Doppler effect
The Doppler effect is named after the Austrian physicist Christian Andreas Doppler (1803 to 1853) and describes a wavelength shift between moving objects.
The Doppler effect occurs wherever a sound source or RF transmitting device shifts in relation to the receiver. The displacement changes the temporal relation of the transmitting waves to the receiving waves. If the reference no longer coincides, then this means a compression or decompression of the wavelengths. The Doppler effect thus occurs in mobile communications, satellite communications and acoustics and manifests itself in a frequency shift. If the transmitter(satellite) and receiver move toward or away from each other, their wavelengths shorten or lengthen, respectively. This means that the received frequency becomes higher or lower.
In mobile communications with UMTS, for example, moving the cell phone at the speed of a moving car (100 km/h) at a UMTS carrier frequency of 2 GHz would result in a frequency shift of 185 hertz.
In acoustics, sound waves contract when a sound source moves toward a listener; they lengthen when the sound source moves away from the listener. In the first case, when the sound source moves toward the listener, the sound waves are virtually compressed, the distance of their zero crossings is reduced, and the listener has the impression that the pitch of the sound shifts toward higher frequencies. If the sound source moves away from the listener, the wavelengths lengthen and the tones shift toward lower tones.
The technique is also used in Doppler radar for speed measurements of moving targets. For example, in radar checks of motor vehicles, but also in short-range radars and long-range radars.