There are many types of Doppler radars like Continuous Wave, Coherent Pulsed, etc. The basic principle behind the working of this device, is to measure the radial velocity of the echoes, that are generated from the target areas.
They are used in several technological aspects like satellite sounding, weather forecasting, traffic surveillance, military services, air traffic control, and radiology.
There are two basic phenomena that are being used, viz., the Doppler effect and the radar. The Doppler effect was discovered by Christian Doppler, an Austrian scientist in 1842. It is the process by which the wave frequency for an observer changes with respect to the direction of movement of the waves source.
The Doppler radar works on the principle of Doppler effect, and was invented in 1904 by Christian Hülsmeyer, while detecting a distant ship in a foggy climate. However, one major drawback of this device was the inability to estimate the object distance. In the early 1940s, modern versions of Doppler radars were discovered.
In case of waves that travel in a medium (for example, sound waves), the overall effect depends on the motion of the source, medium, and the observer. In the case of waves without a medium (for example, light), only the velocity difference between the observer and the source is taken into consideration.
Radars, on the other hand, are systems that make use of transmitters for generating electromagnetic waves: either microwaves or radio waves. These waves after landing on objects, are reflected back in the same direction of the transmitter, which perceives them.
Though the reflected signals are generally weaker than the generated waves, they can be amplified and/or displayed by means of sophisticated machines. Thus, radars can detect objects (either stationary or moving), which are located in far-off areas.
Doppler radars are the best equipment used for detecting rainfall and/or forecasting weather of a particular area. While detecting rain, the Doppler radar sends out radio waves from the antenna, which are reflected back to the transmitter.
The frequency of the reflected waves depends on whether the object(s) (for example, cloud, raindrops, hailstones) is moving away from the antenna or towards it. In the former case, the frequency will be low; whereas, in the latter case, it will be high.
Whatever may be the outcome of the reflected frequencies, all weather radars are installed with equipment that effectively convert the radio waves into pictures, which display the object position, intensity of precipitation, and even a change in the wind direction.
The location of the objects is calculated by determining the time required for the waves, to bounce back to the transmitter.