By the 1900s a German engineer, Chistian Huelsmeyer, proposed the use of radio echoes to avoid collisions. He invented a device he called the telemobiloscope, which consisted of a simple spark gap aimed using a funnel-shaped metal antenna. When a reflection was seen by the two straight antennas attached to the receiver, a bell sounded. Although very simple, the system could detect shipping accurately up to about 3 km. Nevetheless the naval world seemed uninterested in his invention, and it was not put into production.
Nikola Tesla, in August 1917, proposed principles regarding frequency and power levels for primitive RADAR units. Tesla's study of high voltage, high frequency alternating currents led to this development. Tesla had formed the concept of using radio waves to detect objects at a distance. In the 1917 The Electrical Experimenter, Tesla stated the principles in detail.
Tesla stated, "For instance, by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed." Tesla also proposed the use of standing electromagnetic waves along with pulsed reflected waves to determine the relative position, speed, and course of a moving object and other modern concepts of radar.
Tesla had first proposed that radio location might help find submarines (for which it is not well-suited) with a fluorescent screen indicator, though it was first applied successfully to locate aircraft (after their later proliferation) and surface ships during World War II. Emil Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla
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