█ K. LEE LERNER
SONAR, an acronym for Sound Navigation and Ranging, is a technique based on echolocation used for the detection of objects underwater.
Historical development of SONAR. Ancient drawings depict the use of long tubes as non-mechanical underwater listening devices to detect and transmit sound in water. In the late nineteeth century, scientists began to explore the physical properties associated with sound transmission in water. In 1882, a Swiss physicist, Daviel Colladen, attempted to calculate the speed of sound in the known depths of Lake Geneva. Based upon the physics of sound transmission articulated by English physicist Lord Rayleigh (1842–1914), and the piezoelectric effect discovered by French scientist Pierre Curie (1509–1906), in 1915, French physicist Paul Langevin (1872–1946) invented the first system designed to utilize sound waves and acoustical echoes in an underwater detection device.
In the wake of the Titanic disaster, Langevin and his colleague Constantin Chilowsky, a Russian engineer then living in Switzerland, developed what they termed a "hydrophone" as a mechanism for ships to more readily detect icebergs (the vast majority of any iceberg remains below the ocean surface). Similar systems were put to immediate use as an aid to underwater navigation by submarines.
Improved electronics and technology allowed the production of greatly improved listening and recording devices. Because passive SONAR is essentially nothing more than an elaborate recording and sound amplification device, these systems suffered because they were dependent upon the strength of the sound signal coming from the target. The signals or waves received could be typed (i.e. related to specific targets) for identifying characteristics. Although skilled and experienced operators could provide reasonably accurate estimates of range, bearing, and relative motion of targets, these estimates were far less precise and accurate than results obtained from active systems unless the targets were very close—or were very noisy.
The threat of submarine warfare during World War I made urgent the development of SONAR and other means of echo detection. The development of the acoustic transducer that converted electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Although active SONAR was developed too late to be widely used during World War I the push for its development produced enormous technological dividends. Early into World War II, the British Anti-Submarine Detection and Investigation Committee (its acronym, ASDIC, became a name commonly applied to British SONAR systems) made efforts to outfit every ship in the British fleet with advanced detection devices. The use of ASDIC proved pivotal in the British effort to repel damaging attacks by German submarines.
SONAR and RADAR. Although they rely on two fundamentally different types of wave transmission, SONAR and Radio Detection And Ranging (RADAR) and both are remote sensing systems. While active SONAR transmits acoustic (i.e., sound) waves, RADAR sends out and measures the return of electromagnetic waves.
In both systems these waves return echoes from certain features or targets that allow the determination of important properties or attributes of the target (e.g., shape, size, speed, distance to target, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are mostly used for ground or atmospheric observations. Because SONAR signals easily penetrate water, they are ideal for navigation and measurement under water. Within the ocean, the speed of sound varies with changes in temperature and pressure, and these conditions can also be determined by alterations in SONAR signals.
SONAR usually operates at frequencies in the 10,000–50,000 Hz range. Higher higher frequencies provide more accurate location data, but propagation losses (i.e. loss of signal strength over distance) also increase with frequency.
█ FURTHER READING:
Van Trees, Harry L. Radar-Sonar Signal Processing and Gaussian Signals in Noise. Indianapolis, IN: John Wiley & Sons, 2001.
Waite, A. D. Sonar for Practising Engineers. Indianapolis, IN: John Wiley & Sons, 2001.
Canadian Center for Remote Sensing, "History of Remote Sensing." 2001. < http://www.ccrs.nrcan.gc.ca/ccrs/org/history/morleye.htm > (February 1, 2003).