Although they depend on two fundamentally different types of wave transmission, Radio Detection and Ranging (RADAR) and Sound Navigation and Ranging (SONAR) are remote sensing systems with important military, scientific and commercial applications. RADAR sends out electromagnetic waves, while active SONAR transmits acoustic waves (that is, sound). In both systems, these waves return echoes of certain characteristics or objectives that allow the determination of important properties and attributes of the objective (ie, shape, size, speed, distance, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are used mainly for ground or ground observations. As SONAR signals easily penetrate the water, they are ideal for navigation and underwater measurement.
The threat of underwater warfare during the First World War made urgent the development of SONAR and other means of echo detection. The development of the acoustic transducer that converted electrical energy into sound waves allowed for rapid advances in the design and technology of SONAR during the last years of the war. Although active SONAR developed too late to be useful during the First World War, the impetus for its development garnered enormous technological dividends. Not all advances, however, were restricted to military use. After the war, echo devices were placed on many large French ocean liners.