Received: 18 August 2020/Revised: 16 September 2020/Accepted: 18 September 2020/Published: 24 September 2020
Comparison between a tethered remotely operated vehicle (ROV) and a wireless ROV control scenario.
Bạn đang xem: On the feasibility of fully wireless remote control for underwater vehicles
Depiction of the Bellhop ray tracer <27> path loss to lớn present the multipath effect in shallow (a) & deep water (b).
">Nominal bit rate vs. Range for the best among the technologies presented in Section 2.
">Multimodal remote control for ROV: the red fixed node represents the control station, while the yellow nodes represent the medium-frequency (MF) relays used to lớn extend the control range.
">Variation of the attenuation coefficient along the water depth (a) and optical receiving area with the control station modem fixed at a depth of 60.5 m và aligned khổng lồ the x axis by varying the ROV modem installation, namely, in the back of the ROV with the modem aligned lớn the x-axis (b), in the đứng đầu of the ROV pointing lớn the surface (c), & in the bottom of the ROV pointing lớn the seabed (d).
">Complete path that should be followed by the underwater vehicle (a) during the simulated mission: the control station position is marked with a red circle, while the positions of the acoustic relays are marked with red crosses. The zoomed-in section of the path in the proximity of the control station is depicted in (b), while the path that the vehicle should follow at a distance between 200 m and 1400 m from the control station is zoomed-in in (c).
">Instantaneous bit rate of the high-quality và grey-scale clip streams (a) as well as of the very low-quality video clip transmitted via acoustic HF (b), calculated on a time window of 1 s.
">Protocol stack used in the DESERT simulations.
">Average throughput of all data traffics when varying the distance between HROV & control station (CTR): the average throughput was computed for each distance window observed in the step plot.
">Root mean square error (RMSE).
">Transmitted & received instantaneous đoạn phim bit rate calculated on a time window of 1 s (a) và packet delay variation (PDV) obtained with different de-jitter buffer configurations (b) of the VLQ stream.
">Transmitted & received instantaneous video bit rates of VHQC (a) và VHQG (b), calculated on a time window of 1 s.
">PDV & average delay of VHQC & VHQG when using different traffic shaping configurations (a) và PDV when the traffic shaping forces an image packet delay of 40 ms (b).
">Nowadays, the increasing availability of commercial off-the-shelf underwater acoustic and non-acoustic (e.g., optical and electromagnetic) modems that can be employed for both short-range broadband & long-range low-rate communication, the increasing cấp độ of autonomy of underwater vehicles, và the refinement of their underwater navigation systems pave the way for several new applications, such as data muling from underwater sensor networks & the transmission of real-time video streams underwater. In addition, these new developments inspired many companies khổng lồ start designing hybrid wireless-driven underwater vehicles specifically tailored for off-shore operations and that are able lớn behave either as remotely operated vehicles (ROVs) or as autonomous underwater vehicles (AUVs), depending on both the type of mission they are required khổng lồ perform & the limitations imposed by underwater communication channels. In this paper, we evaluate the actual quality of service (Qo
S) achievable with an underwater wireless-piloted vehicle, addressing the realistic aspects found in the underwater domain, first reviewing the current state-of-the-art of communication technologies và then proposing the menu of application streams needed for control of the underwater vehicle, grouping them in different working modes according lớn the level of autonomy required by the off-shore mission. The proposed system is finally evaluated by employing the DESERT Underwater simulation framework by specifically analyzing the Qo
S that can be provided khổng lồ each application stream when using a multimodal underwater communication system specifically designed to tư vấn different traffic-based Qo
Ss. Both the analysis và the results show that changes in the underwater environment have a strong impact on the range và on the stability of the communication link.
underwater multimodal networks; underwater acoustic networks; ROV wireless control; DESERT Underwater network simulations; underwater optical communications
Remotely operated vehicles (ROVs) are unmanned underwater vessels typically operated through the so-called umbilical cable, composed of an optical fiber for a broadband low-latency communication link and a power line lớn supply the vehicle, making it possible to manage the system in real time. However, the umbilical cable inherently limits mobility of the ROV due to cable strain & entanglement risks. Wireless ROV control would help avoid such issues by removing the need for a physical cable at the price of an increased need for ROV power autonomy and smaller data rates, a price that can be accepted only for some possible ROV applications (Figure 1).
Depending on the tasks they are designed khổng lồ accomplish, ROVs can be divided into intervention-class ROVs and inspection-class ROVs <1>. The former type of vehicle, also known as work-class ROVs, is mostly used in the oil & gas industry; can weigh from 100 kg (light work-class) up to lớn 5000 kg (heavy duty work-class); và is used for tasks such as cleaning, stabbing, và drilling as well as for carrying and operating heavy tools và sensors, such as 3 chiều scanners for ship-hull inspection operations <2>. The latter, instead, is easier to lớn deploy và can be used for several applications, including archaeology, coastal monitoring, search and rescue, and ship-hull & shipwreck inspection. Depending on their size, they can weigh from 3 kg (micro-ROVs) up khổng lồ 120 kg (medium-size ROVs) and are typically equipped with a camera plus other optional small operational tools, such as a small manipulator or an imaging sonar. While the power consumption of intervention-class ROVs is more than 50 k
W, the nguồn required by medium-size inspection-class ROVs is less than 6 k
W, and micro-ROVs require less than 1.8 k
W; thus, both medium- và small-size inspection-class ROVs can be powered by lithium batteries. For some ROV applications, such as under-ice research và high-risk environment missions where the tether could be entangled, the availability of on-board batteries may be advantageous: if an emergency maneuver becomes necessary and the cable needs khổng lồ be cut, this is easier thanks to its smaller size (as it does not need to carry a nguồn supply) and the ROV can then safely resurface, behaving like an autonomous underwater vehicle (AUV). In this paper, we inspect how the use of a wireless communication system to substitute the umbilical cable would affect the unique of service (Qo
S) experienced by ROV pilots1 while using a medium-size battery-powered inspection-class ROV during a survey operation. In particular, we start by analyzing the required application data rates for typical services offered by inspection-class ROVs. We identify a number of operational modes which can be chosen as a function of range to support a given set of ROV services, from simple guidance và positioning up to real-time clip streaming. Indeed, with the increase of the communication range, there is an exponential decrease in the achievable bit rate.
Nowadays, it is possible to establish a wireless communication liên kết underwater through radio frequency (RF), optical, or acoustic modems <3>. These three technologies provide different performance & are used for different types of applications. Specifically, underwater RF <4,5,6> và optical communications <7,8,9> can be used for short-range broadband communication links, while acoustic signals are used for long-range low-rate communications <10,11,12>. The best communication performance is obtained when the three technologies are combined together into so-called underwater multimodal networks <3>, where the best performing channels in the experienced condition are used simultaneously to lớn achieve the desired Qo
S. For example, two nodes may communicate with each other via acoustics when they are more than 50 m apart và with optical modems when their distance is less than 50 m. Additionally, an RF modem can be employed to support high-rate communications when the distance between the nodes is less than 5 m và the optical channel cannot be used due lớn poor channel conditions (e.g., high turbidity or ambient light noise).
Also, the concept of designing a wireless hybrid ROV/AUV has already been addressed in the literature. In <13>, the Woods Hole Oceanographic Institute (WHOI) presented an untethered ROV for intervention applications that make use of optical và acoustic modems khổng lồ allow an ROV pilot to carry out the intended mission. In <14>, we proved via simulation the possibility lớn exploit an underwater multimodal acoustic và optical liên kết to pilot an inspection-class hybrid ROV/AUV, và in <15>, we presented how the control range of the vehicle performing an AUV mission can be extended through a multihop acoustic network. Following the same concept, some offshore companies are currently developing commercial hybrid ROVs that can be piloted wirelessly for simple semiautonomous inspection operations <16,17>. Both our system và the WHOI untethered ROV perform best in deep water, where optical signals can reach longer distances and acoustic links are less affected by multipath <18>. On the contrary, in <19>, the authors presented a wireless remote-controlled ROV for very shallow water operations: specifically, they proved through a tank thử nghiệm that, if the ROV is deployed in fresh water at a depth of less than 45 cm, it can be controlled through an RF wireless links in the 420–450 MHz band. In <20>, the authors investigated the acoustic networking of an AUV with Autonomous Surface Vehicles (ASVs) khổng lồ accomplish a common mission. After introducing the vehicle control architecture, the authors described the acoustic communication và ranging capabilities of each node and finally showed some experimental results obtained with two vehicles in the Douro river in Portugal. Recently, in <21>, the authors designed an RF và acoustic wireless remote control for a Blue
ROV <22> vehicle. The used RF liên kết permits a transmission rate of 1.9 kbps & a maximum range of 5 m in sea water <4>. In late 2019, the authors in <23> presented a smart buoy used as a relay for controlling and retrieving data from underwater vehicles. The buoy employs a hybrid surface RF & free-space optical links to communicate with the shore station và an underwater optical links to communicate with the underwater vehicle. The simulation mã sản phẩm used in this paper is quite simplistic, as it does not consider solar light noise in the underwater optical link. Hydromea has recently developed an underwater drone <24> that can be piloted through a tethered system until it reaches a high-risk area, where it unplugs the cable to avoid any possibility lớn get entangled. From that point on, it can be controlled through a wireless optical link: indeed, both the cable connector and the drone are equipped with the LUMA optical modem <25>.
This paper extends the results presented in <14,15>, first by updating the review of available communication technologies, basing the remote control design và simulation results on field measurements và realistic models, & then by using realistic simulated application layers lớn best emulate the behavior of the vehicle và the control station.
The structure of the paper is as follows. In Section 2, we review the state-of-the-art of underwater communication technologies, paying particular attention to lớn experimental results và selecting the most suitable underwater modems lớn be employed for wireless remote control. In Section 3, we analyze the data-rate requirements of an ROV control system, considering each type of traffic that needs lớn be transmitted. In the same section, we also define different working modes based on the level of autonomy required by the vehicle mission. Section 4 presents the simulation settings of an ROV control system designed in accordance with the observations in Section 2 and Section 3, while the whole multimodal optical & acoustic system is analyzed & evaluated in Section 5. Finally, Section 6 draws some concluding remarks.
Nowadays, the technologies used for underwater wireless communications are acoustics, RF, Magneto-Inductive (MI), và optical. In this section, we analyze these technologies by presenting the pro and cons of each system, some details on their expected performance, & the lessons learnt from the actual performance we experienced during sea trials.
Developed in the late 50s of the 20th century <26>, acoustic communication is certainly the most mature underwater telecommunication giải pháp công nghệ available so far. It provides long transmission ranges, up lớn tens of kilometers depending on the carrier frequency và the environmental conditions <10>. The modem bandwidth & its consequent communication rate depend on the carrier frequency, on the characteristics of the modem transducers, as well as on external conditions, such as noise caused by ships, wind, & marine life; the multipath; và the Doppler effect caused by the movements of the submerged nodes. For these reasons, the communication rate of an acoustic links ranges from few tens of bits per second for long-range communications up to few tens of kilobits per second for short-range links. The main disadvantage of acoustic communications are the long propagation delay caused by the low sound speed, the time variation of noise & of the channel impulse response, the presence of asymmetric acoustic links, and the poor performance in shallow water (i.e., when the water column is less than 100 m) due khổng lồ signal reflections. In a mobile network deployed in shallow water, the multipath caused by signal reflections often results in links disruption, where, for instance, the communication between two nodes deployed at a depth of 1 m is established in the range of 0 up lớn 110 m, lost in the range of 110 up to 220 m, and established again in the range of 220 up to 290 m (Figure 2a). The link, instead, is definitely more stable in a deep-water scenario, where communication can be established up to lớn the maximum range of the modem without link disruption (Figure 2b). Depending on the expected conditions và user needs, there is a wide set of acoustic modems in the market that can be employed in a variety of specific scenarios.
For example, to lớn achieve a communication range of more than 4 km, a modem with a carrier frequency below 12 k
Hz should be used, such as Benthos ATM 960 in the low-frequency (LF) band <28>, Evo
Logics S2C 7/17 <29>, Aqua
Se
NT AM-D2000 <30>, Link
Quest UWM3000 <31>, AQUATEC AQUAmodem1000 <32>, Develogic HAM.NODE <11>, the Sercel MATS3G 12 k
Hz <33> modem, the WHOI Micro
Modem <12>, & Kongsberg c
NODE LF <34>. The transducer of most of these devices can be customized to the geometry of the channel2, và the modem bit rate can be adapted accordingly. For this reason, all LF modems can achieve a communication rate up to lớn few kilobits per second in a vertical link in deep water, where the multipath is negligible, while in a horizontal links in very shallow water, they can reach a maximum rate of few hundreds of bits per second. Among the others, WHOI demonstrated that, in good channel conditions, it is possible lớn achieve a communication links of 70 to 400 km at 1 khổng lồ 10 b/s in the arctic <35> by employing a carrier frequency of 900 Hz.
For communication ranges from 1 lớn 3 km, instead, a medium frequency (MF) modem is most suitable, as it can provide a higher bit rate. The carrier frequency of a MF modem is, depending on the manufacturer, selected between 20 and 30 k
Hz. All aforementioned manufacturers that produce LF devices also develop MF acoustic modems. In addition khổng lồ them, other companies also supply commercial off-the-shelf products in this range, such as the Popoto Modem <36>, the Applicon Seamodem <37>, Sonardyne 6G <38>, DSPComm Aquacomm Gen2 <39>, Sub
Nero <40>, and the Blueprint Subsea <41> acoustic modem. In low-noise scenarios, a vertical communication link with an MF modem can reach a bit rate up to lớn tens of kbps <42>, depending on the modem manufacturer, while in horizontal communications in very shallow water, they can provide a communication liên kết with a bit rate spanning from 500 bps up khổng lồ few kbps.
Due lớn the high cost of good-quality LF and MF transducers with a wide bandwidth (the cost of the single transducer can easily exceed 2 k
EUR, i.e., 2000 EURO), research and industrial prototypes of high-performance LF và MF acoustic modems are mostly developed by navy-related research institutes và companies, such as TNO <43>, FOI <44>, FFI <45>, Wärtsilä ELAC <46>, và L3HARRIS <47>, that are specifically interested in very-long-range communications for surveillance applications <48>. Also, the JANUS North Atlantic Treaty Organization (NATO) standard focuses on LF & MF frequency bands <49>. In this frequency domain, some universities and civil research institutes developed some low-cost low-power products for medium- và short-range (few hundreds of meters) low-rate (few hundreds of bits per second) modems for internet of things (Io
T) applications <50,51,52> by employing low-cost narrow band transducers. Some commercial low-cost MF acoustic modems (with a price of less than 2 k
EUR) are also available off the shelf. For instance, the modem recently launched by DSPComm <39> has a maximum transmission rate of 100 bit/s and a nominal range of 500 m. With the same range, the Micron Data Modem developed by Tritech <53> is a low-power compact modem with a maximum data rate of 40 bps. Similar performance can be obtained with the Desert Star SAM-1 modem <54>. Finaly, Deve
NET, a company that mainly produces communication và localization equipment for divers, supplies Sealink <55>, an affordable and low-power acoustic modem that provides a range up to 8 km at a data rate of 80 bps.
To establish communication link of less than 500 m, a high-frequency (HF) acoustic modem with a carrier frequency of at least 40 k
Hz can be used. The market of HF modems is divided into two segments: low-rate acoustic modems và high-rate acoustic modems. While the former has a price below 2 k
EUR và is suitable for low-rate (less than 200 bps) communication in shallow water up to lớn a distance of 200 m <56,57,58>, the latter has the same price as MF & LF acoustic modems (between 8 & 10 k
EUR, depending on depth rate requirements) & can be used for sending a still-frame slide-show-like video clip feedback from an underwater vehicle <43,59,60,61,62> khổng lồ a control station, as they can perform transmissions with a bit rate of more than 30 kbps <29>. In this paper, we focus on this last type of modem in order lớn achieve the best performance possible despite the higher price. Indeed, they can easily achieve a bit rate of more than 30 kbps in vertical links, & in horizontal communications in very shallow water, they can still provide a rate of at least 2 kbps.
Although several high-rate acoustic modems are available in the literature, only a few of them are commercial off-the-shelf products. The maximum rate of a Link
Quest <31> modem is 35.7 kbit/s (with a directional beam pattern), while Evo
Logics S2CM HS <29> is the off-the-shelf modem that provides the highest bit rate (62.5 kbit/s up to 300 m in good channel conditions, although its actual maximum throughput would typically be about 30 kbit/s). This last modem has been used in <61> to lớn perform in-tank low-quality video streaming, where the transmission bit rate selected by the modem during its initial handshake phase <63> was 31.25 kbps and the actual throughput obtained 20 kbps. In good channel conditions, this performance can also be obtained with other Evo
Logics models, such as S2C 48/78 và S2C 42/65. While the former is optimized for horizontal communications, the latter is suitable for vertical links. S2C HS, instead, has an omnidirectional transmitter and, therefore, is more suitable for being installed in an AUV or in an ROV. Modems that can provide a higher bit rate are either university noncommercial systems <59,62,64,65> or company prototypes waiting for a bigger market demand before becoming available off the shelf <66>. The Marecomms Roboust Acoustic Modem (ROAM), recently developed in partnership with Geospectrum, achieves a throughput of 26.7 kbps at a distance of 600 m while operating in the HF band in shallow water. A demo has been performed in the presence of Doppler as the nodes were floating randomly with a tốc độ between 0.5 & 1 knot <67>. With the new version of the modem, the manufacturer expects to lớn achieve 50 kb/s within a range of 1 km. This modem is expected lớn become available on the market by late 2020 lớn early 2021. The ROAM modem can also operate in the MF band, providing a bit rate of 13 kbps.
The most representative underwater acoustic modems with omnidirectional beam patterns that can also be employed for communications in shallow-water scenarios are summarized in Table 1.
Although acoustic modems are the typical solution for underwater communications, their bandwidth is very limited. The need for high-speed communications in an underwater environment has pushed the realization of optical devices that can transmit data within short distances at a bit rate on the order of one or more Mbps (up to few Gbps at very short ranges, depending on the mã sản phẩm and the water conditions). Indeed, unlike acoustic communications, optical communications are more suitable for ranges up to 100 m, especially in deep dark waters, & are not affected by multipath, shipping, và wind noise, as their performance mainly depends on water turbidity and sunlight noise <70>. In fact, high turbidity scatters và attenuates the optical field, whereas ambient light may become a significant source of noise, making transmissions close to lớn the sea surface more difficult. The turbidity coefficient, called the attenuation coefficient, is composed of the sum of scattering and absorption coefficients. The former depends on the quantity of particles, such as plankton, dissolved in the water. The plankton exists due lớn the chlorophyll effect, that happens only where solar light reaches the medium, i.e., in shallow water, up to a depth of 100 m. The latter, instead, is an inherent optical property of the medium. Blue and green lights, which have wavelengths of 470 và 530 nm, respectively, are the most widely used for underwater optical communication <71>, as these wavelengths are the least attenuated in deep and shallow water, respectively. Intuitively, in order khổng lồ understand which of the two wavelengths is less attenuated in a certain scenario, we just need to lớn observe the màu sắc of the water in the presence of white light (e.g., sunlight). If the màu sắc of the water is blue, the best wavelength lớn use is around 470 nm (that is the typical case of a deep water deployment); otherwise, if the water màu sắc is green, wavelengths around 530 nm should be employed (that is the typical case of deployment close to lớn the surface).
Similar to acoustic modems, optical transceivers are also designed lớn perform best in some scenarios; therefore, the best optical modem that outperforms all others in all possible conditions does not exist. Specifically, we can divide the optical modems into models composed of a light emitting diode (LED)-based transmitter designed for hemispherical communications <8,25,72,73> and models composed of a high-directional laser-based transmitter <74,75,76,77,78,79>. Although the latter achieves a throughput from 10 to lớn 100 times higher than the former, we focus on LED-based modems, as laser-based transmitters require perfect alignment between the transmitter and receiver, a condition that a thiết bị di động vehicle can fulfill only during a docking operation or if the modems are equipped with beam-steering capabilities, such as the SA Photonics Neptune modem <80>. The latter, however, is not an off-the-shelf product, i.e., it is custom built to order, with a significantly higher price compared to other commercial products.
We can also divide the optical modems into models tailored for dark water medium range (MR) communications (up lớn 100 m) <7,25> & devices designed for short-range (SR) communications in high ambient light environments <9>. In the MR class, we can find Sonardyne Blue
Comm 200 <7>, equipped with a very sensitive receiver based on a photomultiplier. This modem achieves a hemispherical transmission rate of 10 Mbps up to a distance of 100 m, but only in deep, dark waters. The same modem would perform poorly in the presence of light noise due to lớn saturation of the receiver: for this reason, Sonardyne designed an ultraviolet version of this modem, able khổng lồ achieve a maximum range of 75 m even in the presence of some ambient light. Still, from our experience, both models are unable to lớn establish a communication links when deployed a few centimeters above the sea surface during daytime. Similar issues have been experienced with Hydromea Luma 500ER <25>, able lớn cover, in good conditions, more than 50 m with a bit rate of 500 kbps (beam pattern 120∘); the Ifremer optical modem <73>, that can communicate at a similar range with a bit rate of 3 Mbps; and the early-stage version of the ENEA proof of concept (Po
C) prototype <81>. Also, the Aqua
Optical modem developed by MIT <72> can perform MR communications in low solar noise conditions by reaching a maximum range of 50 m with a rate of 4 Mbps. The beam patterns of both the Ifremer và MIT modems are 100∘, while the ENEA optical modem is omnidirectional. Customized LED-based MR optical modems are developed by Penguin ASI <82,83>; the maximum performance of their system is in the order of 100 s of Mbps at hundreds of meters but comes at the price of very bulky & expensive modems that are only suitable for extremely specialized applications, such as deployment in heavy-size working-class ROVs or similar vehicles.
Models designed for SR communications, instead, typically overcome the ambient light noise issue by employing a noise-compensating mechanism khổng lồ avoid receiver saturation, at the price of lower bit rate and range. This mechanism typically consists of measuring the average noise at the receiver and of injecting a signal with equal intensity but opposite sign at the receiver unit. Blue
Comm 100 <7>, for instance, can be used in all water conditions, including shallow water in daytime to transmit at a rate of 5 Mbps in SR at a maximum distance of 15 m. Its beam pattern is 120∘. Similarly, the Sant’Anna Opto
COMM modem <8> can establish a 10-meter communication link at a speed of 10 Mbps, when both the receiver và transmitter are deployed just half a meter below the sea surface. They use optical lenses to lớn reduce the beam aperture angle lớn 20∘ và to reduce the receiver field of view lớn 70∘ khổng lồ limit sunlight noise. Also, ENEA developed a solar light noise-cancellation mechanism for their new version of the optical modem: preliminary results declared by ENEA proved that their new prototype can now communicate in high ambient light conditions, at the price of a reduced bit rate. Another commercial off-the-shelf optical modem for SR is the AQUAmodem Op1 <9>, which achieves 80 kbps at 1 m, with a beam pattern of 34∘. The company declares that the modem is affected by direct sunlight noise but is generally robust lớn low sources of ambient light noise, as it can be used in the presence of ROV lights without compromising the communication link. The same happens for the Co
Sa optical modem <5>, able to lớn reach 2 Mbps at up lớn 20 m, with a transmitting beam aperture angle of 45∘ và a receiver field of view of 90∘. Another low-cost modem that is quite robust to sunlight noise is the optical modem developed by IST <84> that can reach 200 kbps at a maximum distance of up khổng lồ 10 m and, different from the other modems presented so far, uses green instead of xanh LEDs, as it is specifically tailored for shallow-water operations. This modem uses optical lenses lớn reduce the beam aperture angle to lớn 12∘ và an optical filter to reduce sunlight noise.
Also, electromagnetic radio frequency và magneto-inductive communications can be used underwater. Compared with acoustic & optical waves, RF waves can perform a relatively smooth transition through the air–water interface <85>. This benefit can be used khổng lồ achieve cross-boundary communication: for instance, the authors in <19> used this concept to pilot an ROV deployed up khổng lồ 45 cm below the water surface. Another advantage is that RF & MI are almost unaffected by water turbulence, turbidity, misalignment between transmitters và receivers, multipath, and acoustic và solar noise, that are the main causes of poor performance for either optical or acoustic modems when used in practical scenarios. For these reasons, when in range, RF & MI can provide a much more stable link than optical và acoustic communications, with less disruptions; thus, in our opinion, they should be preferred khổng lồ the other truyền thông media whenever the bit rate và the range required by the application can be supported. However, their communication range is usually limited lớn no more than a few meters. Inductive modems <86,87,88>, for instance, are often deployed in mooring systems <89,90>, as they enable communication over jacketed mooring lines and can be used to retrieve data from instruments such as Conductivity, Temperature, & Pressure sondes (CTDs) and Acoustic Doppler Current Profilers (ADCPs) by substituting physical connectors and the need for dedicated cables for communication. These modems generate a low-frequency signal that travels in the mooring line and can only substitute mechanical connectors for low-rate communications (up khổng lồ 5 kbps). Also RF modems <5,91> can be used lớn replace the mechanical connector of cables in very short ranges, but they provide broadband communications (up lớn 100 s of Mbps) & thus can tư vấn high-rate-demanding applications, such as real-time control and high-quality clip streaming. For example, Hydromea uses an RF connector in the umbilical cable of the EXRAY ROV <24>, where the vehicle’s tether can be disconnected lớn perform an autonomous mission before being reconnected again. Also, in this case, the communication range is in the order of few centimeters. Wi
Sub supplies the Maelstrom connector <92>, able to tư vấn both power and data transfer via RF. Communication based on a microwave links has a rate of 100 Mbps up khổng lồ a distance of 5 cm between the connectors. Similar devices are sold also by blue Logic <93>. Broadband RF modems can also be employed in docking stations lớn quickly tải về data from an AUV <94>. For instance, the WFS Seatooth S500 <91> RF modem provides a bit rate up to 100 Mbps up to lớn a range of 10 cm, và the Lubeck University of Applied Science developed the Co
Sa underwater Wi
Fi <5>, with a rate of 10–50 Mbps up khổng lồ 10 cm.
These examples prove how RF communications can achieve high transmission bit rates underwater, although their communication range is very limited. Indeed, RF communications suffer from RF interference & are prone lớn very strong attenuation in salty waters, where the conductivity of the medium is larger than in fresh waters. A range up to lớn a few meters (SR) can be reached with RF modems, at the price of a lower bit rate. For example, INESC Tec developed a dipole antenna prototype <6> to tư vấn 1 Mbps communication at 1 m, & the Lubeck University of Applied Sciences developed a dipole <5> antenna to communicate with a rate of 0.2 to lớn 1 Mbps & a range of 1-8 m, depending on water conditions (i.e., 1 meter in salty water và 8 m in fresh water).
Although in air ngươi communication is outperformed by RF modems, as the latter can achieve a higher bit rate and a longer range, underwater ngươi modems are almost unaffected by the change of medium while the electrical field is strongly attenuated. Indeed, mày modems are proven khổng lồ reach a bit rate of a few kbps at tens of meters, both in air và underwater <95>. Dalhousie University developed an mày prototype that achieves 8 kbps at 10 m <96>, to lớn perform low-rate low-latency communications. With mi modems, longer distances can be achieved at the price of a lower bit rate. For instance, the authors in <95> established a directional liên kết with a maximum range of 41 m và an omnidirectional links with a range of 26 m. Both links provide data rates of 512 bps. With their new modem design recently presented in <97>, they were able khổng lồ achieve 1 kbps at a 40-m distance with an omnidirectional beam pattern. Nearly 20 years ago, the authors in <98>, instead, demonstrated a 153-bps communication liên kết at a distance of 250 m và a 40-bps communication liên kết at a distance of 400 m.
Very-low (VLF) and extremely-low radio frequency (ELF) signals have been extensively used during the cold war to lớn communicate from inland control stations to lớn submarines <99>. The drawbacks of these systems are the low rate & the need for a very large và high-power-consuming inland antennas. Indeed, VLF can provide a 300 bps one-way communication link from shore to the submarine up to a distance of 20 m below the sea surface và requires a broadcast inland antenna with a form size between 300 m & 2 km <99>. For example, the Sweden Grimeton Radio Station <100> uses a phối of antennas that are 1.9 km long, each with an RF power peak of 200 k
W.
ELF can also be used to communicate from land to lớn submarines (one way): they reach up to 1 bps <101> at a range of several hundreds of meters below the sea surface but require a grounded wire inland antenna (ground dipole) with a kích thước of several tens of kilometers và a transmission power nguồn in the order of millions of watts. Due to lớn the high cost of deployment, US, Russia, India, and china are the only nations known to lớn have constructed ELF communication facilities. For instance, the US ELF system employed a ground dipole antenna 52 km long <102> while the Russian system used an antenna 60 km long <103>. This system has been typically used khổng lồ signal one-way coded messages to the submarine’s commander to resurface khổng lồ receive more information via other means.
Both VLF and ELF technologies are not applicable to ROVs and AUVs but only khổng lồ submarines due to their very large form size and demanding power nguồn consumption.
According khổng lồ the technology comparison presented in this section and summarized in Figure 3, we can conclude that optical technologies are the preferred choice up to lớn a distance of about few tens of meters (100 m in very good conditions) whereas acoustics would be the preferred choice from that point onward. We also note that RF and MI modems are consistently outperformed by optical or acoustic modems, although they have the advantage that their communication is not prone to lớn environmental characteristics (unlike acoustic và optical).
The optical modem considered in this paper is Blue
Comm 200, that has a hemispherical beam pattern and is able to transmit at a tốc độ of 10 Mbps at a range up to 100 m in good channel conditions, such as those considered in this paper. For different scenarios, when choosing which modem to use, it should be considered that the Blue
Comm 200 modem is strongly affected by noise due khổng lồ sunlight và external lights, và for this reason, Sonardyne supplies it together with an ROV lighting system that does not affect the modem performance. Indeed, Blue
Comm 200 can be used only in deep-water scenarios or during night operations in shallow water. If the ROV needs lớn be operated in shallow water during daytime, a different model tailored for shorter ranges in these conditions should be selected, like Blue
Comm 100, that achieves a maximum range of 15 m và a maximum rate of 5 Mbps even in the presence of sunlight, or Blue
Comm 200 UV, that still suffers from direct sunlight but is robust against ROV lighting systems.
One of the most important things to take into trương mục when selecting multiple acoustic modems lớn be used in the same network is to lớn avoid interference between the different devices. For instance, if both LF & MF acoustic modems need khổng lồ be used in the same system, the maximum working frequency of the LF modem must be smaller than the minimum working frequency of the MF modem khổng lồ avoid bandwidth overlap. Moreover, some guard between the bandwidths of the modems should be provided, as the drop of the transducer sensitivity outside the modem bandwidth is usually not vertical: for example, Evo
Logics S2C 7/17 should not be used along with Evo
Logics S2C 18/34 because the bandwidths of the two modems are spaced apart only by 1 k
Hz. In this work, we cannot analyze the characteristics of all transducers of each modem (as most companies vày not provide this information); thus, we assume that two modems can be used together if their bandwidths are spaced apart by at least 5 k
Hz.
When designing deployment with an underwater vehicle, also the interference between modems & the acoustic localization systems, such as ultra-short baseline (USBL) & long baseline (LBL) acoustic positioning systems <104>, used to lớn track the vehicle position along the whole mission must be avoided. A USBL is composed of two components: a transceiver, usually deployed from a control station with a well-known position, such as a ship, & a transponder, installed on the underwater vehicle that needs to lớn be tracked. The former is equipped with an array of at least four hydrophones, used to lớn determine the target position from the range và bearing obtained from the acoustic signal received by the latter. Specifically, the transceiver sends an acoustic pulse (interrogation) to lớn the transponder that responds with another acoustic pulse (reply) immediately, so the transceiver can triangulate the position of the transponder by means of its hydrophone array. In case of bandwidth overlap, the acoustic pulses sent by the USBL may interfere with the acoustic communications: to lớn overcome this issue, some companies <41,105> provide the possibility khổng lồ perform low-rate communication (up to few hundreds of bits per second) with their USBL systems. Some modem manufactures, instead, provide a version of their modem that incorporates a USBL <28,29,31,34,38,47,55>, where the transponder is just a normal unit of the modem programmed to lớn answer the USBL request & the transceiver is a modified version of the modem that includes the hydrophone array into the modem transducer.
An LBL system, instead, uses a network of sea-floor-mounted baseline transponders as the reference point for navigation. The exact coordinates of the baseline transponders are known, & they are used for determining target positions. Baseline transponders reply lớn acoustic interrogation signals from target-mounted transponders with their own acoustic pulses, allowing a target khổng lồ calculate its own position by measuring the distance between itself and each transponder of the baseline array. Although the deployment of an LBL, that typically requires at least four baseline transponders plus the target transponder, is more expensive than the deployment of a USBL, it provides a higher positioning precision and is often used in oil and gas fields. In addition, the LBL transponder can either be specifically manufactured for positioning <105> or can be a regular acoustic modem with positioning capabilities <12,28,29,31,34,38,40,55,56,57>.
The modems equipped with USBL or LBL functionalities are able to lớn switch between positioning and data modes or provide an automatic protocol that performs tracking of the vehicle along with the communication, at the price of a throughput reduction between 10% & 20%. In our design, the longest range acoustic modem should also provide either LBL or USBL capabilities.
One of the main disadvantages of LF acoustic modems when used in ROV và AUV operations is the fact that they are strongly affected by noise caused by the propellers of ships & vessels <106>. From our tests, we indeed discovered that, in a network deployed 40 m from a cargo ship docked in a port, a long-range Evo
Logics S2C 7/17 modem reaches only the same transmission range as an Evo
Logics S2C HS (i.e., 200 m) because the noise level of the former is very close lớn the saturation cấp độ of its transducer while the latter is almost unaffected due khổng lồ its high-frequency bands. Indeed, HF acoustic signals mainly suffer from the noise caused by wind-driven waves and not by shipping noise <10>. Another reason why MF và HF modems are used more often in small AUVs và ROVs than LF modems is because the integration of LF modems in small vehicles can be complex or even impossible due to lớn the large form size of their transducer, that has a diameter of at least 12 cm & a total weight that can easily exceed 5 kg. MF và HF modems, instead, usually have a total weight of less than 2.5 kg và a transducer diameter of less than 6 cm.
The acoustic modems selected for the wireless remote control designed in this paper are the Subnero WNC and the Evo
Logics S2C HS, both equipped with an omnidirectional transducer, the former operating in the MF bandwidth with LBL capabilities & the latter operating in the HF bandwidth.
In this section, we analyze the requirements for operational ROV control. First, we report the raw bit rate measurements of the communication streams used in an inspection-class ROV; specifically, we present the case of the Blue
ROV used by the Fraunhofer Center for Maritime Logistics và Services (CML) <107> during the Martera Robo
Vaa
S project <108>. Although this model of ROV has an extremely low cost & does not motivate the use of expensive underwater modems (the price of one unit of Blue
ROV is less then 5 k
EUR, i.e., about half the price of a commercial acoustic modem), its application streams are similar to the ones employed by more sophisticated inspection-class ROVs, such as the I-ROV used in the same project as the Centre for Robotics và Intelligent Systems (CRIS) of the University of Limerick <109>. After the analysis of the raw data streams, we define different working modes (Table 4), each able khổng lồ provide a different Qo
S at a different working range.
The control station sends, on average, 4.5 kbps khổng lồ Blue
ROV khổng lồ control the vehicle’s position through a joystick-based console as well as the sensors (lighting, cameras, and a small gripper) mounted on board the ROV. The maximum expected control bit rate is 5 kbps. For working-class ROVs, such as the Etain ROV owned by CRIS, the control stream requires higher traffic: a working-class ROV equipped with several sensors, cameras, as well as two heavy manipulators with 7 degrees of freedom each requires a control bit rate up khổng lồ 100 kbps.
Blue
ROV mounts an HD clip camera with a maximum resolution of 1920 × 1080 px at 30 fps, resulting in an average đoạn phim bit rate of 11.5 Mbps. According to the results we presented in <61>, the video bit rate depends on the number of details & items present on the frames và on the level of motion: in general, low-motion quasi-static videos require a lower bit rate than a dynamic video with fast movements, & a đoạn clip representing only one simple object with a trắng background requires a lower bit rate than a đoạn clip showing lots of complex objects and many people with a background composed of mountains, trees, & lakes. For this reason, in the subsea raw video clip sample considered in <61>, the maximum bit rate is related lớn the moment when the ROV performs quick movements in an environment with complex details. The peak bit rate is 24 Mbps, while the average đoạn clip bit rate is 16.5 Mbps. In a real-time video transmission, in order lớn avoid the đoạn clip buffering and causing an undesired delay, we must consider the maximum video clip bit rate as the bit rate khổng lồ be transmitted. Maintaining the same proportion obtained in <61>, given that Blue
ROV streams a đoạn phim with an average bit rate of 11.5 Mbp, we expect a maximum bit rate of 16.5 Mbps.
In addition to đoạn phim streams, the ROV needs to also send information about its status, its estimated position, & the status of its manipulator & of each of its sensors khổng lồ the control station. In general, the traffic requirements for monitoring feedback depend on the types of sensors installed in the ROV & vary from few tens of bits per sensor (such as CTD, turbidity sensors <110>, and single beam sonars) up lớn a few megabits per second, such as the monitoring control required by a 3d laser imaging system <2>, the bathymetry data collected from a multibeam sonar <111>, or the video clip feedback from a heavy manipulator for working-class ROVs. An inspection-class ROV usually mounts small và light sensors with low rate requirements, as more sophisticated tools are often quite heavy (e.g., multibeam sonars) or require carrying by working-class ROVs able lớn maintain a very stable position and move with very high accuracy (e.g., 3d scanners). In this work, we consider a periodic monitoring traffic, where the ROV sends information about its position, speed, và rotation angles as well as a compressed code-word representing the overall status of each component of the system khổng lồ the control station. Given the resolution of the positioning sensors, the fact that the maximum operational range of our system is below 12 km & the fact that the maximum speed of most underwater vehicles is less than 4 m/s, we assume that each component of position (x, y, và depth), tốc độ (u, v, & w), and rotation angles (roll, pitch, & yaw) requires 3 bytes of information. The status code word requires 2 bits per component to lớn indicate whether a certain tool is, for instance, disabled, enabled, working under a warning condition, or disabled due lớn damages. By considering a total number of components equal to lớn 16, the code word requires 4 bytes in total. The monitoring information also includes the last value measured by conductivity, pressure, temperature, turbidity, and pollution sensors. Considering 3 bytes of data for each sensor và a timestamp of 4 bytes, the size of the monitoring packet is therefore equal to 56 bytes, while considering a monitoring traffic period of 6 s, the monitoring traffic rate is 75 bps. In our design, this information is always transmitted with the longest range acoustic modem installed in the ROV, even when the highest rate acoustic modem is in range: in this way, the latter can be used to lớn transmit more demanding traffic, as presented in Section 3.3. In our design, the longest range acoustic modem installed in the ROV also integrates LBL functionalities, used khổng lồ let the ROV identify its own position with respect lớn the control station. Finally, we assume the ROV to be able to lớn capture high-definition 4160 × 2336 px JPEG images with a kích thước of 2.5 Megabytes in places of high interest (e.g., in the correspondence of valves or fittings). These images will be eventually conveyed lớn the control station.
In this section, we describe the short-range full-capacity wireless mode (SRM) with its requirements in terms of supported types of traffic and their respective bit rates. According lớn the review presented in Section 2, the raw video clip produced by the Blue
ROV camera cannot be transmitted with any commercial off-the-shelf underwater communication system for a distance of more than 10 m; therefore, both a lower resolution and video coding should be employed. For example, from our test, a high-definition h264 đoạn clip with resolution of 704 × 578 px at 25 fps can be sent with an average bit rate of 1.3 Mbps và a peak bit rate of 8 Mbps. With h264 compression, the ratio between maximum and average bit rates is higher than in raw videos due to the fact that, while the quasi static parts of the video clip can be compressed with high efficiency, the frames presenting fast motions cannot be compressed as much. This behavior is caused by the fact that the latter has low correlations between each other and, thus, they require more information to lớn be represented in a smooth video. The đoạn phim size can be reduced by lowering the frame rate or the video clip resolution or by forcing a different clip bit rate. A solution lớn achieve a higher compression màn chơi without losing unique and resolution is khổng lồ employ black & white videos: according khổng lồ our tests, in the same environment, a grey-scale video with the same resolution would require 0.6 Mbps on average and a peak bit rate of 2.93 Mbps. Some ROVs, such as the Ageotec models Pegaso and Perseo <112>, in addition to lớn color clip can also transmit black & white videos, which sometimes can provide better contrast than màu sắc streams. For this reason, in the SRM considered in this paper, we account for the transmission of both a high-quality black & white đoạn phim and a high-quality color video with average bit rates of 0.6 Mbps và 1.3 Mbps, respectively.
Ideally, SRM should provide a Qo
S similar lớn the one obtained through the umbilical connector, where an operator can control the ROV lighting intensity, the cameras, & a gripper.
This mode to be supported requires traffic of at least 4.5 kbps from the control station lớn the ROV, & of approximately 2 Mbps from the ROV to lớn the control station. Clip and control delay should be kept as low as possible, & the đoạn phim delay variance should be minimized. Indeed, it is possible khổng lồ control a vehicle in real time even đoạn phim monitoring with a constant delay of up to lớn 1 s, but it is not possible lớn pilot a vehicle if the clip has a variable delay ranging from 0.1 s khổng lồ 0.5 s <113>. This variation on the delay is called jitter & can be analyzed through the packet delay variation (PDV) metric <114>, computed as follows:
where Nv is the number of packets received for the considered đoạn phim stream & d(i) is the delay of the ith packet. If PDV is less than 3 ms <115>, the đoạn clip stream is smooth & can be used for real-time applications; otherwise, a de-jitter buffer <116> is needed lớn artificially introduce a buffering delay và to mitigate jitter: to obtain a zero-delay jitter video, the buffering delay has khổng lồ be mix equal to the maximum jitter expected by the video.
This mode can be supported up khổng lồ a distance of a few tens of meters (depending on the optical range) by the simultaneous use of one optical modem for clip streaming và joystic remote control & one acoustic modem with LBL to support ROV navigation & to send the monitoring status.
In this mode, called mid-range low-capacity wireless mode (MRM), a real-time ROV remote control is no longer supported. Conversely from SRM, in MRM, position control is based on the transmission of way-points và not on a joystick-based trajectory control. A way-point-based remote control consists of sending not only the next position that the vehicle needs to lớn reach but also the speed & rotation angles to the underwater vehicle, the latter used for setting the orientation of the vehicle while proceeding lớn the next position. The size of each component of position (x, y, & depth), speed (u, v, and w), & rotation angles (roll, pitch, và yaw) in a way-point packet is phối equal to lớn 4 bytes; therefore, together with a sequence number used as unique identifier of the way-point, the total size of a way-point packet is 40 bytes. In this mode, we assume that the minimum time lapse between two consecutive way-points is 2 s; thus, the maximum data rate generated by the control traffic is 160 bps.
The operator can observe the progress of the ongoing mission through a very low-quality non-real-time clip feedback, streamed with a high-frequency acoustic modem. The video considered in this mode is a 200 × 96 px 5 fps VP9 đoạn phim that requires an average bit rate of 13.7 kbps & a peak bit rate of 42 kbps. In <61>, we managed khổng lồ stream the same đoạn clip through the Evo
Logics S2CM HS acoustic modem: although the maximum bit rate is higher than the maximum throughput achievable by the modem in that tested conditions (20 kbps), the đoạn phim was not blocked & the receiver was able khổng lồ smoothly reproduce the đoạn clip with few frame losses. Indeed, 14 frames, over a total of 645, were automatically discarded by the application used for streaming to lớn avoid clip jitter and additional delay. Due khổng lồ the codec setup, stream visualization started 10 s after the beginning of transmission.
MRM can be supported by the simultaneous use of an HF acoustic modem for low-quality clip stream plus the use of a long-range LBL acoustic modem for positioning, status monitoring, và sending the control messages up khổng lồ a distance of a few hundred meters.
In this mode, although the gripper cannot be maneuvered due to the long latency of the video monitoring, still, it is possible for the ROV to take some high-definition pictures of some interesting areas. These pictures will be stored in the ROV and buffered until the system switches lớn SRM mode. MRM requires traffic up khổng lồ 160 bps from the control station khổng lồ the ROV (that includes way-point transmission và control of lights và cameras) and of at least 15 kbps from the ROV to the control station (very low-quality đoạn phim and status monitoring).
Also in the long-range minimum control wireless mode (LRM), real-time remote control is not supported. Similarly khổng lồ MRM, the ROV position is controlled through the transmission of way-points but the low-quality video clip stream cannot be transmitted. This mode supports only position monitoring, LBL positioning, và the transmission of status updates. It makes use of an MF underwater multihop network khổng lồ extend the control range up khổng lồ 10 km from the control station. The operator in this case can only receive information about the ROV position và can transmit the next position that the vehicle needs to lớn reach. In this mode, the average time between the transmission of 2 consecutive way-points is 60 s. Also in this mode, the gripper cannot be maneuvered due to lớn the lack of clip monitoring; still, it is possible for the ROV to take some high-definition pictures once it reaches a way-point. Moreover, an automatic identification system can decide to take some pictures in some areas where there might be damage lớn the underwater asset. These pictures will be stored in the ROV và buffered until the system switches khổng lồ SRM mode. LRM requires traffic up to 50 bps from the control station to the ROV (that includes way-point transmission và control of lights và cameras) và at least 75 bps from the ROV to the control station to lớn transmit periodic status information.
In both the MRM and LRM modes, the remote control requires a high cấp độ of autonomy for the underwater vehicle, that behaves more like an AUV than an ROV.
The simulated scenario, depicted in Figure 4, included an underwater hybrid vehicle (HROV) able khổng lồ operate both as an ROV và an AUV, a control station used to pilot the vehicle, và three relays used to extend the communication range between the vehicle and the control station. Both the control station & HROV were equipped with MF, HF acoustic, and optical modems, while the acoustic relays were equipped only with MF acoustic modems.
All simulations were performed with the DESERT Underwater <117> simulation framework. As presented in Section 2.4, we selected the Blue
Comm 200 optical modem, Subnero WNC, và the Evo
Logics S2C HS communication devices; therefore, the simulations were configured according khổng lồ the modem specifications, as will be presented later in this section & is summarized in Table 5.
The performance of the Blue
Comm 200 optical modem was mapped in the form of lookup tables (LUTs), as presented in <70>, by considering a raw bit rate of 5 Mbps and the use of a forward error correction (FEC) mechanism with 80% efficiency (as declared from the manufacturer); hence, the actual data rate was 4 Mbps. Although the modem performs best in a deep, dark water environment, where oil and gas pipelines are typically deployed, in our simulations, we considered the performance of the modem when used in the presence of external lighting sources, such as the light needed for recording đoạn clip streams, that introduced an important source of noise to lớn the receiver, however, without saturating the photomultiplier. A similar effect was found when the Blue
Comm 200 optical modem was used during the night in shallow water, where it was observed that the moon, stars, & port lighting may reduce the maximum range of the optical communication khổng lồ 1/3.
The maximum range of the optical communication also depends on how the attenuation coefficient changes along the water column: in this work, we used the values of the attenuation coefficient depicted in Figure 5a và measured during the ALOMEX’15 research cruise offshore the coast of Morocco (latitude 30∘42.520’ N and longitude 10∘18.680’ W). The ALOMEX’15 cruise was organized by the NATO STO Centre of Marine Research & Experimentation (CMRE), và its dataset was first presented in <18>.
The last parameter that affects the optical range is the alignment between the transmitter and receiver. Indeed, the beam pattern of the Blue
Comm modem is hemispherical and not omnidirectional: this means that the maximum range and the receiving area also depend on the orientation of the two modems. In Figure 5, different cases are analyzed, keeping the position of the control station fixed at a depth of 60.5 m with the modem aligned to lớn the x-axis & with three different setups of the modem installed in the underwater vehicle. First, in Figure 5b, we can observe the receiving area when the modem is installed in the back of the vehicle, that is assumed to lớn be oriented along the x-axis, pointing directly to lớn the modem in the control station. This leads to a symmetric receiving area and maximum range of 75 m; however, in this case, the vehicle can only be controlled by keeping fixed orientation, thus limiting vehicle maneuver. Two possible solutions can overcome this problem: one involves the installation of several modems on the same vehicle, obtaining omnidirectional coverage at the price of a more expensive deployment <24>, và the other requires installation of the modem either in the đứng top or in the bottom of the vehicle, hence orienting the modem along the z-axis. In Figure 5c,d we can observe how the receiving area và the maximum range are reduced to lớn 50 m when the modem is installed in the top & in the bottom of the vehicle, respectively. Despite the shorter range, these solutions are not affected by the vehicle orientation in the x/y plane, thus providing good maneuverability. In this scenario, a larger coverage area is reached in Figure 5c because the attenuation coefficient decreases when increasing the sea depth (Figure 5a).
The Blue
Comm modems use blue wavelengths (470 nm) và a time division multiple access (TDMA) medium access control (MAC) layer that can split its time frame either into two equal time slots or into two time slots, where the former takes 10% of the TDMA frame và the latter takes 90% of the frame. Hence, we configured the optical MAC layer with this last setting, assigning 90% of the frame to the underwater vehicle for transmitting real-time clip streams & 10% of the frame to lớn the control station for the joystick position control.
The Subnero WNC and the Evo
Logics S2C HS acoustic modems were simulated by using two instances of the DESERT acoustic physical layer, called from here onwards MF PHY and HF PHY, respectively. Both PHYs were half duplex, i.e., they can either receive or transmit, and a mean-power interference mã sản phẩm was employed. We implement the empirical underwater sound propagation & noise models described in <10>, with a spreading coefficient g equal to lớn 1.75 in the spreading loss component, shipping activity s equal to lớn 1, and speed of wind w equal to 5 m/s. The tốc độ of sound underwater was assumed lớn be constant and equal lớn 1500 m/s.
The MF PHY layer’s source level was 168 d
B re μPa2 at 1 m: this value was used lớn match in the simulator the maximum range of 3.5 km declared from the modem manufacturer because, in the simulations, we did not consider transducer inefficiencies & multipath (the maximum source màn chơi of the actual modem was 185 d
B re μPa2 at 1 m). The transmission rate was set to 4 kbps, the carrier frequency was 24 k
Hz, and the bandwidth was 12 k
Hz. Actually, a higher rate can be achieved, as a maximum bit rate of 15 kbps can be reached in good conditions: we used a lower bit rate because we considered horizontal transmissions, that often require adaptation of the bit rate to a lower value và the need for FEC và networking headers. In addition, in our design, the MF PHY was also used for LBL positioning, that usually comes at the cost of a lower communication rate, that motivates our bit rate choice.
The HF PHY layer’s source cấp độ was 156 d
B re μPa2 at 1 m: also in this case, this value was used lớn match in the simulator a maximum range of 500 m (the maximum source level of the actual modem was 177 d
B re μPa2 at 1 m). The net transmission rate was 30 kbps (a higher rate can be achieved, as a maximum bit rate of 62.5 kbps can be reached in good conditions, from which the bits needed for FEC và networking headers need to lớn be removed), the carrier frequency was 150 k
Hz, & the bandwidth was 60 k
Hz.
Xem thêm: Không Thấy File Trong Ổ Đĩa Nhưng Vẫn Có Dung Lượng, Không Hiển Thị File Trong Ổ Cứng Là Lỗi Gì
The application layer used to lớn control the HROV position was first presented in <118>, where the controller drove the underwater vehicle along the desired trajectory by sending absolute movement commands in the form of subsequent way-points to be covered & the speed that the HROV should use to reach that way-point. The y-x displacement of the resulting path when all way-points were reached is presented in Figure 6a, while the depth position changed between 9 m and 11 m. The three acoustic relays are represented with red crosses and were deployed 3000 m apart; the first relay was depicted 3000 m from the control station, that was deployed at the origin of the axes & represented with a red circle. Both the relays and the control station were deployed at a depth of 5 m.
To better visualize the path characteristics in different sections, we depict two zoomed-in parts in Figure 6b,c. The former presents the path in the proximity of the control station, and the latter presents the path from a distance of 200 m up khổng lồ 1450 m far from the control station. The joystick control hwas simulated by sending several way-points close lớn each other. The section of the path depicted in Figure 6b from the coordinates (0, −10) và (20, 10) is the desired trajectory of the joystick control and was taken from the real motion of th