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Digital Doppler is a signal processing technique that uses the Doppler effect to calculate the velocity of objects. Originally, the military developed digital Doppler techniques for radars used to track, seek and illuminate targets. As the cost of digital computing decreased, civilian applications of Doppler radars have become common, such as the essential role of Pulse-Doppler radar in weather forecasting. Digital Doppler imaging techniques are also increasingly used in different medical fields.
The Doppler effect is essentially the change in frequency of a signal reflected by a target in motion. The frequency of a signal reflected by an object moving towards an observer will be higher than the frequency of the original signal. The frequency of a signal reflected by an object moving away from an observer will be lower than the frequency of the original signal. This Doppler shift phenomenon can be recorded as a signal's frequency increases or decreases in relation to the original signal over time. The subsequent changes in frequency are used to calculate the velocity of an object in relation to the observer.
Computers are used to digitalize the information collected as each signal is emitted, reflected and received. In its simplest form, a Doppler radar emits an electromagnetic wave at a target. On contact, the wave is dispersed and some of the wave is reflected back to the radar. A digital Doppler receiver computer samples the reflected wave and calculates the phase shift from the emitted wave, determining the change in frequency. The velocity of the object can be calculated from the changes in frequency, although the range and the bearing of the target can not be determined.
As the speed and storage size of computers have improved, so has their ability to process more information available from Doppler shifts. For instance, faster computers can manage the information derived from the rapid emission of microwave pulses instead of a simple continuous wave signal. The time delay for a pulse to bounce back from a target can be calculated as well as the strength of the returned signal. This allows the target’s position and density to be determined in conjunction with its relative velocity. Typically these Pulse-Doppler radars scan 360 degrees around the radar at a variety of elevations, and digital Doppler computers make a composite of the collected data.
Weather Doppler uses Pulse-Doppler radar to study the movement of storms and the intensity of precipitation. Water droplets in clouds and precipitation reflect electromagnetic waves. Digital Doppler processing can thus be used to determine the speed and intensity of an approaching storm system from the velocity of the clouds motion. Waves reflected off of dense hail or heavy rain will be strong, whereas snow and drizzle act more like sieves, attenuating and dispersing the waves and resulting in weaker signals. Using pulse time delay analysis, the exact location of a storm can be determined as well as the type of precipitation.
Computers present the information in two types of Doppler maps. In a reflectivity map, precipitation information is color coded by intensity and superimposed on a geographic map that indicates positioning. A second Doppler map displays a storm’s radial velocity, which can be used to determined wind direction. Severe weather systems such as hurricanes, supercell thunderstorms and tornadoes leave telltale signatures on Doppler velocity maps, allowing forecasters to issue severe weather warnings.
Civilian Doppler manufacturer's innovations have made their technology practical in the medical field. One such application is echocardiographs that test vascular blood flow. Likewise, 3D Doppler fetal sonograms are gaining popularity, since they allow parents and doctors to visualize high-resolution images of a fetus moving inside the womb.