Sensitivity of Altimeter Wave Height Assessment to Data Selection

Thispaperaddressestheissueofhowtheselectionofbuoysandthecalculationofaltimeter averages affect the metrics characterising the errors of altimetric wave height estimates. The use of a 51-point median reduces the sensitivity to occasional outliers, but the quality of this measure can be improved by demanding that there is a minimum number of valid measurements. This had a marked impact in both the open ocean and the coastal zone. It also affected the relative ordering of algorithms’ performances, as some fared poorly when a representative value was gleaned from a single waveform inversion, but had a much better ranking when a minimum of 20 values were used. Validation procedures could also be improved by choosing altimeter-buoy pairings that showed a good consistency. This paper demonstrated an innovative procedure using the median of the differentretrackersanalysed,whichcanbeeasilyextendedtootherdatavalidationexercises. Thisled to improved comparison statistics for all algorithms in the open ocean, with many showing errors less than 0.2 m, but there was only one strong change in the relative performance of the 11 Jason-3 retrackers. For Sentinel-3A, removing the inconsistent coastal buoys showed that all of the new algorithms had similar errors of just over 0.2 m. Thus, although improvements were found in the procedure usedforthe SeaState RoundRobinexercise, the relative rankingsforthe buoycalibrations are mostly unaffected

Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England

DuetothesmallergroundfootprintandhigherspatialresolutionoftheSyntheticAperture Radar (SAR) mode, altimeter observations from the Sentinel-3 satellites are expected to be overall more accurate in coastal areas than conventional nadir altimetry. The performance of Sentinel-3A in the coastal region of southwest England was assessed by comparing SAR mode observations of significant wave height against those of Pseudo Low Resolution Mode (PLRM). Sentinel-3A observations were evaluated against in-situ observations from a network of 17 coastal wave buoys, which provided continuous time-series of hourly values of significant wave height, period and direction. As the buoys are evenly distributed along the coast of southwest England, they are representative of a broad range of morphological configurations and swell conditions against which to assess Sentinel-3 SAR observations. The analysis indicates that SAR observations outperform PLRM within 15 km from the coast. Within that region, regression slopes between SAR and buoy observations are close to the 1:1 relation, and the average root mean square error between the two is 0.46±0.14 m. On the other hand, regression slopes for PLRM observations rapidly deviate from the 1:1 relation, while the average root mean square error increases to 0.84±0.45 m. The analysis did not identify any dependence of the bias between SAR and in-situ observation on the swell period or direction. The validation is based on a synergistic approach which combines satellite and in-situ observations with innovative use of numerical wave model output to help inform the choice of comparison regions. Such an approach could be successfully applied in future studies to assess the performance of SAR observations over other combinations of coastal regions and altimeters

The Roles of the S3MPC: Monitoring, Validation and Evolution of Sentinel-3 Altimetry Observations

The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges

An Overview of Requirements, Procedures and Current Advances in the Calibration/Validation of Radar Altimeters

Analysis of the radar echoes from a spaceborne altimeter gives information on sea surface height, wave height and windspeed, as well as other parameters over land and ice. The first spaceborne radar altimeter was pioneered on Skylab in 1974. Since then, there have been about 20 further missions, with several advances in the sophistication of hardware and complexity of processing with the aim of increased accuracy and precision. Because of that, the importance of regular and precise calibration and validation(“cal/val”) remains undiminished, especially with efforts to merge altimetric records from multiple missions spanning different domains and time periods. This special issue brings together 19 papers, with a focus on the recent missions (Jason-2, Jason-3, Sentinel-3A and HY-2B) as well as detailing the issues for anticipated future missions such as SWOT.This editorial provides a brief guide to the approaches and issues for cal/val of the various different derived parameters, including a synopsis of the papers in this special issue

Nouméa: a new multi-mission calibration and validation site for past and future altimetry missions?

Today, monitoring the evolution of sea level in coastal areas is of importance, since almost 11 % of the world's population lives in low-lying areas. Reducing uncertainties in sea level estimates requires a better understanding of both altimetry measurements and local sea level dynamics. In New Caledonia, the Nouméa lagoon is an example of this challenge, as altimetry, coastal tide gauge, and vertical land motions from global navigation satellite systems (GNSSs) do not provide consistent information. The GEOCEAN-NC 2019 field campaign addresses this issue with deployments of in situ instruments in the lagoon (GNSS buoy, pressure gauge, etc.), with a particular focus on the crossover of one Jason-series track and two Sentinel-3A missions tracks. In this study, we propose a method to virtually transfer the Nouméa tide gauge at the altimetry crossover point, using in situ data from the field campaign. Following the philosophy of calibration and validation (Cal/Val) studies, we derive absolute altimeter bias time series over the entire Jason and Sentinel-3A periods. Overall, our estimated altimeter mean biases are slightly larger by 1–2 cm compared to Corsica and Bass Strait results, with inter-mission biases in line with those of Bass Strait site. Uncertainties still remain regarding the determination of our vertical datum, only constrained by the three days of the GNSS buoy deployment. With our method, we are able to re-analyse about 20 years of altimetry observations and derive a linear trend of −0.2 ± 0.1 mm yr−1 over the bias time series. Compared to previous studies, we do not find any significant uplift in the area, which is more consistent with the observations of inland permanent GNSS stations. These results support the idea of developing Cal/Val activities in the lagoon, which is already the subject of several experiments for the scientific calibration phase of the SWOT wide-swath altimetry mission.</p

The gulf of cadiz as a natural laboratory for the validation and exploitation of coastal altimetry and model data

Hydrodynamic models and satellite altimetry can be complementary tools for the study of sea level variations. Nowadays, users of these tools demand high quality products in coastal zones. In this sense, this doctoral dissertation focusses on the validation of innovative products that entail an advance in the understanding of sea level variation in coastal areas. The study was carried out in the Gulf of Cadiz (GoC), an important region that connects the Atlantic Ocean and the Mediterranean Sea, although other study areas were also used to strengthen the analysis. The research was performed using: in-situ data, sea level altimetry measurements from Sentinel-3 SRAL, and observations from a hydrodynamic model implemented in the main study area. The in-situ data were used to validate the altimetry measurements, as well as to calibrate and validate the numerical model. Different specific objectives were proposed. The first was to determine the quality of altimetric data in coastal areas from the new Sentinel-3 space mission. Altimetry data of Sentinel-3A SRAL (S3A) were validated at the sampling frequency of 80 Hz. The data were obtained from the European Space Agency (ESA) Grid Processing On Demand (GPOD) service over three coastal sites in Spain: Huelva (GoC), Barcelona (Western Mediterranean Sea), and Bilbao (Bay of Biscay). Two tracks were selected at each site: one ascending and one descending. Data were validated using in-situ tide gauge (TG) data provided by the Spanish Puertos del Estado. The altimetry Sea Level Anomaly (SLA) time series were obtained using the corrections available in GPOD. The validation was performed using two statistical parameters, the Pearson correlation coefficient (r) and the root mean square error (rmse). In the 5–20 km segment with respect to the coastline, the results obtained were 6–8 cm (rmse) and 0.7–0.8 (r) for all of the tracks. The 0–5 km segment was also analysed in detail to study the effect of land on the quality of altimetry data. Results showed that the track orientation, the angle of intersection with the coast, and the land topography concur to determine the nearest distance to the coast at which the data retain a similar level of accuracy than in the 5–20 km segment. This ‗distance of good quality‘ to shore reaches a minimum of 3 km for the tracks at Huelva and the descending track at Barcelona. In addition, altimetry sea level data of Sentinel-3A and Sentinel-3B SRAL (S3A and S3B) were also validated at the sampling frequency of 80 Hz. Two tracks of S3A and two of S3B were selected at seven different coasts around the Iberian Peninsula. The altimetry SLA time series obtained were compared with TG in-situ data using the standard deviation of the difference (sdd) and the normalized sdd (sdd_n). Two tidal models were used: TPXO8 and TPXO9. They were previously validated with in-situ data and then used in the S3 assessment. Contrary to expectations, a more current version of the tide model did not always lead to improvements for all of the coasts studied. The same data availability and accuracy results (mean sdd <9cm) were obtained for both satellites, as the radar altimeter on-board S3A and S3B are identical instruments. The sdd_n results were generally ranged between 0.1 and 0.25 cm, higher values were obtained in coastal areas with a complex hydrodynamic. The second specific objective was to implement the Delft3D model in the estuary of the Guadalquivir River and part of the GoC continental shelf with the aim of studying the influence of its discharges on the sea level variability. Details of the Delft3D FLOW module implementation are given in the manuscript. The model was calibrated and validated along the river estuary comparing the output with in-situ observations of water level and current velocities during normal and high-discharge events. In order to obtain the best possible adjustment, the friction coefficient and bathymetry were used as adjustment parameters. The determination coefficients attained mean values of R2= 0.9 and R2=0.8, for calibration and validation, respectively. Moreover, the model was calibrated for the continental shelf during normal discharge conditions using data from three current meters, obtaining good correlation results (rmse= 3.0 cm·s -1 and R2=0.7, on average). The model simulations were able to reproduce the low salinity plume-induced over-elevations at the mouth of the estuary and its adjacent inner shelf during periods of high river discharge from the head dam (> 400 m3 ·s -1 ). These over-elevations were also identified in a qualitative comparison with altimetry data. Despite the good results obtained, there are some improvements that could be made for future work: including wind, coupling the wave module, updating the bathymetry, etc. The aim of this thesis last section was to apply the new Fully Focused SAR (FF SAR) processing technique for the Sentinel-3 altimetry signal. The accuracy and precision of this novel product were analysed in order to provide the best quality product close to the coast (0-5 km track segment). FF SAR processing is similar to SAR altimetry, but with an unprecedented high along-track resolution which goes up to the theoretical limit equal to half the antenna length (~0.5 m). Two FF SAR algorithms still in development were used in this work: FF SAR Back Projection (BP) (S3 prototype version of Kleinherenbrink et al., 2020), and FF SAR Omega-Kappa (WK) (Guccione et al., 2018), as well as different retracking algorithms. Two tracks from Sentinel-3A and two from Sentinel-3B were processed, at 80 Hz. The products were validated by comparing time series of SLA with those obtained from a tide gauge in the Gulf of Cadiz. The accuracy was analysed using the Percentage of Cycles for High Correlation (PCHC) and the standard deviation of the difference (sdd); and the precision was determined by calculating the along-track noise. FF SAR and unfocused SAR products were compared. The results showed improvements using Adaptative Leading Edge Subwaveform (ALES+) retracker for unfocused SAR, although FF SAR BP with ALES+ was the most precise product for all the tracks. In addition, highly accurate SLA measurements were also obtained with FF SAR products. The greatest advantage of FF SAR is that it produces good quality data closer to the coast (1-2 km) than unfocused SAR (3-4 km). Finally, these results highlight the potential of the implementation of validated altimetry data and hydrodynamic models in sea level studies. Furthermore, the methodology described here can be useful to validate altimetry data, as well as to implement the Delft3D model in other coastal areas

Multisatellite altimetry calibration and validation using a GNSS Wave Glider in the North Sea

The concept of in situ multisatellite altimetry calibration and validation in the absolute sense using ocean autonomous surface vehicles as global navigation satellite systems (GNSS) platforms is demonstrated through an experiment in the North Sea during 2016. A Wave Glider (WG) equipped with geodetic GNSS traveled to locations ranging from 21 to 78 km from the coast to be directly under four Jason-series tracks and two CryoSat-2 tracks. 5-Hz sea surface heights (SSHs) were estimated from precise point positioning (PPP) mode processing of GPS+GLONASS data, together with hourly zenith wet tropospheric delays (ZWDs), and used as reference values for altimetry satellite measured SSH, tropospheric delay, and significant wave height (SWH). SSH biases obtained were −30 to −8 mm for Jason-2 using geophysical data record (GDR)-D products, −40 to +1 mm for Jason-3 using GDR-F products, and −29 and +18 mm for CryoSat-2 using SAR mode GOP baseline C products. These biases are almost commensurate with results from previous studies in other regions that used GNSS buoys or onshore GNSS reference stations with geoid and tide extrapolation. The Jason-2 and Jason-3 microwave radiometer (MWR)-measured ZWDs differed, respectively, by −15 and −10 mm on average from those measured by the GNSS WG. Root-mean-square SWH differences of 2–6 cm were obtained between Jason-2/3 and the co-located GNSS WG, and equivalent differences of 19–21 cm for CryoSat-2

Validation of Sentinel-3a Sral Coastal Sea Level Data at High Posting Rate: 80Hz

Altimetry data of two and a half years (June 2016-November 2018) of Sentinel 3A SRAL were validated at the sampling frequency of 80 Hz. The study areas are three coastal sites in Spain: Huelva (HU) (Gulf of Cadiz), Barcelona (BA), and Bilbao (BI). Two tracks were selected in each site: one ascending and one descending. Data were validated using in situ tide gauge (TG) data provided by the Spanish Puertos del Estado. In the 5 to 20 km segment, the results were 6-8 cm (rmse) and 0.7-0.8 (r) for all the tracks. The 0 to 5 km segment was also analyzed in detail to study the land effect on the altimetry data quality. The results showed that the track orientation, the angle of intersection with the coast, and the land topography concur to determine the nearest distance to the coast at which the data retain a similar level of accuracy than in the 5 to 20 km segment. This distance of good quality to shore reaches a minimum of 3 km for the tracks at HU and the descending track at BA