|MadSci Network: Physics|
The Doppler effect was first used to demonstrate the concept of radar early in this century and today Doppler signal processing techniques play a major role in radar, communications and navigation systems. Doppler radars are used to measure automobile speeds (Police radar guns), open super market doors, map the surface of the earth and cloud covered planets, and monitor the weather. There are numerous locations on the Web where Doppler techniques are presented; however, you must know the technical jargon to find them!
To help you understand the concepts of Doppler techniques used in common for acoustic echoing (SONAR), radio echoing (RADAR) and light/laser echoing (LIDAR), I will review some of the physics involved by using a few related thought experiments. Sonar travels well through water, ground, metal and other solids; however, the speed of sound is slow compared to the speed of light. Radar travels at the speed of light and can penetrate weather, clouds, and some building materials, but it cannot penetrate metal or ground. Laser light travels at the speed of light but cannot penetrate weather; however, laser light can be formed into tight beams thousands of times more narrow than radar beams, giving high angular resolution.
Sound waves, radio waves and light waves propagate uniformly in all directions from a stationary point source. When a point source is moving in a straight line the wave fronts are compressed in the direction of motion. The waves are more spread out in the opposite direction. The waves are unaffected in directions normal (at right angles) to the motion. Christian Doppler explained this phenomena mathematically.
Let us have two trombone players, a man and a women, conduct our thought experiments. The trombone can produce a loud fixed frequency (pitch) note or the instruments slide can be moved to change the frequency of the note. Lets say that the trombone players can play a perfect "A" note that is used to tune orchestras. The orchetra A note produces 440 vibrations per second in the air (Scientists label it 440 Hertz in honor of the famous physicist Heinrich Hertz).
Experiment 1: A moving sound source and a stationary listener.
The man rides in an open convertible traveling 30 miles/hr (48 km/hr) constantly playing and hearing an A note and drives past the woman who is standing on the side walk listening. When the car is a block away coming toward the listener she hears a 457 Hz note (the Doppler frequency change is +17.6 Hz). As the car approaches the listener she rotates to face the moving car and when she has to turn 30 degrees , she now hears 448.8 Hz (A Doppler frequency change of 8.8 Hz, 1/2 of 17.6 Hz). As the car passes the listener she rotates 90 degrees to face the side of the car and hears a 440 Hz note (Zero Doppler). As the listener continues to rotate and watch the car leave as she turns 150 degrees she hears 431.2 Hz ( 440 Hz - 8.8 Hz Doppler) and when the car is a block away she has rotated 180 degrees and now hears 422.4 Hz (440 Hz -17.6 Hz Doppler). If the car had been moving at 60 miles/hr the Doppler frequencies would have been twice as large (larger positive and negative numbers) and if the car had driven 15 miles/hr the Doppler frequencies would have been 1/2 as much. The Doppler effect is linear with velocity for the same frequency transmitter.
Experiment 2: Moving listener and stationary sound source.
The same experiment is conducted only now the women stands and faces the car while playing a 440 Hz note on her trombone and the man in the car listens. In this experiment the man in the car hears the same notes as the woman heard in Experiment 1, at the same places along the road, because he is moving relative to the fixed sound source. The women always hears a constant 440 Hz note. This is what physicists call reciprocity in an experiment.
Experiment 3 A repeater experiment:
The car is driving 30 miles/hr and now the standing woman plays a 440 Hz note toward the car while the man in the car listens to the Doppler shifted frequency. He then repeats the note he hears with his horn.. When the man is a block away he hears a 457 Hz Doppler shifted note and plays that note. The man' 457 Hz note is again Doppler shifted 17.6 Hz more on the return path and the woman hears 475.2 Hz
(A double Doppler shift of 35.2 Hz!). At 30 degrees the women hears 457 Hz and at 90 degrees she hears 440 Hz etc.
Experiment 4: An echo experiment
If the player in the car did not play his trombone and instead used a large flat plate to reflect the women's Doppler shifted sound waves back to the woman's ears over the double path to and from the car. THe woman would hear the same double Doppler shifted notes that she heard in Experiment 3 with out the need for the man to repeat the notes that he heard; however, the reflected note would be very weak and hard for the women to hear..
The result of experiments 3 & 4 is that echoing or repeating from a moving body produces a doubling of the Doppler shift. Also if we were using radio waves instead of sound waves we would replace the women with a transmitter and receiver and antennas and call it a Doppler radar. We would replace the trombone player in the car with a radio repeater and we would call this a radar beacon.
When NASA sends very pure microwave signals (notes) derived from an atomic clock from the giant antennas of the Deep Space Network to distant spacecraft traveling in interplanetary space, the spacecraft receive the Doppler shifted microwave signals from earth and then re-transmit (repeat) them back toward earth. This action doubles the relative Doppler shift, just as the sound waves behaved in Experiment 3. One difference is that NASA's microwave signals also carry commands to the spacecraft and the return signals carry data. This information (modulation) carried to and from the space craft does not qffect the Doppler shift of the microwave carrier wave which is monitored on earth to give controllers a very accurate way to calculate the spacecraft's velocity relative to the earth even though it is millions of miles away.
The same techniques as Experiment 4 are used by law enforcement with their radar guns giving them a precise measurement of your cars velocity on a road several blocks away. However, the officers must park along the road so that their radar beams are near zero degrees or 180, degrees to get an accurate Doppler velocity measurement. They cannot park on a side street and use their radar gun on your car as you pass by because at 90 degrees the Doppler shift is ZERO.
The National Weather Service is installing the next generation of weather radars nation wide. The keystone of this modernization is the new Doppler weather surveillance radar (Model WSR-88D). The WSR-88D (also known as NEXRAD) excels in detecting the severe weather events that threaten life and property, from early detection of damaging winds to estimating amounts of rainfall for use in flood forecasting. This radar and a typical radar display are presented on the National Weather Service's Web site located at:
Weather Doppler radars of this type are so sensitive that they can also monitor bird and insect migrations for scientists to study!
You can monitor the Twin Cities weather radar in near real time at the following Web site:
Doppler techniques are also used by astronomers to determine the velocity of distant stars and galaxies relative to the earth. The most distant objects we can detect in the universe are moving away from us at tremendous velocities because of the big bang. Both light and radio signals from these distant objects are Doppler shifted toward the longer wavelengths. Astronomers call this Doppler shift in light waves the RED SHIFT. Some stars in our galaxy are moving toward the earth and the radio and light from these objects are Doppler shifted toward shorter wavelengths. Astronomers call this a BLUE SHIFT. The degree of the radio waves or light waves Doppler shift gives a very accurate measure of the relative velocity of these objects.
The airborne radars that were used in the Gulf War in F-14, F-15, F-16 fighter planes are called pulse - Doppler radars which can be used in dozens of different modes such as aircraft detection, ground mapping, missile tracking etc. The pulses give distance and angle information while the Doppler processing gives velocity information. Large airborne pulse Doppler radars are now monitoring the peace in Bosnia using the Airborne Warning and Control System (AWACS) and the Joint Surveillance Target Attack Radar System (STARS). Radar waves enable the peace keepers to precisely detect the location and speed of military movements 24 hours a day at night and in bad weather. Pictures and a description of the AWACS and JSTARS can be found at:
Perhaps the most sophisticated Doppler processing techniques have been used to map the surface of the earth and Venus through their cloud cover. These systems are called Synthetic Aperture Radars (SAR) and dozens of high resolution radar pictures of earth and Venus are presented at the NASA/JPL Imaging Radar Home Page :
This web site also has a tutorial discussion of how the spaceborne SARs operate along with many fascinating 3D pictures and movies made from Doppler radar data.
Typically these SAR imaging systems work by continuously recording the millions of Doppler shifted signal tracks recorded as the vehicle moves through the Zero Doppler angle at 90 degrees (as in our experiments). These systems , such as JSTARS, have also been called Side Looking Array Radars (SLAR). The SAR or SLAR receive and record echoes from buildings, mountains, trees etc. while the aircraft or spacecraft travels many miles. By using a mathematical process called FOURIER ANALYSIS the Doppler recordings can be processed to give highly detailed images that would have required a conventional antenna that was several miles long (As long as the path over which the data was recorded). This is the synthetic array. In the past the aircraft and spacecraft had to transmit to transmit the raw Doppler data signals to an earth terminal for processing. Today airborne computers now have the power to do on board processing in near real time! Because of power and weight limitations spacecraft SARs still transmit the raw Doppler data to earth for processing.
Also side looking SONAR systems are now used to image sunken ships such as the Titanic and the Civil War ironclad Monitor.
Doppler radar processing is a facinating field and there is much more than I could cover here. However if you have any additional, more specific questions or if anyone your class wants to do some simple doppler radar experiments with a transistor radio, send me an E-mail note.
Regards - your Mad Scientist
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