Nowadays, most fishing vessels are equipped with high-resolution commercial echo sounders. However, many instruments cannot be calibrated and missing data occur frequently. These problems impede the collection of acoustic data by commercial fishing vessels, which are necessary for species classification and stock assessment. In this study, an automatic detection and classification model for echo traces of the Pacific saury (Cololabis saira) was trained based on the algorithm YOLO v5m. The in situ measurement value of the Pacific saury was measured using single fish echo trace. Rapid calibration of the commercial echo sounder was achieved based on the living fish calibration method. According to the results, the maximum precision, recall, and average precision values of the trained model were 0.79, 0.68, and 0.71, respectively. The maximum F1 score of the model was 0.66 at a confidence level of 0.454. The living fish calibration offset values obtained at two sites in the field were 116.30 dB and 118.19 dB. The sphere calibration offset value obtained in the laboratory using the standard sphere method was 117.65 dB. The differences between in situ and laboratory calibrations were 1.35 dB and 0.54 dB, both of which were within the normal range.

In the field of signal processing such as system identification, the affine projection algorithm (APA) is extensively implemented. However, running such algorithms in a non-Gaussian scenario may degrade its performance, since the second-order moment cannot extract all information from the signal. To prevent performance degradation of the algorithm in system identification tasks, we propose a novel APA based on least mean fourth (LMF) algorithm. The new algorithm, namely affine projection least mean fourth algorithm (APLMFA) is based on the high-order error power (HOEP) criterion and as such, can achieve improved performance. We also provide a convergence analysis for APLMFA. Numerical simulation results verify the presented APLMFA achieves smaller steady-state error as compared with the state-of-the-art algorithms.

This paper described the architecture and working mechanism of the content centric network, and proposed the idea of introducing it into the sea-sky information network. The corresponding simulation model was built by taking the ship based wireless video monitoring system as the research target. The relationship among the frequency of the user request, the number of different content types and the node cache capacity on the system performance was summarized through a large number of experiments. Then the system performance under given parameters was evaluated to verify the feasibility and advantages for the application of the content centric network.

Presently, most of the long-reach underwater wireless optical communication (UWOC) systems employ laser diodes rather than light-emitting diodes (LEDs) as the transmitters. Due to lasers' smaller divergence angles, those UWOC systems could realize longer communication links. Although LED-based long-reach UWOC has distinguished advantages, it suffers from very low received optical power. Fortunately, highly sensitive multipixel photon counter (MPPC) and energy efficient pulse position modulation (PPM) pave a way towards LED-based long-reach UWOC. In this paper, we investigated the underlying working principle of an MPPC and proposed a UWOC system based on single LED as well as a lens-free MPPC with digital output. The relationship between the MPPC p.e. threshold and its received optical power was theoretically studied and further proved by experimental results. Single 2.3-MHz, 3-W blue LED was used as the transmitter to generate 8-PPM to 64-PPM signals with a 5-MHz slot frequency. After a 46-m underwater transmission, the measured BERs were all below the forward error correction (FEC) limit, which were achieved with less than 100 incident photons during each pulse slot.

A underwater wireless optical communication (UWOC) system using a multi-pixel photon counter (MPPC) as the receiver and orthogonal frequency division multiplexing (OFDM) was proposed and experimentally investigated. The MPPC with high sensitivity combined with quadrature amplitude modulation (QAM) OFDM with high spectral efficiency can potentially support a data rate of hundreds of Mbps under a relatively long underwater transmission distance. Although the photoelectric response of each individual pixel in the MPPC is nonlinear due to its intrinsic dead time, the MPPC consisting of thousands of pixels can be operated like a linear photodetector due to the statistical effect. A net data rate of 312.03 Mbps, with a bit error rate (BER) below the forward error correction (FEC) limit, was successfully achieved over a 21-m underwater channel exploiting 32-QAM OFDM modulation.

The adaptive algorithms have been widely studied in Gaussian environment. However, the impulsive noise and other non-Gaussian noise may largely deteriorate the performance of algorithm in practical applications. To address this problem, in this paper, we propose two novel adaptive algorithms for system identification problem with mixed noise scenarios. Both proposed algorithms are based on the framework of the affine projection (AP) algorithm. The first proposed algorithm, termed as VS-APMCCA, combines variable step-size (VS) strategy and maximum correntropy criterion (MCC) to obtain improved performance. For further performance improvement, the VC-VS-APMCCA is developed, which is based on the variable center (VC) scheme of MCC. The convergence analysis of the VC-VS-APMCCA is conduced. Finally, simulation results demonstrate the superior performance of the VS-APMCCA and VC-VS-APMCCA.

An efficient target tracking algorithm based on an imaging sonar was proposed to solve the problem of underwater multi-target tracking. The echo signal model based on signal intensity was established for each pixel point in the acoustic image according to the imaging features of the sonar in order to extract the individual target from the images. The sequential Monte Carlo probability hypothesis density (SMCPHD) filtering was applied to the target states. The Auction track recognition algorithm was used to associate the filtered target states with the identified tracks, so that the multi-target tracking was realized. The simulation analysis of the algorithm showed that the proposed method was more efficient than the multi-target tracking algorithms based on data correlation, eg. joint probabilistic data association (JPDA) and multiple hypothesis tracking (MHT). A field experiment was conducted to collect the sonar data. The tracking trajectories of all the targets were obtained after the target extraction and tracking.

为准确估计整片水域中的鱼群数量,提出一种利用成像声呐进行数量估计的方法.将成像声呐固定在调查船下,并使波束发射方向与船前进方向一致,通过走航调查方式采集水下信息,对采集的数据进行声呐图像构建、噪声去除、目标提取,其中噪声去除采用固定数据窗口的迭代最小二乘法,目标提取采用基于三倍标准差准则的阈值分割法.接着利用扩展卡尔曼滤波结合最近邻的多目标跟踪算法对图像中的个体目标进行一一计数,同时统计声呐扫描过的水域面积,获得目标个数的平均面密度值,最后结合水域占地面积,估算出整片水域中的鱼群数量.利用该方法实现对滴水湖鱼群数量的估计,通过与人工计数结果比较,发现基于声呐图像处理的数量统计方法具有较高精度,两者的统计值相差约10％.

We investigate and experimentally demonstrate the impact of continuous wave (CW) injection from a malicious ONU on the upstream transmission in a power splitting-based PON. Both OOK signal as used in a TDM-PON and OFDM signal in an OFDM-PON are considered. In our experiment, 10-Gb/s NRZ-PRBS and OFDM signals were used to test the effect of CW injection in TDM-PON and OFDM-PON, respectively. A bit error rate floor is observed for the upstream OOK signal when the signal-to-crosstalk ratio is reduced to 7.7 dB. Similarly, the CW injection from a malicious ONU also impose severe power penalty to the upstream OFDM signal. To overcome this security issue, we propose a novel protection scheme via harvesting the optical energy at the remote node (RN) that is originally wasted in a common power splitting-based PON. The proposed protection scheme can maintain the passive nature of the RN.

This paper introduces a methodology applying an imaging sonar for three-dimensional (3D) target tracking underwater. The key process in this work involves obtaining the target’s position in space using two images of the same scene, acquired by an adaptive resolution imaging sonar (ARIS) at different positions. A data association algorithm was designed to connect the same target in image sequences. The goal of this work was to track multiple targets in 3D space. The ARIS provides sequences of bi-dimensional images from the backscattered energy according to the range and azimuth. The challenge involved determining the missing elevation information for the observed object within the sonar detection range. By computing the geometrical transformation between the acquisition planar images and the cubical space, using only the sonar information that included the posture and moving speed of the ARIS, the target’s elevation information was obtained. To evaluate the performance of the proposed method, an indoor experiment was conducted using the ARIS. On the basis of the experimental results, we confirmed that the proposed method effectively obtained the target’s position in 3D space. A moving target simulation was also conducted, and the results showed that this method was effective for moving targets. Finally, a field experiment was performed to obtain the vertical distribution and track the 3D trajectories of fish.

Seafloor deformation or displacement in methane hydrate production areas is a significant environmental problem that can cause considerable damage. This paper describes a conceptual design for monitoring seafloor deformation that differs from existing monitoring methods that include acoustic and pressure sensors, which have high costs and are only suitable for detecting vertical deformation. The proposed monitoring system is vertically mounted in each monitoring well to detect the displacement of submarine soil layers at different depths and to provide visual feedback by displaying three-dimensional images in real time. To reduce the drift error and to obtain more reliable angle estimates, a Kalman filter algorithm is used to combine the data that are measured using a gyroscope and a digital compass. The stability and accuracy of the system are tested both in real-time test and off-line experiment, and the total errors of the azimuth and tilt angle are less than 0.03° and 0.19° in the static off-line experiment, respectively.

In this work, we propose an underwater wireless optical communication (UWOC) system using an arrayed transmitter/receiver and optical superimposition-based pulse amplitude modulation with 4 levels (PAM-4). At the transmitter side, we design a spatial summing scheme using a light emitting diode (LED) array, which is divided into two groups in a uniformly interleaved manner. With on-off keying (OOK) modulation for each group, optical superimposition-based PAM-4 can be realized. It has enhanced tolerance to the modulation nonlinearities of LEDs. We numerically investigate the feasibility of the proposed spatial summing scheme in various underwater channels via Monte Carlo simulation. With the increase of divergence angle of LEDs and link distance, the optical power distribution tends to be more uniform at the reception plane. It can significantly relax the requirement on the link alignment. Furthermore, we conduct a proof-of-concept experiment employing two blue LEDs. A multi-pixel photon counter (MPPC), containing an array of single-photon avalanche diodes (SPADs), is used as the detector. It has a much higher sensitivity and can further relax the requirement for pointing. Over a 2-m tap water channel, data rates of 6.144 Mb/s, 8.192 Mb/s, and 12.288 Mb/s were achieved by using the PAM-4 signal generated by optical superimposition, within a 2.5-MHz system bandwidth. With 0.570-mg/L Mg(OH)2, the measured optical power is just 12.890 µW after a 2-m underwater channel. The corresponding bit error rate (BER) of the 12.288-Mbs PAM-4 signal is 2.9 × 10-3, which is still below the forward error correction (FEC) limit of 3.8 × 10-3. It implies that the UWOC system based on the high-sensitivity MPPC with array structure has superior power efficiency and robustness.

In this paper, we first propose that self-powered solar panels featuring large receiving area and lens-free operation have great application prospect in underwater vehicles or underwater wireless sensor networks (UWSNs) for data collection. It is envisioned to solve the problem of link alignment. The low-cost solar panel used in the experiment has a large receiving area of 5 cm2 and a receiving angle of 20°. Over a 1-m air channel, a 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal at a data rate of 20.02 Mb/s is successfully transmitted within the receiving angle of 20°. Over a 7-m tap water channel, we achieve data rates of 20.02 Mb/s using 16-QAM, 18.80 Mb/s using 32-QAM and 22.56 Mb/s using 64-QAM, respectively. By adding different quantities of Mg(OH)2 powders into the water, the impact of water turbidity on the solar panel-based underwater wireless optical communication (UWOC) is also investigated.

In order to obtain the three-dimensional (3D) distribution of fish in a marine ranching, a method using imaging sonar to calculate the 3D coordinates of fish is proposed in this paper. The imaging sonar used in this re-search is a Dual-frequency Identification sonar (DIDSON). It is a multi-beam sonar that uses acoustic lens to form individual beams. It constructs a high-definition two-dimensional image for target detection by transmitting ultra-sonic beams underwater and receiving the echo signals. It sets a new standard for excellence in underwater vision in black and turbid waters due to obtaining near-video quality dynamic images for the identification of objects un-derwater. In addition, split-beam echo-sounders have also been used on some occasions to monitor the movements of fishes. Despite the improvements achieved in fish monitoring techniques, the interpretation and classification of the data collected by the traditional acoustic techniques are often challenging and require extensive experience and effort. The DIDSON bridges the gap between existing fisheries-assessment sonar and optical systems. In a DID-SON, 96 transducer elements constitute a linear array and each element both transmits and receives acoustic beams such that echo amplitude is determined by the intensity of the reflected signal. It can obtain the distance and azi-muth of the target from sonar images, but it is unable to acquire the elevation of the target from the images. To overcome this difficulty and obtain the fish distribution, a new method is proposed. Firstly, the sonar is fixed on the outside of the ship′s rail and submerged in the water to collect fish′s information through the investigation on navigation. At the same time, the beam emission direction is on the same plane with the sonar′s moving direction. After data collection, image processing is conducted, including image construction, background elimination and target extraction from horizontal field-of-view. Target association based on Interacting Multiple Model Joint Prob-abilistic Data Association Filtering (IMMJPDAF) is carried out to deal with the extracted targets, thus the rela-tions of one target in different frame images can be obtained. The 3D target coordinates are acquired according to the spatial geometric relationship between the positions in two consecutive frame images. Finally, multiple target trajectories in 3D space and the depth distribution of targets are obtained through the correlation algorithm. The ex-periment was carried out in Dishui Lake which is located in Shanghai. Experimental results showed that the pro-posed method can effectively acquire the fish movement tracks in 3D space underwater and the distributions in depth. It also showed that most fish swam in the depth 3-5 meters. It will help to analyze the fish behavior and provide technical support for fishery resource assessment in a marine ranching.

This paper proposes an underwater visual simultaneous localization and mapping system using a stereo camera. In front-end, the image data is used to execute feature detection and matching. And the vehicle motion is estimated by the feature matching result. In back-end, a global optimization method represented by a graph, is used to eliminate the errors. The mapping result is in the from 3-D point cloud, which is established by the information from color image and depth image.

The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10-3, 2.0 × 10-3 and 2.3 × 10-3, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10-3.

In view of the shortage of the means for water environment monitoring,a monitoring method using four-rotor drones is proposed.The drone can fly above the water to obtain aerial videos,and also hover and ski over the water with the camera and monitoring sensors submerged underwater for real-time monitoring.An attitude controller based on robust compensation and a position controller based on PID method are designed.With the constructed prototype drone,hovering and water skiing tests were conducted.A waterproof camera and other sensors combined with wireless modules were used to realize real-time monitoring of the underwater environment.

This paper describes a method to reconstruct the three-dimensional aspects of underwater objects using side-scan sonar images. The sonar images are segmented into three kinds of regions: echo, shadow and background. A two-dimensional intensity map is estimated from the echoes and a two-dimensional depth map is computed from the shadow information. With the transformation model, the final reconstruction merges these two maps to generate three-dimensional point cloud images of underwater objects.

We experimentally demonstrate a high-speed air-water optical wireless communication system with both downlink and uplink transmission employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) and a single-mode pigtailed green-light laser diode (LD). This work is an important step towards the future study on optical wireless communications between underwater platforms and airborne terminals. Over a 5-m air channel and a 21-m water channel, we achieve a 5.3-Gbps transmission without power loading (PL) and a 5.5-Gbps transmission with PL in the downlink. The corresponding bit error rates (BERs) are 2.64×10-3 and 2.47×10-3, respectively, which are below the forward error correction (FEC) criterion. A data rate of 5.5 Gbps with PL at a BER of 2.92×10-3 is also achieved in the uplink.