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Doppler Shift

You have probably heard an example of the Doppler shift on many occasions. For example, we have all been at a railroad crossing when a train is approaching the road intersection. The sound of the train's horn starts with a higher pitch as it approaches. As the train enters the intersection, the pitch of the horn levels out (becomes constant). As the train moves beyond the intersection, the pitch again changes -- this time it becomes lower.

What you have heard in this case is referred to as the Doppler effect. The Doppler effect is a shift in the frequency of a wave (a sound wave in this example) reflected by an object in motion. The motion of the train towards the intersection causes the sound wave to be compressed (a higher pitch). As the train enters the intersection, the distance between the train and an individual in the road intersection becomes constant (or a 90 degree angle to the person). At this point in time, the sound wave is no longer compressed since there is NO RELATIVE movement between the horn and the individual (yes, the train is moving; however, for one brief moment, it is NOT moving toward the individual OR away from the individual). When the train is in the intersection, its bearing is 90 degrees to the individual. At this point there is no change in the distance between the train and the person and therefore NO Doppler effect. As the train moves beyond the intersection, the distance begins to increase and the Doppler effect returns. This time, the movement of the train away from the person makes the sound of the horn wane as the sound waves are expanded.

Applications of the Doppler Shift

One job of the APG-70 is to locate aircraft flying close to the ground while the F-15E is flying well above them (20,000 - 30,000 feet above them for example). A pulse radar looking down on the earth would see EVERYTHING -- mountains, buildings, lakes, and the aircraft. This would make it difficult (or impossible) to find an aircraft flying at low altitude. A continuous wave radar (or other radar using Doppler technology) will only "see" objects that are moving (the radar's computer will filter out the speed of the F-15E). Thus, the Doppler shift gives advanced radars like the APG-70 the ability to see aircraft flying at very low altitudes.

Another example of radars using the Doppler shift is in detecting wind shear (a rapid change in the direction and speed of wind, a by-product of thunderstorms, that can be deadly to aircraft during takeoff and landings). New radars using computer and Doppler technology are able to see the wind shear and provide warnings to pilots. Older radars were unable to provide this life saving information.

Funded in part by

Used with permission From
90th Fighter Squadron
"Dicemen" Aviation

Updated: February 24, 1999