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Although they rely on two fundamentally different types of wave transmission, Radio Detection and Ranging (RADAR) and Sound Navigation and Ranging (SONAR) both are remote sensing systems with important military, scientific and commercial applications. RADAR sends out electromagnetic waves, while active SONAR transmits acoustic (i.e., sound) waves. In both systems, these waves return echoes from certain features or targets that allow the determination of important properties and attributes of the target (i.e., shape, size, speed, distance, 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.

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 useful during World War I, the push for its development reaped enormous technological dividends. Not all of the advances, however, were restricted to military use. After the war, echo sounding devices were placed aboard many large French ocean-liners.

During the early battles of 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 upon both British warships and merchant ships keeping the island nation supplied with munitions and food.

While early twentieth century SONAR developments proceeded, another system of remote sensing was developed based upon the improved understanding of the nature and propagation of electromagnetic radiation achieved by Scottish physicist James Clerk Maxwell (18311879) during the 19th century. Scottish physicist and meteorologist Sir Robert Alexander Watson-Watt (18921973) successfully used short-wave radio transmissions to detect the direction of approaching thunderstorms. Another technique used by Watson-Watt and his colleagues at the British Radio Research Station measured the altitude of the ionosphere (a layer in the upper atmosphere that can act as a radio reflector) by sending brief pulses of radio waves upward and then measuring the time it took for the signals to return to the station. Because the speed of radio waves was well established, the measurements provided very accurate determinations of the height of the reflective layer. In 1935, Watson-Watt had the ingenious idea of combining these direction and range finding techniques and, in so doing, he invented RADAR. Watson-Watt built his first practical RADAR device at Ditton Park.

Shortly thereafter, without benefit of a test run, WatsonWatt and Ministry scientists conducted an experiment to test the viability of RADAR. Watson-Watt's apparatus was found able to illuminate (i.e., detect) aircraft at a distance of up to eight miles. Within a year, Watson-Watt improved his RADAR systems so that it could detect aircraft at distances up to seventy miles. Pre-war Britain quickly put Watson-Watt's invention to military use and by the end of 1938, primitive RADAR systems dotted the English coast. These stations, able to detect aircraft regardless of ground fogs or clouds , were to play an important role in the detection of approaching Nazi aircraft during World War II. By the end of the war, the British and American forces had developed a number of RADAR types and applications including air interception (AI), air-to-surface vessel (ASV), ground controlled interception (GCI), and various gun sighting and tracking RADARs.

Regardless of their application, both RADAR and SONAR targets scatter, deflect, and reflect incoming waves. This scattering is, however, not uniform, and in most cases a strong echo of the image is propagated back to the signal

transmitter in much the same way as a smooth mirror can reflect light back in the specular direction. The strength of the return signal is also characteristic of the target and the environment in which the systems are operating. Because they are electromagnetic radiations, RADAR waves travel through the atmosphere at the speed of light (in air). SONAR waves (compression waves) travel through water at much slower pace, the speed of sound. By measuring the time it takes for the signals to travel to the target and to return echoes, both RADAR and SONAR systems are capable of accurately determining the distance to their targets.

Within their respective domains, both RADAR and SONAR can operate reliably under a wide variety of adverse conditions to extend human sensing capabilities.

RADAR technology also had a dramatic impact on the fledgling science of radio astronomy . During the Second World War, British officer, J.S. Hey correctly determined that the Sun was a powerful source of radio transmissions. Hay discovered this while investigating the causes of system wide jamming of the British RADAR net that could not be attributed to enemy activity (Hey attributed the radio emission to increased solar flare activity). Although kept secret during the war, British RADAR installations and technology became the forerunners of modern radio telescopes as they recorded celestial background noise while listening for the telltale signs of enemy activity.

Remote sensing tools such as RADAR and SONAR also allow scientists, geologists, and archaeologists to map topography and subsurface features on Earth and on objects within the solar system . SONAR readings led to advances in underwater seismography that allowed the mapping of the ocean floors and the identification of mineral and energy resources.

See also Sound transmission