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RADARs are used to track an aircraft. RADAR stands for RAdio Detection And Ranging. Radio waves are used in RADAR for detection purpose. Radio waves are electromagnetic waves. Mainly two properties of radio waves are used for detection purpose. One is reflection property and other is doppler effect.

Reflection
Radio waves reflect back when they come in contact with any obstacle. It is same as reflection of sound waves. Echo is the best example for reflection of sound waves. Sound waves produced by sound source will reflect back when they collapse on obstacle kept in front of them. In this case we will hear an echo. Sound waves & Radio waves are completely different wave forms. But property of reflection is same in both the cases. Of course you cannot hear Radio waves. Then how can we find that radio waves are reflected back?
In such a way Aircraft can be detected by using reflection property of Radio wave. Radio waves will reflect back from aircraft. Reflected waves are detected by Radio receiver. According to amount of radio waves received & time taken for reflection even type of aircraft can be detected.

Now we have detected that aircraft is flying! What about its speed? How can we obtain its speed?
For that second property of Radio waves i.e. Doppler effect(doppler shift) is used.

Doppler Effect
Speed of aircraft can be obtained by this property of radio waves. When radio waves collapse on fixed obstacle (obstacle which is not moving) radio waves will simply reflect back without any change. But when radio waves collapse on moving object, the frequency of radio waves will change according to motion of an object. If object is moving towards the RADAR, Frequency of Radio waves reflected back will increase. On the other hand if object is moving away from RADAR frequency of  Radio waves reflected back will reduce.
So based on change in frequency of radio waves computer automatically detects the speed of an Aircraft.

In such a way using two properties of Radio wave Aircraft can be detected.