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Abstract
The development of multiple autonomous underwater vehicles (AUVs) has revolutionized the traditional reliance on a single, costly AUV for conducting underwater surveys. This shift has garnered increasing interest among marine researchers. Communication between AUV fleets is an urgent concern due to the data rate limitation of underwater acoustic communication. Laser-based underwater wireless optical communication (UWOC) is a potential solution once the link-establishing requirement between AUVs can be met. Due to the limited coverage area of the laser beam, the previous pointing, acquisition, and tracking (PAT) method is to quickly adjust the beam direction and search for the target according to the set scanning path. In response to these challenges, we propose a scalable laser-based link establishment method that combines the maneuvering of the AUV, the acoustic positioning, and the control of the optical system. Our proposed approach has consistently outperformed the existing PAT method in simulated environments, effectively establishing laser links. Importantly, we have successfully implemented our approach in machine experiments, confirming its practical applicability.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
The investigation of the marine environment is almost inseparable from autonomous underwater vehicles (AUVs). As a desirable mobile platform, AUV can flexibly cruise to the target area and complete a variety of underwater tasks, such as seafloor mapping [1], environmental monitoring [2], and coral conservation [3]. A joint survey approach based on fleets of AUVs could change how surveys rely on a single, large, and expensive underwater vehicle to cover an entire area [4]. However, communication limitations in the fleet prevent real-time or near-real-time data sharing between AUVs. Compared with acoustic communication, which dominates underwater scenarios, emerging underwater wireless optical communication (UWOC) technology offers a larger bandwidth and higher data rates [5]. With the progress of UWOC technology, optical communication systems have emerged as a promising solution for facilitating information sharing among AUV fleets [6–8].
Considering how UWOC technology can be applied to AUV fleets in underwater exploration is attractive. The limited propagation distance and directivity of UWOC are the main challenges in establishing line-of-sight (LOS) links [9]. To relax the strict link-establishing requirements of UWOC systems, LED-based transmitters capable of emitting optical beams with broader coverage are used in underwater scenarios [10]. Laser-based UWOC systems have the advantages of long transmission distances, high data rates, and good power efficiency due to their ability to be focused into the directive beam [11,12]. Due to the limited effective range of the above two methods, researchers typically use coordinated control algorithms to ensure beam alignment by keeping the AUVs close in parallel and moving at a standard speed [13,14]. Acoustic communication still dominates in multi-AUV cooperation, as the risk of collision increases if the AUVs are in close proximity [15,16].
AUVs with laser communication in the fleet can maintain a wider relative distance to perform survey tasks and exchange data. In 2020, Chen et al. reported a 2-Mbps laser-based transmission over a distance of 117 m [17]. Due to the limited coverage area of the laser beam, a pointing, acquisition, and tracking (PAT) system is usually used along with scanning algorithms to search and establish the LOS link [18,19]. Hardy et al. demonstrated a PAT system with a spiral acquisition pattern in the pool [20]. Yang et al. proposed a sector scanning method with multiple laser-based transmitters to reduce the acquisition time [21]. In addition, the following issues should be considered or optimized when establishing the LOS link in a real environment: (1) how the AUVs in the fleet identify the initial position for tracking; (2) how to observe the pointing error caused by environmental disturbance and platform uncertainty; (3) how to optimize scanning efficiency during link establishment.
To tackle the challenges mentioned above, we consider combining the maneuvering of the AUV, the acoustic positioning, and the control of the optical system during the establishment of the laser link. A scalable laser communication mechanism suitable for AUV fleets is proposed in this study. Acoustic positioning technology is used to measure and share the state of AUVs to guide link establishment. Based on the estimation results, AUVs need to reduce the relative depth and maintain the distance and orientation available for point-to-point communication. A PAT system quickly scans the target with a one-dimensional acquisition pattern to establish the LOS link. An acoustic channel is used to provide link-quality feedback and share acknowledgment information. To validate our approach, we developed a simulation environment for comparative analysis against existing methods. In the simulated environment, our approach consistently outperformed the existing PAT approach. Furthermore, the results from actual machine experiments confirm the practical applicability of our proposed method, demonstrating its readiness for deployment in practical scenarios.
The rest of this article is organized as follows. The underwater optical communication process between AUV fleets is modeled in Section 2. The method for establishing communication links is presented in Section 3. Simulation verification is given in Section 4. Detailed experiments are conducted and discussed in Section 5. The conclusions are given in Section 6.
2. PROBLEM FORMULATION
A. Data Sharing between AUV Fleets
In the joint investigation, AUV fleet operations are suggested to overcome the shortcoming of insufficient sampling coverage of a single AUV [22]. As shown in Fig. 1, the AUV in the fleet explores its own survey area independently. Our proposed laser-based communication solution allows AUVs to share the collected data in time through high-speed optical links. The AUV that transmits the laser beam is regarded as the transmitting AUV, represented by the blue-yellow vehicle in the figure. The AUV that receives optical signals is defined as the receiving AUV and represented by red-yellow color. Two-way communication will be established between the transmitting and receiving AUVs, using optical communication from the transmitting AUV to the receiving AUV for sharing large amounts of survey data and acoustic communication from the receiving AUV to the transmitting AUV for transmitting parameters such as position, orientation, and signal status. Once the data are shared, the receiving AUV can leverage information perceived from the whole survey area to look ahead and plan subsequent actions for all AUVs [23]. Additionally, the receiving AUV can surface to upload the collected data onshore.
Fig. 1. Underwater joint investigation mission performed by AUV fleets. Each AUV is assigned a survey area for exploring and data sampling. AUVs can share the collected data through high-speed optical links, which are represented by blue dashed lines.
We propose a laser-based UWOC strategy to establish LOS links between AUV fleets. Transmitting AUVs are equipped with directional emitters to emit laser beams. The receiving AUV is usually fitted with an omnidirectional detector, which can simultaneously receive optical signals from different AUVs using a multiple-input multiple-output orthogonal frequency division multiplexing technology [24].
In our method, all AUVs can receive optical signals from multiple AUVs simultaneously, and each AUV can transmit the laser beam independently without being affected by the state of other AUVs. We introduce our link establishment method using one transmitting AUV and one receiving AUV. As shown in Fig. 2, the link establishment process is mainly divided into three steps.
- (1) In Stage 1, the transmitting AUV cruises to the depth where the receiving AUV is located and remains at the same depth. The depth of the receiving AUV can be shared through acoustic communication. The same depth can reduce the scanning area of the PAT system from a three-dimensional space to a two-dimensional plane.
- (2) In Stage 2, the transmitting AUV identifies the initial position of the receiving AUV and continuously observes the relative position and orientation through acoustic positioning. A scalable acoustic positioning method is used, allowing all the transmitting AUVs to monitor the states simultaneously. The estimated location of the receiving AUV will be the starting point of the scanning process.
- (3) In Stage 3, the scanning device of the transmitting AUV quickly adjusts the pan angle of the laser beam relative to the vehicle’s frame. The light intensity detected by the receiving AUV is fed back to the transmitting AUV through acoustic communication to establish and maintain the stability of the LOS link.
B. Laser Beam
Laser beams have strong directivity, making it challenging to achieve omnidirectional coverage underwater. During the link establishment process, the transmitting AUV needs to emit a laser beam to cover the receiver of the receiving AUV. The propagation of laser beams in seawater is affected by absorption, scattering, and turbulence [25,26].
The beam spread function (BSF) [27] is used here to model the laser beam propagation process between the transmitting AUV and the receiving AUV:
C. Link Establishment
It is necessary to model the motion of AUV platforms and the scanning device that can steer the laser beam. The variables of the transmitting AUV are denoted by the superscript $T$, while the variables of the receiving AUV are denoted by the superscript $R$. The variables related to the scanning device are denoted by the superscript $P$. We present the motion of the AUVs in the vertical and horizontal planes. The roll and pitch motions of the AUVs are ignored since they are statically stable for the hovering-type AUVs. The state of the AUV is defined in the model, including the position in three-dimensional space $[x,y,z]$, yaw orientation $\psi$, surge velocity $u$, sway velocity $v$, heave velocity $w$, and yaw angular velocity $r$.
The vehicle’s movement in the vertical plane is shown in Fig. 3. A black arrow with a dotted line indicates the roll axis of the AUV. The orange sector represents the laser beam emitted from the transmitting AUV, and the beam divergence angle is defined as ${\theta _{\rm{Tx}}}$. The laser beam radius near the receiving AUV is defined as ${r_{{\rm Tx},R}}$. The depth difference between the transmitting AUV and the receiving AUV is defined as ${z^{\rm{TR}}}$, which needs to be reduced in the first stage.
Fig. 3. Motion of AUVs in the vertical plane. The transmitting AUV is marked with blue and yellow, while the receiving AUV is marked with red and yellow. The orange sector represents the laser beam.
The state of the AUVs and the scanning device on the horizontal plane is shown in Fig. 4. In the second stage, the transmitting AUV must utilize acoustic positioning to measure the relative bearing angle ${\alpha ^{\rm{TR}}}$ and estimate the relative distance ${l^{\rm{TR}}}$. A scanning device is mounted on the transmitting AUV to search and track targets. A blue arrow with a dotted line indicates the roll axis of the scanning device. We define the angle between the AUV’s roll axis and the scanning device’s roll axis as the pan angle ${\theta ^{\rm{TP}}}$. The angular pointing error from the scanning device to the receiving AUV is defined as ${\theta ^{\rm{PR}}}$ in the horizontal plane.
Fig. 4. Motion of AUVs in the horizontal plane. The transmitting AUV is marked with blue and yellow, while the receiving AUV is marked with red and yellow. The orange sector represents the laser beam. The blue dotted arrow indicates the direction of the laser beam.
3. METHODOLOGY
A. Depth Control
Current PAT schemes typically scan in three-dimensional space to acquire targets [21]. In our approach, the transmitting AUV will be guided to the same depth as the receiving AUV. Establishing links on the same horizontal plane reduces the scanning range from three-dimensional space to a two-dimensional plane, which improves the efficiency of the scanning process.
All AUVs are equipped with pressure sensors that accurately measure depth based on the standard equations for seawater properties [28]. Typically, quartz crystal pressure sensors can achieve an overall accuracy of about 0.01% of full scale or better, and the overall resolution can reach 0.0001% of full scale [29]. AUV fleets conduct surveys in the same area, so the factors that affect depth measurements, such as seawater properties, tide, and weather, can be considered the same.
The depth value ${z^R}$ measured by the receiving AUV is broadcast via acoustic communication and can be received by any transmitting AUV. To establish and maintain the optical link, the transmitting AUV first needs to reduce the depth difference ${z^{\rm{TR}}}$ by heavy control to maintain the same horizontal plane as the receiving AUV.
The laser beam emitted by the transmitting AUV needs to cover at least part of the receiver of the receiving AUV, which means that in the vertical plane, the depth difference ${z^{\rm{TR}}}$ is required to be less than the radius of the beam ${r_{{\rm Tx},R}}$.
B. Acoustic Positioning
When the transmitting AUV and the receiving AUV are kept on the same plane, the transmitting AUV needs to know the receiving AUV’s position to point to the laser beam. The ultra-short baseline (USBL) device is equipped on each AUV to transmit and receive acoustic signals. A bearing-only acoustic positioning is used in this research, motivated by the following.
- (1) The effective transmission distance of acoustic signals can reach several kilometers.
- (2) Bearing-only is scalable; its one-way transmission mechanism allows it to serve multiple clients simultaneously.
- (3) Frequent updates of relative angles can guide the scanning of the laser beam.
The receiving AUV continuously broadcasts the acoustic signal in the fleet with a period ${T_{{\rm bear}}}$, and the transmitting AUV listens to the signal to observe the state of the receiving AUV. The USBL device on the transmitting AUV can detect the angle of the incident signal and thus measure the relative bearing angle ${\alpha ^{\rm{TR}}}$. The state of the receiving AUV, such as position and orientation, can be shared with the transmitting AUV through broadcast acoustic signals. Based on bearing-only acoustic positioning, the transmitting AUV can also estimate the relative distance ${l^{\rm{TR}}}$ by a state estimator, such as a particle filter.
The particle filter method can estimate the current state based on the measurement results. Each particle is denoted as ${\xi ^i} = \{{s^{\prime i}},{W^i}\}$, where $i = 1,2, \cdots ,{\rm N}$ is the identification, ${s^{\prime i}}$ is the estimated state vector, and ${W^i}$ is the weight. The estimated state vector includes the north and east positions:
The state transition is as follows:
The estimated bearing angle is
Finally, the estimated range between the transmitting and receiving AUVs is
where ${x^T}$, ${y^T}$ are the mean values of the particle filter state estimates.C. Scanning and Tracking
As shown in Fig. 5, a scanning device is used to steer the laser beam relative to the transmitting AUV’s frame of reference. The laser transmitter is mounted on a waterproof servo actuator module that can adjust the pan angle ${\theta ^{\rm{TP}}}$. Note that ${T_d}$ is the dwelling time of the scanning device at each point.
Fig. 5. Scanning device for carrying the laser transmitter. The servo actuator module enables precise and fast adjustment of the pan angle. The black dotted arrow indicates the direction of the AUV, and the blue dotted arrow indicates the direction of the laser beam. The orange dotted line represents the scanning range during the acquisition phase.
The scanning process is shown in Fig. 5. The scanning device sets a range to search for the optimal pointing position, where the receiving AUV can detect the strongest light intensity for optical communication. The scanning algorithm proposed in this research is listed in Algorithm 1. The transmitting AUV can estimate the relative angle ${\alpha ^{\rm{TR}}}$ to the receiving AUV through acoustic positioning and the particle filter estimator. The transmitting AUV determines the half-angle of the scanning range ${\theta _{\rm{st}}}$ and starts scanning from ${\alpha ^{\rm{TR}}} - {\theta _{\rm{st}}}$ to ${\alpha ^{\rm{TR}}} + {\theta _{\rm{st}}}$ along clockwise. The interval angle of scanning is set to ${\theta _{\rm{si}}}$. The laser signals with different pan angles $\theta _i^{\rm{TP}}$ are marked with the scanning index ${i_s}$:
where $\theta _0^{\rm{TP}}$ is the initial pan angle during the scanning process.On the receiving AUV side, the detector records the intensity of the light signal and feeds it back through acoustic communication. The detection ranges ${i_{s,{\rm st}}},{i_{s,{\rm ed}}}$ at which the detector can receive the light signal, and the beam index ${i_{s,{\rm max}}}$ at which the light intensity is strongest, will be shared with the transmitting AUV. Once the transmitting AUV receives the feedback, the pan angle will be adjusted to the optimal position:
The error between the pointing angle $\theta _{\rm{max}}^{\rm{TP}}$ obtained by scanning and the relative angle ${\alpha ^{\rm{TR}}}$ estimated by the acoustic method is recorded for link maintenance:
After the link is established, the scalable acoustic positioning method is still used for tracking. The transmitting AUV continuously listens to the acoustic signal from the receiving AUV to measure relative angle changes due to external environment or platform uncertainty. The scanning device will adjust the pan angle according to the acoustic positioning results:
When the detected light intensity becomes too low, the transmitting AUV starts re-scanning to find the optimal position.
4. SIMULATION IMPLEMENTATION
A. Simulation Environment
A simulation environment is developed to evaluate the proposed scanning method. We define the scanning experiments that can be efficiently repeated by the simulator. The main parameters of the simulation environment are listed in Table 1. After initializing all parameters, the receiving AUV is placed at the origin, and the transmitting AUV is randomly placed at a distance of ${L_{\rm{init}}}$ m from the receiving AUV. The simulator randomly generates the initial yaw angles of the two AUVs. The transmitting and receiving AUVs move with the velocity command $[u,v,r]$ in each time step. The surge, sway, and yaw angular velocities are mixed with Gaussian noises, whose standard deviations are ${\sigma _u}$, ${\sigma _v}$, and ${\sigma _r}$, respectively. The particle filter algorithm is used to estimate the states of the transmitting AUV based on the measurements.
Table 1. Simulation Parameters
The transmitting AUV searches for the receiving AUV by manipulating the scanning device. Once the transmitting AUV has measured the relative angle ${\alpha ^{\rm{TR}}}$ by acoustic positioning, the vehicle can reduce the pointing error by the scanning algorithm defined in Algorithm 1. The transmitting AUV starts scanning from ${\alpha ^{\rm{TR}}} - {\theta _{\rm{st}}}$ to ${\alpha ^{\rm{TR}}} + {\theta _{\rm{st}}}$ along clockwise, and the scan interval is ${\theta _{\rm{si}}}$. The receiving AUV records the intensity of the light signal and feeds it back through acoustic communication. In the simulation, all the parameters at each time step are collected for evaluation.
Algorithm 1. Scanning Algorithm
B. Results and Comparison
To benchmark the performance of our scanning strategy, we compared it with an existing scanning method proposed by Yang et al. [21]. The basic concept of this scanning method is to quickly adjust the laser beam direction and search according to the sector pattern. Once the receiving AUV detects the laser beam, it will use the LED system to send back the acknowledgment signal. This method can employ multiple laser devices for cooperative scanning to improve the scanning efficiency.
The researchers developed a simulation environment for the scanning device to search targets using the proposed sector scanning method. Three-dimensional space is constructed for scanning, with horizontal scanning ranging from 0 to $2\pi$ rad and vertical ranging from 0 to $\pi /2$ rad. The initial relative distance ${L_{\rm{init}}}$ is set to 15 m, and the maximum coverage radius is set to 0.2 m. Time ${T_d}$ and ${T_{\rm{step}}}$ are set to 0.01 s and 0.01 s, respectively. The attenuation coefficient is set to 0.275. The scan interval is set to $\pi /100$, $\pi /50$, and $\pi /10$ rad. The parameter ${P_u}$ denotes the probability of the laser beam covering the target, and the acquisition time ${T_a}$ represents the time spent in the scanning process. The results show that the probability ${P_u}$ and acquisition time ${T_a}$ decrease as the scan interval increases. To fulfill the requirement of acquisition probability ${P_u} \ge 0.6$ and ${T_a} \le 12s$, the researchers propose to choose scan intervals ${\Delta _\theta}$ and ${\Delta _\alpha}$ to be $(\pi /50)$ rad [21].
In order to compare with the sector scanning method, we configure the same parameters in the simulation environment. The interval of acoustic positioning is set to 2 s, which can be realized in practical experiments. The standard deviation of relative angle measurements in bearing-only acoustic positioning is set to 4° for standard sensor condition and 20° for low-specification sensor condition [30]. Similarly, the scan interval ${\theta _{\rm{si}}}$ is set to $\pi /100$, $\pi /50$, and $\pi /10$ rad. The Gaussian noises in motion [${\sigma _u}$, ${\sigma _v}$, ${\sigma _r}$] are set to 0.1 m, 0.1 m, and 1°, respectively.
We tested our approach 100 times each under different scan intervals ${\theta _{\rm{si}}}$ and acoustic measurement conditions ${\sigma _{\rm{ac}}}$. The probability ${P_u}$ and average acquisition time ${T_a}$ under different conditions are listed in Table 2. When the scan interval was $\pi /10$ rad, the scanning device could not effectively search for the target, and the probability ${P_u}$ was 0.41. Since the maximum detection radius ${r_{{\rm Tx},R}}$ was assumed to be 0.2 m, at a link distance of 15 m, the detection radius was smaller than the scan interval. The scanning device needed to repeat the scanning process continuously to catch the target by accident. A large scan interval can save scanning time and improve efficiency when the unknown area is too large. Nevertheless, considering that we utilize acoustic positioning to reduce the scan range, the effect of a large scan interval is not obvious to our approach. When the scan interval was decreased to $\pi /50$ rad, the probability that the scanning device searches for the target reached one. Even in low-specification sensor conditions, the probability can reach 0.91. When the scan interval was further reduced to $\pi /100$ rad, our approach maintained the acquisition probability of one, and the average acquisition time decreased to 2.36 s. Under the low-spec sensor condition, the acquisition probability and time are 1 and 5.85 s, respectively.
Table 2. Simulation Results
Our proposed scanning algorithm further improves the alignment accuracy based on the approximate orientation estimated by acoustic positioning. We define the pointing error as the difference between the actual value of the relative angle ${\alpha ^{\rm{TR}}}$ and the optimized pan angle $\theta _{\rm{max}}^{\rm{TP}}$. The pointing error in different conditions is shown in Fig. 6. We set the standard deviation of relative angle measurements in bearing-only acoustic positioning to 4° for standard sensor conditions and 20° for low-specification sensor conditions. In the simulation experiments, the pointing errors under the listed six conditions are all limited to 0.8°.
Fig. 6. Pointing error in (a) standard sensor condition ${\sigma _{\rm{ac}}} = 4$ and (b) low-specification sensor condition ${\sigma _{\rm{ac}}} = 20$ for different scanning intervals ${\theta _{\rm{si}}}$.
Our approach outperformed the sector scanning method in the simulation environment [21]. The sector scanning method selected a strategy with a scanning interval of $\pi /50$ rad, and its acquisition probability and time were 0.65 and 12.50 s, respectively. Under the same parameter conditions, our approach successfully found the target in all 100 episodes with an average acquisition time of 5.08 s. In addition, the space for scanning is compressed with the help of AUV control and acoustic technology. Using smaller scanning intervals, the acquisition probability and time are instead better.
C. Selected Episodes
Two simulation episodes using the proposed scanning algorithm are presented for discussion. The first episode shown in Fig. 7 used a scanning interval of $\pi /100$ rad, with the standard deviation ${\sigma _{\rm{ac}}}$ set to 4°.
Fig. 7. Scanning process between two AUVs. The standard deviation of relative angle measurements in bearing-only acoustic positioning to 4°. The scanning interval is set to $\pi /100$ rad.
In the beginning, the receiving AUV, represented by the black triangle, was placed at the origin, and the transmitting AUV, represented by the blue triangle, was randomly deployed and kept an initial relative distance of 15 m from the receiving AUV. The sharp corner of the triangle is the head of the vehicle. The particle filter estimation results of the transmitting AUV are depicted by blue dots. The scanning device represented by the yellow triangle was initialized with a pan angle of 0°. The green dotted line indicates the direction of the laser beam during the scanning process, and the orange dotted line represents the scanning range. The parameters listed in the bottom left corner are the time in the experiment, the position of the transmitting AUV (${x^T},{y^T}$), the position of the receiving AUV (${x^R},{y^R}$), the pan angle of the scanning device ${\theta _{\rm{TP}}}$, the real relative angle ${\alpha _{\rm{TR}}}$, and the estimated relative angle ${\hat \alpha _{\rm{TR}}}$.
As shown in Fig. 7(a), the relative angle estimated by the acoustic positioning ${\hat \alpha _{\rm{TR}}}$ was 43.41°, while the actual value ${\alpha _{\rm{TR}}}$ was 35.43°. The scanning device used the estimation as a reference to determine the scanning range. The orange dotted line represents the scanning range during this scanning process, while the green dotted line indicates the direction of the laser beam. The transmitting AUV started clockwise scanning from 31.95°s to 54.86°. After feedback from the receiving AUV through the acoustic channel, the scanning device adjusted the pan angle to 35.53° with a pointing error of 0.12°. The results show that our proposed scanning algorithm further optimizes the estimation results obtained by the acoustic positioning method and improves the alignment accuracy.
Figure 8 shows another episode with standard deviation ${\sigma _{\rm{ac}}}$ set to 20°. As the sensor performance of the acoustic device degrades, the relative angle measurement can be mixed with a larger error. If the receiving AUV is out of scan range due to poor acoustic positioning, the transmitting AUV cannot successfully search for the target. Therefore, the transmitting AUV did not get feedback from the receiving AUV during the first scanning process. The difference between the estimation ${\hat \alpha _{\rm{TR}}}$ and the actual value ${\alpha _{\rm{TR}}}$ was 26.03°. At the 4.01 s of this simulation, the transmitting AUV began to scan for the second time and finally got the feedback to optimize the pan angle. The scanning device adjusted the pan angle to 204.62° with a pointing error of 0.25°. In low-specification sensor conditions, the scanning range ${\theta _{\rm{st}}}$ should be larger to tolerate errors in the acoustic estimation results.
Fig. 8. Scanning process between two AUVs. The standard deviation of relative angle measurements in bearing-only acoustic positioning to 20°. The scanning interval is set to $\pi /100$ rad.
5. EXPERIMENTS
A. Experimental Setup
The transmitting AUV successfully scanned and captured the target in the simulation environment using the proposed method. The feasibility of this scanning method needs to be evaluated in actual machine experiments. According to the proposed strategy for establishing a laser link, we sequentially conduct the following experiments in a real environment: (1) depth control, (2) acoustic positioning, and (3) scanning.
As shown in Fig. 9, the hovering-type AUV Tri-TON is used as the transmitting AUV to carry an optical scanning device in actual machine experiments. This underwater platform can hover in the sea and has a duration of up to 8 h and a maximum depth of 800 m. The weight and maximum speed of the platform are 230 kg and 0.5 m/s. The main structure of Tri-TON is three waterproof pressure hulls; the upper one is used to carry instruments, and the lower two contain batteries. Five thrusters are equipped for surge, sway, yaw, and heavy motion. The SeaTrac X150 is mounted as a USBL device and acoustic modem for underwater acoustic positioning and communication. A Doppler velocity log (DVL) Teledyne RDI Navigator is mounted on the bottom of the platform to measure ground velocity.
Fig. 9. Sea experiments at Hiratsuka, Japan. The yellow platform is the hovering-type AUV Tri-TON, and the red platform is the autonomous surface vehicle BUTTORI.
The autonomous surface vehicle BUTTORI is deployed as the receiving AUV to provide the acoustic positioning for Tri-TON. BUTTORI has a dynamic positioning ability and can deal with environmental disturbances such as waves and wind. For underwater acoustic communication and positioning, the SeaTrac X150 is mounted as the USBL device.
B. Sea Experiments
We deployed Tri-TON and BUTTORI in the sea environment to evaluate the ability of underwater platforms to perform the proposed scanning algorithm. The transmitting AUV needs to stay at the set depth and estimate the direction in which the receiving AUV is.
In sea trials, Tri-TON was required to remain at depths of 1 and 2 m. Depth data from Tri-TON were recorded by a depth sensor Mensor DPT6000 with a sampling frequency of 5 Hz. The depth maintained by Tri-TON during the depth control stage is shown in Fig. 10. At a depth of 1 m, the average depth of Tri-TON is 1.00 m with a standard deviation of 0.02 m. At a depth of 2 m, the average depth of Tri-TON is 2.03 m with a standard deviation of 0.02 m.
Fig. 10. (a) Depth and (b) pitch maintained by Tri-TON and (c) the deviation of the beam center from the receiving AUV in the vertical distance.
Pitch data from Tri-TON were recorded by a gyroscope Tamagawa AU7428N1100 with a sampling frequency of 10 Hz. The pitch angle of the platform is affected by the structure and the buoyancy configuration, and can be kept near 0° with further buoyancy adjustments. Note that the pitch angle of Tri-TON in the experiments was not adjusted to 0°. In the 1-m experiment, the pitch angles have a mean of 2.17° and a standard deviation of 0.46°. In the 2-m experiment, the pitch angles have a mean of 2.18° and a standard deviation of 0.58°. The pitch angle maintained by Tri-TON during the depth control stage is shown in Fig. 10.
The proposed method requires the transmitting and receiving AUVs to be on the same horizontal plane to reduce the space to be scanned. The maintenance of the transmitting AUV at depth and pitch angle can affect whether the laser beam can cover the receiving AUV. We assume two AUVs are 15 m apart and calculate the deviation of the beam center from the receiving AUV in the vertical distance, shown in Fig. 10. Pitch angle shifts due to the buoyancy configuration are not considered. In the 1-m experiment, the deviation values have a mean of 0.00 m and a standard deviation of 0.13 m. In the 2-m experiment, the deviation values have a mean of 0.03 m and a standard deviation of 0.16 m. Experimental results show that this deviation is smaller than the maximum detectable radius. The maximum detectable radius of the 15-m link calculated by Yang et al. is 0.20 m [21].
Fig. 11. Trajectory of the (a) transmitting AUV and (b) receiving AUV in the experiment. The trajectories of both platforms are drawn using the same coordinate system.
In the second stage, the transmitting AUV needs to measure the direction in which the receiving AUV is located through acoustic positioning. The estimated relative angles are provided to the optical system for scanning. We adjusted the relative positions of the two platforms to measure the acoustic positioning performance under different conditions.
The trajectory of the transmitting AUV and the receiving AUV is shown in Fig. 11. The receiving AUV maintained dynamic positioning at six locations, marked in six colors, and the other paths are marked in gray. The corresponding trajectories of the transmitting AUV are plotted in the same color during the period when the receiving AUV maintains dynamic positioning.
The relative angles estimated by acoustic positioning are compared with the Global Navigation Satellite System (GNSS) results. The Pearson correlation coefficient of acoustic positioning data is shown in Fig. 12. The results measured by acoustic techniques are linearly correlated with the results obtained by GNSS. The standard deviations of the relative angle measurements for the six positions are 5.63, 2.56, 4.87, 6.63, 7.10, and 4.88°. The experiment was tested in shallow seawater, where the acoustic channel is affected by multiple paths. The performance of the acoustic measurements is also influenced by the relative distance, as shown in the results of points 4 and 5. Acoustic positioning is designed to quickly determine the direction in which the target is located. If the measurement produces errors within acceptable limits, the pointing accuracy can be improved by scanning in the third stage.
C. Beam Detection
We tested the scanning and detection of the laser beam in a water tank. During the scanning stage, the scanning device with the laser beam needs to adjust its pan angle according to the algorithm. The receiver on the target then feeds back the pan angle at which the maximum optical power can be detected based on the received light signal.
As shown in Fig. 13, the length of the water tank is 1.2 m. On the right side of the water tank, a laser transmitter SN-LDM-T-P-450 was deployed. The beam used in the experiment is a collimated laser beam with a wavelength of around 450 nm. On the left side, an optical receiver was placed to detect the intensity of the light received. The receiver consists of a 2-inch lens focusing the light on a photodetector. The detector used is a silicon-based detector DET36A2 with a ${3.6}\;{\rm mm} \times {3.6}\;{\rm mm}$ active area. The laser was operated at 150 mA (5.56 V) with an optical power of 114.8 mW. The water used for the experiment is tap water. The attenuation in this experiment does not play a big role since all the power values are normalized.
Fig. 13. Experimental setup for beam scanning and detection. The detected optical power at different pointing angles is shown on the left side.
In the beginning, the transmitter and receiver were deployed in alignment. We adjusted the angle of the receiver to test the laser beam at different pointing angles. The detected light intensity at different pointing angles is shown in Fig. 13. The full width ${-}3 \;{\rm dB}$ field of view (FOV) is estimated to be around 5°. Based on the detected light intensity, the receiver can give feedback on the angle of the maximum optical power that can be detected.
In this case, the angles that can be detected depend on the transmitter’s divergence angle, its distance from the receiver, and the receiver’s field of view. As the distance between the transmitting AUV and receiving AUV increases, the range of angles becomes narrower. The beam’s divergence can be increased by moving the lens closer to the laser diode to compensate for that.
D. Discussion
A scalable approach is proposed that consists of three stages to establish the laser link between the AUV fleets: depth control, acoustic positioning, and scanning. All three stages were tested in real experiments, and the results showed that the proposed strategy could be applied to actual machines. This method has no special requirements for the structure of the underwater platform and can, therefore, be implemented on the usual underwater platforms. PAT systems have been widely used in space laser communications but are difficult to utilize underwater. The complexity and variability of the marine environment have an impact on the propagation of laser beams. Our method combines AUV operation and acoustic techniques to improve scanning efficiency compared to previous methods that relied only on PAT systems to scan and track targets. This combination paves the way for the future use of PAT systems underwater.
The scanning area may not cover the target if significant errors occur in acoustic positioning or AUV manipulation. This method can cover the target based on the results in Fig. 10 and the maximum detectable radius given in the previous research [21]. Absorption and scattering by seawater can affect the laser beam’s propagation direction. Therefore, this method does not fully trust the results of acoustic positioning but further locates the target within the scanning area. In practice, to ensure the stability of the scanning and link establishment process in unknown environments, it is advisable to consider an appropriate increase in the beam radius or detectable radius. The method of adjusting the beam divergence is discussed in the beam detection experiment.
The bearing-only acoustic positioning was successfully used to share the state of AUVs in sea experiments. We tested relative distances of about 10–50 m, and the platform effectively received acoustic signals for position estimation. The effective long-distance transmission of acoustic signals ensures that the transmitting AUV can quickly reduce the scanning area during the initial position identification. Its one-way transmission mechanism allows the receiving AUV to serve multiple clients simultaneously. For example, as shown in Fig. 11, all six positions can simultaneously receive the same acoustic signal. This scalable approach allows AUV fleets to share data to one platform simultaneously.
Environmental disturbances and platform uncertainties have an impact on the underwater link establishment. In the experiments, depth and pitch angle affect the pointing of the laser beam. The sensors on the AUV platform can measure these data. Currently, the optical system scans the target based on the acoustic positioning and adjusts the pan angle according to the received optical power from the receiving AUV. From actual machine experiments, more AUV states, such as depth and orientation, can be shared with the optical system to further improve scanning efficiency in the future.
In addition, experimental results show that this method can be extended to bi-directional optical communication when a large amount of data needs to be sent from the receiving AUV to the transmitting AUV. As shown in Fig. 4, when the orientation of the receiving AUV ${\phi ^R}$ is shared with the transmitting AUV through acoustic communication, the transmitting AUV can calculate the relative angle ${\alpha ^{\rm{RT}}}$ and transmit it to the receiving AUV via the optical communication. Thereafter, the receiving AUV can emit a laser beam pointing toward the transmitting AUV according to the relative angle ${\alpha ^{\rm{RT}}}$.
6. CONCLUSION
The underwater laser communication is of great significance for achieving high data rate information sharing between AUV fleets in underwater investigations. Despite its significance, there have been limited prior investigations into the practical application of laser communication within AUV fleets in real-world scenarios. This study introduces a scalable laser-based link establishment method that integrates AUV maneuvering, acoustic positioning, and optical system control. Our approach was evaluated in a simulated environment and confirmed in actual experiments that it can be deployed in real scenarios. The results show that the dynamic positioning and acoustic techniques of the underwater vehicle effectively limit the space to be scanned. Acoustic and optical cooperative scanning algorithms can improve the accuracy and stability of link pointing.
Our approach provides a solution for future applications of underwater laser communication among AUV fleets. The onboard sensors of AUVs can effectively sense the environment and observe the target. Acoustic technology can measure and share the status of the platform even at long distances. Considering these data and techniques in the PAT method will make it more efficient, stable, and accurate than the original PAT method, which relies only on optical techniques. It also demonstrates that more AUV states can be utilized to establish the laser link and further optimize the PAT method. As we consider future improvements, an exciting direction for optimization lies in extending our current approach, which demands dynamic platform positioning, to platforms in motion. This would expand the horizons of underwater laser communications to more scenarios.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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