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Whenever a signal source moves towards or away from a receiver, the
frequency of the signal as observed at the receiver increases or decreases respectively.
This is known as the Doppler effect. The degree of frequency shift is a linear function of
the speed of motion and the carrier signal frequency. For example, a source moving at
70 mph using a carrier frequency of 900 MHz will experience a Doppler shift of up to +/100 Hz
at the receiver.
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Correcting for Doppler shift can be very difficult, particularly in a multipath environment, where signals arriving from different angles experience different Doppler shifts. |