The adaptive algorithm used to retrack the nadir waveforms is particularly accurate for SWH estimation and thus for coastal studies. A good correlation with the bathymetry has been shown in Nouvelle-Calédonie (on atoll to detect coral barrier) and in Guyane (Maroni estuary). In addition, the 2D wave spectrum can also be exploited with a ribbon approach to get closer to the coast. The information will not be complete on 360° but accurate guess can be reached.

In open ocean, in addition to the classical three main parameters of the wave spectra (SWH, dominant direction, dominant wavelength), the Stokes drift can be estimated from the directional spectra (integrating the contribution of waves up to wavelength of 30 m). long term validation of Stokes drift computed from SWIM wave spectra has been implemented and has indicated a great interest to force oil spill and drifting models. Moreover, Stokes components as given by CFOSAT can be used directly as wave forcing in ocean model which will lead to better estimate of surface currents and also improved mixing processes in upper ocean layers. This recent development will give for the first time a Stokes drift estimate from satellite and opens the use to NRT applications related to marine pollution and maritime safety.


The presentation during the conference will highlight the updated results related to coastal, Stokes drift and sea-ice applications.


4. CONCLUSION
SWIM instrument fulfills its objective by providing at the global scale, new observations with directional wave spectra, nadir parameters and NRCS profiles. Performances of SWIM products and coupling with CFOSAT SCAT wind scatterometer or other sensor measurements open the field for improvements in ocean surface characterization and modeling. New perspectives are emerging by exploiting SWIM advanced capacities such as SWH and wind obtained at a 5 Hz sampling along-track for coastal applications, sea ice detection and characterization through the analysis of NRCS [9], wave field studies in the marginal ice zone and in extreme events [10] or global estimation of additional wave-related parameters (like e.g. the Stokes drift). CFOSAT is thus a new and original source of observations for many studies and applications.

REFERENCES
[1] Hauser D. et al., “Overview of the CFOSAT mission”, IGARSS’2016, Beijing (China), July 2016
[2] Liu Jianqiang, Wenming Lin, Xiaolong Dong, et al, « First Results From the Rotating Fan Beam Scatterometer Onboard CFOSAT », 10.1109/TGRS.2020.2990708, 2020
[3] Hauser D., et al, SWIM: the first spaceborne wave scatterometer, 10.1109/TGRS.2017.2658672, 2017
[4] W. R. Alpers and C. Brüning, “On the relative importance of motion related contributions to the SAR imaging mechanism of oceansurface waves,” IEEE Trans. Geosci. Remote Sens., vol. GE-24, no. 6, pp. 873–885, Nov. 1986
[5] Hauser D. et al, “New observations from The SWIM radar on board CFOSAT; instrument validation and ocean wave measurement assessment”, doi 10.1109/TGRS.2020.2994372, 2020
[6] C. Tourain et al., "Benefits of the Adaptive Algorithm for Retracking Altimeter Nadir Echoes: Results From Simulations and CFOSAT/SWIM Observations," in IEEE Transactions on Geoscience and Remote Sensing, doi: 10.1109/TGRS.2021.3064236.
[7] Gressani V., D. Nouguier and A. Mouche, “Wave Spectrometer Tilt Modulation Transfer Function Using Near-Nadir Ku-and Ka-Band GPM Radar Measurements”, Proceedings of the 2018 IEEE International Geoscience and Remote Sensing Symposium, Valencia (Spain), 2018
[8] C. Tourain et al., "Evolutions and Improvements in CFOSAT SWIM Products," 2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021, pp. 7386-7389, doi: 10.1109/IGARSS47720.2021.9553274
[9] Peureux C. et al., Sea-ice detection from near-nadir Ku-band echoes of1CFOSAT/SWIM scatterometer. Journal of Geophysical Research: Oceans: Submitted
[10] Shi, Y., Du, Y., Chu, X., Tang, S., Shi, P., & Jiang, X. (2021). Asymmetric wave distributions of tropical cyclones based on CFOSAT observations. Journal of Geophysical Research: Oceans, 126, e2020JC016829. https://doi.org/10.1029/2020JC016829

SWIM instrument onboard CFOSAT wind and wave satellite is measuring the ocean surface wave related modulations in Ku-band, using a rotating instrument that provides a directional 1D wave spectra every 7 degrees azimuth for each of its 5 beams at 2 4 6 8 and 10° incidence angle, resulting in a very special cycloid ground footprint geometry. The L2S SWIM product is a level 2 product that provides the wave spectra along this cycloid. Details about this L2S product geometry, content, format and access, including the algorithms behind will be discussed. Exploiting this continuous measurements along this cycloid allows to estimate wave systems properties at the highest possible resolution of about 20 km.
Comparison of this L2S wave spectra measurements with in-situ drifting wave buoy measuring 2D wave spectra at the same time and location of the satellite overpass will be shown. Modulation transfer function between SWIM cross-section modulation spectra in wavenumber space and wave buoy or WaveWatchIII wave model spectra in frequency space will be discussed.
Application of this L2S product for swell tracking across the ocean basins and into the sea ice will be demonstrated and compared to Sentinel1 wave mode complementary capabilities. Demonstration of the capability of SWIM instrument to detect wave signal in the marginal Ice Zone using the lower incidence angles will be done. Finally so called firework swell tracking animations will be demonstrated.

In July of 2020, the orbit of CryoSat-2 was modified to allow for repeated overlaps with ICESat-2. Following a year of coincident orbits with parallel observations by radar from CryoSat-2, and lidar from ICESat-2 allows for direct comparison between these systems. Using 136 orbit segments from the northern hemisphere, constrained to the Pacific and Atlantic oceans as well as the Bering Sea, we compare the significant wave height (SWH) observations. By utilizing the coincident orbits, we can compare observations between altimeters of the same sea state within a constrained time lag (less than four hours), along longer stretches of the orbits. This is crucial to assess the level of agreement between observations, owing to the high variability of the ocean surface. With the comparison between the systems, as well as discussing the inherent benefit of each system, we can assess the possibilities of alternate methods for ocean surveying. From the available data, SWH up to 10 m has been used for the analysis, enabling comparison at various sea states.

We have used three methods with the ICESat-2 data in the comparison, with the first being the standard ocean data output (ATL12) as produced by the ICESat-2 team. This is compared with a method where modeling of the individual surface waves is used as an assessment for the SWH. It has been shown before to be possible to use the geolocated photons from ICESat-2 to assess these waves, which would be beneficial to compare with the radar altimeter of CryoSat-2. Functioning as a baseline for the wave approach, we are using the standard deviation of the ocean surface, the same method as ATL12, however with the same filtering as the wave-based model.

From this, we have described the differences between the altimeters and show a high correlation, with correlations between the models and CryoSat-2 SWH of 0.97 for ATL12, 0.95 for the observed waves model, and 0.97 for the standard deviation model. There has been found a mean deviation relative to the observed SWH for each model, deviating more at SWH larger than 2.5 m, but generally between -10 cm and 16 cm for SWH smaller than 2.5 m for all models. Compared with CryoSat-2 there was found an increasing deviation along with increasing SWH, along with a larger variance. In general, the SWH observed from ICESat-2 is found to agree with observations from CryoSat-2, within limitations due to cloud coverage. Observing the individual surface waves from ICESat-2 is therefore seen to provide additional observed properties of the sea state for global observations.