Doppler Radar
Danielson et al
¥Doppler can detect motion toward or away from radar by the frequency  of the return beam
¥Higher - toward radar
¥Lower - away from radar
¥Doppler effect explains why pitch of whistle changes as a train approaches then moves away
Frequency of return beam changes when reflective object is moving either toward or away from radar. Velocity can be determined from frequency shift.
Lower frequency
Higher frequency