Ultra-short baseline (USBL) underwater positioning systems are widely used in marine scientific research and ocean engineering. Angle misalignment is a main error that reduces the accuracy of USBL underwater positioning. The conventional angle-calibration method assumes that the transponder position obtained by USBL positioning is an errorless coefficient matrix. However, errors inevitably exist in the estimation of the transponder’s position via USBL positioning, and the precision varies at different epochs. Ignoring the error in the transponder’s position will significantly reduce the precision of the angle misalignment estimation. In this paper, a new angle-calibration method is proposed for precise USBL underwater positioning. The angle alignment model is derived by treating the transponder’s position obtained by USBL positioning as an observation, and the stochastic model is then established according to the bearing angles. Robust estimation is likewise applied to further improve the precision of the angle misalignment estimation. To verify the performance of the proposed method, a sea experiment was performed. The results show that the new method has high calibration accuracy and robustness. The estimation precision of this method is improved by 0.0457°~0.6896° in heading, 0.0125°~0.8072° in roll, and 0.0077°~0.9436° in pitch, compared with that of the conventional angle alignment method.

Due to the extremely complex geological conditions in the Bohai Strait, active faults is very developed and earthquake activity occurs frequently. It is of great importance to evaluate the regional crustal stability for the planning and construction of the Trans-straits Passage. In this study, the authors carry out the evaluation by means of buffer analysis, topology analysis and overlay analysis based on ArcGIS software. The evaluation mainly analyzed six assessment factors of Bohai Strait including neotectonic activity, active faults distribution, seismicity, tectonic stress fields, Bouguer gravity anomaly and geological disasters. In brief, the crustal stability of the eastern North Yellow Sea Basin is best, while that of the Bohai Basin and southwest Bohai Strait is poor. Meanwhile, small and frequent earthquakes in the southern Strait and geological disasters developed in the northern and central Strait such as tidal ridge and shallow gas which may adversely impact on the Trans-straits Passage construction.

The accuracy of GNSS/Acoustic seafloor geodetic calibration is greatly influenced by the temporal variation of sound velocity, especially in the vertical direction. Aiming at correcting of the unknown parameters related to both the positions and the sound velocity, this paper proposes a step-by-step inversion scheme based on moving survey data. The proposed method firstly estimates the horizontal normalized travel time delay with sound ray tracing strategy and then computes the horizontal position with circle line observations. We reconstructed an inversion scheme for extracting the surface sound velocity disturbance (SSVD) and corrected the vertical position from cross line data. The SSVD is decomposed into a sum of different period disturbances, and a new SSVD is reconstructed by combining the long period disturbance and short period disturbance. The proposed algorithm is verified by the South China Sea experiment for GNSS/Acoustic seafloor geodetic calibration. The results demonstrate that the new method can take the effects of sound velocity variation into consideration and improve the precision of the vertical position, which is superior to the least squares (LS), the single-difference LS for seafloor geodetic calibration.

The accurate classification of seabed sediment and benthic covers in shallow-water environments remains a key challenge for marine activities and oceanographic research. However, coastal areas of shallow waters are influenced by complex dynamic environments, making it difficult to obtain authentic sediment and benthic-cover samples. Therefore, to address the problem of few-shot classification of seabed sediment and benthic covers, a few-shot classification algorithm of seabed sediment and benthic covers based on the fusion model of airborne LiDAR bathymetry (ALB) and multispectral images is proposed in this article. Based on the extracted features, a scale-invariant feature transform-progressive sample consensus (SIFT-PROSAC) algorithm and perspective transform model were constructed to achieve feature fusion. Then, multi-modal feature selection is realized using a formal concept analysis-Relief-F (FCA-Relief-F) algorithm. Finally, a graph attention network-prototype network (GAT-PN) model was established to classify five types of sediment and benthic cover (coral reef, stone, sand, vegetation, and coastal zone). To validate the effectiveness of the proposed method, experimental data from actual measurements at Ganquan Island in the Xisha Islands of China were used. Compared to other classical classifiers, the GAT-PN algorithm achieves a higher classification accuracy, with an overall accuracy (OA) and Kappa coefficient of 97.50% and 0.97, respectively. The findings of this study provide effective technical support for marine engineering and related fields.

High-rate Global Navigation Satellite System (GNSS) observations enable real-time single-receiver velocity determination in marine environments. This study systematically evaluates raw Doppler velocity estimation (RDVE), time-differenced pseudorange (TDPR), and time-differenced carrier phase (TDCP) using 5 Hz buoy and shipborne data, broadcast ephemerides, and observations from the Global Positioning System (GPS), BeiDou Navigation Satellite System (BDS), and combined GPS+BDS. A unified processing strategy with consistent stochastic modeling, quality control, and method-specific reference alignment is applied. RDVE is evaluated against the same-epoch reference velocity, whereas TDPR and TDCP are evaluated against the corresponding 0.2 s interval-average reference. For statistical evaluation, finite synchronized estimates are screened using a consistent case-specific residual criterion, with the corresponding finite and retained epoch counts reported separately. In the buoy experiment, GPS+BDS RDVE achieved horizontal and vertical root-mean-square errors (RMSEs) of 0.0160 and 0.0344 m/s, respectively, while GPS+BDS single-frequency TDCP (TDCP-SF) achieved 0.0111 and 0.0237 m/s. In the shipborne experiment, GPS+BDS TDCP using the narrow-lane combination (TDCP-NL) achieved horizontal and vertical RMSEs of 0.0635 and 0.0551 m/s, respectively, with 99.86 % availability. TDCP showed the most stable performance under shipborne dynamics, whereas TDPR was more sensitive to pseudorange noise, unfavorable satellite geometry, and occasional large errors. These results indicate that the practical benefit of multi-GNSS velocimetry depends on observation type, signal availability, and reference-time alignment rather than constellation count alone.