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SeaSTAR is a satellite mission candidate for ESA Earth Explorer 11 that proposes to measure small-scale ocean dynamics below 10 km at ocean/atmosphere/land/ice interfaces of the Earth System. SeaSTAR products consist of high-resolution images of total surface current vectors and wind vectors of unprecedented resolution (1 km) and accuracy over a wide swath. A key objective of SeaSTAR is to characterize, for the first time, the magnitude, spatial structure, regional distribution and temporal variability of upper ocean dynamics on daily, seasonal and multi-annual time scales, with particular focus on coastal seas, shelf seas and Marginal Ice Zone boundaries. The mission addresses an urgent need for new measurements of small-scale ocean processes to help understand and model their impacts on air-sea interactions, horizontal water pathways, vertical mixing and marine productivity. High-resolution imaging of total currents with collocated wind and waves data would bring new means of validating and developing models to improve operational forecasts and climate projections. The presentation will outline the key elements of the mission and the latest status of the mission concept evolution, with the technical solutions and trade-offs that are being considered. We will also present the latest results of the SEASTARex airborne campaign in Iroise Sea using the OSCAR (Ocean Surface Current Airborne Radar) demonstrator.

OSCAR (Ocean Surface Current Airborne Radar) is a new airborne instrument which provides unique 2D synoptic views of ocean and atmosphere dynamics (currents, waves, winds) below km-scale. OSCAR is a Ku-band (13.5 GHz) SAR system with Doppler and scatterometry capabilities in three azimuth look directions. The OSCAR instrument features an along-track interferometric (ATI) baseline in two lines-of-sight squinted 45° fore and aft from the broadside direction. The fore and aft antenna pairs provide interferometric Doppler measurements in two views angularly separated by 90 degrees. This ensures two orthogonal measurements of the ocean surface motion velocity that enable the retrieval of the total ocean surface current vector. In addition, backscatter measurements from the broadside antenna in the zero-Doppler direction serve to retrieve wind direction and wind speed, which are critical to correctly measure total ocean surface currents.In each line-of-sight, the ocean surface motion sensed by the microwave radar (after correcting for navigation and geometry) has two constituents: the total ocean surface current – consisting of all currents contributing to actual horizontal transport of water – and a measurement bias associated with the Doppler signature of the surface scatterers responsible for the backscatter, a term known as Doppler wave bias or Wind-wave induced Artefact Surface Velocity — WASV (Martin et al., 2016). The WASV is caused by the phase velocity of the surface scatterers responsible for the microwave backscatter (e.g. Bragg waves) and the effect of the orbital motion of longer ocean waves. The magnitude of the WASV can reach 0.5-1 m/s and is, at first order, a function of the wind direction. A number of geophysical model functions (GMFs) have been published in recent years to correct this effect.In May 2022, OSCAR was flown during the SEASTARex campaign over Iroise Sea (French Brittany). The campaign consisted of three flights on three different days, including acquisitions over a well-instrumented site with ground truth measurements of total ocean surface current fields from a WERA HF radar, supported by data from an X-band marine radar, stereo-video and a down-looking ADCP. Here, we present the first results of the validation of the OSCAR retrieved current fields against data from independent ground truth sensors and models.

The growing satellite record of sea state observations is becoming increasingly important for climate change research, to improve ocean and weather forecasts and to inform climate change mitigation and investment strategies. In this context, coastal processes and impacts are of particular concern, driving a yet stronger research imperative. The Copernicus Sentinel-6 Michael Freilich (S6-MF) mission was launched in November 2020 by the European Space Agency to succeed Jason-3 (J3) as the long term satellite altimetry reference mission. S6-MF commissioning involved a unique 12-month Tandem Experiment during which S6-MF flew approximately 30 seconds behind J3 on the same ground tracks, resulting in an unprecedented global dataset of quasi-simultaneous collocated altimeter sea state measurements in Low-Resolution Mode (LRM) and Synthetic Aperture Radar (SAR) mode.In this work, this unique dataset is examined to evaluate uncertainties in altimeter significant wave height (Hs) observations from the two missions in different operating modes and different sea state conditions. A particular focus is placed on the evaluation of uncertainties in the coastal zone by exploiting the large number of moored buoys located near the coast of the U.S. S6-MF and J3 data are compared with buoy measurements and reanalysis data using, amongst other methods, triple collocation (TC) analysis. Attention is paid to both the collocation methodology and possible correlation of random errors. Results indicate that, over both global and coastal oceans, J3 and S6-MF Low-Resolution Hs are almost identical, with near-zero bias, low RMS difference and very high correlation. This very high correlation precludes the use of triple collocation to the J3/S6-MF-SAR/buoy triplets. Comparing S6-MF SAR with J3 LRM and buoys confirms the positive sea-state dependent bias in SAR Hs. Triple collocation of J3, S6-MF and buoys reveals the sensitivity of measurement uncertainty to collocation criteria, particular in coastal areas. Further, we show how sea state dependence of measurement uncertainty varies between oceanic and coastal settings. In general, we find that steeper spatial gradients of sea state typically associated with coastal regions can hamper interpretation of TC analyses without undue consideration. These findings demonstrate the value of the Tandem Experiment to evaluate uncertainty and provide evidence of the stability and/or enhancements of new mission data contributing to the growing satellite climate record.

OSCAR (Ocean Surface Current Airborne Radar) is a new airborne instrument which provides unique 2D synoptic views of ocean and atmosphere dynamics (currents, waves, winds) below km-scale. OSCAR is the airborne demonstrator of SeaSTAR, an innovative satellite mission concept currently under study in Phase 0 of ESA Earth Explorer 11. SeaSTAR aims to observe ocean submesoscale dynamics and small-scale atmosphere-ocean processes in all coastal, shelf and polar seas by providing simultaneous measurements of current and wind vectors at 1 km resolution with accuracy better than 0.1 m/s and 2 m/s respectively. A key objective of SeaSTAR is to characterize, for the first time, the magnitude, spatial structure, regional distribution and temporal variability of upper ocean dynamics on daily, seasonal and multi-annual time scales, with particular focus on coastal seas, shelf seas and Marginal Ice Zone boundaries.OSCAR was flown over the Iroise Sea (West of Brittany, France) in May 2022 during the SEASTARex campaign. The OSCAR operations and products are representative of the spaceborne concept, with geophysical parameters and accuracies that directly relate to those of the SeaSTAR satellite mission. In itself, OSCAR provides a new observing capability that will improve our understanding of microwave Doppler sensing of the ocean thanks to its unique Doppler and scatterometry capabilities in three azimuth directions. OSCAR’s high-resolution images (8 metres pixels resolution) over a 5km swath provide 2D synoptic views of ocean and atmosphere dynamics below km-scales that are highly relevant to support and complement scientific investigations of fine-scale ocean-atmosphere processes based on in-situ, satellite and model data.In this paper, we give an overview of the OSCAR system, of the SEASTARex campaign over the Iroise Sea in May 2022 and present the main preliminary results about the performance and imaging capability of the instrument.

International audience; This review paper reports on the state-of-the-art concerning observations of surface winds, waves and currents from space and their use for scientific research and subsequent applications. The development of observations of sea state parameters from space dates back to the 1970s, with a significant increase in the number and diversity of space missions since the 1990s. Sensors used to monitor the sea-state parameters from space are mainly based on microwave techniques. They are either specifically designed to monitor surface parameters or are used for their abilities to provide opportunistic measurements complementary to their primary purpose. The principles on which is based the estimation of the sea surface parameters are first described, including the performance and limitations of each method. Numerous examples and references on the use of these observations for scientific and operational applications are then given. The richness and diversity of these applications are linked to the importance of knowledge of the sea state in many fields. Firstly, surface wind, waves and currents are significant factors influencing exchanges at the air/sea interface, impacting oceanic and atmospheric boundary layers, contributing to sea level rise at the coasts, and interacting with the sea-ice formation or destruction in the polar zones. Secondly, ocean surface currents combined with wind-and wave-induced drift contribute to the transport of heat, salt and pollutants. Waves and surface currents also impact sediment transport and erosion in coastal areas. For operational applications, observations of surface parameters are necessary on the one hand to constrain the numerical solutions of predictive models (numerical wave, oceanic or atmospheric models), and on the other hand to validate their results. In turn, these predictive models are used to guarantee safe, efficient and successful offshore operations, including the commercial shipping and energy sector, as well as tourism and coastal activities. Long time series of global sea-state observations are also becoming increasingly important to analyze the impact of climate change on our environment. All these aspects are recalled in the article, relating to both historical and contemporary activities in these fields. Article Highlights • The different techniques and satellite missions to monitor winds, waves and currents at the ocean surface from space are described • Sea-state observations from space are widely used in operational meteorology, as well as for research on ocean, atmosphere and climate processes • Sea-state observations from space are also actively used by the offshore industry, as well as in coastal zone management

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km2 island in the northwestern Mediterranean). Forty-six repeat passes were analyzed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly, one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modeling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges but also greatly enhance the study of storm surges and other coastal phenomena.

The “Air-Sea Observations for a Safe Ocean” satellite event to the UN Decade Safe Ocean Laboratory was held on April 7, 2022 at 0000 CEST with a total number of 39 participants. The 2-hour virtual workshop, also referred to on the Observing Air-Sea Interactions Strategy (OASIS) website as “OASIS for a Safe Ocean” (https://airseaobs.org/oasis-for-a-safe-ocean), included a 30-minute poster/social session in the interactive Gather.Town platform (Figure 1). Overall, the event was interactive and productive, fostering constructive discussions about the OASIS strategy. With a focus on Small Island Developing States (SIDS), three of the four speakers and one moderator were from island states. Overall, the group was diverse and demonstrated the strong interest of the global air-sea interactions community to promote a Safe Ocean, particularly for SIDS. Participants included many Early Career Ocean Professionals (ECOP), representing the stake they have in the future, and had active women participation.

Reflectometry is one of many new applications made possible by freely available global navigation satellite system (GNSS) signals. Reflectometry measurement geometry has some features that are significantly different from conventional radar or radiometry. To understand the potential utility of GNSS reflectometry (GNSS‐R), first this chapter reviews some of the broader principles of remote sensing and their application to Earth observation. Following a brief history of the field, it describes the fundamental principles and basic observables used in GNSS‐R. This is followed by applicable scattering models and their numerical evaluation. Before developing a comprehensive electromagnetic model for the Delay‐Doppler map (DDM), the chapter explores the basic geometry of bistatic radar, showing the relationship between the DDM and surface coordinates. This is used to define the essential features of the DDM which can be used to extract information about the roughness and reflectivity of the surface.

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from Global Navigation Satellite Systems (GNSS), which have been reflected off the Earth’s surface. GNSS-R is particularly promising as it does not require a dedicated transmitter, thereby reducing mass and power requirements of the instrument, which gives an opportunity to build a constellation of low-cost sensors providing short revisit times and unprecedented sampling capabilities.Ocean data has been collected regularly from GNSS-R instruments since 2014 from technology demonstration missions, e.g. TechDemoSat-1 (TDS-1, 2014 – 2018) and DoT-1 (2019 – Present), as well as operational missions, e.g. CyGNSS (2016 – Present). These missions have had different aims and setups, providing different perspectives on the capabilities of GNSS-R. TDS-1 and DoT-1 are polar orbiting, allowing the additional collection of data over sea-ice, where GNSS-R has shown strong signal sensitivity in coherent scattering conditions. The CyGNSS mission is the first GNSS-R constellation and consists of eight small satellites in an orbit that provides revisit times of only a few hours between the latitudes of ±35º, achieving a much higher sampling rate and faster revisit than TDS-1. Additionally, DoT-1 demonstrates onboard processing of signals originating from both GPS and Galileo satellites. The strong signal sensitivity to geophysical parameters such as ocean wind speed, when using signals from the Galileo system, shows the potential for further improvement in sampling when using signals from multiple Global Navigation Satellite Systems.Both the CYGNSS and TDS-1 missions have shown consistent performance in the retrieval of ocean wind speed once instrument calibration steps have been taken to mitigate some of the challenges inherent to GNSS-R technology. However, a number of geophysical variables theorised to be impacting the GNSS-R observables were investigated over a range of ocean wind speeds. The major dependencies affecting wind speed retrieval appear to be significant wave height and precipitation, which have their greatest impact at low wind speeds, with sea surface temperature having a weaker impact but across all wind speeds. These geophysical dependencies, additional to wind speed, can have a significant impact on retrievals and may need to be isolated for accurate retrievals. GNSS-R has shown strong capabilities for ocean remote sensing of multiple variables, using platforms and instruments that are still advancing to most effectively utilise the technology. A number of such advancements will be employed in the forthcoming ESA HydroGNSS mission, where the National Oceanography Centre (NOC) are leading the development of ground segment processors for Level-1 signal calibration and Level-2 ocean surface wind speed and sea-ice extent products.

High-resolution satellite images of sea surface temperature and ocean colour reveal an abundance of ocean fronts, swirls, vortices and filaments at horizontal scales below 10 km that permeate the global ocean, especially near mesoscale jets and eddies, in coastal seas and close to sea ice margins. These small-scale ocean features are the fingerprints of dynamic atmosphere-ocean interactions and intense ocean vertical processes that mediate exchanges across all the fundamental interfaces of the Earth System – between the atmosphere, the ocean surface, the ocean interior, the cryosphere and land – and impact major aspects of the global climate system.Numerous research studies and high-impact scientific publications confirm the key role of submesoscale processes in air-sea interactions, upper-ocean mixing, lateral transports and vertical exchanges with the ocean interior. Small-scale processes also visibly dominate in coastal, shelf seas and polar seas - regions of disproportionally high strategic and societal value as hosts to numerous human activities and natural resources. This paper will review some of the evidence about the fundamental role of small-scale ocean dynamics in the Earth System, making the case for new observations from space to characterise these important phenomena. The contribution ends by outlining the science drivers and objectives of the SEASTAR satellite candidate mission currently under study as one of four candidates to the European Space Agency Earth Explorer 11 programme.

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. We make use of products presented by Ribal et al. (2019), and the recently released products developed through Sea State CCI. In particular, making use of long term and continuous time series from moored data buoys, we demonstrate the impact of steadily increasing altimeter sample density on trend estimation. 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, thus 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 CCI Sea State altimetry and imaging SAR products.

Global Navigation Satellite System Reflectometry (GNSS-R) is a rapidly developing Earth observation technology that makes use of signals of opportunity from Global Navigation Satellite Systems that have been reflected off the Earth’s surface. The Cyclone Global Navigation Satellite System (CyGNSS) is a constellation of eight small satellites launched by NASA in 2016, carrying dedicated GNSS-R payloads to measure ocean surface wind speed at low latitudes (±35° North/South). The ESA ECOLOGY project evaluated CyGNSS v3.0 products, which were recently released following various calibration updates. This paper examines the performance of the new calibration by evaluating CyGNSS v3.0 Level-1 Normalised Bistatic Radar Cross Section (NBRCS) and Leading Edge Slope (LES) data from individual CyGNSS units and different GPS transmitters under constant ocean wind conditions. Results indicate that L1 NBRCS from individual CyGNSS units are well inter-calibrated and remarkably stable over time, a significant improvement over previous versions of the products. However, prominent geographical biases reaching over 3 dB are found in NBRCS, linked to factors including the choice of GPS transmitter and the bistatic geometry. L1 LES shows similar anomalies as well as a secondary geographical pattern of biases. These findings provide a basis for further improvement of CyGNSS Level-2 wind products and have wider applicability to improving the calibration of GNSS-R sensors for the remote sensing of non-ocean Earth surfaces.

This paper presents an in-depth study concerning the development of a sea state bias (SSB) model designed with three parameters exclusively derived from altimeter data and globally applied to all reference altimeter missions. The proposed technique, first tested for the Jason-1 mission, proves to have a good performance for a wide range of ocean conditions when compared with the state-of-the-art SSB corrections currently in use. In addition to the significant wave height ( $H_{s}$ ) and wind speed ( $U~_{{10}}$ ), a third predictor acting as a mediator parameter gathered by the mean wave period ( $T_{z}$ ) has been used. Two different empirical algorithms for altimeter ocean wave period have been tested and implemented, improving the SSB model performance in some ocean regions. The methodology relies on nonparametric modulation and statistical techniques based on smoothing splines embedded in a generalized additive model. This SSB modeling approach shows good performance when applied to all reference missions, in particular to TOPEX and Jason-2 missions, slightly reducing the explained variance of sea-level anomaly (SLA) when compared with the established SSB models. The approach is computationally efficient, capable of generating a stable SSB model using a small training data set when little information is available, as is the case with the recent Jason-3 mission. Model performance is assessed by comparison with existing SSB corrections for each reference mission, intercomparisons during the period of the tandem phases, and by SLA variance analysis, providing a consistent set of SSB corrections for the four reference missions.

Direct estimate of ocean surface motion sensed by the Doppler shift of the surface includes ocean surface current and a wind-wave induced artefact surface velocity (WASV). The Sentinel-1 (S1) C-band SAR mission includes direct ocean surface motion estimates as an operational Level-2 Ocean (OCN) Radial VeLocity (RVL) product. The existing operational RVL products suffer from significant uncorrected platform and instrument effects that presently prevent exploitation of the data. This paper proposes a simple method to calibrate and correct for these effects and evaluate the benefit of these corrections over 2.5 years S1A acquisition against ground truth measurements. A specific geometry for S1 has been chosen for S1-A over the HF radar (HFR) instrumented site in the German Bight. The 78 S1A snapshots end in 56 match-ups within 20 minutes of HFR measurements. HFR velocity fields were projected in the same radial direction as S1A. Land calibration corrects for constant snapshot biases of the operational products up to 2 m/s. Besides these constant biases there is persistent relative biases within snapshots between up to 0.4 m/s in addition to the TOPSAR uncorrected scalloping effect with an amplitude of 0.1 m/s. After calibration, corrected RVL are compared against HFR with various WASV correction. Applying WASV correction with a reduced 70% C-Dop model, gives the best results with a precision of 0.25 m/s and correlation in time of 0.9. This might be due to C-Dop amplitude in up/downwind being too strong for a coastal environment as encountered in the German Bight. Quadratic mean of all 78 S1A snapshots after all corrections applied exhibits coastal current jets in good agreement with bathymetry channels and is promising as a cheap way to infer local bathymetry channels.

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from Global Navigation Satellite Systems, which have been reflected off the Earth’s surface. CYGNSS is a constellation of 8 satellites launched in 2016 which use GNSS-R technology for the remote sensing of ocean wind speed. The ESA ECOLOGY project aims to evaluate CYGNSS data which has recently undergone a series of improvements in the calibration approach. Using CYGNSS collections above the ocean surface, an assessment of Level-1 calibration is presented, alongside a performance evaluation of Level-2 wind speed products. L1 data collected by the individual satellites are shown to be generally well inter-calibrated and remarkably stable over time, a significant improvement over previous versions. However, some geographical biases are found, which appear to be linked to a number of factors including the transmitter-receiver pair considered, viewing geometry, and surface elevation. These findings provide a basis for further improvement of CYGNSS products and have wider applicability to improving calibration of GNSS-R sensors for remote sensing of the Earth.

Measurements of significant wave height from satellite altimeter missions are finding increasing application in investigations of wave climate, sea state variability and trends, in particular as the means to mitigate the general sparsity of in situ measurements. However, many questions remain over the suitability of altimeter data for the representation of extreme sea states and applications in the coastal zone. In this paper, the limitations of altimeter data to estimate coastal Hs extremes (&lt, 10 km from shore) are investigated using the European Space Agency Sea State Climate Change Initiative L2P altimeter data v1.1 product recently released. This Sea State CCI product provides near complete global coverage and a continuous record of 28 years. It is used here together with in situ data from moored wave buoys at six sites around the coast of the United States. The limitations of estimating extreme values based on satellite data are quantified and linked to several factors including the impact of data corruption nearshore, the influence of coastline morphology and local wave climate dynamics, and the spatio-temporal sampling achieved by altimeters. The factors combine to lead to considerable underestimation of estimated Hs 10-yr return levels. Sensitivity to these factors is evaluated at specific sites, leading to recommendations about the use of satellite data to estimate extremes and their temporal evolution in coastal environments.

Accurately resolving coastal Total Water Levels (TWL) is crucial for socio-economic and environmental reasons. Recent efforts in satellite altimetry and numerical modeling have improved accuracy over near-shore areas. In this study we used data from tide gauges (TGs), SAR-mode altimetry from two satellites [Sentinel-3A (S3) and CryoSat-2 (C2)], and a state-of-the-art high-resolution regional coupled environmental prediction model (Amm15) to undertake an inter-comparison between the observations and the model. The aim is to quantify our capability to measure TWL around the United Kingdom coast, and to quantify the capacity of the model to represent coastal TWL. Results show good agreement between the satellite and TG data [the mean correlation (R) over seventeen TGs between June 2016 and September 2017 is 0.85 for S3 and 0.80 for C2]. The satellite-model comparison shows that the variability is well captured (R = 0.98 for both satellite), however, there is an offset (−0.23 m for S3, −0.15 m for C2) between the satellite and model data, that is near-constant across the domain. This offset is partly attributed to the difference in the reference level used by the satellites and the model, and residual differences linked to the temporal resolution of the model. The best agreement between model and satellite is seen away from the coast, further than 3–4 km offshore. However, even within the coastal band, R remains high, ∼0.95 (S3) and ∼0.96 (C2). In conclusion, models are still essential to represent TWL in coastal regions where there is no cover from in-situ observations, but satellite altimeters can now provide valuable observations that are reliable much closer to the coast than before.

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from GNSS, which have been reflected off the Earth's surface. This technology has been demonstrated to be applicable to the remote sensing of a number of geophysical surface parameters including ocean wind speed and sea-ice. Using data collected by the UK TechDemoSat-1 mission between 2014 and 2018, the National Oceanography Centre (NOC) has developed a GNSS-R signal processing scheme called the NOC Calibrated Bistatic Radar Equation (C-BRE) processor that features an ocean wind speed inversion algorithm incorporating radiometric calibration submodules and several corrections steps that mitigate effects related to the GNSS system, instrumentm and geometry. The latest version of the NOC GNSS-R processor additionally features updated data quality control mechanisms that include the flagging of radio frequency interference (RFI) and sea-ice detection based on the GNSS-R waveform.

The UK’s TechDemoSat-1 (TDS-1), launched 2014, has demonstrated the use of global positioning system (GPS) signals for monitoring ocean winds and sea ice. Here it is shown, for the first time, that Galileo and BeiDou signals detected by TDS-1 show similar promise. TDS-1 made seven raw data collections, recovering returns from Galileo and BeiDou, between November 2015 and March 2019. The retrieved open ocean delay Doppler maps (DDMs) are similar to those from GPS. Over sea ice, the Galileo DDMs show a distinctive triple peak. Analysis, adapted from that for GPS DDMs, gives Galileo’s signal-to-noise ratio (SNR), which is found to be inversely sensitive to wind speed, as for GPS. A Galileo track transiting from open ocean to sea ice shows a strong instantaneous SNR response. These results demonstrate the potential of future spaceborne constellations of GNSS-R (global navigation satellite system–reflectometry) instruments for exploiting signals from multiple systems: GPS, Galileo, and BeiDou.

This paper presents the temporal evolution of Global Navigation Satellite System Reflectometry (GNSS-R) ocean wind speed retrieval performance during three years of the UK TechDemoSat-1 (TDS-1) mission. TDS-1 was launched in July 2014 and provides globally distributed spaceborne GNSS-R data over a lifespan of over three years, including several months of 24/7 operations. TDS-1 wind speeds are computed using the NOC Calibrated Bistatic Radar Equation algorithm version 0.5 (C-BRE v0.5), and are evaluated against ERA5 high resolution re-analysis data over the period 2015–2018. Analyses reveal significant temporal variability in TDS-1 monthly wind speed retrieval performance over the three years, with the best performance (~2 m∙s−1) achieved in the early part of the mission (May 2015). The temporal variability of retrieval performance is found to be driven by several non-geophysical factors, including TDS-1 platform attitude uncertainty and spatial/temporal changes in GPS transmit power from certain satellites. Evidence is presented of the impact of the GPS Block IIF Flex mode on retrieved GNSS-R wind speed after January 2017, which results in significantly underestimated ocean winds over a large region covering the North Atlantic, northern Indian Ocean, the Mediterranean, the Black Sea, and the Sea of Okhotsk. These GPS transmit power changes are shown to induce large negative wind speed biases of up to 3 m∙s−1. Analyses are also presented of the sensitivity of TDS-1 wind speed retrieval to platform attitude uncertainty using statistical simulations. It is suggested that a 4° increase in attitude uncertainty can produce up to 1 m∙s−1 increase in RMSE, and that TDS-1 attitude data do not fully reflect actual platform attitude. We conclude that the lack of knowledge about the GNSS-R nadir antenna gain map and TDS-1 platform-attitude limits the ability to determine the achievable wind speed retrieval performance with GNSS-R on TDS-1. The paper provides recommendations that accurate attitude knowledge and a good characterisation of GNSS-R nadir antenna patterns should be prioritised for future GNSS-R missions.

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from Global Navigation Satellite Systems, which have been reflected off the Earth’s surface. Using GNSS-R data collected by the UK TechDemoSat-1 (TDS-1) between 2014 and 2018, the National Oceanography Centre (NOC) has developed a GNSS-R wind speed retrieval algorithm called the Calibrated Bistatic Radar Equation (C-BRE), which now features updated data quality control mechanisms including flagging of radio frequency interference (RFI) and sea-ice detection based on the GNSS-R waveform. Here we present an assessment of the performance of the latest NOC GNSS-R ocean wind speed and sea-ice products (NOC C-BRE v1.0) using validation data from the ECMWF ERA-5 re-analysis model output. Results show the capability of spaceborne GNSS-R sensors for accurate wind speed retrieval and sea-ice detection. Additionally, ground-processed Galileo returns collected by TDS-1 are examined and the geophysical sensitivity of reflected Galileo data to surface parameters is investigated. Preliminary results demonstrate the feasibility of spaceborne GNSS-R instruments receiving a combination of GNSS signals transmitted by multiple navigation systems, which offers the opportunity for frequent, high-quality ocean wind and sea-ice retrievals at low relative cost. Other advancements in GNSS-R technology are represented by future mission concepts such as HydroGNSS, a proposed ESA Scout mission opportunity by SSTL offering support for enhanced retrieval capabilities exploiting dual polarisation, dual frequency, and coherent reflected signal reception.

Long‐term changes in ocean surface waves are relevant to society and climate research. Significant wave height climatologies and trends over 1992‐2017 are intercompared in four recent high‐quality global datasets using a consistent methodology. For two products based on satellite altimetry, including one from the European Space Agency Climate Change Initiative for Sea State, regional differences in mean climatology are linked to low and high sea states. Trends from the altimetry products, and two reanalysis and hindcast datasets, show general similarity in spatial variation and magnitude but with major differences in equatorial regions and the Indian Ocean. Discrepancies between altimetry products likely arise from differences in calibration and quality control. However, multi‐decadal observations at two buoy stations also highlight issues with wave buoy data, raising questions about their unqualified use, and more fundamentally about uncertainty in all products. Plain Language Summary Changes to ocean waves over decades and longer are of considerable importance to climate, society and the marine economy. Accurate observations of waves spanning many decades are required to understand long‐term changes, but the challenges and cost of measuring waves worldwide with devices like buoys means that alternatives like Earth‐orbiting satellites become attractive. We compare two recently published global wave products derived from the same satellite observations, with two high quality products from computer simulations, and buoy measurements. Using a consistent methodology, we find important differences between the satellite products, and the simulations, in the reported average global wave conditions, and their evolution in time. The disagreement between the satellite products points to complex differences in the way satellite data are corrected, which raises questions about uncertainty in these products, and more generally, about what is our most reliable long‐term observational record of sea state.

CryoSat’s ability to operate in different operating modes over water surfaces led to the first in-orbit evidence of the value of SAR-mode altimetry for oceanography, with the mission continuing to provide high-quality data and information not just over ice but also over the open ocean, polar waters and coastal regions. Approaching ten years in orbit, CryoSat routinely delivers a number of oceanographic products for global ocean applications. A dedicated operational CryoSat ocean processor (COP) has existed since April 2014 generating data products available in near real time (FDM/NOP), within ~3 days (IOP) and a geophysical ocean product (GOP) available within a month. An improved processing baseline was introduced in late 2017 and the same processing chain has now been applied to provide consistent ocean data products from the start of the mission. Within the ESA funded CryOcean-QCV project, the National Oceanography Centre (NOC) in the UK is responsible for routine quality control and validation of CryoSat Ocean Products. Activities include the production of daily and monthly reports containing global assessments and quality control of sea surface height anomaly (SSHA), significant wave height (SWH), backscatter coefficient (Sigma0) and wind speed, as well as a suite of validation protocols involving in situ data, model output and data from other satellite altimetry missions. This presentation will review some of the metrics and results obtained for CryoSat Ocean Products for SSHA, SWH and wind speed when assessed against data from tide gauges, wind and wave buoys, WaveWatch III wave model output, HF radar surface current data and comparisons with Jason-2 and Jason-3. Example metrics include SSHA along-track power spectra and the characterisation of offsets and variability regionally and in different sea states. In this presentation, we demonstrate the quality and scientific value of the CryoSat data in the open ocean where the altimeter operates mainly in conventional low-resolution-mode (LRM) but also over selected ocean regions where CryoSat operates in SAR-mode. Finally, scientific exploitation of the CryoSat data for oceanographic studies will be illustrated, focusing on CryoSat sea surface height anomalies. We will present examples of the benefits of CryoSat ocean products for oceanographic studies based on a dedicated Level 3 gridded product, featuring investigations of propagating ocean features (e.g. Rossby-type wave propagation) and their signatures in CryoSat in comparisons with data from other sources including SMOS, Sentinel 3A and 3B.

SCOOP (SAR Altimetry Coastal & Open Ocean Performance) is a project funded under the ESA SEOM (Scientific Exploitation of Operational Missions) Programme Element, to characterise the expected performance of Sentinel-3 SRAL SAR mode altimeter products, and then to develop and evaluate enhancements to the baseline processing scheme in terms of improvements to ocean measurements. Another objective is to develop and evaluate an improved Wet Troposphere correction for Sentinel-3.The SCOOP studies are based on two 2-year test data sets derived from CryoSat-2 FBR data, produced for 10 regions. The first Test Data Set was processed with algorithms equivalent to the Sentinel-3 baseline, and the second with algorithms expected to provide an improved performance.We present results from the SCOOP project that demonstrate the excellent performance of SRAL at the coast in terms of measurement precision, with noise in Sea Surface Height 20Hz measurements of less than 5cm to within 5km of the coast.We then report the development and testing of new processing approaches designed to improve performance, including, for L1B to L2:Application of zero-padding Application of intra-burst Hamming windowing Exact beam forming in the azimuthal direction Restriction of stack processing to within a specified range of look angles. Along-track antenna compensation And for L1B to L2Application of alternative re-trackers for SAR and RDSAR. Based on the results of this assessment, a second test data set was generated and we present an assessment of the performance of this second Test Data Set generated, and compare it to that of the original Test Data Set.Regarding the WTC for Sentinel-3A, the correction from the on-board MWR has been assessed by means of comparison with independent data sets such as the GPM Microwave Imager (GMI), Jason-2, Jason-3 and Global Navigation Satellite Systems (GNSS) derived WTC at coastal stations. GNSS-derived path Delay Plus (GPD+) corrections have been derived for S3A. Results indicate good overall performance of S3A MWR and GPD+ WTC improvements over MWR-derived WTC, particularly in coastal and polar regions. Based on the outcomes of this study we provide recommendations for improving SAR mode altimeter processing and priorities for future research.

Direct 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. Among those, WaCM (Rodriguez et al., 2019), SKIM (Ardhuin et al., 2019) and the ESA Earth Explorer candidate Harmony (ESA, 2020) and SEASTAR (Gommenginger et al., 2019).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.