Years ago when a sailor was approaching a rocky cliff at night, he would sometimes set off a whistle blast and count the number of seconds from the time of the blast until the time he heard the echo from the cliff. By nothing the number of seconds that had elapsed, he could determine roughly how far off the coast was. This crude method is based on the fact that sound travels in air at the rate of roughly 330 meters per second.
The mariner knew that the sound waves generated by the whistle blast had to go out to the shore and back. Hence the actual distance to the shore was 165 meters for every second from the time the original blast was emitted to the time when the echo was received. A cliff 1,650 meters away turned an echo about 10 seconds after the blast sounded.
Like sound waves. Radio waves have echoes, reflected from metals, wood, glass, plastics and other objects. The reflected radio energy will be scattered in many different directions, including the direction from which the radio signals originally emanated. This phenomenon has been put to use in the device called radar (from Radio Detection And Ranging).
A radar antenna transmits radio-frequency energy focused into a beam, which is made to scan a wide area. Reflections, or echoes, from a target in this area then make their way to a radio receiver. Since radio waves travel at a constant speed – about 300,000 kilometers per second – the distance of the target can be determined by measuring the time that has elapsed from the transmission of the radio energy to the reception of the echo from the target. The latter’s position can be indicated by nothing the position of the beam that yields the strongest echoes. In some cases, one can also calculate the velocity of the target.