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雷达[Radar] [2005-8-2]
iamet 发表在 Ω〖物理〗
| Radar is a type of electromagnetic radiation whose name is an acronym for "Radio Detection And Ranging." Although originally invented many years ago, it wasn't developed and made into a very useful technology until World War II, where it was used to detect and locate airplanes and ships. Radar typically consists of a high frequency radio pulse sent out for a short interval of time. This low duty cycle allows the pulse amplitude to be high, so that when it encounters a conducting object, sufficient energy is reflected back to allow the radar system to detect it. Since radio waves travel in air at close to their speed in a vacuum, one can calculate the distance d of the detected object from the round-trip time t between the transmitted and received pulses as
where c is the speed of light and the factor of 1/2 accounts for the fact that t is the round-trip time.
Additionally, correlating the time delay with the angular velocity of a rotating radar antenna will give the operator the bearing of the detected object.
Radar also has the very useful property that radio signals of the proper frequency aren't blocked by clouds or fog. This allows ships and airplanes to navigate better in bad weather. Over the years, many new techniques have been developed for radar. One notable example is phased-array radar, where hundreds of small antennas can be geometrically positioned so that their outputs and received signals are correlated in terms of phase.
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