Define Doppler shift.

30. Define Doppler shift.

If the receiver is moving towards the source, then the zero crossings of the signal appear faster and the received frequency is higher.The opposite effect occurs if the receiver is moving away from the source. The resulting chance in frequency is known as the Doppler shift (fD). 

FD = fr – f0 = -f0V/C 

Where f0 -> transmission frequency 

fr -> received frequency