Underwater Applications Research Articles

Fiber-Based Noncontact Sensor with Stretchability for Underwater Wearable Sensing and VR Applications.

The rapid development of artificial intelligent wearable devices has led to an increasing need for seamless information exchange between humans, machines, and virtual spaces, often relying on touch sensors as the primary interaction medium. Additionally, the demand for underwater detection technologies is on the rise owing to the prevalent wet and submerged environment. Here, a fiber-based capacitive sensor with superior stretchability and hydrophobicity is proposed, designed to cater to noncontact and underwater applications. The sensor is constructed using bacterial cellulose (BC)@BC/carbon nanotubes (CNTs) (BBT) helical fiber as the matrix and methyltrimethoxysilane (MTMS) as the hydrophobic modified agent, forming a hydrophobic silylated BC@BC/CNT (SBBT) helical fiber by the chemical vapor deposition (CVD) technique. These fibers exhibit an impressive contact angle of 132.8°. The SBBT helicalfiber-based capacitive sensor presents capabilities for both noncontact and underwater sensing, which exhibits a significant capacitance change of -0.27 (at a distance of 0.5 cm). We have achieved interactive control between real space and virtual space through intelligent data analysis technology with minimal interference from the presence of water. This work has laid a solid foundation of noncontact sensing with attributes such as degradability, stretchability, and hydrophobicity. Moreover, it offers promising solutions for barrier-free communication in virtual reality (VR) and underwater applications, providing avenues for smart human-machine interfaces for submerged use.

Day and night continuous high-resolution shallow-water depth detection with single-photon underwater lidar.

Single-photon lidar has emerged as a strong technology for bathymetric measurements. However, its heightened sensitivity additionally makes it susceptible to solar radiation noise, particularly in the green light wavelength where solar radiation is strong, posing challenges for its daytime operation. To address this issue, a single-photon underwater lidar system is proposed and demonstrated. This scheme has these features. 1) Underwater applications not only mitigate the impact of the air-water interface on laser transmission but also significantly attenuate solar radiation reaching the lidar due to the absorption and scattering properties of water. 2) The telescope is designed with a small aperture and narrow field of view to significantly suppress solar radiation. 3) A combination of a narrowband laser and narrowband filter technique is effectively employed to minimize residual solar radiation, thus enabling continuous bathymetric observation capabilities during both day and night. 4) After acquiring the backscattered signal from the bottom, a water depth extraction algorithm utilizing bi-Gaussian fitting is proposed. To demonstrate the robustness of the lidar and the effectiveness of the algorithm, the underwater single-photon lidar system is deployed on a ship to conduct cruise surveys of two bays in the nearshore area, as well as a full-day stationary observation experiment. The lidar measurements are highly consistent with the synchronized sonar observations. The full-day stationary observation experiment showcased its capability to deliver continuous measurements throughout the day and night. These results demonstrate the potential of the system in various applications, including high-precision underwater terrain mapping, obstacle avoidance for underwater platforms, and underwater target imaging.

Self-Healing Polymers Designed for Underwater Applications

Polymeric materials have been employed in a wide range of applications in both dry and wet environments. When these materials suffer damage, it becomes crucial to initiate repairs in order to mitigate further losses. The use of self-healing materials emerges as a promising strategy not only to address this issue but also possess the advantage of prolonging the product’s lifespan. Nevertheless, the development of self-healing materials tailored for wet environments presents a set of obstacles and complexities. The review examines the current state of research in the field and highlights the challenges associated with developing self-healing materials that can effectively repair damage in such environments. We discuss the self-healing mechanisms and various polymers that are extensively employed in the advancement of self-healing materials. We study the progress made in the research and development of self-healing materials specifically designed for wet environments. Furthermore, it provides a summary of various applications of self-healing materials in wet environments.

Waterproof and Flexible Aquatic Tactile Sensor with Interlocked Ripple Structures for Broad Range Force Sensing

AbstractUnderwater perception of a broad range of force holds great importance in aquatic explorative activities, while flexible tactile sensors face technical challenges to realize. This paper presents a novel flexible aquatic tactile sensor based on waterproof graphene (GR)/carbon nanotube (CNT)/Polydimethylsiloxane (PDMS) composites. The prepared GR/CNT/PDMS composites possess excellent hydrophobic and electromechanical properties with a water contact angle over 134° and an ultrahigh gauge factor of 2296, making them an ideal piezoresistive sensing material for underwater broad‐range force sensing. The proposed tactile sensor has 3 × 3 sensing units and uses dual interlocked water‐ripple structures to improve its sensitivity and force detection range. The fabricated tactile sensor is characterized by two distinct sensitivities: a high sensitivity of 0.0338 kPa−1 at 0.062–150 kPa and a low sensitivity of 0.00357 kPa−1 at 150–450 kPa. Further, the sensor exhibited excellent resistance response, fast dynamic recovery, and both mechanical and electrical stability in aquatic environments. Then, the aquatic tactile sensor is worn on the palm of the human hand to detect the distribution and variation of contact forces when grasping objects with different shapes and hardness, demonstrating the potential underwater applications of the developed aquatic tactile sensor for broad‐range force sensing.

Demultiplexing of 40 acoustic orbital angular momentum wavelength-division multiplexing (OAM-WDM) channels by an improved virtual rotating receiver method

Orbital angular momentum (OAM) multiplexing/demultiplexing technology is crucial in increasing the data transmission rate for acoustic communication. However, the existing acoustic OAM multiplexing/demultiplexing is still limited to eight channels, and its combination with other communication techniques has not been verified experimentally. Here, we experimentally demonstrate the demultiplexing of up to 40 data channels using OAM multiplexing combined with wavelength-division multiplexing (WDM). The proposed demultiplexing method is improved from the virtual rotating receiver method used to detect the rotational Doppler effect of the OAM waves by a static array of microphones. The improved method has overcome the challenges of insufficient response, cross-talk, and signal aliasing, which often hinder the existing demultiplexing methods in the low-frequency region. The proposed demultiplexing method can be used to quickly decode the massive information concealed in a large number of acoustic OAM-WDM channels. Our work also shows practical prospects in underwater communication applications, especially in long-range communication using acoustic waves at low frequencies.

Fabrication of versatile lignocellulose nanofibril/polymerizable deep eutectic solvent hydrogels with anti-swelling, adhesive and low-temperature resistant properties via a one-pot strategy

Lignocellulosic nanofibril (LCNF) is indispensable in numerous potential applications because of its unsurpassed quintessential characteristics. While it still remains a challenge to assemble LCNF in a facile and environmental economy-first manner. In this work, a simple and green one-step synthetic approach was reported to prepare a series of LCNF-containing versatile hydrogels using deep eutectic solvent (DES). In particular, the LCNF5% hydrogel (namely LCNF5%-gel) in this work perfectly integrated superior stretchability (∼643 %), and displayed a dramatically improved anti-swelling ability (25 %) compared to the control sample (neat DES hydrogel, 2252 %). Simultaneously, the LCNF5% hydrogel presented underwater adhesiveness and outstanding long-term low-temperature resistance (stable at −25 °C for a month). This novel multifunctional hydrogel, prepared by a facile and eco-friendly strategy, is potentially useful in wet adhesion or underwater applications.

Design and Implementation of a Multi-Function Hydrophone for Underwater Acoustic Application

In recent years, underwater acoustic applications have attracted much attention, for example, for underwater environmental monitoring, underwater exploration, etc. Hydrophones play a particularly important role. Although hydrophone design has been in multifarious application forms, it still needs to consider increasing demand for low-cost, low-consumption, and multiple-function devices, as well as issues around miniaturization, lossless data collection, etc. In this paper, we design a compact underwater acoustic device that has the capability of underwater acoustic signal storage, underwater acoustic signal transmission via the Internet, and decoding based on the direct sequences spread spectrum (DSSS). The key problem is how to implement multiple functions in only one micro-controller unit (MCU). The hardware and software of the proposed multi-function hydrophone are described in detail. In particular, the MCU, the pre-amplifier with gain control, and the analog-to-digital integrated chip are introduced. Moreover, underwater acoustic data storage, underwater acoustic transmission, and the DSSS receiver are depicted in terms of software. The different functions of the hydrophone are verified in sea trial experiments. The results show that the proposed multi-function hydrophone is able to sample underwater acoustic data at high quality. In addition, to demonstrate configurable parameters, the DSSS receiver with different carrier frequencies is provided. The proposed multi-function hydrophone realizes zero bit error rate (BER) when carrier frequency fc=9 kHz, and the BER with 10−3 order of magnitude when carrier frequency fc=15.5 kHz. The results show that the proposed multi-function hydrophone has great potential to explore the ocean.

Fabrication of a twisted sensing yarn for multifunctional wearable applications

Nanofibers are ideal for the fabrication of fiber-based sensors owing to the wide applicability of such sensors in various fields. However, achieving high sensitivity and excellent tensile properties while maintaining high electrical conductivity in such sensors is difficult. This study proposes a simple fabrication method for a helical conductive yarn, core–sheath helical yarn (CSHY), in which the electrospun thermoplastic polyurethane (TPU) /boron nitride (BN) film is used as the fibrous material and coated with a liquid metal (LM), EGaIn. The fiber is then stretched and twisted to obtain the CSHY with high sensitivity, tensile stability, and durability as well as fast response and recovery. Experimental results show that the sensor fabricated by weaving the CSHY has a water contact angle of 143.2°, indicating that it can be used for human motion monitoring and underwater applications. In addition, the LM has excellent electrical conductivity and thermal conductivity and exhibits a synergistic effect with BN that is uniformly arranged along the fiber length. Thus, the fabric-based sensor exhibits excellent heat-dissipation, temperature-sensing, and electric-heating performances. Consequently, the proposed multifunctional smart yarn exhibits excellent strain sensing, self-controlled thermal management, and hydrophobic performance, with potential applications in future wearable electronics.

Deep-Neural-Network-Based Receiver Design for Downlink Non-Orthogonal Multiple-Access Underwater Acoustic Communication

The excavation of the ocean has led to the submersion of numerous autonomous vehicles and sensors. Hence, there is a growing need for multi-user underwater acoustic communication. On the other hand, due to the limited bandwidth of the underwater acoustic channel, downlink non-orthogonal multiple access (NOMA) is one of the fundamental pieces of technology for solving the problem of limited bandwidth, and it is expected to be beneficial for many modern wireless underwater acoustic applications. NOMA downlink underwater acoustic communication (UWA) is accomplished by broadcasting data symbols from a source station to several users, which uses superimposed coding with variable power levels to enable detection through successive interference cancellation (SIC) receivers. Nevertheless, comprehensive information of the channel condition and channel state information (CSI) are both essential for SIC receivers, but they can be difficult to obtain, particularly in an underwater environment. To address this critical issue, this research proposes downlink underwater acoustic communication using a deep neural network utilizing a 1D convolution neural network (CNN). Two cases are considered for the proposed system in the first case: in the first case, two users with different power levels and distances from the transmitter employ BPSK and QPSK modulations to support multi-user communication, while, in the second case, three users employ BPSK modulation. Users far from the base station receive the most power. The base station uses superimposed coding. The BELLHOP ray-tracing algorithm is utilized to generate the training dataset with user depth and range modifications. For training the model, a composite signal passes through the samples of the UWA channel and is fed to the model along with labels. The DNN receiver learns the characteristic of the UWA channel and does not depend on CSI. The testing CIR is used to evaluate the trained model. The results are compared to the traditional SIC receiver. The DNN-based DL NOMA underwater acoustic receiver outperformed the SIC receiver in terms of BER in simulation results for all the modulation orders.

Anti-swelling and adhesive γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogels for underwater wearable sensors

Unnecessary swelling and lack of interfacial adhesion in underwater environments will lead to distortion of sensing signal based on the conductive hydrogel. However, achieving the anti-swelling and underwater adhesive capabilities of conductive hydrogels is still challenging. Based on the strategies of the Hofmeister effect and catechol chemistry, polyglutamic acid (γ-PGA) and polyvinyl alcohol (PVA) were used as the hydrogel matrix, and poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) and tannic acid (TA) were performed as a conductive filler and modifier, respectively. The γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel was successfully prepared by freezing-crystallization (FC γ-PGA/PVA/PEDOT:PSS/TA). The resultant composite hydrogel reveals high anti-swelling and underwater adhesive properties, the equilibrium water absorption rate is 13.0 ± 0.9 %, and the underwater adhesive strength to human skin is up to 11.2 kPa. In addition, the composite hydrogel shows high tensile mechanical strength (0.9 ± 0.1 MPa) at break and tensile breaking elongation (1495.5 ± 15.5 %), and its electrical conductivity is 0.94 ± 0.05 S·m−1. Most importantly, the FC γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel can maintain long-term underwater mechanical and sensing stability. Underwater strain sensors of the composite hydrogel can be employed for motion signal detection (breathing, drinking, finger bending) and underwater communication (Morse code). The research provides ideas for designing conductive hydrogel sensors with anti-swelling and underwater adhesion, which can be utilized for long-term underwater applications.

An Improved SIFT Underwater Image Stitching Method

Underwater image stitching is a technique employed to seamlessly merge images with overlapping regions, creating a coherent underwater panorama. In recent years, extensive research efforts have been devoted to advancing image stitching methodologies for both terrestrial and underwater applications. However, existing image stitching methods, which do not utilize detector information, heavily rely on matching feature pairs and tend to underperform in situations where underwater images contain regions with blurred feature textures. To address this challenge, we present an improved scale-invariant feature transform (SIFT) underwater image stitching method. This method enables the stitching of underwater images with arbitrarily acquired images featuring blurred feature contours and that do not require any detector information. Specifically, we perform a coarse feature extraction between the reference and training images, and then we acquire the target image and perform an accurate feature extraction between the reference and target images. In the final stage, we propose an improved fade-in and fade-out fusion method to obtain a panoramic underwater image. The experimental results show that our proposed method demonstrates enhanced robustness, particularly in scenarios where detecting feature points is challenging, when compared to traditional SIFT methods. Additionally, our method achieves higher matching accuracy and produces higher-quality results in the stitching of underwater images.

Investigating the effects of Coating systems on the degradation behavior of 3D-Printed pressure vessels

This study investigates the long-term performance of 3D-printed pressure vessels subjected to accelerated aging through water mass infusion. Thermoplastic composites were used to construct the pressure vessels using Material extrusion 3D printing, and subsequent submersion in an ocean water solution at elevated temperatures was employed to simulate accelerated aging. The effectiveness of various coating systems in minimizing the long-term degradation of these 3D-printed pressure vessels was also examined. Implosion experiments were conducted on both aged specimens with and without coating systems using a high-pressure tank facility. The results demonstrate that the application of polyurethane coating systems significantly slows the degradation process of 3D-printed vessels, with aged specimens exhibiting a reduced critical operational depth by 10.3 % compared to those without coatings. The pressure differential between unweathered specimens with and without coatings was 3.61 %, increasing to 23.3 % for weathered specimens. Coatings proved effective in reducing mass absorption and increasing critical collapse pressure. The analytical collapse pressure prediction closely aligns with experimental results, enabling performance generalization for other coating systems. This research contributes insights into the use of coating systems to enhance the longevity and performance of 3D-printed pressure vessels, highlighting their potential for mitigating degradation and improving structural integrity in long-term underwater applications.

A Novel Hybridized Cluster-Based Geographical Opportunistic Routing Protocol for Effective Data Routing in Underwater Wireless Sensor Networks

Underwater wireless sensor nodes comprise hundreds to thousands of battery-operated sensor nodes with limited bandwidth. These networks are employed to transmit the data with enhanced quality of service (QoS). However, efficient data routing is the most challenging obstacle in many underwater applications. To solve the issues in underwater sensor nodes, the hybridized cluster-based geographical opportunistic routing protocol with distance vector establishment has been proposed to transmit the data efficiently. Primarily, the proposed methodology finds out the shortest path with minimal hop count whereas the void node can be updated with infinite hop count. Thereafter, the sleep/wake scheduling and waiting mechanism and periodic beaconing algorithm are incorporated into the proposed model to attain a higher packet delivery ratio with minimal energy consumption. This proper scheduling and optimal cluster routing enhance the continuous data transmission in underwater applications. The simulation result reveals that the proposed method achieves better energy efficiency and higher network lifetime when compared with the existing clustering methods.

Underwater Single-Photon Lidar Equipped with High-Sampling-Rate Multi-Channel Data Acquisition System

Lidar has emerged as an important technology for the high-precision three-dimensional remote sensing of the ocean. While oceanic lidar has been widely deployed on various platforms, its underwater deployment is relatively limited, despite its significance in deep-sea exploration and obstacle avoidance for underwater platforms. Underwater lidar systems must meet stringent requirements for high performance, miniaturization, and high integration. Single-photon lidar, by elevating the detection sensitivity to the single-photon level, enables high-performance detection under the condition of a low-pulse-energy laser and a small-aperture telescope, making it a stronger candidate for underwater lidar applications. However, this imposes demanding requirements for the data acquisition system utilized in single-photon lidar systems. In this work, a self-developed multi-channel acquisition system (MCAS) with a high-resolution and real-time histogram statistics capability was developed. By utilizing field-programmable gate array (FPGA) technology, a method that combines coarse counters with multi-phase clock interpolation achieved an impressive resolution of 0.5 ns and enabled a time of flight duration of 1.5 μs. To address counting instability, a dual-counter structure was adopted in the coarse counter, and real-time histogram statistics were achieved in the data acquisition system through a state machine. Furthermore, the non-uniform phase shift of the clock was analyzed, and a correction algorithm based on code density statistics was proposed to mitigate the periodic modulation of the backscattered signal, with the effectiveness of the algorithm demonstrated through experimental results. The robustness and stability of the MCAS were validated through an underwater experiment. Ultimately, the development of this compact acquisition system enables the implementation of underwater single-photon lidar systems, which will play a crucial role in underwater target imaging, obstacle avoidance in underwater platforms, and deep-sea marine environment monitoring.

Hydrophobic deep eutectic solvent-based eutectogels for underwater sensing

Eutectogels derived from deep eutectic solvents (DESs) exhibit great potential for the fabrication of flexible sensors. However, the hygroscopicity of eutectogels hinders their applications in underwater sensing. In this work, a hydrophobic eutectogel with exceptional long-term underwater stability is produced through one-step polymerization of lauryl methacrylate and glycidyl methacrylate in a hydrophobic DES. The hydrophobic gel network and hydrophobic DES fulfill the eutectogel with outstanding water resistance (water contact angle > 110°) and excellent mechanical properties in an aqueous environment, thus leading to extraordinary durability (over 1000 testing cycles). Additionally, based on this eutectogel, underwater strain and pressure sensors with high sensitivity, rapid responsiveness, and superior durability were fabricated for accurate real-time monitoring of human activity. Furthermore, it has been demonstrated that the eutectogel sensor can transmit information through Morse code, performing as a wearable underwater communicator. This research provides an exemplary way for a demonstration method of hydrophobic eutectogel for durable underwater applications.

Performance Evaluation of Underwater (UW) Welding System with Friction Stir Welding (FSW) Method on 316L Stainless Steel Exposed to Sea Water

Friction Stir Welding (FSW) is a solid-state welding technique that can produce high-quality joints without defects that commonly occur in liquid welding. However, FSW has limitations in underwater applications due to the friction between the tool and the material that causes a decrease in heat efficiency and shear force. To overcome this problem, this study proposes an underwater welding (UW) system with the FSW method on 316L stainless steel exposed to seawater. This UW-FSW system uses a high-speed rotating tool surrounded by an inert gas shroud to prevent direct contact between the tool and seawater. The aim of this study is to evaluate the performance of this UW-FSW system in terms of mechanical properties, microstructure, and corrosion resistance of 316L stainless steel joints. The results of this study show that the UW-FSW system can produce joints with better tensile strength, hardness, and corrosion resistance than conventional FSW. This is due to the rapid cooling, reduction of oxidation, and formation of a thicker passive layer on the joint surface. This study provides a contribution to the development of environmentally friendly and efficient underwater welding technology.

A new artificial lateral line attitude perception method based on mixed activation function-multilayer perceptron (MAF-MLP)

Attitude perception is significant for state recognition and navigation of underwater applications. However, few studies have combined pressure sensors with gyroscopes for attitude sensing alone, and even fewer have addressed how to obtain the pitch angle, roll angle and travel speed at the same time. To make up for these shortcomings, an artificial lateral line attitude perception method based on mixed activation function-multilayer perceptron (MAF-MLP) model was proposed to recognize various attitudes of underwater vehicles. The artificial lateral line carrier model is created based on the lateral line structure of fish, and its surface pressure distribution characteristics are obtained by simulation analysis. The artificial lateral line (ALL) system and the data collection board equipped with a gyroscope module are used to conduct underwater experiments, and the gyroscope is used to collect the carrier attitude information in real-time to improve model prediction accuracy. Finally, the experimental data agree with the theoretical analysis results, and the MAF-MLP model is used as the data source to predict the carrier attitude. The results show that the MAF-MLP model can accurately predict the pitch angle, roll angle and travel speed, which provides a theoretical basis for realizing the state identification and navigation of the underwater vehicle.

Hybrid Composites for the Design and Development of Pressure Vessel for Underwater Applications

The study’s main objective is to design and develop pressure vessels in underwater applications using Hybrid composites-Fibre Metal Laminates (FML) so that the weight will be reduced. The proposed pressure vesselaccommodates electronics in the underwater ambient noise measurement system under an external hydrostatic pressure of 1 MPa (10 bar). The research study aims initially to design and develop a pressure vessel with stainless steel 316 L and subsequently design a pressure vessel with hybrid composites with a combination of composite materials of E-glass and carbon/epoxy materials with a metal alloy stainless steel 316 L. The pressure vessel has been optimised with varying metal and composite percentage combinations. The cylinder’s wall thickness has been pivotal in optimizing pressure vessel design. Classical Laminate Theory (CLT) transforms the FML pressure vessel or cylinder into a rectangular plate. As preliminary measures, FML specimen with a size of 0.45 m square laminate and 0°orientation has been developed with 50 % metal layer and 50 % fibre composites, and corresponding mechanical tests have been carried out as per the standards. The tensile strength of the developed FML is 420 MPa compared to base metal (SS316 L) strength of 556 MPa, and similarly, Flexural and Impact properties have shown a higher level when compared to other types of FMLs.

Factors Governing the Failure of Subsea Critical Connector Bolts

Impressed current cathodic protection (ICCP) systems are commonly used to shield offshore drilling rigs, pipelines, and subsea equipment in the oil and gas industry. In underwater service conditions, water temperature, salinity and velocity play a major role in the longevity of subsea applications. Interactions between the preceding factors can induce catastrophic failure to critical systems while the underlying cause is unclear. This paper proposes an approach for elucidating the corrosion process accompanying underwater applications. The service conditions of underwater application are simulated in a multidisciplinary system that records various parameters such as water temperature, reference-electrode potential, and electric current at five-minute intervals during the 21 d of the experiment. This novel, experimental, and inexpensive ICCP system was developed on an “Arduino” microcontroller and applied to an actual ASTM A193 B7 bolt tightened on an ASTM A105 flange at different torque levels. Experimental results indicate a direct relationship between the water day-night temperature profile and the cathodic protection performance. Specifically, the ICCP performance declines with increasing temperature. When the ICCP system was activated, gas bubbles are generated on the metal surface. Presumably these bubbles could induce hydrogen embrittlement cracks which were observed in scanning electron microscope images of the bolt cross-sections.

Design, Implementation, and Evaluation of an External Pose-Tracking System for Underwater Cameras

To advance underwater computer vision and robotics from lab environments and clear water scenarios to the deep dark ocean or murky coastal waters, representative benchmarks and realistic datasets with ground truth information are required. In particular, determining the camera pose is essential for many underwater robotic or photogrammetric applications and known ground truth is mandatory to evaluate the performance of, e.g., simultaneous localization and mapping approaches in such extreme environments. This paper presents the conception, calibration, and implementation of an external reference system for determining the underwater camera pose in real time. The approach, based on an HTC Vive tracking system in air, calculates the underwater camera pose by fusing the poses of two controllers tracked above the water surface of a tank. It is shown that the mean deviation of this approach to an optical marker-based reference in air is less than 3 mm and 0.3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}. Finally, the usability of the system for underwater applications is demonstrated.