In a radar speed trap the pulse emitted from the gun reflects from a
moving car and when a bat locates an insect by a pulse of ultrasound the reflected pulse will be
Doppler shifted twice because of the relative motion of the source and reflector.
Take the
case of the car and the radar speed trap. (Figure 1).The emitted frequency of the police radar gun
is f, the car acts like a moving observer receiving it at f'. It reflects it back at f', acting now as a
moving source and the police receive it back shifted once again as f''.
The total change in
frequency (f'' - f) is twice what it was in the simple case above.
