Doppler frequency shift

(Also called Doppler effect.) In general, the change in frequency of a signal reaching a receiver when the receiver and the transmitting source are in motion relative to one another.

This phenomenon was first noted for sound waves by the Austrian physicist Christian Johann Doppler (1803–53) in 1842. In meteorology, this effect is successfully employed with remote sensors, such as Doppler radars and Doppler lidars, in which the receiver (collocated with the transmitter) is fixed and only the scatterers (upon which transmitted power impinges and is reradiated) are moving. The frequency shift, f, induced by a scatterer having a radial component of motion v_r relative to the radar may be expressed as

where λ is the wavelength of the transmitter, f is positive for motion toward the radar, and, by the usual convention, v_r is positive for motion away from the radar.
See also Doppler velocity.

An instrument used to detect the presence of and to determine the information carried by electromagnetic radiation.

(Coined word for radio detection and ranging.) An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy.