Urban lakes play a critical role in urban ecosystems, providing essential services such as water supply, flood control, and recreational spaces. However, monitoring urban lake levels using traditional in-situ methods is often challenging due to high costs, data continuity issues, and restricted access. Satellite altimetry has emerged as a promising alternative, offering global coverage and consistent measurements. Despite its success in monitoring large inland water bodies, applying satellite altimetry to urban lakes remains challenging due to their small size, shallow depth, and complex surrounding environments, which seriously distort altimeter waveforms. This study evaluates the performance of Jason-3 satellite altimetry data for monitoring urban lake levels, focusing on East Lake in Wuhan, China. We propose a novel waveform retracking method, the MP3 retracker, designed to address the complexities of urban lake environments by analyzing the first three maximum peaks in the waveform. The MP3 retracker is compared with traditional retracking algorithms, including ICE, Offset Center of Gravity, and 50% threshold method, using in-situ gauge data for validation. Results show that the MP3 retracker significantly outperforms traditional methods, achieving a root mean square error of 7.21 cm and a correlation coefficient of 0.94, with a valid data rate exceeding 70%. In contrast, traditional retrackers exhibited lower accuracy and higher rates of invalid data. This study demonstrates the potential of satellite altimetry, combined with advanced retracking techniques, for accurate and reliable monitoring of urban lake levels, contributing to improved water resource management and environmental conservation in urban settings.

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.

In this letter, the clutter rank estimation formulas for end-fire array airborne radar (EFAAR) in two typical scenarios are derived. In comparison to the airborne radar with a traditional broadside array, the clutter rank of the EFAAR in the forward-looking scenario still maintains a reasonably accurate estimation criterion. The effectiveness of the proposed clutter rank formulas is validated, and the impacts of certain parameters on the clutter rank are analyzed through simulations. Simulation results indicate a direct proportionality between clutter rank and the number of array elements, coherent processing pulses, and clutter ridge slope.

The space geodetic technologies (e.g., Global Navigation Satellite System (GNSS) and Gravity Recovery and Climate Experiment (GRACE)/GRACE Follow-on (GFO)) provide effective tools to infer terrestrial water storage (TWS) change, which is an important indicator of the hydrological cycle and climate change. This study investigated the optimization approaches of the Slepian basis function (SBF) method in recovering TWS changes in the Southwest River Basin of China (SWRB) with sparse data and conducted a comprehensive analysis of TWS changes in SWRB using GNSS and GRACE/GFO. The results show that using the Shannon number criterion and even-distributed GNSS observations can obtain reliable TWS changes based on the SBF method. The GNSS-inverted TWS changes generally maintain good consistency with GRACE/GFO and hydrological model results in SWRB, but the ground-based daily GNSS estimates present stronger signal amplitudes and more high-frequency information. In the upper SWRB, the GRACE/GFO well reveals the continuous water loss (-8.19±1.76 mm/yr) induced by human activity, while the GNSS does not reveal the long-term linear changes of TWS, because it is difficult to separate them from the GNSS vertical displacement time series. In the lower SWRB, both GNSS and GRACE/GFO can reveal the increasing and decreasing trends of TWS changes within the study period, and precipitation is the dominant driving factor that causes the secular changes in TWS. Additionally, the ENSO generally presents relatively stronger effects on TWS changes than PDO in SWRB. Our results contribute to understanding the regional hydrological cycle and managing the water resources in SWRB using geodetic data such as GNSS and GRACE/GFO.

The outburst of Zhuonai Lake (ZL) in 2011 changed the local hydro-ecological system in northeast Hoh Xil (HX). This study focuses on the impact of the outburst on lake water storage changes (LWSCs) of four lakes in this region. Monthly LWSC from 2009 to 2016 were constructed using Landsat and Cryosat-2 data. We determined and validated lake levels and areas derived from Cryosat-2 and Landsat data, respectively. The two satellite-derived sequences were merged by interpolation to compute LWSC, based on the level-area curve derived from the digital elevation model (DEM). Results reveal the apparent impact of the outburst on the hydrological system. Before the outburst, water volumes in all four lakes continuously grew from January 2009 to September 2011, with a total increase of 1.49 ± 0.08 Gt. Most increased water was distributed in ZL and Kusai Lake (KL). The LWSC of each lake was strongly correlated (>0.9) with those of the whole basin. However, from October 2011 to December 2016, there was no ascending trend in LWSC of the three upper stream lakes, although the water volume in the basin was still increasing. More than 80% of the increased water flowed into Salt Lake (SL). Only the LWSC of SL kept a high correlation (0.91) with those of the whole basin.

This paper is concerned with the problem of the robust sampled-data H∞ control for an offshore steel jacket platform subject to the self-excited wave force and the external disturbance. First, by using the input delay method, the corresponding closed-loop system with the sampling measurements is transformed into a continuous-time system. Then, based on a Lyapunov functional, the H∞ performance is established and the stability criteria for the closed-loop system is derived. Finally, the effectiveness of the proposed robust sampled-data H∞ controller is demonstrated by a simulation example. The simulation results show that the designed controller is effective to control the offshore platform, Moreover, to obtain almost the same control performance, the required control force under the robust sampled-data H∞ controller is smaller than the one under the robust H∞ controller.

Measuring sea surface height (SSH) using satellite altimetry in coastal ( from coasts) and shallow water region has long been a challenge since the radar altimeter waveforms are often contaminated by complex coastal topography and do not conform to theoretical Brown waveform shapes. The land contamination or surface variation due to ocean dynamics induce spurious peaks in altimeter waveforms that deviate from Brown's theoretical model as the altimeter footprint approaches or leaves the shoreline. These spurious peaks should be mitigated to minimize the error in the determination of the leading edge and associated track offset in the waveform retracking process. Here, we introduce a novel algorithm to modify coastal waveforms (0.5-7 km from coasts, using 20 Hz altimetry data), thus improving coastal data coverage and accuracy. We apply our processing algorithm and use various retrackers to compare retrieved coastal SSHs in four study regions in North America, using both Envisat and Jason-2 altimetry. The retrieved altimetry data in the 1-7 km coastal zone indicate that the 20% Threshold retracker with modified waveform has a RMSE of 21 cm as compared with in situ tide gauge data, which corresponds to a 63% improvement in accuracy compared to the use of the original deep-ocean waveform retracker.

This paper presents a three-DOF permanent spherical actuator, with precise measurement system. As magnetic model is very useful to torque model and control model, magnetic model is the basis of spherical actuator study. A new analytical magnetic field model is presented for the proposed 3-DOF spherical actuator in this paper. This analytical magnetic field method combines the Laplace equation with the equivalent magnetic charge law, which is simple and accurate. Finally, we apply the finite-element magnet analysis method to verify the accuracy of the proposed analytical method.