This study aims to systematically quantify the effects of fin-hull geometric configuration on the propulsion performance of bionic undulating-fin vehicles employing media and/or paired fin propulsion (MPF). It addresses the lack of a unified analysis of geometric parameters across different bionic underwater vehicles in existing research.

To this end, a universal parametric geometric model incorporating the hull and a pair of undulating fins was developed. The model innovatively introduces the ratio of fin width to hull width β as the core dimensionless geometric parameter. Based on this model, high-fidelity CFD numerical simulations were conducted to analyze the propulsion performance and flow field structure of the vehicle under different β values.

The results indicate that β has a nonlinear and significant influence on propulsion performance, and that an optimal range of β values exists for maximizing propulsion efficiency. Excessively small β values lead to insufficient thrust generation, whereas excessively large β values increase drag due to intensified fin-hull interactions that induce flow separation. Furthermore, β significantly modulates the magnitude of the pitching moment, imposing a critical constraint on the vehicle's attitude stability.

This study clarifies the design trade-off between efficiency and stability governed by the β parameter. The established parametric model and the identified underlying mechanisms provide a quantitative theoretical basis for the shape design of bionic underwater vehicles and lay a solid foundation for future research on multi-parameter coupling optimization and self-propulsion performance.

This paper addresses the path tracking control problem for underactuated unmanned surface vehicles (USVs) under the conditions of lumped disturbances, input saturation, and limited onboard energy. These factors complicate the path tracking process and reduce the effectiveness of traditional control methods. The aim of this study is to propose an event-triggered fixed-time path tracking control strategy that improves robustness, energy efficiency, and tracking precision in complex environments.

The proposed control strategy integrates several key components to address the challenges mentioned. First, a longitudinal speed guidance law and a fixed-time line-of-sight (SGFTLOS) guidance law are designed to provide the desired longitudinal speed and heading angle for the USV, ensuring it follows the trajectory with optimal speed and heading. Next, to handle model uncertainties and external disturbances (such as wind and current), a Fixed-Time Extended State Observer (FESO) is introduced. The FESO estimates and compensates for lumped disturbances, improving the system's robustness in uncertain environments. To address input saturation, an auxiliary dynamic system is designed to smooth inputs and maintain stable path tracking, even when saturation occurs. Finally, to overcome onboard energy limitations, a periodic event-triggered mechanism based on relative threshold is proposed. This mechanism adjusts control signal update frequency based on system states, minimizing unnecessary actuator activity and energy consumption.

The stability of the system is proven to be fixed-time stable using Lyapunov's fixed-time stability theory, which also eliminates Zeno behavior (infinite triggering in finite time) that could otherwise cause instability. SimuNPS simulation results demonstrate that the tracking error converges within a fixed time, verifying the effectiveness of the proposed method. Compared to existing methods, the proposed strategy exhibits faster transient response, smaller steady-state errors, and superior robustness in the presence of lumped disturbances. Furthermore, the introduction of the FESO provides accurate real-time disturbance estimation, allowing the controller to compensate for disturbances and maintain precise path tracking. Additionally, the event-triggered mechanism significantly reduces the number of control signal updates and actuator actions, improving the system's energy efficiency.

The proposed event-triggered fixed-time path tracking control strategy effectively addresses the challenges of lumped disturbances, input saturation, and limited onboard energy in underactuated USVs. By integrating event-triggered mechanisms, innovative guidance laws, and robust disturbance compensation, the strategy provides a reliable solution for path tracking in complex and uncertain environments. The fixed-time convergence property ensures that the USV achieves desired performance within a fixed time, making the strategy suitable for real-time applications requiring stability, precision, and energy efficiency. This method offers a robust, efficient, and reliable solution for USV path tracking control under difficult operational conditions.

This paper introduces a novel data-driven approach for generating realistic and hazardous overtaking scenarios. These scenarios are crucial for rigorously evaluating the autonomous collision avoidance capabilities of autonomous ships. Existing methods often struggle to balance scenario diversity, realism, and the representation of hazardous situations. To overcome this limitation, our method leverages the rich information embedded in automatic identification system (AIS) data to generate diverse and realistic overtaking encounters.

Specifically, we propose a hybrid model that integrates a sequence generative adversarial network (SeqGAN) with a self-attention mechanism (SAM). The SeqGAN captures the complex patterns and dynamics in AIS-based ship trajectories, enabling the generation of novel, yet plausible, overtaking maneuvers. The incorporation of a SAM further enhances the model's ability to capture long-range dependencies in ship trajectories, resulting in more realistic and nuanced simulations. To ensure that the generated scenarios accurately reflect hazardous situations, we have developed a constraint model based on longitudinal and lateral safety distances between vessels to define realistic initial conditions. This model dynamically adjusts the initial positions and velocities of both the target vessel and the autonomous ship under test, ensuring that each generated scenario presents a genuine collision risk.

The results show that the effectiveness of our approach is validated through extensive simulations. A total of 500 hazardous overtaking scenarios were generated, significantly improving the coverage of test scenarios. Notably, 97.3% of these generated trajectories fall within a predefined buffer zone that encompasses real-world trajectories, demonstrating the high fidelity of our model. Furthermore, the speed distributions of the generated target vessels closely match those observed in real-world AIS data, further validating the realism of our approach.

The enhanced realism and diversity of scenarios generated by this method significantly improve the efficiency of autonomous collision avoidance testing. This allows for a more precise definition of safety performance boundaries and accelerates the development and optimization of autonomous collision avoidance algorithms. Ultimately, this work contributes to the development of safer and more reliable autonomous maritime systems capable of navigating the complexities of modern maritime environments.

This research focuses on the energy characteristics of false targets generated by time-modulated adaptive jamming technology, aiming to investigate the significant variations in these characteristics caused by different modulation schemes and parameter settings. It aims to provide a comprehensive understanding of how modulation parameters influence the energy distribution of false targets, thereby offering practical insights for electronic warfare applications.

First, theoretical interference models were established for different modulation schemes against linear frequency modulation (LFM) pulse radar. These models elucidate the mapping relationship between modulation timing and the amplitude of false targets. Second, a Ku-band jamming system was designed and built to experimentally validate the theoretical findings. The system incorporates 1-bit modulation and control modules to generate time-modulated signals. Numerical simulations were conducted to evaluate the energy characteristics of false targets under various duty cycles and modulation schemes. Additionally, experimental measurements were performed in a controlled environment to compare the performance of various modulation modules and to verify the accuracy of the simulation results.

The results demonstrate that 1-bit modulation effectively conceals the target's energy at the fundamental frequency, making it difficult for radar systems to detect the true target. Under a fixed modulation scheme, it was observed that decreasing the duty cycle of the modulation signal reduces the amplitude difference between each harmonic and the fundamental frequency. When the harmonics approach the fundamental frequency in amplitude, the radar's ability to distinguish between true and false targets is significantly compromised. This finding highlights the importance of optimizing the duty cycle to enhance the effectiveness of time-modulated adaptive jamming. The experimental results closely matched the numerical simulations, validating both the theoretical models and the effectiveness of the proposed jamming system.

By employing 1-bit modulation and carefully adjusting the duty cycle of the modulation signal, it is possible to effectively shape the energy profile of the real target, thus improving the jamming effectiveness against modern radar systems. This research provides both qualitative and quantitative analysis of the energy characteristics of false targets, and offers practical guidance for the development and implementation of time-modulated adaptive jamming systems. Future work may focus on extending this study to multi-target jamming scenarios, and on incorporating artificial intelligence algorithms to optimize jamming strategies in real time, as well as exploring countermeasures against emerging radar technologies.

Complex backgrounds, significant target size variations, and severe sea clutter in maritime infrared imagery often result in missed or false detections. To address this challenge, an improved method based on YOLOv8n, termed maritime infrared target detection-YOLO (MITD-YOLO), is proposed to enhance target detection accuracy in maritime infrared images.

MITD-YOLO incorporates a diverse branch module (DBB) and enhanced multi-scale convolution (EMSConv) to leverage multi-scale convolutions, enabling the model to more effectively capture complex features. A triple attention mechanism is employed to facilitate spatial and channel-wise feature interaction, thereby improving key feature extraction. Additionally, the powerful-IoUv2 (PIoUv2) loss function is introduced to address the anchor box expansion problem, leading to improved detection accuracy and enhanced model robustness.

Experimental results show that the improved model significantly enhances the efficiency of maritime infrared target detection, with a 2.3% increase in precision and a 1.7% increase in recall. The model achieves an average precision of 88.9%, and 132.8 FPS, outperforming the original model.

MITD-YOLO enhances maritime infrared target detection performance and provides a more reliable target detection technology for applications such as maritime surveillance and ship navigation, contributing to the advancement of intelligent maritime systems.

To suppress hydrodynamic noise at the source, a noise reduction method for pump-jet propulsors based on porous media is proposed.

By replacing the metallic leading edges of the stator blades of the pump-jet propulsor with porous materials, the interaction between the blade wake and the inner wall of the duct can be effectively modulated, thereby reducing wall pressure fluctuations. Large eddy simulation (LES), combined with acoustic analogy analysis, was employed to investigate the flow characteristics and noise control performance of the stator blades with porous leading edges. The mechanisms by which the porous media modulates the flow field and suppresses noise were analyzed, and the effects of key parameters, such as porosity and advance coefficient, on hydrodynamic noise control were examined.

Comparative results indicate that the porous leading edges of the stator significantly reduce the low-frequency sound pressure level components on the duct wall and the far-field radiation noise. The maximum reduction in the sound pressure level (SPL) reaches 5.52 dB in the direction perpendicular to the rotation axis of the pump-jet propulsor.

The findings of this study provide useful guidance for flow control and hydrodynamic noise reduction in pump-jet propulsors.

To accurately evaluate the impact of blanking on pulse interception, this study examines the loss probabilities of pulses caused by the blanking.

The influence of blanking on pulse sorting was analyzed. Based on the pulse sorting method used by reconnaissance equipment, the criteria for identifying the loss pulses were established. The output waveform of a pulse erased by the blanking gate was then examined under various time-sequence relationships between the pulse and the blanking gate, allowing the varying law of the residual pulse to be obtained. The expressions were derived for the average loss probabilities of the pulses and the loss probabilities of the periodic pulses both caused by the periodic blanking gates, as well as for the average loss probabilities of the pulses under blanking gates with time-varying duty cycles. Finally, each pulse loss probability was validated by comparing the calculation results with numerical simulation. The maximum absolute deviation between the two was approximately 5.8×10⁻⁴, which is negligible.

The mathematical models for the pulse loss probabilities and the average loss probabilities are obtained, and their accuracy has been verified.

The research provides a quantitative evaluation of the influence of the blanking on the loss of the intercepted pulse. It provides the input for accurately analyzing the missed alarm probability in pulse interception with the blanking gate and supports decision-making regarding the use of blanking measures.

With the increasing diversification of application requirements for unmanned underwater vehicles (UUVs), traditional design methods centered on text-based documentation have revealed numerous limitations in practice, such as scattered design documents, difficulty in maintenance, and low iteration efficiency among systems. Therefore, it is necessary to introduce a novel overall design methodology.

In this study, the model-based systems engineering (MBSE) methodology was incorporated into the design process of UUVs and integrated with traditional design approaches to establish a model-driven design and verification framework. Using the M-Design collaborative research platform, graphical system modeling language (SysML) was employed to construct a comprehensive system model, including the requirements model, logical architecture model, and physical architecture model, thereby forming an integrated design framework. To further validate the feasibility of the framework, multi-system co-simulation technology was adopted, and a distributed simulation platform was developed to perform performance simulation and verification for typical mission scenarios of UUVs. Based on these efforts, a conceptual scheme for an agile design and verification prototype system has been proposed to support agile development requirements.

The results demonstrate that the model-based design methodology can significantly improve the design efficiency and verification capability of UUVs, enabling a closed-loop development process from requirements definition to design implementation.

The proposed design methodology provides effective guidance for the design and specification verification of various manned and unmanned underwater platforms.

To improve the overall operational capability of autonomous underwater vehicles (AUVs) and address the critical issue of collision risks during the dynamic docking process with towed recovery docks (TRDs), this study conducts a systematic investigation into the collision mechanisms and control strategies of the docking system. Reliable docking and recovery technology is essential for extending AUV operational endurance, enhancing data transmission efficiency, and enabling long-term underwater deployment. However, in real marine environments, limitations in sensor accuracy, external disturbances, and the dynamic response of the docking system often lead to unavoidable contact or collision between AUVs and TRDs, which may result in mission failure or structural damage to the equipment. Therefore, this study aims to clarify the influence of key initial operating conditions on docking-induced collisions and to propose an effective control strategy for optimizing the dynamic docking process, thereby providing theoretical and technical support for the engineering application of AUV towed recovery systems.

Based on dynamic analysis, a simulation model incorporating contact and collision dynamics was developed using the ADAMS-MATLAB co-simulation platform. First, rigid body dynamic models of AUV and TRD were constructed. The AUV model accounts for gravity, buoyancy, viscous hydrodynamic drag, inertial hydrodynamic drag, thrust, and environmental disturbances. The TRD model adopts a frame-cage structure with a bell-mouth guiding cover, and a discrete flexible body method is used to model the towing cable. Subsequently, a nonlinear contact model based on Hertz theory was employed to calculate the collision forces between AUV and TRD, which more accurately captures the transient impact characteristics of the collision process compared with the linear contact model. On this basis, the effects of initial operating conditions including eccentric angle, eccentric distance, relative initial velocity, and mother vessel acceleration on docking collisions were systematically analyzed using the control variable method. To mitigate attitude disturbances induced by collisions, a multi-stage coordinated control strategy based on PID control was proposed, which realizes active attitude adjustment of AUV by switching control modes across different docking phases.

The simulation results indicate that increases in eccentric angle and eccentric distance primarily prolong the docking time while exerting only a limited influence on the peak collision force, which remains within the range of 1 000–2 000 N under most working conditions. In contrast, increasing the relative initial velocity can shorten the docking time but significantly amplifies the peak collision force, showing a positive correlation between them. Further analysis of mother vessel acceleration reveals the complex, non-monotonic relationship between collision force and docking efficiency. As the mother vessel's acceleration increases, the amplitude of the TRD attitude variations intensifies, leading to greater uncertainty in the collision position, and the peak collision force reaches its maximum value when the acceleration is 0.2 m/s². Moreover, the proposed multi-stage coordinated control strategy enables effective post-collision attitude adjustment of the AUV. In the case of uniform motion of the mother vessel, the strategy reduces the peak collision force by up to 74.5% and shortens the docking time from 7.56 s to 5.93 s. Even under the complex working condition of uniform acceleration of the mother vessel, the peak collision force is reduced by 19.6%, and the docking time is shortened by 16.7%, effectively optimizing the dynamic docking process and ensuring both docking safety and efficiency.

This study systematically clarifies the effects of key initial operating conditions on the docking collision between AUV and TRD. The research findings indicate that controlling the initial eccentric angle and eccentric distance can improve docking efficiency, whereas adjustments to the relative initial velocity and mother vessel acceleration require a careful balance between collision risk and docking speed. The proposed multi-stage coordinated control strategy can significantly reduce the peak collision force while maintaining docking efficiency, achieving reductions of 14%–74.5% under different working conditions. This strategy exhibits superior robustness and stability compared with the traditional position tracking control strategy, effectively addressing the limitations of passive control methods that rely solely on the dock structure. Overall, this study provides a reliable simulation basis and design reference for the design and stability control of AUV towed recovery systems. In addition, the research framework and methods provide guidance for the collision analysis and control in similar underwater docking systems.