Sirens A
siren on a passing
emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer
John Dobson explained the effect thus: In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial speed does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity: v_\text{radial} = v_\text{s} \cos(\theta) where \theta is the angle between the object's forward velocity and the line of sight from the object to the observer.
Astronomy of
spectral lines in the
optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left) The
Doppler effect for electromagnetic waves such as light is of widespread use in
astronomy to measure the speed at which
stars and
galaxies are approaching or receding from us, resulting in so called
blueshift or
redshift, respectively. This may be used to detect if an apparently single star is, in reality, a close
binary, to measure the rotational speed of stars and galaxies, or to
detect exoplanets. This effect typically happens on a very small scale; there would not be a noticeable difference in visible light to the unaided eye. The use of the Doppler effect in astronomy depends on knowledge of precise frequencies of
discrete lines in the
spectra of stars. Among the
nearby stars, the largest
radial velocities with respect to the
Sun are +308 km/s (
BD-15°4041, also known as LHS 52, 81.7 light-years away) and −260 km/s (
Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial speed means the star is receding from the Sun, negative that it is approaching. The relationship between the
expansion of the universe and the Doppler effect is not simply caused by the source moving away from the observer. In cosmology, the
redshift of expansion is considered separate from redshifts due to gravity or Doppler motion. Distant galaxies also exhibit
peculiar motion distinct from their cosmological recession speeds. If redshifts are used to determine distances in accordance with
Hubble's law, then these peculiar motions give rise to
redshift-space distortions.
Radar , an application of Doppler radar, to catch speeding violators The Doppler effect is used in some types of
radar to measure the velocity of detected objects. A radar beam is fired at a moving target – e.g. a motor car, as police use radar to detect speeding motorists – as it approaches to or recedes from the radar source. In the case of a car moving away from the source, each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gaps between the wave crests increase, increasing the wavelength of the radiation returning to the radar. In the opposite case, when the radar beam is fired at the moving car as it approaches, each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's speed. For instance, a police radar operating at 24.15 GHz (K-band) detecting a vehicle traveling at 30 m/s (108 km/h) will measure a Doppler shift of approximately 4.83 kHz, a change easily detected by modern digital signal processing. Moreover, the
proximity fuze, developed during
World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc. Bats use
echolocation in a similar way to locate
moths. The Doppler shift affects the frequency of the wave incident upon the target (moth). When the wave is reflected back from the moth to the bat, the moth acts as the wave emitter and the bat as the wave receiver. The frequency of the reflected wave is again Doppler-shifted. A bat, emitting a wave at the frequency f and flying at v_\textrm{b} towards a moth flying at v_\textrm{t} will detect a final reflected wave with a frequency: Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (
when the received signal arrives). Velocity measurements of blood flow are also used in other fields of
medical ultrasonography, such as
obstetric ultrasonography and
neurology. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular problems like
stenosis.
Flow measurement Instruments such as the
laser Doppler velocimeter (LDV),
Acoustic Doppler current profiler (ADCP), and
acoustic Doppler velocimeter (ADV) have been developed to measure velocities in a fluid flow. The LDV emits a
light beam, and the ADCP and ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and phase. This technique allows non-intrusive flow measurements, at high precision and high frequency.
Velocity profile measurement Developed originally for velocity measurements in medical applications (blood flow), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique is fully non-invasive.
Satellites Satellite navigation The Doppler shift can be exploited for
satellite navigation such as in
Transit and
DORIS.
Satellite communication Doppler also needs to be compensated in satellite communication. Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station. The speed, thus magnitude of Doppler effect, changes due to earth curvature. Dynamic Doppler compensation, where the frequency of a signal is changed progressively during transmission, is used so the satellite receives a constant frequency signal. After realizing that the Doppler shift had not been considered before launch of the
Huygens probe of the 2005
Cassini–Huygens mission, the probe trajectory was altered to approach
Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift. Doppler shift of the direct path can be estimated by the following formula: f_{\rm D, dir} = \frac{v_{\rm mob}}{\lambda_{\rm c}}\cos\phi \cos\theta where v_\text{mob} is the speed of the mobile station, \lambda_{\rm c} is the wavelength of the carrier, \phi is the elevation angle of the satellite and \theta is the driving direction with respect to the satellite. The additional Doppler shift due to the satellite moving can be described as: f_{\rm D,sat} = \frac{v_{\rm rel,sat}}{\lambda_{\rm c}} where v_{\rm rel,sat} is the relative speed of the satellite.
Audio The
Leslie speaker, most commonly associated with and predominantly used with the famous
Hammond organ, takes advantage of the Doppler effect by using an electric motor to rotate an acoustic horn around a loudspeaker, sending its sound in a circle. This results at the listener's ear in rapidly fluctuating frequencies of a keyboard note.
Vibration measurement A
laser Doppler vibrometer (LDV) is a non-contact instrument for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.
Robotics Dynamic real-time path planning in robotics to aid the movement of robots in a sophisticated environment with moving obstacles often take help of Doppler effect. Such applications are specially used for competitive robotics where the environment is constantly changing, such as robosoccer. ==Inverse Doppler effect==