what is SONAR?

Question

its uses& properties

Answer 1

GOLDSMITH
OR
SONAR (SOund Navigation And Ranging) — or sonar — is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. There are two kinds of sonar — active and passive. Sonar may be used as a means of acoustic location.

Acoustic location in air was used before Radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. Acoustic emission measurements can also be made for crack detection.

The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic.

The study of underwater sound is sometimes known as hydroacoustics.


SH

Answer 2

SONAR (SOund Navigation And Ranging) — or sonar — is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. There are two kinds of sonar — active and passive. Sonar may be used as a means of acoustic location.

Acoustic location in air was used before Radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. Acoustic emission measurements can also be made for crack detection.

The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic.

Answer 3

Extract from local newspaper, Cornish Guardian. CONCERN is mounting that Royal Naval exercises could be responsible for the deaths of 26 dolphins off the Roseland peninsula on Monday. Despite initial denials of involvement by the Royal Navy, the Ministry of Defence confirmed it was carrying out live firing exercises in Falmouth Bay just hours before the dolphins beached themselves in the Fal Estuary. Marine experts, who say it is one of the worst cases of mass stranding ever in the UK, said the real reason for the disaster may never be known, but the animals could have been panicked by an underwater disturbance. Suspicion centres on sonar activity and firing by Royal Navy training ships and a submarine which were known to be in the area. Live firing exercises had been heard by Roseland residents for two to three weeks, day and night. There were also reports of an explosion in Falmouth Bay over the weekend

Answer 4

Uday is correct...but as a former antisubmarine warfare (ASW) aviator I thought I might add some experential insight to his answer.

On ASW aircraft (e.g., P-3 and S-2) we typically used something called Julie and Jezebel...both are SONAR based sub detection systems. Jule was active sonar in that the aircraft would drop practice depth charges (PDC's) of about 1.1 lbs TNT explosive power that created the sound wave. If you are in a submarine when one of these goes off nearby, it makes a lot of noise.

The sound wave traveled out from the PDC and intersected sonobuoys that hear the sound and transmitted it to a receiver on the aircraft. Some of that sound traveled directly to one or more of the several sonobuoys laid out in very specific patterns on the water.

But some of the sound traveled out from the PDC and bounced off solid submerged objects, like a submarine. That bounced sound wave is called an "echo," for obvious reasons.

If we were lucky to get one echo each from three or more sonobuoys, we were able to triangulate or fix the position of that submarie relative to the sonobuoys receiving the echos. However, due to the inaccuracies of the positions of the sonobuoys, we were lucky to have a fix plus or minus several hundred yards of uncertanty.

Unlike the active (PDC) sounds, which are called pings when submarines create the sound, Jezebel is passive. This means that the sounds are created by the target...which could be shrimp schools, whales in love, nuclear submarines, or anything else that makes underwater noise.

With Jez, multiple frequencies of sound are listened to. Each frequency has distinct sources (like propellers, engine turbines, auxillary equipment, and even people talking). And each frequency has different levels of energy that travel at different speeds through water.

These levels can be traced out on a form of graph paper or on an electronic screen into a pattern called a finger print. Some Jez operators are so good at reading these finger prints, they can actually tell the pilot in command which sub they are tracking and who the sub's skipper is. Angle and distance information is calculated from the different frequencies and their spreads on the finger print.

SOSUS is a fixed base passive sonar located up and down our ocean coastlines. Like Jez, they create finger prints for tracking and identifying submarines. Because they have much larger receiver apertures, their ranges of detection are much longer than Jez ranges based on relatively small microphones of the sonobuoys.

Submarines carry both active and passive sonar capabilities. The "ping" in "Red October" the movie, was from an active sonar of the Soviet submarine. The major disadvantage of active sonar is that the ping gives the submarine's position away. It is unlikely that a submariner would use active sonar in a combat situation. They are good for navigation around underwater obsticals however.

Some green protesters object to active sonar because the very big ones pour a lot of energy into the water...sometimes for hundreds if not thousands of miles around. The protesters claim this energy is harming the ecosystem in which whales, dolphins, and other sea life live. That claim has yet to be proved by third party, non-political parties having no agenda or bias in the issue.

Although I've been out of the ASW business for a number of years, I've been told by current ASW aviators that the aircraft are now equipped with water penetrating radar rather than sonar systems to detect submarines. The major advantage of subs is their ability to hide. Once that is removed, subs are sitting ducks and easily destroyed.

Answer 5

SONAR originates (from “sound navigation ranging”),technique for detecting and determining the distance and direction of underwater objects by acoustic means. Sound waves emitted by or reflected from the object are detected by sonar apparatus and analyzed for the information they contain.

Sonar systems may be divided into three categories. In active sonar systems an acoustic projector generates a sound wave that spreads outward and is reflected back by a target object. A receiver picks up and analyzes the reflected signal and may determine the range, bearing, and relative motion of the target. Passive systems consist simply of receiving sensors that pick up the noise produced by the target (such as a ship, submarine, or torpedo). Waveforms thus detected may be analyzed for identifying characteristics as well as direction and distance. The third category of sonar devices is acoustic communication systems, which require a projector and receiver at both ends of the acoustic path.

Sonar was first proposed as a means of detecting icebergs. Interest in sonar was heightened by the threat posed by submarine warfare in World War I. An early passive system, consisting of towed lines of microphones, was used to detect submarines by 1916, and by 1918 an operational active system had been built by British and U.S. scientists. Subsequent developments included the echo sounder, or depth detector, rapid-scanning sonar, side-scan sonar, and WPESS (within-pulse electronic-sector-scanning) sonar.

The uses of sonar are now many. In the military field are a large number of systems that detect, identify, and locate submarines. Sonar is also used in acoustic homing torpedoes, in acoustic mines, and in mine detection. Nonmilitary uses of sonar include fish finding, depth sounding, mapping of the sea bottom, Doppler navigation, and acoustic locating for divers.

A major step in the development of sonar systems was the invention of the acoustic transducer and the design of efficient acoustic projectors. These utilize piezoelectric crystals (e.g., quartz or tourmaline), magnetostrictive materials (e.g., iron or nickel), or electrostrictive crystals (e.g., barium titanate). These materials change shape when subjected to electric or magnetic fields, thus converting electrical energy to acoustic energy. Suitably mounted in an oil-filled housing, they produce beams of acoustic energy over a wide range of frequencies.

In active systems the projector may be deployed from an air-launched sonobuoy, hull-mounted on a vessel, or suspended in the sea from a helicopter. Usually the receiving and transmitting transducers are the same. Passive systems are usually hull-mounted, deployed from sonobuoys, or towed behind a ship. Some passive systems are placed on the seabed, often in large arrays, to provide continuous surveillance.

Answer 6

It is a representation of physical spaces determined by emitting soundwaves and collecting their reflections. By analyzing the information that is received, you can calculate distances between you and the objects the soundwaves bounced off of.

A number of animals employ this technique, including cetaceans (whales and dolphins) and bats.

The Navy also uses this technology to detect submarines, which has led to a lot of controversy recently.

(See below for link to controversy)

Answer 7

SONAR (SOund Navigation And Ranging) — or sonar — is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. There are two kinds of sonar — active and passive. Sonar may be used as a means of acoustic location.

Acoustic location in air was used before Radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. Acoustic emission measurements can also be made for crack detection.

The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic.

WORKING
Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound speed is slower in fresh water than in sea water. In all water sound speed (sometimes called velocity though this is incorrect) is affected by density (or the mass per unit of volume). Density is affected by temperature, dissolved molecules (usually salinity), and pressure. The speed of sound (in feet per second) is approximately equal to:

4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet) + salinity (in parts-per-thousand)).
This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters 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 may be present in shallower coastal waters, however, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation. Increased pressure increases the density of the water and raises the sound speed. Increases in sound speed cause the sound waves to refract away from the area of higher sound speed. 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. The ocean must be at least 6000 feet (1850 meters) deep, or the sound waves will echo off the bottom instead of refracting back upwards, and the loss at the bottom reduces performance. Under the right conditions these sound waves will then be focused near the surface and refracted back down and repeat another arc. Each focus at the surface is called a convergence zone (CZ). This CZ is in the form of an annulus about the sonar. The distance and width of the CZ depends on the temperature and salinity of the water. In the North Atlantic, for example, CZs are found approximately every 33 nautical miles (61 km), depending on the season, 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. With powerful sonars 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, i.e. can detect despite low signal-to-noise ratios.

If the sound source is deep and the conditions are right, propagation may occur in the 'deep sound channel'. This provides extremely low propagation loss to a receiver in the channel. This is because of sound trapping in the channel with no losses at the boundaries. Similar propagation can occur in the 'surface duct' under suitable conditions. However in this case there are reflection losses at the surface.

In shallow water propagation is generally by repeated reflection at the surface and bottom, where considerable losses can occur.

Sound propagation is also affected by absorption in the water itself as well as at the surface and bottom. This absorption is frequency dependent, with several different mechanisms in sea water. Thus sonars requiring to operate over long ranges tend to utilise low frequencies to minimise absorption effects.

The sea contains many sources of noise that interfere with the desired target echo or signature. The main noise sources are due to waves and shipping. The motion of the receiver through the water can also cause low frequency noise, which is speed dependent.

Answer 8

Sonar means sound.
http://en.wikipedia.org/wiki/Sonar