Ocean Observations

1. Introduction

Our Oceans cover more than 70% of the Earth’s surface, and thus various ocean engineering projects have been undertaken to utilize these vast resources effectively. Ocean observations are fundamental for the planning, designing, and implementing such projects. These observations are crucial in mitigating risks, enhancing early warning capabilities, and ensuring the ocean engineering infrastructure’s structural integrity and long-term sustainability. By systematically monitoring and analyzing the dynamic changes in the marine environment, these observations provide essential data that support engineering safety and inform the development of robust design parameters [1,2].

The marine environment is characterized by complex and multifaceted variability, including fluctuations in wave height, tide, ocean current, temperature, salinity, and other physicochemical factors. Each of these variables directly influences the performance, stability, and safety of ocean engineering structures. Consequently, accurate and comprehensive oceanographic data are indispensable for engineers to anticipate environmental loads and design resilient structures capable of withstanding extreme marine conditions [3].

Advancements in ocean observation technologies, such as data buoys or floats equipped with multi-parameter sensors [4,5], autonomous underwater vehicles [6,7], satellite remote sensing platforms [8,9], and high-frequency radar systems [10,11,12], have significantly expanded spatial and temporal coverage for marine data acquisition. These technologies enable the real-time, wide-area, and continuous monitoring of oceanographic parameters, thereby enhancing the scientific rigor and operational efficiency of engineering design and management processes. Integrating diverse observational platforms facilitates a holistic understanding of marine dynamics, which is critical for optimizing engineering solutions and adaptive management strategies [13].

Moreover, the development of novel ocean observation instruments is also committed to achieving long-term, cost-effective monitoring. A key challenge in designing these instruments is balancing power consumption and data storage capacity to maximize continuous observation time. Therefore, innovations to reduce energy consumption and improve battery efficiency have become the core of developing autonomous ocean monitoring equipment. These autonomous systems provide unprecedented flexibility and durability for continuous ocean data collection, thereby supporting more informed decision-making in marine engineering [14].

This Special Issue highlights original research and innovative methodologies aimed at fostering interdisciplinary collaboration and inspiring new approaches to address the technical and scientific challenges in ocean observation. The contributions made to this Special Issue will benefit the global marine science community, support evidence-based decision-making, and accelerate our understanding of the oceans in a more actionable way. This Editorial provides a comprehensive overview of the articles featured in this Special Issue. For clarity, the articles are organized into four thematic sections: Ocean Observation Technology, Ocean Environment Observation, Energy for Sustained Observation, and Ocean Ecological Observation.

2. Overview of Contributions

2.1. Ocean Observation Technology

Corner reflectors are widely used as calibration targets for radar systems, especially in synthetic aperture radar (SAR) and other remote sensing applications. Wu et al. (Contribution 1) examined the dynamic Doppler characteristics of maritime airborne corner reflectors. Understanding the Doppler characteristics of these reflectors is essential for accurate radar calibration, target detection, and maritime surveillance. The authors analyzed several factors that affect the Doppler characteristics, such as flight speed, altitude, trajectory, and sea state. They established an analytical model to describe the Doppler frequency changes caused by the dynamic motion of the reflector. This model helps to clarify the relationship between the dynamic characteristics of the reflector and the Doppler effect observed in the radar echo. The results show that the model can enhance the calibration of maritime radar systems, improve the reliability of SAR data interpretation, and contribute to developing sophisticated ocean monitoring and surveillance technologies.

Underwater single-photon LiDAR imaging faces challenges due to a low signal-to-noise ratio and decreased image clarity caused by water scattering and absorption. Rong et al. (Contribution 2) proposed a sequential dual-mode fusion algorithm aimed at improving image reconstruction and depth accuracy. This algorithm leverages the strengths of two distinct imaging modes: photon counting mode, which offers high sensitivity in low light conditions, and intensity mode, which provides enhanced detail in areas with stronger signals. By enabling adaptive switching and information sharing during the image reconstruction process, the system proves to be robust even in turbid water conditions, where traditional LiDAR imaging often struggles. This innovation is beneficial for various applications, including marine exploration, archeology, environmental monitoring, and operations involving autonomous underwater vehicles (AUVs).

Accurate motion compensation is essential to maintain the quality of synthetic aperture radar (SAR) images, especially when platform motion (such as aircraft or satellite) introduces errors in the long aperture range. Zhang et al. (Contribution 3) proposed a combined motion compensation method using sub-aperture processing. The SAR data is divided into multiple sub-apertures to more finely estimate and correct the phase error caused by motion within each sub-aperture. This method significantly improves the image focus and resolution of long-aperture SAR systems and effectively reduces the distortion caused by motion. It can provide high-quality SAR imaging for ocean and coastal monitoring, ship detection, and environmental monitoring.

To address the issue of subswath overlap potentially leading to geometric and radiometric inconsistencies in coastal areas, making it difficult to create seamless, high-quality stitched images from GF-3 ScanSAR images, Wang et al. (Contribution 4) proposed a dedicated method for the stitching and correction of GF-3 ScanSAR images in coastal areas to address the challenges posed by the limited range of subswath overlap. The method includes the precise alignment, radiometric normalization, and correction of seam artifacts in the overlap area. Quantitative evaluation shows that the method improves image consistency and reliability compared to traditional stitching methods. This method can be applied to other multi-band SAR datasets, thereby supporting a wider range of applications in remote sensing and marine science.

Reverberation is the persistence of sound waves due to scattering from boundaries and inhomogeneities. It is a key challenge for underwater acoustic systems, especially in shallow waters. The study conducted by Kosteev et al. (Contribution 5) used a vertical antenna array to study the short-range reverberation characteristics in a shallow marine environment. It quantified how the reverberation pattern changes with the distance from the sound source and environmental conditions, thereby improving sonar performance and acoustic monitoring. The study showed that the intensity and decay rate of short-range reverberation were significantly affected by the roughness of the seabed and the type of sediment, with more complex substrates producing reverberations with longer duration. Due to dynamic scattering at the air–sea interface, surface waves introduce changes in the reverberation signal, especially for higher frequencies. These findings are valuable for marine habitat monitoring and underwater navigation in coastal areas.

2.2. Ocean Environmental Observation

Tropical cyclones significantly impact the marine environment, making it important to consider the cumulative effects of consecutive storms in coastal management and disaster preparedness. Chen et al. (Contribution 6) studied how these cyclones influence shallow water temperature and significant wave height (SWH) in the northeastern Beibu Gulf, a region prone to storms in the South China Sea (SCS). Using data from buoys, coastal stations, remote sensing, and numerical simulations, they found that the first cyclone typically causes a rapid drop in shallow water temperature due to mixing and upwelling. However, if a second cyclone follows closely, the cooling effect is reduced, as the first cyclone has already mixed the upper ocean. Wave heights also sharply increase during cyclonic events, peaking as the cyclones approach. These cumulative effects can prolong heightened wave activity, highlighting the need for better predictions and resilient coastal infrastructure.

To understand the spatiotemporal variations in the marine environment of the Ocean Energy Test Site northeast of Taiwan, Ho et al. (Contribution 7) used an acoustic Doppler current profiler to measure the ocean currents in the area. Data analysis included the velocity, direction, vertical shear, and their changes over time. The results showed that the measured current profiles revealed significant vertical and temporal variations, which were affected not only by the tidal cycle but also by larger-scale ocean dynamic processes such as the Kuroshio intrusion. The results provide valuable reference data for marine operations, offshore infrastructure planning, and environmental impact assessments in the region.

Understanding this wind–wave relationship is crucial for ocean forecasting, coastal engineering, navigation safety, and disaster prevention, especially in areas with frequent typhoons and monsoons. Cheng et al. (Contribution 8) used buoy data to explore the relationship between wind conditions and sea surface wave characteristics in the waters around Taiwan. The authors analyzed the correlation between wind speed, direction, significant wave height, and other parameters. The results showed a clear and quantifiable relationship between wind speed and significant wave height, which varies with season (monsoon vs. non-monsoon) and location (open sea vs. nearshore). This finding provides a scientific basis for improving wave forecast models and coastal risk assessment in Taiwan. This will aid in the design and operation of offshore structures, port facilities, and maritime activities, and provide valuable regional data for the broader field of sea–air interaction research.

Physical mechanisms such as internal waves, upwelling, and tropical cyclones affect the ocean mixing process and change the turbulent mixing parameters. The MacKinnon-Gregg (MG) model used by Hu et al. (Contribution 9) estimates the turbulent dissipation rate to address the turbulent mixing parameterization. The authors used multi-year turbulent dissipation rate observations from different cruises in the northwestern South China Sea to evaluate the performance of the original MG model. The study found that the key parameter (ε₀) of the MG model varies significantly with region and season, resulting in its low reliability for the northwestern South China Sea. By analyzing the importance of various physical parameters, the authors found that the normalized depth (D) is a key predictor of turbulent mixing. They proposed an improved MG (IMG) model incorporating depth dependence, making the parameterization more physically consistent and regionally applicable. The IMG model can accurately capture the spatiotemporal distribution of turbulent mixing during and after tropical cyclone events. The new parameterization method helps to better understand the physical processes and changes in turbulent mixing in the North Sea.

Wang et al. (Contribution 10) conducted a study comparing ocean gravity models derived from satellite altimetry with high-precision gravity measurement data collected by advanced platform-based ocean gravimeters. The aim was to evaluate the accuracy and reliability of various global and regional ocean gravity models that use satellite altimetry data. These findings indicate that the new platform gravimeter provides stable and accurate gravity measurements suitable for marine geophysical research. While gravity models based on satellite altimetry generally perform well in open ocean areas, significant discrepancies are noted in regions with complex water depths or near coastal zones. The results of the study demonstrate that integrating advanced shipboard measurements with satellite data can enhance multidimensional models.

2.3. Energy for Sustained Observation

Battery safety and life are very important for underwater instruments. Ho (Contribution 11) developed an advanced capacitor-based battery balancer that can redistribute charge between battery cells in a battery pack to improve the performance and life of underwater vehicle battery systems. The results show that the system exhibits lower energy loss, better thermal management performance, and can be easily integrated into existing battery management systems. Enhanced battery balancing can extend the operating time, safety, and battery life of underwater vehicles. This technology is particularly important for AUVs and other submersible platforms, where battery reliability and efficiency are critical. This method is also applicable to other applications that require robust and energy-efficient battery management in harsh or remote environments.

2.4. Ocean Ecological Observation

To address the challenges of underwater tracking and data retrieval, Tian et al. (Contribution 12) developed a marine animal behavior recording tag system that integrates positioning and recovery technologies to enhance data collection on marine animal activities and behaviors. This tag system combines various positioning technologies, including the Global Positioning System for surface tracking and acoustic or other underwater positioning methods, allowing for monitoring animal activities both underwater and at the surface. It captured detailed behavioral patterns and movement trajectories of tagged marine animals in their natural habitats, providing valuable support for advanced marine biology research. The technology has potential applications in conservation efforts, fisheries management, and ecological research, helping to improve our understanding and protection of marine species.

Gwak (Contribution 13) conducted an analysis of mitochondrial DNA (mtDNA) to explore the population genetic structure of mackerel (Scomber japonicus) off the coast of Korea. Understanding the genetic diversity and population structure of this commercially important species is crucial for effective fisheries management and conservation. The authors collected mackerel samples from various locations along the Korean coast. They sequenced and analyzed the mtDNA control region to evaluate genetic diversity, population differentiation, and gene flow between regional populations. The results indicate that mackerel in the Korean coastal waters can be managed as a single genetic population, which simplifies management strategies. The higher connectivity between populations may enhance their resilience to environmental changes and fishing pressures. This study establishes a genetic baseline for future monitoring and conservation efforts.

3. Conclusions

Incorporating advanced ocean observation technologies into engineering practices enhances the safety and reliability of marine structures and supports sustainable development in the marine environment. Continuous innovation in observation instruments and data collection methods is essential for addressing the emerging challenges posed by the dynamic oceanic environment. It has been a pleasure to compile these insightful articles, and I would like to express my gratitude to the authors for their high-quality contributions to this Special Issue.