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The height of the mean sea surface above the reference ellipsoid is a time independent reference model that can be removed from radar altimeter measurements to isolate the oceanographic signals.  This document explains how the CLS mean sea surface model is updated at short wavelengths using the 30-years of sea surface slope measurements that have been archived and computed at Scripps Institution of Oceanography.  The data are available at https://topex.ucsd.edu/pub/ .

Abstract Satellite altimetry is the main tool for constructing global or regional marine gravity fields. To improve the accuracy and spatial resolution, it is necessary to fuse multi-mission altimeters. How to determine the weights of multi-mission altimeters is a crucial issue, making the conventional calculation process very complex. In addition, traditional satellite inversion methods are often independent of shipborne gravity, which is used only as validation data, thus not take full advantages of high accuracy and resolution of shipborne gravity. In this study, we introduce a convolutional neural network (CNN) to merge the vertical deflections (DOVs) obtained from multi-altimeter missions to construct a marine gravity model in the South China Sea. High-accuracy shipborne gravity and a dataset comprising DOVs and geo-locations are employed as input data for neural network training. For the validation of CNN method, the gravity model is also computed by conventional Inverse Vening Meinesz (IVM) method. Independent shipborne gravity measurements and SIO V32.1, DTU17 models are used as validation data. The evaluation results show that the CNN-derived model achieves a higher level of accuracy, yielding a standard deviation (STD) of 3.21 mGal, with an improvement of 36.56% compared to IVM-derived model. More than 92% of the differences between the CNN-derived model and shipborne gravity are less than 5 mGal. In addition, spectral analysis results further show that the CNN-derived model has stronger energy at short wavelengths (less than 25 km) compared to other models. These findings reveal that CNN method is feasible for marine gravity recovery and the CNN-derived model can achieve higher accuracy. The CNN method can improve the accuracy and spectral characteristics of the constructed gravity model by taking advantage of the high accuracy and high resolution of shipborne gravity. Graphical Abstract

Satellite altimetry is the main tool for constructing global or regional marine gravity fields. To improve the accuracy and spatial resolution, it is necessary to fuse multi-mission altimeters. How to determine the weights of multi-mission altimeters is a crucial issue, making the conventional calculation process very complex. In addition, traditional satellite inversion methods are often independent of shipborne gravity, which is used only as validation data, thus not take full advantages of high accuracy and resolution of shipborne gravity. In this study, we introduce a convolutional neural network (CNN) to merge the vertical deflections (DOVs) obtained from multi-altimeter missions to construct a marine gravity model in the South China Sea. High-accuracy shipborne gravity and a dataset comprising DOVs and geo-locations are employed as input data for neural network training. For the validation of CNN method, the gravity model is also computed by conventional Inverse Vening Meinesz (IVM) method. Independent shipborne gravity measurements and SIO V32.1, DTU17 models are used as validation data. The evaluation results show that the CNN-derived model achieves a higher level of accuracy, yielding a standard deviation (STD) of 3.21 mGal, with an improvement of 36.56% compared to IVM-derived model. More than 92% of the differences between the CNN-derived model and shipborne gravity are less than 5 mGal. In addition, spectral analysis results further show that the CNN-derived model has stronger energy at short wavelengths (less than 25 km) compared to other models. These findings reveal that CNN method is feasible for marine gravity recovery and the CNN-derived model can achieve higher accuracy. The CNN method can improve the accuracy and spectral characteristics of the constructed gravity model by taking advantage of the high accuracy and high resolution of shipborne gravity. [ABSTRACT FROM AUTHOR]

Several satellite gravity anomaly models are freely available to calculate the free‐air gravity anomaly in areas where shipborne gravity measurements are scarce. Two models produced by the Technical University of Denmark (DTU17) and the Scripps Institution of Oceanography (SIOv32.1), respectively, were selected to compute the free‐air anomalies over the Cosmonaut Sea, East Antarctica. A statistical comparison analysis was performed to evaluate the resolution of satellite gravity anomaly models by comparing them with the shipborne surveying date. The radially averaged energy spectra of free‐air anomaly from different sources were calculated and compared over two selected regions to further evaluate the reliability of the data derived from satellite gravity anomaly models. The satellite gravity anomaly models have a better resolution in the ocean basin than in the area near the continental shelf. The comparison analysis revealed that the precision of both DTU17 and SIOv32.1 is close to the shipborne gravity data, but on average, SIOv32.1 is a little bit better than DTU17. The spectral analysis showed that the shipborne measurements may provide higher resolution than the satellite gravity anomaly model at wavelengths shorter than 20 km, and the free‐air data derived from SIOv32.1 have better resolution than the one from DTU17. These shipborne datasets will provide contributions for the updates of the Antarctic gravity anomaly and enable new high‐resolution combined Earth gravity models to be derived in Antarctica. [ABSTRACT FROM AUTHOR]

Marine gravity has been modeled using satellite radar altimetry for more than two decades. However, the accuracy of the predicted gravity field is constrained by the range precision and spatial coverage of the altimetry observations. The objective of this research is to enhance the gravity field over the Red Sea by utilizing the Least Square Collocation (LSC) algorithm with SARAL/AltiKA mission observations. The marine gravity field in the Red Sea was determined by using observed Sea Surface Heights (SSHs) obtained from the SARAL/AltiKA mission. The Remove-Compute-Restore (RCR) technique, the Residual Terrain Modelling (RTM) reduction method, and the LSC technique are used to estimate the marine gravity. To evaluate the accuracy of the SARAL/AltiKA predicted model, it was compared against two global altimetry gravity models (DTU21 and SSv29.1) using 42331 shipborne gravity measurements over the Red Sea. The SARAL/AltiKA predicted model showed the best results, with root mean square error and standard deviation values of 6.29 and 5.93 mGal, respectively. The impact of bathymetry depths on the SARAL/AltiKA predicted gravity model accuracy was also investigated. The SARAL/AltiKA predicted gravity model produced better results for water depths up to 1000 m when compared with shipborne data, while the SSv29.1 model performed better for depths between 1000 and 3000 m due to the performance of the residual slopes of the SSH estimation technique used in the deep areas.

The Hawaiian Ridge, a classic intraplate volcanic chain in the Central Pacific Ocean, has long attracted researchers due to its origin, eruption patterns, and impact on lithospheric deformation. Thought to arise from pressure‐release melting within a mantle plume, its mass‐induced deformation of Earth's surface depends on load distribution and lithospheric properties, including elastic thickness (Te). To investigate these features, a marine geophysical campaign was carried out across the Hawaiian Ridge in 2018. Westward of the island of O'ahu, a seismic tomographic image, validated by gravity data, reveals a large mass of volcanic material emplaced on the oceanic crust, flanked by an apron of volcaniclastic material filling the moat created by plate flexure. The ridge adds ∼7 km of material to pre‐existing ∼6‐km‐thick oceanic crust. A high‐velocity and high‐density core resides within the volcanic edifice, draped by alternating lava flows and mass wasting material. Beneath the edifice, upper mantle velocities are slightly higher than that of the surrounding mantle, and there is no evidence of extensive magmatic underplating of the crust. There is ∼3.5 km of downward deflection of the sediment‐crust and crust‐mantle boundaries due to flexure in response to the volcanic load. At Ka'ena Ridge, the volcanic edifice's height and cross‐sectional area are no more than half as large as those determined at Hawai'i Island. Together, these studies confirm that volcanic loads to the west of Hawai'i are largely compensated by flexure. Comparisons to the Emperor Seamount Chain confirm the Hawaiian Ridge's relatively stronger lithospheric rigidity. Plain Language Summary: The Hawaiian Ridge, a seafloor volcanic chain in the Pacific, has long fascinated scientists for its origin, eruptions, and impact on Earth's surface layers. It is believed to form from melted rock originating in the Earth's mantle. As lava builds up each volcanic edifice, it presses down on the Earth's surface, deforming it by a few kilometers vertically, with the degree of deformation depending on its weight and the properties of the Earth's outer layers. Our geophysical study has imaged the interior of the volcanic ridge and the underlying crust and mantle. The volcano contains an interior core of highly crystalline rocks draped by volcanic material, and contributes to a broad lithospheric flexural response of up to ∼3.5 km, vertically. This study sheds light on volcano formation, load‐induced lithospheric flexure, and their implications for understanding Earth's subsurface processes. Key Points: The Hawaiian Ridge at Ka'ena consists of volcanic edifices ∼7 km thick, sitting on Pacific oceanic crust ∼6 km thickSeismic and gravity data reveal an inner volcanic core draped by layers of lava flows and volcanic debrisBroad lithospheric flexure (peak deflection ∼3.5 km) suggests a flexural rigidity of the 90 Myr old lithosphere equivalent to Te = ∼25.6 km [ABSTRACT FROM AUTHOR]

The Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, Te, of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher Te beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge. Plain Language Summary: The Hawaiian Islands are one of Earth's best examples of a volcanic chain that formed on a tectonic plate that is moving over a fixed hotspot in the deep mantle. They are a "natural laboratory" for the study of intraplate volcanism and their impact on the large‐scale deformation of the plates. We carried out a seismic imaging experiment along a ∼535‐km‐long profile that intersected the chain between the islands of Maui and Hawai'i. The seismic velocity image reveals a high velocity, high density, "core" within part of the chain and that the combined weight of the edifices that make up each island has flexed the Pacific oceanic plate down by up to ∼6–7 km over distances of up to 400–500 km. There is evidence that the elastic thickness of the Pacific lithosphere may be higher for Hawai'i than for the older islands in the Hawaiian ridge, suggesting that the adjustment to volcanic loading is still on‐going at the youngest end of the chain. Key Points: The Hawaiian Ridge consists of volcanic edifices over 9 km thick, sitting on Pacific oceanic crust of 4.5–6 km thickBroad flexure of Pacific crust (extending ∼200–250 km to each side of the ridge, up to 6–7 km vertical) indicates a Te value of 26.7 kmThe upper mantle exhibits modest velocity changes (7.85–8.1 km/s), with no evidence of substantial magmatic underplating [ABSTRACT FROM AUTHOR]

Marine gravity has been modeled using satellite radar altimetry for more than two decades. However, the accuracy of the predicted gravity field is constrained by the range precision and spatial coverage of the altimetry observations. The objective of this research is to enhance the gravity field over the Red Sea by utilizing the Least Square Collocation (LSC) algorithm with SARAL/AltiKA mission observations. The marine gravity field in the Red Sea was determined by using observed Sea Surface Heights (SSHs) obtained from the SARAL/AltiKA mission. The Remove-Compute-Restore (RCR) technique, the Residual Terrain Modelling (RTM) reduction method, and the LSC technique are used to estimate the marine gravity. To evaluate the accuracy of the SARAL/AltiKA predicted model, it was compared against two global altimetry gravity models (DTU21 and SSv29.1) using 42331 shipborne gravity measurements over the Red Sea. The SARAL/AltiKA predicted model showed the best results, with root mean square error and standard deviation values of 6.29 and 5.93 mGal, respectively. The impact of bathymetry depths on the SARAL/AltiKA predicted gravity model accuracy was also investigated. The SARAL/AltiKA predicted gravity model produced better results for water depths up to 1000 m when compared with shipborne data, while the SSv29.1 model performed better for depths between 1000 and 3000 m due to the performance of the residual slopes of the SSH estimation technique used in the deep areas. [ABSTRACT FROM AUTHOR]

The wide-swath altimetry mission, Surface Water and Ocean Topography (SWOT), makes it possible to further break through the accuracy and resolution of marine gravity field recovery. However, as its main payload, the Ka-band radar interferometer (KaRIN) is easy to be affected by the spacecraft attitude, mechanical deformations and dual antenna signal reception status, which generates instrumental errors including roll errors, phase errors, baseline dilation errors, timing errors, and KaRIN noise. Together with ocean temporal variations, the instrumental errors have large effects on the SWOT Sea Surface Height (SSH) observations and hence the marine gravity field recovery. Here, taking the sea area around Japan as an example, we investigated the calibrations of instrumental errors and ocean temporal variations on the recovery of the marine gravity field. The SSH observations of SWOT are first simulated by Mean Sea Surface (MSS), Absolute Dynamic Topography (ADT), Mean Dynamic Topography (MDT) and instrumental errors. Next, the influence of instrumental errors on determining vertical deflection within single-cycle data was analyzed. Then, three calibration methods (KaRIN/KaRIN cross-calibration, Representative KaRIN/KaRIN cross-calibration, and KaRIN/Nadir cross-calibration) are used to reduce the instrumental errors; the experimental results show KaRIN/KaRIN cross-calibration is the optimal one. Last, for the multi-cycle observations containing ocean temporal variations, crossover calibration is done in single cycle and then followed by collinear adjustment in multi-cycles. This approach is verified by considering 18 simulated cycles that cover almost 1 year. Our result indicates an improvement in the accuracy of marine gravity anomaly by about 45% compared to that of one cycle. The calibration strategy of instrumental errors and ocean temporal variations can be used for high-precision marine gravity field recovery with abundant SWOT observations in the near future. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the effect of geographical location on Cyclone Global Navigation Satellite System (CYGNSS) observables is demonstrated for the first time. It is found that the observables corresponding to the same wind speed vary with geographic location regularly. Although latitude and longitude information is included in the conventional method, it cannot effectively reduce the errors caused by geographic differences due to the non-monotonic changes of observables with respect to latitude and longitude. Thus, an improved method for Global Navigation Satellite System Reflectometry (GNSS-R) wind speed retrieval that takes geographical differences into account is proposed. The sea surface is divided into different areas for independent wind speed retrieval, and the training set is resampled by considering high wind speed. To balance between the retrieval accuracies of high and low wind speeds, the results with the random training samples and the resampling samples are fused. Compared with the conventional method, in the range of 0–20 m/s, the improved method reduces the Root Mean Square Error (RMSE) of retrieved wind speeds from 1.52 to 1.34 m/s, and enhances the correlation coefficient from 0.86 to 0.90; while in the range of 20–30 m/s, the RMSE decreases from 8.07 to 4.06 m/s, and the correlation coefficient increases from 0.04 to 0.45. Interestingly, the SNR observations are moderately correlated with marine gravities, showing correlation coefficients of 0.5–0.6, which may provide a useful reference for marine gravity retrieval using GNSS-R in the future.

Since 2010, a slew of new satellite altimetry missions has started providing gravity data generated from altimetry with worldwide coverage and high quality that rivals shipborne gravity measurements in several areas. As a result, worldwide offshore high-resolution gravity fields have considerably improved. This paper aims to compare two altimetry gravity models i.e. DTU21 and SSv29.1 with shipborne gravity measurements to evaluate their accuracies over the Red Sea. At first, the DTU21 and SSv29.1 altimetry models were compared with the shipborne data at different water depths to evaluate the impact of bathymetry depths on the accuracy. The corresponding results revealed that the DTU21 gravity model gave the best results in the comparison of all shipborne with a standard deviation (s.d.) of 7.37 mGal and a Root Mean Square (RMS) of 8.73 mGal, while the SSv29.1 model achieve an s.d. of 8.50 mGal and an RMS of 8.81 mGal. In water depths less than 1000 m the DTU21 model gives the best results in terms of s.d. and RMS, while the SSv29.1 model achieves better results at water depths ranging between 1000 m and 3000 m.

In this paper, the effect of geographical location on Cyclone Global Navigation Satellite System (CYGNSS) observables is demonstrated for the first time. It is found that the observables corresponding to the same wind speed vary with geographic location regularly. Although latitude and longitude information is included in the conventional method, it cannot effectively reduce the errors caused by geographic differences due to the non-monotonic changes of observables with respect to latitude and longitude. Thus, an improved method for Global Navigation Satellite System Reflectometry (GNSS-R) wind speed retrieval that takes geographical differences into account is proposed. The sea surface is divided into different areas for independent wind speed retrieval, and the training set is resampled by considering high wind speed. To balance between the retrieval accuracies of high and low wind speeds, the results with the random training samples and the resampling samples are fused. Compared with the conventional method, in the range of 0–20 m/s, the improved method reduces the Root Mean Square Error (RMSE) of retrieved wind speeds from 1.52 to 1.34 m/s, and enhances the correlation coefficient from 0.86 to 0.90; while in the range of 20–30 m/s, the RMSE decreases from 8.07 to 4.06 m/s, and the correlation coefficient increases from 0.04 to 0.45. Interestingly, the SNR observations are moderately correlated with marine gravities, showing correlation coefficients of 0.5–0.6, which may provide a useful reference for marine gravity retrieval using GNSS-R in the future. [ABSTRACT FROM AUTHOR]

Near-axis seamounts provide a unique setting to investigate three-dimensional mantle processes associated with the formation of new oceanic crust and lithosphere. Here, we investigate the characteristics and evolution of the 8˚20’N Seamount Chain, a lineament of seamounts that extends ~ 175 km west of the East Pacific Rise (EPR) axis, just north of the fracture zone of the Siqueiros Transform Fault. Shipboard gravity, magnetic, and bathymetric data acquired in 2016 are utilized to constrain models of seamount emplacement and evolution. Geophysical observations indicate that these seamounts formed during four distinct episodes of volcanism coinciding with changes in regional plate motion that are also reflected in the development of intra-transform spreading centers (ITSCs) along the Siqueiros transform fault (Fornari et al. 1989; Pockalny et al. 1997). Although volcanism is divided into distinct segments, the magnetic data indicate continuous volcanic construction over long portions of the chain. Crustal thickness variations along the chain up to 0.75 km increase eastward, inferred from gravity measurements, suggest that plate reorganization has considerably impacted melt distribution in the area surrounding the Siqueiros-EPR ridge transform intersection. This appears to have resulted in increased volcanism and the formation of the 8˚20’N Seamounts. These findings indicate that melting processes in the mantle and subsequently the formation of new oceanic crust and lithosphere are highly sensitive to tectonic stress changes in the vicinity of fast-spreading transform fault offsets. [ABSTRACT FROM AUTHOR]

Haiyang-2 (HY-2) missions have accumulated sea surface height (SSH) observations on a global scale for more than 10 years. Four satellites, HY-2A, HY-2B, HY-2C and HY-2D, provide even but differently distributed data, which play a complementary role in marine gravity studies with other missions. Therefore, this paper evaluates the performances of HY-2 altimetric data in marine gravity modeling from the following four perspectives: SSH accuracy, geoid signal resolution ability, vertical deflections and gravity anomaly. First, the centimeter-magnitude accuracy level of HY-2 data is proved by analyzing SSH discrepancies at crossover points within a certain time limit. Second, the spectral analysis of repetitive along-track data sequences in a time domain shows a geoid resolution range from 18 to 24 km. Taking HY-2 exact repeat missions (ERM), for example, the resolution could be remarkably enhanced by stacking repetitive cycles. Third, validation with an XGM2019 model showed that vertical deflections were reliably computed for all HY-2 missions, but HY-2A performed slightly worse than the other HY-2 missions. Meanwhile, HY-2C and HY-2D with a ~66° orbital inclination obviously had an improved ability to capture east–west signals compared to HY-2A and HY-2B. Finally, we constructed global marine gravity results based on three input datasets, HY-2 dataset only, multi-satellite dataset without HY-2 and multi-satellite dataset with HY-2. Validations were performed using published models and shipborne gravimetric data. The results showed that the HY-2 dataset is capable of improving marine gravity anomaly recoveries and that the accuracy of NSOAS22 with incorporated HY-2 data is comparable to DTU21 and SS V31.1. Furthermore, HY-2 observations should not be the only input dataset to construct a 1' × 1' resolution marine gravity model. [ABSTRACT FROM AUTHOR]

Since 2010, a slew of new satellite altimetry missions has started providing gravity data generated from altimetry with worldwide coverage and high quality that rivals shipborne gravity measurements in several areas. As a result, worldwide offshore high-resolution gravity fields have considerably improved. This paper aims to compare two altimetry gravity models i.e. DTU21 and SSv29.1 with shipborne gravity measurements to evaluate their accuracies over the Red Sea. At first, the DTU21 and SSv29.1 altimetry models were compared with the shipborne data at different water depths to evaluate the impact of bathymetry depths on the accuracy. The corresponding results revealed that the DTU21 gravity model gave the best results in the comparison of all shipborne with a standard deviation (s.d.) of 7.37 mGal and a Root Mean Square (RMS) of 8.73 mGal, while the SSv29.1 model achieve an s.d. of 8.50 mGal and an RMS of 8.81 mGal. In water depths less than 1000 m the DTU21 model gives the best results in terms of s.d. and RMS, while the SSv29.1 model achieves better results at water depths ranging between 1000 m and 3000 m. [ABSTRACT FROM AUTHOR]

Objectives: The laser interference absolute gravimeters can only work in static conditions until now, and absolute gravity measurement in a dynamic environment is one of the hotspots for future techno⁃ logical development. The shipboard absolute gravity measurement can overcome the problems of zero drift, calibration, and error accumulation of the marine relative gravity instruments, and improve their efficiency and reliability.Methods: We present an absolute gravity measurement system from a ship, which consisted of an absolute gravity instrument, a gyro⁃stabilized platform, a force balance accelerometer and GPS (global positioning system).Firstly, Four types of interference sources that the vertical fluctuation, the pitch and roll, the horizontal fluctuation and the Eötvös effect, which cause the measurement errors of the shipboard gravity measurement system, were analyzed. Then, the limits of dynamic measurement, the error correc⁃ tion methods and their accuracy were given.Results and Conclusions: Under current technical conditions, the precision of the shipboard absolute gravimeter measurement system can be better than ±1.1 mGal. Therefore it provides theoretical support for further shipborne absolute gravity measurement experiments. [ABSTRACT FROM AUTHOR]

The marine gravity field is vital for mapping various submarine geological and tectonic structures, also for computation of high-resolution gravimetric geoid. This study aims to evaluate the accuracy of two latest high-resolution marine gravity models derived from satellite altimetry (DTU17 and SSv27.1) using shipborne gravity data and to pruduce high-precision gravity field over the Gulf of Guinea. The gross-errors affecting the shipborne gravity data have been removed by cross-validation technique to ensure better evaluation of gravity field models. The standard deviation σ of the differences between the measured and model gravity data drops from 9.96 mGal before the cross-validation to 6.28 mGal after this process. The comparison between the DTU17 and SSv27.1 gravity field models has been done in order to detect significant differences between them. The differences between the two models are quite small with a mean of 1.73 mGal and σ of 6.55 mGal. The discrepancies between them are found around coastal areas and along islands. This shows the poor accuracy of satellite altimetry near coastal areas. Afterwards, the accuracy of each marine gravity field models was evaluated using shipborne gravity data free of gross-errors. The SSv27.1 model fits better to the shipborne gravity data with a mean of −4.88 mGal and σ of 7.18 mGal. Hence, the SSv27.1 model has a better performance than the DTU17 model on the Gulf of Guinea. Finally, we used the least-squares collocation technique associated to the Markov model of second-order covariance to combine the SSv27.1 model with the shipborne gravity data. We produced here a marine gravity field of good accuracy around the Gulf of Guinea with no data gaps. The precision of this combined gravity field is estimated to be 5.54 mGal with a spatial resolution of 1 arc-minute. [ABSTRACT FROM AUTHOR]

For the purpose of marine geoid modeling and many other geodetic and geophysical applications, a marine gravity map around Egypt is established by the integration of gravity data provided by satellite altimetry and shipborne gravimetric observations. Firstly, the collected shipborne data were compared with GO_CONS_GCF_2_TIM_R6 and XGM2019e GGMs and with SSv29.1 and DTU17 altimetry models. Then, a pre-refinement of ship marine surveys was done with a rigorous condition, in which a number of 6525 points have been removed from the dataset. After that, 87709 points were deducted from the pre-filtered shipborne dataset to fit the study area and the cross-validation approach with the kriging interpolation algorithm were applied. A rigorous level of confidence was decided in this step where the points which have differences between the estimated and the observed values more than twice the STD of the residuals were removed until the STD reached a value less than 1 mGal. Finally, the filtered shipborne gravity data were combined with DTU17 (the best evaluation model) using the least-squares collocation technique (LSC). The final gravity map was tested using 8000 randomly chosen shipborne stations, which were not included when applying LSC, revealing the significant enhancement gained after the integration process. [ABSTRACT FROM AUTHOR]

Satellite radar altimetry collected during a number of geodetic missions has provided a new understanding of the topography and tectonics of the deep oceans. As altimeter performance and coverage improves, smaller structures are revealed. Here we investigate the contribution of six altimeter missions that have been placed into geodetic mapping phases for more than one year. Two types of evaluations are performed. We first compare the composite (all six altimeters) grids of east and north vertical deflection to matching grids where one altimeter has been omitted evaluate their contribution versus latitude. We then estimate the noise in each altimeter by computing the median absolute deviation of the profiles with the best composite grid. Both analyses show that SARAL/AltiKa provides the greatest contribution and ERS-1 no longer provides any significant improvement. The major limitation for recovering small scale gravity features is the sea surface roughness from ocean waves. There have been steady improvements in instrumentation and processing methods that will continue into the future with higher frequency radars and interferometric swath altimeters planned for future missions. [ABSTRACT FROM AUTHOR]

Traditionally, geoid models have been validated using GNSS-levelling benchmarks on land only. As such benchmarks cannot be established offshore, marine areas of geoid models must be evaluated in a different way. In this research, we present a marine GNSS/gravity campaign where existing geoid models were validated at sea areas by GNSS measurements in combination with sea surface models. Additionally, a new geoid model, calculated using the newly collected marine gravity data, was validated. The campaign was carried out with the marine geology research catamaran Geomari (operated by the Geological Survey of Finland), which sailed back and forth the eastern part of the Finnish territorial waters of the Gulf of Finland during the early summer of 2018. From the GNSS and sea surface data we were able to obtain geoid heights at sea areas with an accuracy of a few centimetres. When the GNSS derived geoid heights are compared with geoid heights from the geoid models differences between the respective models are seen in the most eastern and southern parts of the campaign area. The new gravity data changed the geoid model heights by up to 15 cm in areas of sparse/non-existing gravity data. [ABSTRACT FROM AUTHOR]

SARAL/AltiKA (SRL) is the first altimetry satellite with a Ka-band altimeter. To validate the advantages of the Ka-band altimeter over traditional Ku-band altimeters in marine geodetic applications, a comprehensive analysis is carried out over the South China Sea (SCS) (0–30° N, 105–125° E) from three aspects, namely the influence of load on waveforms, the precision of sea surface heights (SSHs), and the precision of altimeter-derived marine gravity field. Coastal waveforms of SRL, Jason-2, and CryoSat-2 are separately compared with corresponding ocean-type waveforms. The radius of coastal influence on SSHs of SRL/exact repeat mission (SRL/ERM) is the smallest, being about 3 km. Crossover discrepancies, global mean sea surface models, and tide gauge data are used to assess the precision of altimetric SSHs. Compared with the SSH precision of Ku-band Jason-2/ERM, the SSH precision of Ka-band SRL/ERM is 4.6% higher over the SCS and 10% higher in offshore areas. Gridded gravity anomalies are derived from measurements of SRL/drifting phase (SRL/DP) and CryoSat-2 through the inverse Vening-Meinesz formula, respectively. According to the assessment by shipborne gravity data and global marine gravity models, the precision of SRL/DP-derived gravity is higher than that of CryoSat-2-derived gravity over the SCS, especially in offshore areas. In some cycles, ground tracks of SRL/ERM have large drifting of more than 10 km from nominal tracks. The results show that the Ka-band altimeter of SRL has better precision in SSHs and marine gravity recovery than the Ku-band altimeter over the SCS, particularly in offshore areas. [ABSTRACT FROM AUTHOR]

Deriving marine gravity is one of the important applications of satellite altimetry in earth science. The quality of marine gravity derived from each altimeter data always is limited by the range measurement on the certain ground track spacing. We assess the performance of Jason-2/geodetic-mission (GM) altimeter in deriving marine gravity by the improved range from waveform retracking. The singular value decomposition was applied to denoise waveforms, and the waveform derivative retracking was employed to improve the sea surface heights (SSHs). Finally, Jason-2/GM-derived gravity anomalies on 1′ × 1′ grids were determined by the least-squares collocation method based on the resample 5 Hz SSHs around the South China Sea (SCS). Assessed by ship-borne data from different institutions, the accuracy of Jason-2/GM-derived gravity is consistent with that of the V27.1 and DTU13 models. Moreover, with the application of waveform derivative retracking, the accuracy of gravity anomalies has improved from 6.6 mGal before retracking to 5.5 mGal after retracking over the open ocean, and from 8.9 to 7.4 mGal over the coastal area. It is better than the results obtained by traditional retracking methods, including off-center of gravity (OCOG), 5-β, threshold, Ice-1, and maximum likelihood estimator-4 (MLE4). Furthermore, evaluated by V27.1 and DTU13, the result shows root mean square (RMS) difference of 2.0 mGal and 3.0 mGal, respectively. Therefore, the waveform derivative retracking is an effective method for altimeter data, and the accuracy of derived gravity is an improvement by retracked SSHs. [ABSTRACT FROM AUTHOR]

The resolutions of current global altimetric gravity models and mean sea surface models are around 12 km wavelength resolving 6 km features, and for many years it has been difficult to improve the resolution further in a systematic way. For both Jason 1 and 2, a Geodetic Mission (GM) has been carried out as a part of the Extension-of-Life phase. The GM for Jason-1 lasted 406 days. The GM for Jason-2 was planned to provide ground-tracks with a systematic spacing of 4 km after 2 years and potentially 2 km after 4 years. Unfortunately, the satellite ceased operation in October 2019 after 2 years of Geodetic Mission but still provided a fantastic dataset for high resolution gravity recovery. We highlight the improvement to the gravity field which has been derived from the 2 years GM. When an Extension-of-Life phase is conducted, the satellite instruments will be old. Particularly Jason-2 suffered from several safe-holds and instrument outages during the GM. This leads to systematic gaps in the data-coverage and degrades the quality of the derived gravity field. For the first time, the Jason-2 GM was “rewound” to mitigate the effect of the outages, and we evaluate the effect of “mission rewind” on gravity. With the recent successful launch of Sentinel-6 Michael Freilich (S6-MF, formerly Jason CS), we investigate the possibility creating an altimetric dataset with 2 km track spacing as this would lead to fundamental increase in the spatial resolution of global altimetric gravity fields. We investigate the effect of bisecting the ground-tracks of existing GM to create a mesh with twice the resolution rather than starting all over with a new GM. The idea explores the unique opportunity to inject Jason-3 GM into the same orbital plane as used for Jason-2 GM but bisecting the existing Jason-2 tracks. This way, the already 2-years Jason-2 GM could be used to create a 2 km grid after only 2 years of Jason-3 GM, rather than starting all over with a new GM for Jason-3.

HaiYang-2A (HY-2A, where ‘Haiyang’ means ‘Ocean’ in Chinese) has provided reliable sea surface height observations for gravity with uniform ocean data coverage on a global scale for more than 8 years, particularly with denser across track sampling during the geodetic mission since March 2016. This paper aims at modeling and evaluating the regional marine gravity field at 1′×1′ resolution by incorporating HY-2A altimeter waveform data from 7 complete 168-day cycles in the geodetic mission phase. Initial evaluation indicates that, firstly, the measurements in the geodetic mission stay at a consistent accuracy level with observations at the start-of-life stage according to statistics of discrepancies at crossover points cycle by cycle. Secondly, range precision improvement can be achieved using a two-pass weighted least-squares retracker. Thirdly, a downsampling procedure combined with a low-pass filter is designed for HY-2A 20 Hz data to obtain 5 Hz measurements with enhanced precision. We calculate the 1′×1′ marine gravity field model over the South China Sea area by using the EGM2008 model as a reference field with the remove/restore method. The verifications with published models and shipborne gravimetric data show that HY-2A GM data is capable of improving marine gravity field modeling. Results show slightly higher accuracy than other models with similar input datasets but not including HY-2A. The accuracy is also compared with the latest DTU17 and SIO V27.1 model.

Marine gravity anomaly derived from satellite altimetry can be computed using either sea surface height or sea surface slope measurements. Here we consider the slope method and evaluate the errors in the slope of the corrections supplied with the Jason-1 geodetic mission data. The slope corrections are divided into three groups based on whether they are small, comparable, or large with respect to the 1 microradian error in the current sea surface slope models. (1) The small and thus negligible corrections include dry tropospheric correction, inverted barometer correction, solid earth tide and geocentric pole tide. (2) The moderately important corrections include wet tropospheric correction, dual-frequency ionospheric correction and sea state bias. The radiometer measurements are more preferred than model values in the geophysical data records for constraining wet tropospheric effect owing to the highly variable water-vapor structure in atmosphere. The items of dual-frequency ionospheric correction and sea state bias should better not be directly added to range observations for obtaining sea surface slopes since their inherent errors may cause abnormal sea surface slopes and along-track smoothing with uniform distribution weight in certain width is an effective strategy for avoiding introducing extra noises. The slopes calculated from radiometer wet tropospheric corrections, and along-track smoothed dual-frequency ionospheric corrections, sea state bias are generally within ±0.5 microradians and no larger than 1 microradians. (3) Ocean tide has the largest influence on obtaining sea surface slopes while most of ocean tide slopes distribute within ±3 microradians. Larger ocean tide slopes mostly occur over marginal and island-surrounding seas, and extra tidal models with better precision or with extending process (e.g. Got-e) are strongly recommended for updating corrections in geophysical data records. [ABSTRACT FROM AUTHOR]

The evolution of seafloor spreading of Africa, South America, and Antarctica is key to understanding the initial break-up of Gondwana. Vector geomagnetic surveys were conducted in the Natal Valley and Mozambique Ridge, off South Africa. We summarize the nature of the crust using the results of dense vector geomagnetic anomaly data, as well as satellite gravity data. Based on both inversion and forward analytical results, we identified areas of stretched continental crust, with basaltic magma intrusion in parts, as the northern Natal Valley, north part of the Mozambique Ridge, and north part of the southern Natal Valley. Oceanic crust was identified in the south part of the southern Natal Valley and south part of the Mozambique Ridge. Magnetic isochrons M0–M10 were identified in the south part of the southern Natal Valley. Clear magnetic lineations were observed in the south part of the Mozambique Ridge, where some areas were distorted by hotspot volcanism. The location of the continental ocean boundary in the Natal Valley, along with a four-stage model of tectonic evolution of the study area since about 183 Ma, are newly proposed. [ABSTRACT FROM AUTHOR]

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