Range & Dopple

7/29/2019 Range & Dopple

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Doppler Effect

Outline

Definition

Basic concept

Relation b/n Source and Observer

How a Doppler Radar works

Range and accuracy relation


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Doppler Shift:A frequency shift in electromagnetic waves due to the motion of scatters toward or away from

Analogy: The Doppler shift for sound waves is the change in frequency one detects as race caapproach and then recede from a stationary observer

Doppler Radar:

A radar that can determine the frequency shift through measurement of the phase change

that occurs in electromagnetic waves during a series of pulses


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Basic Concepts The apparent change in the frequency due to the relative motion between th

and the observer is known as Doppler effect.

Three main things are our consideration here

Source

Observer

Whether the source is moving towards or away or stationary from

observer


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Stationary Sound SourceSound waves are produced at a constant frequency f

0, and the wavefront

symmetrically away from the source at a constant speed v, which is the

sound in the medium. The distance between wavefronts is the waveleng

observers will hear the same frequency, which will be equal to the actua

of the source.


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Source moving with vsource < vsoundHere also the same sound source is radiating sound

waves at a constant frequency in the same medium.However, now the sound source is moving to the

right. The wave fronts are produced with the same

frequency as before. However, since the source is

moving, the center of each new wave front is now

slightly displaced to the right. As a result, the wave

fronts begin to bunch up on the right side (in front

of) and spread further apart on the left side (behind)

of the source. An observer in front of the source

will hear a higher frequencyf >f0, and an observer

behind the source will hear a lower frequencyf


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Source moving with vsource = vsound

Now the source is moving at the speed of sound in the medium. The

speed of sound in air at sea level is about 340 m/s or about 750 mph.The wavefronts in front of the source are now all bunched up at the

same point. As a result, an observer in front of the source will detect

nothing until the source arrives.The pressure front will be quite

intense (a shock wave), due to all the wavefronts adding together, and

will not be percieved as a pitch but as a "thump" of sound as the

pressure wall passes by.

Jet pilots flying at Mach 1 report that there is a noticeable

"wall" or "barrier" which must be penetrated before

achieving supersonic speeds. This "wall" is due to the intense

pressure front, and flying within this pressure front produces

a very turbulent and bouncy ride.

(Mach 1breaking the
http://www.acs.psu.edu/drussell/Demos/doppler/mach1.html


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Source moving with vsource > vsound

The sound source has now broken through the sound speed barrier, and is traveling at 1.4 times

speed of sound .Since the source is moving faster than the sound waves it creates, it actually leads

the advancing wavefront. The sound source will pass by a stationary observer before the observer

actually hears the sound it creates.

. The figure shows a bullet travelling at velocity greater than velocity of sound. The mach cone

wave fronts are very noticeable.

(Mach 1.4 -
http://www.acs.psu.edu/drussell/Demos/doppler/bullet-3.gif


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The picture at the left shows the shock wave front generated by

a T-38 Talon, a twin-engine, high-altitude, supersonic jet trainer

(below).

This picture shows a sonic boom created by the

team car as it broke the land speed record (and a

sound barrier on land).
http://www.af.mil/news/factsheets/T_38_Talon.htmlhttp://www.af.mil/news/factsheets/T_38_Talon.htmlhttp://www.af.mil/news/factsheets/T_38_Talon.htmlhttp://www.af.mil/news/factsheets/T_38_Talon.htmlhttp://www.radartutorial.eu/11.coherent/pic/doppler8.big.gif


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How can we relate Doppler Effect to a RADAR ?Doppler Radar:

A radar that can determine the frequency shift through measurement of the phase change that oelectromagnetic waves during a series of pulses

If both the source and the receiver of the sound remain stationary, the receiver will hear the sam

produced by the source. This is because the receiver is receiving the same number of waves per

source is producing.

Now, if either the source or the receiver or both move toward the other, the receiver will perceiv

sound. This is because the receiver will receive a greater number of sound waves per second and

number of waves as a higher frequency sound. Conversely, if the source and the receiver are moving apart, the receiver will receive a smaller num

of sound waves per second and will perceive a lower frequency sound. In both cases, the frequen

produced by the source will have remained constant.
http://www.radartutorial.eu/11.coherent/pic/doppler8.big.gif


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Calculation of Doppler frequency formulaeFor example, the frequency of the whistle on a fast-moving car sounds increasingly higher in pitch as

approaching than when the car is departing. Although the whistle is generating sound waves of a con

and though they travel through the air at the same velocity in all directions, the distance between the and the listener is decreasing. As a result, each wave has less distance to travel to reach the observer

than the wave preceding it. Thus, the waves arrive with decreasing intervals of time between them.

Range & Dopple

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