chapter  15
Doppler navigation
Pages 4

This will have the effect of increasing the tone

(above the fixed frequency) as heard by the

observer. At the instant when the train is adjacent

to the observer, the true fixed-frequency will be

heard. When the train travels away from the

observer, fewer cycles per second will be

received and the tone will be below the fixed-

frequency as heard by the observer. The

difference in tone is known as the Doppler shift;

this principle is used in Doppler navigation

systems. Doppler shift is, for practical purposes,

directly proportional to the relative speed of

movement between the source and observer. The

relationship between the difference in frequencies

and velocity can be expressed as:

where FD = frequency difference, v = aircraft

velocity, f = frequency of transmission, and c =

speed of electromagnetic propagation (3 × 108

metres/second).