Page 20: of Maritime Reporter Magazine (January 2014)
Ship Repair & Conversion Edition
20 Maritime Reporter & Engineering News • JANUARY 2014
Sonar & The USCG
The USCG Plans to Widen its use of the subsea surveillance technology. The question is: is the investment warranted?
The US Coast Guard is planning to widely use sonar to support its maritime security and marine environmental pro- tection missions. The agency currently has limited capability to detect objects below the water’s surface and relies on others (such as the US Navy or the com- mercial sector) when such detection is needed.
Sonar is an acronym for Sound Navi- gation and Ranging. The principle is similar to radar, used in the atmosphere.
While radar uses radio waves, sonar re- lies on sound waves, which propagate much better than radio waves do in wa- ter. There are two major types of so- nar – passive and active. Passive sonar only listens, detecting sounds made by vessels, persons, fi sh, and other things in the water, or the water itself (i.e., waves). Active sonar transmits a sound signal that then is refl ected off an object within range and returns to the trans- ducer. Measuring the time difference between transmission and reception provides an indication of the range of the target. Measuring the angle of the received signal provides an indication of the direction of the target.
Sonar transmissions are made in a va- riety of frequencies, depending upon the intended purpose. Low frequency transmissions have the advantage of very long range. On the other hand, these sonar transmissions can only de- tect very large objects and with mini- mal accuracy relative to range or bear- ing. Mid-frequency transmissions have a range measured in miles and are able to detect objects such as submarines and large whales with good accuracy relative to range and bearing. This is the type of sonar utilized almost exclu- sively by the military, having virtually no commercial application. High fre- quency and ultra-high frequency sonar transmissions have a short range, but are able to detect small objects. This type of sonar is utilized commercially and is the type under consideration by the Coast Guard.
Sonar in the form of echo-sounders has been utilized for years by ships to determine the depth of water under the keel. A signal is transmitted straight down. The signal refl ects back when it hits the seabed. The time difference provides the operator with an accurate measure of the depth. More sensi- tive versions are in use as fi sh fi nders.
These devices differentiate between the strong signal generated by refl ections off the seabed and weaker signals gen- erated by refl ections off objects in the water column. Sophisticated versions provide more detailed information, al- lowing determination of the size of the fi sh school, etc. Sonar transmitters can even be attached to trawl nets, allowing for better placement of the nets relative to the target species.
Stronger and more sophisticated sonar can be used to penetrate the upper layer of the seafl oor, allowing determination of characteristics such as bottom type (i.e., mud, sand, gravel) and depth to hard strata. This information can prove valuable for determination of anchor- age grounds and for laying of subma- rine cables and pipelines.
Sonar is used commercially to exam- ine hulls, pilings, and underwater struc- tures such as offshore platforms. This type of sonar is sometimes handheld by a diver or mounted on the end of a pole.
Increasingly, it is mounted on remotely operated vehicles (ROVs) or on un- manned underwater vehicles (UUVs).
ROVs have greater capabilities regard- ing power and data transmission, since they are tethered to a shore location or to a manned vessel. UUVs have less power and data transmission capabili- ties, but can get into tighter spaces and have longer potential range.
Diver detection sonar is used for detec- tion of divers and submerged swimmer delivery systems, such as those used in several of the James Bond movies. This sonar provides detection, classifi cation, and tracking information on human un- derwater incursions that could endanger lives or property. It is being employed around some marine facilities world- wide, but not yet to a great extent.
Limpet mine imaging sonar is used for detection of small underwater objects.
Originally used for detection of limpet mines potentially attached to the hulls of ships, it can now be used to detect caches of drugs and other contraband.
This type of sonar may also be used to detect hull damage and underwater structural damage.
The Coast Guard proposes to utilize commercially-available sonar equip- ment to broaden its capability to locate, image, and classify submerged and un- derwater targets of interest (TOIs). This would include such things as terrorist
BY DENNIS L. BRYANT
A civilian contractor steadies a M18
Mod 2 Kingfi sh Unmanned Underwater
Vehicle (UUV) as it is lifted with a crane onto the deck of an 11-meter rigid-hull infl atable boat. The Kingfi sh uses side scan sonar to search and discover ob- jects of interest. This marks the fi rst time these UUVs have been added to mine countermeasure operations in the
U.S. 5th Fleet area of responsibility. (U.S. Navy photo by Mass Communication Specialist 2nd Class Blake Midnight/Released)
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