About Doppler principle: Doppler theory is based on a time basis, when a radio wave hits an object during traveling, the waves will be reflected, the reflected wave frequency will be changed with the movement state of the encountered objects while. If the radio wave is encountered in the position of the object is fixed, then the frequency and the frequency of the transmission wave of the reflected wave should be equal. If the object is moved, the direction of the emitted waves reflected back is compressed, that the frequency of the reflected wave will increase; Conversely, when an object moving toward the direction away from the emitted, reflected back wave frequency will follow decreases, this is the Doppler effect. This phenomenon is often encountered in everyday life, such as a whistle by police car at high speed from your side, the frequency of the sound you hear is the change: When the police car high speed when close to you, (and still sound source compared) voice transmission time is shortened, the frequency increases. When the police car away from you, lengthen the time of the transmission of sound, the frequency decreases.



