Long-Range Underwater Acoustic Communication
Long-range underwater communications rely mostly on acoustic communications, which are characterized by very low bitrates, thus making the transfer of large amounts of data too slow. Underwater acoustic systems are the onlyway to provide geo-location and telemetry in ice-covered regions. The acoustic solutions for basin- and region-scale tomography and communicationsrequire low-frequency and reliable broadband deepwater sound transmitters. The existingunderwater navigation systems are based on the traditional 260 Hz and 780 Hz RAFOS narrow band organ-pipe sound sources with temperature-compensated quartz clocks. Suchsystems appear less reliable for under-ice use than desired. Frequencies below 50 Hz are recommended for ranges more than one thousand kilometres in under-ice propagation conditions in order to reduce scattering loss from surface ice.
The higher the frequency, the faster the sound is absorbed. Even though acoustics and optics share many fundamental similarities, their practical applications have very different environmental constrains. For long-range underwater communication, optical waves suffer from strong scattering, while radiofrequency has strong absorption. Therefore, acoustic wave is considered a viable method for long-range underwater communications. However, current acoustic communication using frequency modulation is rather primitive because the allowed narrow-frequency bandwidth limits the data transmission rate.
One of the main classes of problems solved using the AUV are survey and search tasks. They consist in covering a certain area under water for the purpose of searching and examining given objects. In Russia, studies on the creation of more effective and reliable methods for solving survey and search tasks in relation to underwater vehicles are conducted at IPMT, FEB, RAS and other organizations.
In the past underwater unmanned vehicles were connected by cable to a surface vessel or ship, from the board of which they are controlled. The cable length limits the range of the apparatus, and when working at great depths, its length and weight are constrained by maneuverability. In addition, cable hooks are likely due to bottom irregularities and various kinds of obstacles, which threatens with the loss of the apparatus. That is why in recent years the emphasis has shifted more and more towards the AUV, especially in the naval sphere. They dive deeper and swim further. Through sonar communication, they can also be controlled from the board of a surface vessel, as well as receive color video images from the depths in real time. True, sonar capabilities are also limited. Its stability depends on the range of the control object, salinity of sea water, temperatures at different depths, and other factors. Therefore, the most promising military vehicles are recognized as fully autonomous, that is, operating according to a given program, which can be adjusted when the AUV emerges to the surface.
Long range underwater acoustic communication is a decisive milestone for very long cruising AUVs deployment (>1,000 km). Due to their multipath interactions, underwater acoustic (UWA) channels are considered one of the most challenging wireless communication environments. The slow sound speed, the sound absorption and the variability of water can produce serious interference in UWA signals. Especially in shallow water channel, specific multipath interference is strong enough to cause severe inter-symbol interference (ISI) resulting in symbol errors in UWA communication. Time reversal mirror (TRM) can be used for time compression and spatial focusing, making channel equalization and multipath interference reduction come true, in the absence of any prior knowledge, so it is widely studied in the field of UWA communications.
Underwater detection of objects by echo-ranging principles poses the problem of discovery. It would be desirable to employ a passive technique for detection by making triangulation measurements from two listening underwater stations, such as submarines. For triangulation ranging, however, it is necessary that the two submarines be in communication with each other and this communication must be undetectable to the object to preserve security. The development of a secure underwater communication system would thereby greatly increase the effectiveness of submarines.
Small, maneuverable underwater vehicles are in extensive use. Such craft are normally transported to a deployment site on a surface control craft, placed in the water, and submerged and operated in the vicinity of the surface craft. Command control of such vehicles by the surface craft is desirable from the standpoints of operational safety and effectiveness. A two-way communication link is necessary to effect the command and control capability.
Communication links which satisfy this objective have been provided either through a direct umbilical link between the surface and submerged crafts or through a sonar link. The tethered, umbilical link limits the range and maneuverability of the undersea vehicle as does the directionality of the sonar link. A two-way communication link utilizing radio broadcast techniques is obviously desirable from the standpoint of enhancing the freedom of movement of the undersea vehicle as well as providing omni-directionality to the communication link.
Most equipment operating under water is currently being implemented with either hydraulic or electronic/hydraulic systems which require the use of either electrical cables or hydraulic pilot control cables, extending from the sea to a remote control center which, in turn, is controlled remotely from either a submarine, a surface platform or shore station. Since all of these systems depend upon maintaining the integrity of a "hard wire" in the water, there is increasing concern that catastrophic failures may occur if this link parts for any reason. An acoustic communication link that propagates control commands directly through the water would provide a method of maintaining control in the event of failure of the hard wire link. Other examples of activities and operating conditions under which an acoustic communication link could be used to advantage will be evident.
Concomitant with increased interest and importance in marine resources exploration, marine environmental surveillance and underwater military defenses, demand on underwater communication capable of collecting multifarious underwater information from oceans has recently increased. The underwater communication is performed using ultrasonic waves because of physical properties of media. A communication network for underwater information transmission may be implemented by installing a sensor node capable of performing transmission/reception of underwater information under underwater environments, and by obtaining and controlling underwater information from the sensor node.
Very long-range underwater acoustic communication (UAC) is crucial for long cruising (>1000?km) autonomous underwater vehicles (AUVs). Very long-range UAC source for AUV must exhibit high electro-acoustic efficiency (>60%) and compactness. The Janus-Hammer Bell (JHB) transducer has been designed for this purpose and meets those requirements. The transducer works on the 450–550 Hz bandwidth and reaches source level above 200 dB (ref. 1µPa at 1m) with 1kW excitation and full immersion capability. JHB source has been used for communication experiments by the Japanese institute for marine technology (Japan Agency for Marine-Earth Science and Technology) achieving a baud rate of 100 bits/s at 1000km.
Because of underwater communication environments using ultrasonic waves, the underwater communication network is relatively smaller in bandwidth of signal that is transmitted than the land communication, and signal attenuation relative to distance is also very large. That is, frequencies used in underwater communication network have to be very limited in order to perform a reliable communication at a distance of several kilometers to several scores of kilometers.
Moreover, when the demand on underwater information obtainment using underwater communication network increases, the number of sensor nodes performing communication in the underwater also increases. However, the conventional underwater communication network failed to efficiently control the sensor nodes due to limitation of frequencies useable at the underwater channel environments. That is, when only one frequency is used to perform the communication in the conventional underwater communication network, and when a relevant frequency is allocated to one sensor node, all other sensor nodes cannot transmit or receive a signal.
Furthermore, when communication is performed using a plurality of frequencies in the conventional underwater communication network, and when the number of sensor nodes wanting to perform a communication in the water is greater than the allocated frequency, the underwater sensor nodes as many as the number exceeding the allocated frequency cannot transmit or receive a signal. In addition, in this case, all sensor nodes must continuously inspect what frequency an ambient sensor node uses by being allocated, such that battery consumption greatly increases in the water to greatly decrease an operation period of underwater sensor node.
Thus, the conventional underwater communication network has limited the number of communicable sensor nodes because a plurality of sensor nodes cannot be efficiently managed. On top of that, it has become inevitable to increase the number of sensor nodes due to increases in various demands on marine information, and therefore, the trend is that an efficient control of underwater communication network in various areas is greatly required.
Sea water can propagate very low frequency acoustic energy over very long distances. For example, acoustic energy 300-400 cycles per second generated at the transducer at a power level over 100 kilowatts can be transmitted over 1000 miles. The speed of sound in sea water is on the order of 0.8 nautical miles per second or about 20 minutes for 1,000 miles. If time is an important factor as in transmitting messages to submarines, it is not practical to design for transmission distances longer than 1,000 miles though it is possible to do so.
A major problem in sending messages acoustically through sea water is the noise level sensed in this band by the receiver on the submarine. For the most part the noise level is the self-noise of the submarine itself. This noise includes many spectral components in the very low frequency band generated by the multitude of machinery components within the submarine. Besides self noise, there is considerable noise in the sea originating from a variety of sources that can interfere with, mask, or drown out a very low frequency signal. In Fundamentals of Sonar, by J. W. Horton, published by U.S. Naval Institute, Annapolis, Md., pages 57-72, there is a discussion of various kinds of acoustic energy in the sea. Therefore, any intelligence imparted to the sea on very low frequency acoustic wave energy for long distance transmission must be of a character clearly discernible from self noise at the receiver and other interfering acoustic energy present in the sea in the vicinity of the receiver.
In United States patent application Ser. No. 285,556, filed May 31, 1963, by Warren A. Tyrrell for Underwater Low Frequency Sonic Communication, now U.S. Pat. No. 3,811,106, assigned to the United States Government, there is described acoustic signals that may be described as slow FM. In that case, messages are made up of a succession of signal characters each of which occupies frequency bands several cycles wide and below 500 cycles per second. The several cycle frequency band for each signal character is swept at a slow linear rate, e.g., two cycles per second per minute, for a period one to five minutes duration. Two signal characters can be transmitted in each band. Sweeping the band in the direction of increasing frequency provides one signal character and sweeping the band in reverse direction provides another signal character. A band on the order of six cycles is suitable for this purpose but may be narrower or considerably wider. A signal character of the type described can be detected in the presence of considerable noise. A submarine is equipped with a low frequency omnidirectional hydrophone, amplifiers and an equipment. The recorder operates with intensity modulation. A six cycle band swept linearly in three minutes is clearly distinguishable from background noise even of comparable or higher intensity.
High-frequency, coherent underwater communications modem performance is impacted by signal-to-noise ratio at the receiver and reverberation levels determined by scattering from the sea surface and seafloor. Waves impact both of these processes by radiating noise when they break and by focusing and scattering surface-reflected sound. They also influence surface reverberation by injecting micro-bubbles into the upper ocean boundary layer that scatter and attenuate sound.
The team of Ultra Electronics Ocean Systems Inc, ERAPSCO, and Lockheed Martin Marion, Inc designed three types of two-way communications devices and associated submarine and shore equipment. Two of the devices – the tethered expendable communications buoy (TECB) Iridium system and the TECB–UHF system – are launched from submarines. The third is an acoustic-to-RF Gateway (A2RF) system that can be launched from both submarines and aircraft. This new fleet capability will enable secure, two-way communications between submarines operating below periscope depth and at tactical speeds with surface ships, aircraft and land-based assets. All classes of U.S. Navy submarines will be equipped with this transformational capability.
The contractor Ultra Electronics is a world leader in underwater acoustic communications technology. It has collective experience with Proteus, Hail, Deep Siren, and USN ACOMMS, packaged in a variety of retrievable and expendable submarine launched SATCOM offerings. Ocean Systems offerings include underwater telephone, advanced acoustic communications sonar, expendable two way submarine satellite communications, situational awareness buoys, and emergency rescue beacons. New lightweight, broadband, highly efficient sonar offerings have been designed to support the next generation of transformational distributed netted sensors and unmanned undersea vehicles.
Wärtsilä ELAC UT 3000 is the latest version of underwater communication systems designed and built by Wärtsilä Elac Nautik. The system is a milestone in the history of underwater communication as it offers proven digital data transmission for the first time. The Wärtsilä ELAC UT 3000 is the first proven system to offer digital data transmission in addition to analogue voice communication. It is already installed in an increasing number of submarines and surface ships worldwide. For submarines, digital communication offers numerous new applications at speed and depth, such as fast exchange of tactical, operational and navigation data. Now, also encoded communication can be realised effectively.