# Introduction to Doppler Effect

Doppler effect is used to measure the relative speed of a moving object. This principle is widely used in Radio Navigation. Radio aids like Airborne Doppler and Doppler VOR use Doppler principle. When you stand in the railway platform and hear a train’s whistle, you would realise that the volume increases sharply as it nears you and reduces drastically as it moves away from you. This happens because of the change in frequency of sound due to relative motion between you and the moving train. This is called doppler effect which is widely used in radio navigational aids.

### Doppler Shift Principle

When there is a relative motion between the transmitter and the receiver, the transmitted frequency varies from the received frequency. As the transmitter and receiver come closer, the frequency increases and as they move away the received frequency decreases. This is called as Doppler Shift.

### Effect of Doppler Shift

We have seen that, doppler shift is the change in frequency due to relative movement between the transmitter and receiver. The amount of shift or change depends on the relative velocity between the transmitter and receiver. Doppler equipments actually are interested in the relative velocity between the transmitter and receiver. This is measured using the observed Doppler shift.

### Relative Velocity and Doppler Shift

As we had seen earlier, the frequency varies due to the Doppler shift. Radio Navigational aids obtain the Relative velocity by measurement of frequency or phase difference. Doppler shift is a determined by a formula. Doppler shift is the product of transmitted frequency and relative velocity divided by speed of light.