Modern sonar.

#modern sonar

Modern sonar.

Zakharov Alexander Ivanovich,
Krivtsov Alexander Pavlovich,
Sedov Maxim Vyacheslavovich,
Sknarya Anatoly Vasilyevich,
Trusilov Vladimir Tarasovich,
Sharov Vladimir Sergeevich

MODERN SONAR  

First steps in hydroacoustics.

The origins of many modern areas of technology originate in centuries distant from us, when the first important fundamental discoveries in science and technology were made.

Hydroacoustics was no exception to this rule.

At the end of the 15th century, Leonardo da Vinci established that on a stationary ship, using a long hollow tube, lowered at one end into the water and the other end placed to the ear, one can hear ships located at a great distance. He also established that sound in water travels at a certain speed.

The next important step in the development of hydroacoustics was the measurement of the speed of sound in water, which was done by D. Colladon and C. Sturm in 1827. As a result of the accumulated knowledge in this area, in 1877 Lord Rayleigh published a fundamental work on the issues of radiation, propagation and reception of sound — «The Theory of Sound».

In 1880, Pierre and Jacques Curie made an important discovery, which ultimately led to the development of the modern ultrasonic transducer. They noticed that if you press on quartz crystals, an electric charge is generated, the value of which is directly proportional to the force applied to the crystal.

In addition, they demonstrated the opposite effect — when a rapidly changing potential was applied to the crystal, it began to vibrate.

The vacuum triode invented in 1907 by Lee de Forest marked, on the one hand, the beginning of the century of the modern electronics industry, and on the other, the independence of the hydroacoustic system from the sensitivity of the human ear. The sinking of the Titanic in 1912 became a powerful catalyst for the further development of hydroacoustics.

Immediately after this event, L.F. Richardson filed an application with the British Patent Office for the invention of a method for determining distance using an echo signal propagating in water.

In 1916, in France, the Russian electrical engineer Konstantin Shilovsky and the French physicist Paul Langevin, in experiments with condenser resonators and carbon microphones, were able to obtain echo signals from the bottom and a steel plate at a distance of 200 m, that is, for the first time they created a new class of hydroacoustic equipment — the first sonar.

During the First World War, sonars were widely used by the warring parties to detect submarines and surface ships. These were passive noise finders. However, they were not always effective in detecting submarines. In addition, these systems did not allow for a sufficiently accurate determination of the distance to the target.

In 1917, the French physicist P. Langevin demonstrated an active sonar operating at a frequency of 38 kHz to detect submarines. The sonar had a narrow beam pattern and made it possible to determine both the bearing of a target and the distance to it with sufficient accuracy for practical purposes.

In Russia, at the end of the 19th – beginning of the 20th centuries, intensive work was also carried out to create hydroacoustic equipment.

A whole galaxy of famous scientists worked in the field of acoustics and hydroacoustics: F.F. Petrushevsky, A.G. Stoletov, N.E. Zhukovsky and others, and the first successful practical steps in applied hydroacoustics were made by the admiral of the Russian Navy S.O. Makarov.

Thanks to his work, as well as the work of M.N. Beklemishev, R.N. Nirenberg and A.N. Krylov, the Russian fleet had hydroacoustic equipment that allowed it to conduct successful combat operations against the ships of the German Navy. However, for the further development of a systematic approach to the design and analysis of hydroacoustic systems, a more in-depth and comprehensive study of the acoustic environment was necessary.

Features of the propagation of acoustic waves in water

The beginning of comprehensive and fundamental research on the propagation of acoustic waves in water was laid during the Second World War, which was dictated by the need to solve practical problems of wartime.

Experimental and theoretical work continued in the post-war years and was summarized in a number of monographs.

As a result of these studies, some features of acoustic wave propagation in water were identified and clarified: absorption, attenuation, divergence, reflection and refraction. The absorption of acoustic wave energy in seawater is caused by two processes: internal friction of the medium and dissociation of salts dissolved in it.

The first process converts the energy of the acoustic wave into thermal energy, and the second, by converting into chemical energy, takes the molecules out of equilibrium, and they disintegrate into ions. This type of absorption increases sharply with increasing frequency of acoustic oscillation.

The presence of suspended particles, microorganisms and temperature anomalies in water also leads to the attenuation of the acoustic wave in water. As a rule, these losses are small and are included in the total absorption, but sometimes, as in the case of scattering from a ship's wake, these losses can amount to 90%.

The presence of temperature anomalies leads to the acoustic wave entering the acoustic shadow zones, where it can undergo multiple reflections.

The presence of water-air and water-bottom interfaces results in the reflection of an acoustic wave from them, and if in the first case the acoustic wave is reflected completely, then in the second case the reflection coefficient depends on the bottom material: a muddy bottom reflects poorly, while a sandy and rocky bottom reflects well.

At shallow depths, due to multiple reflections of the acoustic wave between the bottom and the surface, an underwater sound channel arises, in which the acoustic wave can propagate over long distances.

A change in the speed of sound in water causes the sound “rays” to bend. This is refraction. Refraction of acoustic waves in water determines the formation of specific conditions for their propagation, which lead to the formation of four types of special zones: a sound channel, an isothermal layer, a surface with a negative gradient, and a surface with a positive gradient.

In addition, there is a phenomenon of divergence in space of acoustic radiation, as a result of which its intensity weakens proportionally to the square of the distance from the emitter.

Current state of sonars.

The last decade has been marked by further development of sonar systems (SSS), which was based on the successes achieved in a number of areas of science, in particular, in the field of digital methods of signal generation and processing.

The development of the element base also had a significant impact on the development of SSS. Sonars have become “smarter”, their weight and size characteristics have significantly decreased, and their functional capabilities have expanded.

The very concept of sonar has also changed. If earlier sonars were understood as echolocation devices for detecting submarines, now they are hardware systems for determining the position of underwater and floating objects using acoustic signals, that is, in a broader sense.

A wide variety of systems can be classified as sonar devices: sonars with a sharp directional pattern, echo sounders, circular scanning sonars lowered from a helicopter, towed sonars, hydroacoustic radio buoys, coastal stations for acoustic direction finding and echo signal processing.

Currently, sonars are successfully used to solve both military and purely peaceful, civilian tasks: searching for, detecting and classifying underwater objects, ensuring communication between objects, detecting and studying underwater deposits, ensuring navigation safety, etc.

Today, there are many large and small companies in the world that produce a variety of sonar devices in large quantities, and it is not possible to give a full overview of them in this article. We will limit ourselves to describing only a small number of sonars, adhering to the main varieties listed above.

We will begin the review with side-scan sonars (SSS).

These locators have a narrow antenna pattern in the horizontal plane (1 — 3 degrees) and a wide pattern in the vertical plane (40 — 60 degrees). As a result, the pattern of the receiving and transmitting antenna has a knife-shaped form and is directed perpendicular to the line of motion.

The receiving and transmitting antenna can be located either on board the vessel or on a special towed apparatus. As the carrier moves, a strip is “lit” on the bottom, the dimensions of which in width (perpendicular to the line of motion) are 10 — 14 depths on both sides.

SSS allows obtaining high-quality acoustic images of the seabed and is widely used primarily in searching for objects such as sunken ships, cables, minerals, etc.

These sonars are manufactured by EdgeTech (USA).

It produces several models of towed sonar equipment, among which we will highlight two — DF-1000 Townfish and 272 Townfish. The first model operates at two frequencies of 100 and 500 kHz, and the second — at a frequency of 100 kHz. These sonar equipment are used for bottom surveys at depths of up to 1000 meters. The presence of a 500 kHz channel allows you to distinguish objects on the bottom that are several centimeters in size.

To ensure safe navigation in difficult conditions, as well as for the rapid search for underwater floating objects, circular and sector sonars are used.

Interphase (USA) produces a whole series of scanning sonars — Twinscope, Probe, Outlook (photo 1), Sea Scout, Vista, PC View, PC 180.

These sonars differ from each other in the way they scan the surrounding underwater space and their range.

Twinscope scans in the vertical and horizontal planes, has a range of 365 m forward and 244 m in depth. The scanning beam has an opening angle of 1 degree. The Probe locator scans only in the vertical plane and has the same parameters for range and depth.

The latest model, the PC 180, scans a 180-degree field of view in the horizontal plane ahead of a moving vessel with a 12-degree beam and has a range of 365 m and 244 m in depth.

Photo 1. Scanning sonar
Outlook by Interphase, USA

In addition to Interphase, there are two other large companies that also produce a whole range of circular and sector scanning sonars: Furuno and Simrad.

These sonars are designed to search for and detect fish aggregations, but this does not prevent them from being used to search for other underwater objects, since their potential allows them to detect even a single specimen.

These models operate successfully at short (100 m) and long (2800 m) distances, differ in their frequency range and method of viewing space.

The CSH-5 MARK-2 model from Furuno (photo 2) allows you to monitor the underwater environment in a 360-degree sector with the ability to scan the beam angularly. The CH-26 model (photo 3) from the same company allows for a survey in a sector with a step of 6 degrees, depending on the situation, at one of three frequencies – 60, 88 or 150 kHz.

The SP70 model from Simrad (photo 4) is an omnidirectional low-frequency sonar.

It allows the user to select one of nine frequencies in the range from 22 to 30 kHz and view the surrounding space simultaneously in both the vertical and horizontal planes.

Photo 2. CSH-5 MK-2 sonar
Furuno, USA

Photo 3. Model SP70
by Simrad, Norway

Circular scanning sonars have also found wide application for submarine detection.

Thus, for these purposes, the first all-weather American anti-submarine helicopter S-61/SH-3 was equipped with a dipping sonar AN/ASQ-14, and the Russian helicopter KA 27 — VGS-3. The presence of all-round sonars on board significantly increases the efficiency of submarine searches and allows for the exploration of an area of ​​up to 2,000 km2 in 1 hour.

Recently, echo sounders, which have long been used to ensure navigation safety, have also been further developed.

The main purpose of single-beam echo sounders is to determine the depth under the keel of a vessel.

Along with the development of single-beam echo sounders, a new class of devices has now been developed – multi-beam echo sounders, which allow obtaining depth values ​​not only under the keel of a vessel, but also to the side of it in a strip of up to 3 – 4 depths.

These sonars are widely used in constructing depth maps, to ensure navigation safety, selecting routes for laying communication cables, pipelines, for conducting survey work during the construction of port facilities, etc.

In such acoustic systems, with the help of a special design of the receiving and transmitting antenna and processing of echo signals, many (more than a hundred) narrow beams are obtained, fanned out in the direction to the side of the line of motion of the antenna carrier (usually this is the vessel itself).

The most famous multi-beam echo sounders are the Sea Beam and Seabat echo sounders.

These echo sounders have approximately identical characteristics, with the only difference being that the latter have models operating at higher frequencies — up to 455 kHz. (Seabat 9001, 9003), while Sea Beam echo sounders operate in the frequency range from 12 to 180 kHz.

As an example, we can consider the Sea Beam 2112 model (photo 5), which operates at a frequency of 12 kHz, forms 149 beams with a beam pattern of each beam of about 1 degree, has a range of operating depths from 700 m to 11,000 m and provides a survey band depending on the depth from 2 to 3 depths.

At one time, the USSR paid great attention to the development of hydroacoustics, which was dictated by ensuring the country's security and solving national economic problems. Large scientific and production centers for the development and manufacture of sonars were created in the country, and fundamental research in this area was carried out in large quantities. However, the collapse of the USSR and the events that followed had a negative impact on the development of hydroacoustics.

But, despite all the difficulties, these works are currently continuing in our country: a new generation of sonars is being developed. However, the lack of sufficient information about domestic developments in this area leads to the fact that many consumers of these products turn to foreign, usually more expensive developments, although today many domestic models are in no way inferior to them in terms of the most important parameters.

It is also necessary to note another very important factor: operational support. As a rule, foreign systems are completely “closed”, and their “tuning to the customer requires significant time and additional funds.

Among domestic developments to date, it is worth noting the developments of the Far Eastern Branch of the Russian Academy of Sciences, where several projects have been implemented:

• development of a towed side-scan sonar with a two-side coverage of up to 1,500 m, a range resolution of 30 cm and an angle of 1.5 degrees,
• development of a sector-scan sonar with a coverage sector from 10 to 360 degrees, a range resolution of 10 cm and an angle of 1 degree, and a range of up to 75 meters.

TsNII Gidropribor has also developed a high-frequency towed side-scan sonar with a 120-meter swath and 3 cm resolution.

The Gidra sonar, developed and manufactured jointly by Ekran and the V.V. Tikhomirov Research Institute of Instrument Engineering, is another example. As noted above, high-quality acoustic images of the bottom can be obtained using the GBO.

However, since the acoustic image depends on both the bottom relief shape and the type of soil, there is ambiguity in interpreting the acoustic image of the bottom. This ambiguity can be resolved by comparing the acoustic image of the bottom and the data on its relief.

In this regard, it is of great interest to develop and create a single complex that would be able to simultaneously obtain both an acoustic image of the bottom and its relief.

In particular, such a complex may include SSS with phase or interferometric channels.

In this case, to obtain high measurement accuracy, it is necessary to take into account a number of destabilizing factors, such as the accuracy of the vessel and towed apparatus coordination, the speed of propagation and refraction of acoustic waves in water, the list and trim, and the accuracy of measuring echo signal parameters.

Large volumes of processed information and the complexity of processing algorithms for solving this problem require the widespread use of automation tools of the complex using digital methods for generating and processing signals.

The Hydra sonar is designed to obtain high-quality acoustic images of the bottom and its relief and combines a side-scan sonar and an interferometer.

It is the first of a series of similar devices planned for release, designed to solve both hydrographic problems when studying the bottom at depths from a few to 1,500 meters, and problems of searching for and detecting underwater objects.

When developing this sonar, the goal was to create a compact, easy-to-use automated complex for work on rivers and the shelf using digital methods of signal generation and processing and a modern element base. The complex can be installed both on board small boats and on board large-displacement vessels.

The sonar includes: an antenna unit (photo 4), a power amplifier unit, a signal receiving and conversion unit, and a PC.

All units, except for the PC, as well as the board for generating probing signals and pulse sequences, were developed anew.

A standard IBM PC is used as a PC. The entire sonar, except for the PC, fits into a container measuring 700x700x300 mm.

The complex is powered either by a 220 V AC network or by on-board batteries.

Photo 4. Antenna unit
of the Hydra sonar, Russia

The antenna unit consists of the left and right side transmitting and receiving antennas operating at a frequency of 240 kHz, and the receiving antennas of the interference channels.

The total weight of the antenna unit together with the cable is about 10 kg.

The antenna unit provides the ability to change the angle of the antenna plane relative to the vertical in the range from 0 to 30 degrees.

The power unit includes power amplifiers for both sides, a protection system and power supplies.

The unit weighs about 6 kg, and its dimensions are 300x300x160 mm.

The processing unit is based on a PC with a special generator board that generates probing signals and pulse sequences that synchronize the operation of the entire complex.

To increase the energy potential required for offshore operations, the complex provides for the use of a probing signal with linear-frequency modulation (LFM).

The sonar software runs under the WINDOWS system and allows the sonar to be controlled in interactive mode.

The software consists of programs for primary and secondary (office) processing.

Primary processing provides for the display of information on the monitor screen in real time, its archiving on a hard drive and allows the operator to carry out both control and diagnostics of the complex in interactive mode.

The presence of high-resolution side-scan channels in the sonar allows the system to be used for solving problems of searching for and classifying small objects, such as cables, sunken boats and ships, etc. on the bottom.

During secondary processing, taking into account destabilizing factors, such as navigation data, data on the distribution of acoustic wave velocity by depth, roll-trim, a calculation is made for each depth line in the survey strip, geometric distortions of the acoustic image of the bottom are eliminated, individual lines are stitched together, and the processing results are laid out in a single tablet.

As an example, Fig. 1 shows one of the options for presenting data obtained after secondary processing.

Fig. 1. One of the options for presenting the results of secondary data processing. The numbers in the figure indicate the distance in meters.

Table 1

Technical characteristics of the Hydra sonar

The sonar can be used for different purposes:

— seabed exploration for the purpose of preparing the laying of pipelines and cables, construction of bridges and other underground structures;
— exploration of fairways;
— exploration of the condition of underwater structures;
— search for sunken and other underwater objects;
— monitoring of underwater moving objects;
— observation of the underwater situation when moving in unknown waters on a boat, yacht or large vessel.

There are several ways to recommend using a sonar to solve various problems.

So, if the task is to study the bottom relief, then the most suitable is the information from the interferometer.

The user can get a three-dimensional image of the bottom relief (see Fig. 1), which can be used to assess the possibilities of navigation.

With such a tool for quickly obtaining a bottom map, it is easy to obtain information about the possibility of mooring in an unfamiliar area during a boat or yacht trip.

If the task is to detect a small object, it is more convenient to use mainly an acoustic image, which provides a contrast picture from objects of different densities.

Small objects include not only some objects, such as cables, stones, boxes, etc., but also, for example, cracks in an underwater structure.

Combining both information arrays can provide additional information. This can be used to detect swimmers or large marine animals.

On this basis, it becomes possible to build systems to protect beaches from sharks or various objects from underwater saboteurs.

As noted above, side-view systems operate from a moving vessel.

However, there are a number of tasks that would be desirable to solve from a stationary point, for example, from the shore. These are security tasks and tasks of monitoring the state of the water area.

Systems with scanning antennas, which are currently under development, can be proposed for such tasks.