The Fully-focused Synthetic Aperture Radar (FFSAR) altimetry technology has greatly enhanced the accuracy of coastal sea surface height (SSH). However, due to its pulse-limited mode in the cross-track direction, FFSAR is still susceptible to land interference. Additionally, the spatiotemporal resolution of in-situ data limits the evaluation of altimetric SSH in both coastal and open ocean regions. This study proposes a waveform interference detection and removal algorithm (WIDA), which utilizes waveform differencing and azimuthal constant false alarm rate techniques to identify and reject interference signals in island areas. Tide gauge data, particularly a seamless and high-resolution airborne gravimetric quasi-geoid model, are employed to validate the performance of WIDA, providing a new perspective for the continuous and consistent evaluation of SSH profiles. Validation of five years of data from six Sentinel-3A/B tracks over the Xisha Islands in the South China Sea demonstrates that FFSAR_WIDA (FFSAR waveforms retracked by WIDA), with 80 Hz sampling, significantly outperforms FFSAR_CORAL (Coastal Retracker for SAR Altimetry), UFSAR (Unfocused SAR), and UFSAR_WIDA within 5 km of the tide gauge station. The standard deviations (SD) for FFSAR_WIDA, FFSAR_CORAL, UFSAR, and UFSAR_WIDA are 0.153 m, 0.192 m, 0.367 m, and 0.173 m, respectively. Moreover, the evaluation with the quasi-geoid also reveals a marked improvement over FFSAR_CORAL and UFSAR within 5 km from the islands. The spatial distribution assessment shows that as the distance from the island decreases, the SD for FFSAR_CORAL (UFSAR) increases from 0.010 m (0.011 m) at 15 km to 0.061 m (0.089 m) at 3 km, while FFSAR_WIDA (UFSAR_WIDA) remains consistently below 0.011 m (0.014 m). Index Terms—FFSAR, Island Areas, Land Interference, Validation, Sea Surface Height.

The potential of using airborne gravity gradient tensor (GGT) for coastal quasi-geoid (QG) refinement is explored, and the contributions introduced from individual GGT components and their combinations are quantified and evaluated. High-resolution QGs, with a spatial resolution of ~0.5 km, are computed over St. George’s Bay in southwestern Newfoundland, Canada. The findings indicate that fully focused Synthetic Aperture Radar (FFSAR) and Surface Water and Ocean Topography (SWOT) altimetry data are effectively in differentiating the performance of various QGs in coastal areas. The application of the vertical gravity gradient obtains the highest quality QG when utilizing individual GGT components. The combination two or more components results in improved QGs compared to the results derived from individual components. The integration of full GGT yields the best QG, with standard deviation (SD) of misfits against Sentinel-3A FFSAR (SWOT) altimetry data being 1.13 (2.49) cm, representing reductions of 28.48–53.50% (5.32–15.02%) compared to results derived from individual GGT components. Comparisons of the QG computed by fusing full GGT with the Canadian gravimetric QG CGG2013 and high-degree global geopotential models further underscore the advantages of using GGT in QG modeling, revealing SD reductions of 55.16–65.02% (7.55–33.95%) against FFSAR (SWOT) altimetry data. These findings underscore the effectiveness of using airborne GGT in coastal QG modeling, particularly in recovering short-wavelength signals and addressing challenges in satellite altimetry over coastal environments. Additionally, this study highlights the superiority of using full GGT over individual components in QG modeling.

Bathymetry provides instrumental information for studying sedimentary processes, global climate change, and benthic morphologies. The advantages and applicabilities of different techniques for bathymetry detection vary. We propose a framework for bathymetry enhancement from multisource data based on spherical radial basis functions (SRBFs). A case study is conducted over the Paracel Islands in South China Sea (SCS), where Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) measurements, airborne gravimetric observations, echo soundings, and the reference model DTU18BAT are merged. Numerical results suggest that the fusion of ICESat-2 observations dramatically enhances the quality of the computed bathymetry model near the islands, the root-mean-squared error (RMSE) of which is reduced by 45.35%–67.95% compared to existing models when validated against the satellite-derived bathymetry (SDB) with decimeter-level accuracy. By additionally fusing the airborne gravimetric data, bathymetry is further enhanced by ~22.49%, particularly over islands with sparse ICESat-2 trajectories. Comparisons with surveyed airborne bathymetric lidar data over the northern Antelope Reef yielded results consistent with those obtained from the SDB, suggesting that SDB is possible to serve as control data in waters devoid of ground truth data. Further analysis reveals that the models constrained by echo soundings performed better than existing models in deep waters, with reductions of 17.79%–44.99% in terms of RMSE. By fusing airborne gravity data, bathymetry is improved by ~10%, highlighting the utilization of airborne gravimetry in both shallow and deep waters. The proposed SRBF approach offers an effective way to merge heterogeneous data for high-quality bathymetry determination.

The lack of marine gravimetric measurements and the presence of severely contaminated altimetry data pose multiple challenges in high-quality quasi-geoid (QG) and mean dynamic topography (MDT) determination over islands, where the application of nadir altimetry alone is inadequate. We explore the potential for regional enhancement using wide-swath data from the Surface Water and Ocean Topography (SWOT) mission. Numerical experiments over the Paracel Islands in South China Sea underscore the superiority of using SWOT data in QG/MDT computation. Comparisons with the airborne gravimetry-derived QG reveal that the Root Mean Square Errors (RMSEs) of QGs derived from the SWOT data are reduced by 38.24–60.90% compared to those derived solely from nadir altimetry. The QG profiles retrieved from the Sentinel-3A/B altimetry using the fully-focused SAR technology are effective in discriminating the quality of different QGs near islands. The RMSEs of SWOT-derived QGs constitute reductions of 6.27–17.52% compared to those computed from nadir altimetry alone. Notable improvements up to 4 cm are observed when Sentinel-3A/B tracks approached islands. The mutual comparison of the QGs computed from the SWOT gravity anomaly (GRA) and vertical gravity gradient (VGG) data suggests that the VGG-derived QG has improved quality, and the utilization of VGG recovers more small-scale signals. The SWOT-derived MDTs reduce the bubble-like errors up to several centimeters compared to those computed exclusively using nadir altimetry. Our findings highlight that utilizing SWOT data enables the acquisition of an accurate QG/MDT with an RMSE less than 1 cm compared to that derived from airborne gravimetry.

Gravity anomaly over shallow waters is one of the fundamental data sources for studying sea level change, ocean currents, and water exchanges between coastal areas and open seas. However, the acquirement of gravity data over shallow waters faces multiple challenges due to the degraded quality of satellite altimetry data and scarcity of surveyed gravimetric observations. To alleviate this problem, we establish a framework for marine gravity anomaly refinement by using satellite-derived bathymetry (SDB). We use a cosine-tapered band pass filter to extract high-frequency gravity signals from the SDB data, which compensate for the unresolved signals in satellite altimetric gravity data. Numerical experiments over the Discovery Reef and an offshore region near the Port Hedland demonstrate that the utilization of SDB effectively strengths marine gravity anomaly. By combining the SDB data, the fits between the enhanced gravity anomaly models and surveyed airborne gravity data are improved, by 5.3–15.7% in comparison to an altimetric gravity model DTU21GRA. The SDB calculated from the linear band model has slightly better performances in gravity anomaly modeling than that computed from the band ratio model and physical-based approach, agreeing well with the SDB validation results. Our results verify the feasibility of using the SDB computed from the physical-based approach for gravity anomaly augmentation, which is of great value in areas devoid of ground truth depths. This study cements a way for the augmentation of marine gravity anomaly worldwide, especially in remote regions characterized by the scarcity of ground-based gravity data.

We study the role of airborne gravimetry for seamless bathymetry modeling over the Paracel Islands in northern South China Sea, and investigate the possibility of using ICESat-2 data and satellite-derived bathymetry (SDB) to evaluate bathymetry models over shallow waters. We use ICESat-2 data for training Sentinel-2 imagery and derive the SDB data with a Root Mean Squared Error (RMSE) of 0.29-0.50 m, which is lower than 10% of the maximum depths. The local bathymetry is modeled by using a modified version of S&S band-pass filter, and a partition-wise scheme is applied for determining the scaling factors. Numerical experiments verify the feasibility of using ICESat-2 and SDB data to assess bathymetry models. By utilizing the airborne gravity data, the fit between the computed bathymetry and the SDB data is significantly improved, by 18.7-58.0% over different shallow waters compared to recently released bathymetry models. The bathymetry predicted from the airborne data has also higher performance in deep water areas, which performs best in all these depth ranges from 500 to 3000 m. In comparison to the existing models, the RMSEs of the misfits between the computed bathymetry and the National Oceanic and Atmospheric Administration depths are reduced by tens to hundreds of meters in different depth ranges. Our study highlights that using airborne gravimetry for bathymetry modeling over island areas is advantageous, in both shallow and deep waters; and that ICESat-2 and SDB data can largely alleviate the lack of in-situ depths over shallow waters.

GEROS-ISS (GEROS hereafter) stands for GNSS REflectometry, Radio Occultation and Scatterometry onboard the International Space Station. It is a scientific experiment, proposed to the European Space Agency (ESA) in 2011 for installation aboard the ISS. The main focus of GEROS is the dedicated use of signals from the currently available Global Navigation Satellite Systems (GNSS) for remote sensing of the System Earth with focus to Climate Change characterisation. The GEROS mission idea and the current status are briefly reviewed.

The availability of Cryosat-2 with its coverage throughout the Arctic Ocean up to 88N is a quantum leap forward for altimetric gravity field modeling and here we have tried to quantify the improvement of Cryosat-2 to global and particularly Arctic altimetric gravity field modeling through a comparison with highly accurate marine gravity observations.

HY-2A (`HaiYang' denotes Ocean) was launched in August 2011. It payloads Ku and C bands radar altimeters with repeat cycles of 14 days (for three years) and 168 days. In the present study, we preliminary evaluate the HY-2 satellite altimetric data against SARAL/AltiKa and CryoSat-2 data in the Arctic Ocean. The results demonstrates that the HY-2 data shows higher standard variation and mean sea level than AltiKa and CryoSat-2 data during HY-2 cycle 49 (20130803 and 20130817) with more available sea level measurements than CryoSat-2 satellite altimetry. Moreover, consistent sea level variation is observed from AltiKa and CryoSat-2 monthly sea level time series.

The assessment of altimetric data is crucial for investigating the regional sea level variability. Few works has been performed to validate the altimetric data [1, 2] in the Baltic Sea. The exploring of multi-mission altimetric data in the Baltic Sea has yet to be published. The number of available altimetric measurements increases of 96% by replacing the radiometer wet troposphere correction with model based correction. The results indicate the high quality of the along-track altimetry measurements in the semi-closed sea, which shows good agreement with tide gauge data except in the shallow waters and ice-covered regions, such as Danish Straits and the Gulf of Bothnian.

European Space Agency's Cryosat-2 comes with the first ever SAR (Synthetic Aperture Radar) altimeter onboard a satellite. In this work precise sea surface heights and gravity fields are determined using Cryosat-2 SAR data. These determinations through satellite altimetry are difficult in the Arctic because of the presence of sea ice and coastal areas. Traditional retrackers generate erroneous results due to the superposition of echoes from the sea surface and sea ice. This work establishes an adaptation of the SAMOSA3 retracker as a suitable candidate; this model is improved and customized for the Arctic. Through this research it has been demonstrated that the SAMOSA3 retracker has a better performance as compared to other SAR retrackers when sea surface height and gravity field determination needs to be done. The performance evaluation of the SAMOSA3 retracker as compared to other retrackers has been done using sea surface height anomaly method and gravity field anomaly method.

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Satellite Eye for Galathea 3 (www.satelliteeye.dk) contains education at the internet for secondary and upper secondary schools and the public. The Galathea 3 ship expedition circumnavigated the globe starting from Denmark 11 August 2006, visiting Greenland, Azores, South Africa, Australia, Solomon Islands, New Zealand, Antarctica, Chile, Galapagos, the Caribbean, the Northeastern USA and finishing in Denmark 25 April 2007. During the entire expedition satellite images were ordered along the ship track, downloaded, processed, archived and used for education. The satellite images are displayed in Google Earth along with 10 minute observations of air and sea parameters observed at the ship (http://galathea.oersted.dtu.dk). This material forms basis for 9 running projects along the entire route and 24 site-specific cases. Observations from several science projects onboard will be used in the site-specific cases. One of the continuous study cases is chlorophyll observed from Envisat MERIS. As an example: in the upwelling zone near Namibia a very high level of chlorophyll was observed from MERIS. The ship route consequently was changed slightly the day after in order to traverse the area with the highest amount of chlorophyll. Chlorophyll observed onboard (in-situ) two days later show high amounts. In addition, in the science project on the carbon cycle a significant emission of CO2was observed. The students can use the data online in the classroom. Students from several classes were onboard for part of the expedition and these classes in particular used the Satellite Eye teaching material. In Google Earth satellite images of many themes are shown. These include sea ice, sea surface temperature, ocean wind, wave height, sea surface level, ozone, clouds and radar images of ocean and land. Also high spatial resolution land cover mapping of the visited harbors and their surroundings are included. The case studies uses the image processing software LEO Works developed through the ESA project EDUSPACE, the European Earth Observation web site for Secondary Schools (http://www.eduspace.esa.int). For each theme a thorough educational material has been developed in Danish and English. ESA has granted a large amount of Envisat as well as PROBA images and third mission data from SPOT and Landsat. Also data from NOAA, NASA, JAXA and QUICKBIRD were used.