EUMETSAT Invitation To Tender 14/209556: JASON-CS SAR Mode Sea State Bias Study. Final report

This document represents the final report of a study funded by EUMETSAT about SAR mode Sea State Bias (SSB) for the Sentinel-6/Jason-CS mission. The study comprises a critical review of SSB estimation methods in conventional (low-resolution mode or LRM) altimetry, theoretical considerations about the effect of swell on SAR altimeter waveforms and empirical investigations with Cryosat-2 SAR mode data to detect swell effects in L1B and Level 2 Sea Surface Height (SSH). The report concludes by summarising the basis for the selection and derivation of the SAR altimeter sea state bias correction algorithm and the methods available to calibrate and validate SAR mode SSB corrections. Theoretical considerations based on simple SAR waveform modelling indicate that multipeaked waveforms could occur in the presence of swell, but that effects become clearly detectable only when swell height exceeds 4 meters, which is relatively rare. In the case of the Cryosat-2 data examined in this study, only 2% of samples satisfied this condition. Experimental investigations of Cryosat-2 SAR mode data in different swell conditions produce no consolidated evidence of swell effects. Although anomalous 20Hz waveforms are occasionally observed, no statistically detectable effect of swell is obtained in the overall results for average L1B waveform shapes and L2 1Hz SSH biases and precisions. However, it is stressed that analyses in this study were limited geographically by the availability of Cryosat-2 SAR mode acquisitions over the ocean that could be collocated with Envisat ASAR swell data. It is strongly advised that analyses should be repeated with a broader geographical scope, including data from the central Pacific and the Southern Ocean where high sea state and swell conditions are more prevalent. It is suggested that this could be achieved using Sentinel-3 SRTM and Sentinel-1 L2 swell products, should such data be available. Empirical SSB estimation methods offer the only viable way forward at present to estimate SAR mode SSB. Parametric, non-parametric and hybrid methods are all relevant, noting that hybrid methods may provide more robust estimates in those high sea state and swell conditions that are less densely populated and where effects will be more significant. The development of SAR mode SSB corrections should include additional dependence on sea state development, which would be consistent with the tendency in LRM towards three-parameters SSB models (e.g. Tran et al., 2010b; Pires et al., 2016). The challenges of calibrating and validating SAR mode SSB corrections are the same - i.e. no better, no worse - than for conventional altimetry. For SAR mode altimetry however, P-LRM offer a unique way of calibrating and validating SAR mode SSB against conventional altimetry by providing coincident range measurements that have been shown to be unbiased against conventional LRM. In the case of Sentinel-6/Jason-CS, interleaved SAR mode will deliver true LRM data that make it possible to tie the Jason-CS SAR mode mission to the long-term altimetric data record without the issues linked to the loss of precision seen for SAR burstmode P-LRM

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

BALTIC+ Theme 3 Baltic+ SEAL (Sea Level) Product Handbook

This handbook is designed to support both novice and more advanced users. It is a reference guide for users of the ESA Baltic SEAL suite of products. It provides fundamental information on the theory underpinning the products, and the technical specifications of the data you can access. It also provides links to the more in-depth literature and information on the theory and technical aspects of the product you are using. Newcomers to satellite altimetry data can find basic information on altimetry, and how to interpret and understand the data files you have obtained. There are also helpful basic codes to display and explore your newly acquired data. For more expert users, the overview information is presented here, with more technical, and in-depth information available in the referenced literature

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

The Development of the Joint NASA GSFC and the National Imagery and Mapping Agency (NIMA) Geopotential Model EGM96

The NASA Goddard Space Flight Center (GSFC), the National Imagery and Mapping Agency (NIMA), and The Ohio State University (OSU) have collaborated to develop an improved spherical harmonic model of the Earth's gravitational potential to degree 360. The new model, Earth Gravitational Model 1996 (EGM96), incorporates improved surface gravity data, altimeter-derived gravity anomalies from ERS-1 and from the GEOSAT Geodetic Mission (GM), extensive satellite tracking data-including new data from Satellite Laser Ranging (SLR), the Global Postioning System (GPS), NASA's Tracking and Data Relay Satellite System (TDRSS), the French DORIS system, and the US Navy TRANET Doppler tracking system-as well as direct altimeter ranges from TOPEX/POSEIDON (T/P), ERS-1, and GEOSAT. The final solution blends a low-degree combination model to degree 70, a block-diagonal solution from degree 71 to 359, and a quadrature solution at degree 360. The model was used to compute geoid undulations accurate to better than one meter (with the exception of areas void of dense and accurate surface gravity data) and realize WGS84 as a true three-dimensional reference system. Additional results from the EGM96 solution include models of the dynamic ocean topography to degree 20 from T/P and ERS-1 together, and GEOSAT separately, and improved orbit determination for Earth-orbiting satellites

A Conceptually Simple Modeling Approach for Jason-1 Sea State Bias Correction Based on 3 Parameters Exclusively Derived from Altimetric Information

A conceptually simple formulation is proposed for a new empirical sea state bias (SSB) model using information retrieved entirely from altimetric data. Nonparametric regression techniques are used, based on penalized smoothing splines adjusted to each predictor and then combined by a Generalized Additive Model. In addition to the significant wave height (SWH) and wind speed (U10), a mediator parameter designed by the mean wave period derived from radar altimetry, has proven to improve the model performance in explaining some of the SSB variability, especially in swell ocean regions with medium-high SWH and low U10. A collinear analysis of scaled sea level anomalies (SLA) variance differences shows conformity between the proposed model and the established SSB models. The new formulation aims to be a fast, reliable and flexible SSB model, in line with the well-settled SSB corrections, depending exclusively on altimetric information. The suggested method is computationally efficient and capable of generating a stable model with a small training dataset, a useful feature for forthcoming missions

Investigating the optimal integration of airborne, ship-borne, satellite and terrestrial gravity data for use in geoid determination

Each gravity observation technique has different parameters and contributes to different pieces of the gravity spectrum. This means that no one gravity dataset is able to model the Earth’s gravity field completely and the best gravity map is one derived from many sources. Therefore, one of the challenges in gravity field modelling is combining multiple types of heterogeneous gravity datasets. The aim of this study is to determine the optimal method to produce a single gravity map of the Canterbury case study area, for the purposes of use in geoid modelling. This objective is realised through the identification and application of a four-step integration process: purpose, data, combination and assessment. This includes the evaluation of three integration methods: natural neighbour, ordinary kriging and least squares collocation. As geoid modelling requires the combination of gravity datasets collected at various altitudes, it is beneficial to be able to combine the dataset using an integration method which operates in a three-dimensional space. Of the three integration methods assessed, least squares collocation is the only integration method which is able to perform this type of reduction. The resulting product is a Bouguer anomaly map of the Canterbury case study area, which combines satellite altimetry, terrestrial, ship-borne, airborne, and satellite gravimetry using least squares collocation