Doppler Effect RNAV

Doppler effect is a change of frequency caused by either the transmitter or the receiver moving relative to the other. This can happen when one, or the other, or both are moving provided they are not both moving in the same direction at the same speed.

A very common example is the sound of emergency vehicle sirens as they travel past you at speed. The vehicle is not changing the siren pitch it is emitting, but it sounds to you as if they are because of their movement. As they come towards you, each “peak” of the wave is emitted closer and closer to you. This means the peaks are closer together – which means the frequency is increased – which means the siren sounds higher because of the increased frequency. Transmitter or receiver moving towards the other = higher frequency. Conversely, as the vehicle travels away, each peak is emitted further away from you, reducing the frequency and causing the siren to sound at a lower pitch. Transmitter or receiver moving away from the other = lower frequency. This results in the typical “yeee-yumm” sound.

This phenomenon is made use of for moving target indicators, and detection of turbulence, in the radar section.

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