http://www.sina.com.cn 2007年12月20日 11:12
Radar is a new development in radio science.It plays an increasingly important part in meteorology for cloud，precipitation，hail and thunderstorm detection as well as the navigation of aircraft and ships.
Let'sbegin with the reflection of sound waves and light waves as well as their uses in finding objects.
Suppose that you are standing on a flat piece of open land，on which there is a house some distance away.You face it shout aloud “Hello”and a little later an answering “Hello”is heard.It is an echo and the house act the relector of the sound.This phenomenon offers a way to measuring the distance and range of a target which plays its part here as a reflector.Nevertheless the speed at which sound travels is too slow for such a purpose.Thus this soundwave echo method has its limitation in practical use.
Now，turn our attention to the reflection oflight waves.When you switch your flashlamp，the light waves sent out by the flashlamp return from the object on which the light falls and reach your eyes.Since the speed of light is about a million times as great as that of sound waves，the reflection of light waves may very well be applied in detecting a target and its distance.However，we must not forget that this very method is affected to a great extent by weather conditions such as fog，cloud，rain，snowand the like.
Astothe wireless waves with travelling speed as great as that of light waves，for all practical purposes，not only be reflected but also completely unaffected by temperature，barometric pressure，darkness，daylight or other weather conditions.So wireless waves are just the most suitable to be used in detecting a distant target and measuring its distance.And this is exactly what a radar set，the equipment that sends out wireless signals and receives their echoes，bases on.
Tofind a target and determine its position，any radar station has to basically consist of a transmitter，a receiver，an antenna，an antenna duplexer，a display and a timer.
Theradar signal，usually in the form of a repetitive train of short pulses，is generated by a transmitter and radiated into space by an antenna.Reflecting objects or targets intercept and reradiate a portion of the radar signal；asmall amount returns in the direction of the radar and is collected by the antenna.Detection of echo energy could reveal the presence of a target and its location，size，shape，density and intensity.