The frequency and wavelength of an electromagnetic field are affected by relative motion. This is known as the Doppler effect. Only the radial (approaching or receding) component of motion produces this phenomenon. The Doppler effect also occurs with acoustic waves. The Doppler effect is significant in low-earth-orbit (LEO) satellite systems. All LEO satellites are constantly moving relative to each other and to points on the surface. This causes variations in the frequencies and wavelengths of received signals. In geostationary satellite systems, Doppler effect is not a factor unless the end user is on board a spacecraft or high-speed aircraft. If the source of an EM field is approaching an observer, the frequency increases and the wavelength decreases. If the source is receding, the frequency decreases and the wavelength increases. If there are several observers, each moving radially at different speeds relative to the source of an EM field, every observer will perceive a unique frequency and wavelength for the EM field produced by the source. A special type of RADAR, called Doppler radar, uses the Doppler effect to ascertain wind velocity in heavy thundershowers, tornadoes, and hurricanes. In a rotating storm such as a hurricane or tornado, the maximum sustained wind speed can be found by measuring the difference in frequency of echoes returned from approaching rain droplets and receding rain droplets. A more primitive form of Doppler radar is used by law enforcement personnel to enforce traffic speed limits