Sonar

Sonar (sound navigation and ranging) is a technique that uses sound propagation under water to navigate or to detect other watercraft. There are two kinds of sonar, active and passive.

History

In 1906, Lewis Nixon invented the very first Sonar type listening device, as a way of detecting icebergs. During World War I, with the need to detect submarines, interest in Sonar was increased. Paul Langevin invented the first sonar type device for detecting submarines in 1915. His work influenced future of sonar designs. These first sonar devices were passive listening devices.

By 1918, both the United States and Britain had built active systems. In World War II, the American term Sonar (an acronym for SOund, NAvigation and Ranging) for this system was used. The British called their system ASDICS (an acronym for Anti-Submarine Detection Investigation Committee).

Active sonar

Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse.
To measure the distance to an object, one measures the time from emission of a pulse to reception. To measure the bearing, one uses several hydrophones, and measures the relative arrival time to each in a process called beamforming.

The first active sonar technology was originally called ASDIC after the "Allied Submarine Detection Investigation Committee".

The pulse may be at constant frequency or a chirp of changing frequency. For a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total energy were emitted. In practice, the chirp signal is sent over a longer time interval; therefore the instantaneous emitted power will be reduced, which simplifies the design of the transmitter. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound.

The most useful small sonar looks roughly like a waterproof flashlight.
One points the head into the water, presses a button, and reads a distance. Another variant is a "fishfinder" that shows a small display with shoals of fish. Some civilian sonars approach active military sonars in capability, with quite exotic three-dimensional displays of the area near the boat. However, these sonars are not designed for stealth.

When active sonar is used to measure the distance to the bottom, it is known as echo sounding.

Active sonar is also used to measure distance through water between two sonar transponders. A transponder is a device that can transmit and receive signals ('pings') but when it receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transponder transmits an interrogation signal and measures the time between this transmission and the receipt of the other transponder's reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two transponders. This technique, when used with multiple transponders, can calculate the relative positions of static and moving objects in water.

Sonar and marine animals

Some marine animals, such as whales and dolphins, use echolocation systems similar to active sonar to locate predators and prey. It is feared that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating. A recent article on the BBC website (see below) reports findings published in the journal Nature to the effect that military sonar may be inducing some whales to experience decompression sickness (and resultant beachings).

High-powered sonar transmitters can kill marine animals. In the Bahamas in 2000, a trial by the US Navy of a 230 decibel transmitter in the frequency range 3 to 7 kHz resulted in the beaching of sixteen whales, seven of which were found dead. The Navy accepted blame in a report published in the Boston Globe on 2002-01-01. However, at low powers, sonar can protect marine mammals against collisions with ships.

Passive sonar

Passive sonars listen without transmitting. They are usually military (although a few are scientific).

Sonar used in fresh water lakes is different in operation from sonar used at sea. In salt water, sonar operation is affected by temperature, pressure, and salinity. Ocean temperature varies with depth, but at between 30 and 100 metres there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or refracted, off the thermocline. The thermocline is not present in shallower coastal waters. Water pressure also affects sound propagation by increasing the density of the water and raising the sound velocity. Increases in sound velocity cause the sound waves to refract away from the area of higher velocity. The mathematical model of refraction is called Snell's law.

Sound waves that are radiated down into the ocean bend back up to the surface in great arcs due to the effect of pressure on sound. Under the right conditions these waves will then reflect off the surface and repeat another arc. Each arc is called a CZ . CZs are found approximately every 33 nautical miles (61 km), forming a pattern of concentric circles around the sound source. Sounds that can be detected for only a few miles in a direct line can therefore also be detected hundreds of miles away. Typically the first, second and third CZ are fairly useful; further out than that the signal is too weak, and thermal conditions are too unstable, reducing the reliability of the signals. The signal is naturally attenuated by distance, but modern sonar systems are very sensitive.

Identifying sound sources

Military sonar has a wide variety of techniques for identifying a detected sound. For example, U.S. vehicles usually operate 60 Hz alternating current power systems. If transformers are mismounted (without proper vibration insulation from the hull), or flooded, the 60 Hz sound from the windings and generators can be emitted from the submarine or ship, helping to identify its nationality. In contrast, most European submarines have 50 Hz power systems. Intermittent noises (such as a wrench being dropped) may also be detectable to sonar.

Passive sonar systems usually have large sonic databases. A computer system frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released), and even particular ships. Most navies continuously update the information in these databases.

Noise

Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. For this reason, many submarines operate nuclear reactors that can be cooled without pumps, using silent convection, or fuel cells or batteries, which can also run silently. Vehicles' propellers are also designed and precisely machined to emit minimal noise. High speed propellers often create tiny bubbles in the water, and this cavitation has a distinct sound.

The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself. Towed units also combat the thermocline, as the unit may be towed above or below the thermocline.

For many years, the United States operated a large set of passive sonar arrays at various points in the world's oceans, collectively called SOSUS. As permanently mounted arrays in the deep ocean, they were very quiet.

In war-time, emission of an active pulse is so compromising for a submarine's stealth that it is considered a very severe breach of tactics.

The display of most passive sonars used to be a two-dimensional waterfall display. The horizontal direction of the display is bearing. The vertical is frequency, or sometimes time. Another display technique is to colour-code frequency-time information for bearing. More recent displays are generated by the computers, and mimic radar-type plan position indicator displays.

Sonar in warfare

Active sonars are extremely useful since they give the exact position of an object. Active sonars work the same way as radars: a signal is emitted. The sound wave then travels in many direction from the emitting object. When it hits an object, the sound wave is then reflected in many other directions. Some of the energy will travel back to the emitting source. The echo will enable the sonar system or technician to calculate, with many factors such as the frequency, the energy of the received signal, the depth, the water temperature, etc., the position of the reflecting object.
Using active radars is somewhat hazardous however, since it does not allow the sonar to identify the target, and any vessel around the emitting sonar will detect the emission. Having heard the signal, it is easy to identify the type of sonar (usually with its frequency) and its position (with the sound wave's energy).
Morever, active sonars, similarly to radars, allow the user to detect objects at a certain range, but also enable other platforms to detect the active sonar at a far greater range.
That is due to the power of the signal: for instance, an actve radar emits a signal of 20 W and is capable of detecting an echo of 4 W (these values are hypothetical).
A target is at 20 km. If that target receives a signal of 8 W (20-4)/2, 20 km is the maximum range of the sonar, since a farther object will radiate less than 4 W.
In that case, a target at 30 km will "hear" the sonar, but the sonar will not be in range and will not detect it. That example is over-simplified, since there are losses due to the soundwave's dispersion in the water, the water's absorption, the size of the target, and the depth, which all affect the reception of the echo.

Since active radars do not allow an exact identification and are very noisy, this type of detection is used by fast platforms (planes, helicopters) and by noisy platforms (most surface ships) but rarely by submarines.

Passive sonars have fewer drawbacks. Most importantly, they are silent. Generally, they have a much greater range than active sonars, and allow an identification of the target. Since any motorized object makes some noise, it may be detected eventually.
It simply depends of the amount of noise emitted and the amount of noise in the area, as well as the technology used.
To simplify, passive sonars "see" around the ship using it.
On a submarine, the nose mounted passive sonar detects in directions of about 270°, centered on the ships alignment, the hull-mounted array of about 160° on each side, and the towed array of about 300°. The no-see areas are due to the ship's own interference. Once a signal is detected in a certain direction (which means that something makes sound in that direction, this is called broadband detection) it is possible to zoom in and analyse the signal received (narrowband analysis).
This is generally done with the help of an FFT fast fourier transform, the sum of sines at different frequencies which make up the sound. Since every engine makes a specific noise, it is easy to identify the object.
Another use of the passive sonar is to determine the target's trajectory (TMA Target Motion Analysis). That is done by marking from which direction the sound comes at different times. By using another tool, which allows the sonar to count the number of turns per knot of the vessel, it is possible to plot a ship's trajectory.

Passive sonars are stealthy and very useful. However, they require high-tech components (band pass filters, receivers) and are costly. They are generally deployed on expensive ships in the form of arrays to enhance the detection.
Surface ships use them to good effect, they are even better used by submarines, and they are also used by planes and helicopters (mostly to a "surprise effect" since submarines can hide under thermal layers.
If the submarine believes it is alone, it may go closer to the surface and be easier to detect, or go deeper but also faster, and therefore make more sound).

In the United States Navy, a special badge known as the Integrated Undersea Surveillance System Badge is awarded to those who have been trained and qualified in sonar operation and warfare.

See also