SODAR
Encyclopedia
SODAR also written as sodar, is a meteorological instrument used as a wind profiler
to measure the scattering of sound waves by atmospheric turbulence. SODAR systems are used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere
.
Sodar systems are like radar
(radio detection and ranging) systems except that sound
waves rather than radio
waves are used for detection. Other names used for sodar systems include sounder, echosounder and acoustic radar.
signals. A mono-static system uses the same antenna for transmitting and receiving, while a bi-static system uses separate antennas. The difference between the two antenna systems determines whether atmospheric scattering is by temperature fluctuations (in mono-static systems), or by both temperature and wind velocity fluctuations (in bi-static systems).
Mono-static antenna systems can be divided into two categories: those using multiple axis, individual antennas and those using a single phased array
antenna. The multiple-axis systems generally use three individual antennas aimed in specific directions to steer the acoustic beam. One antenna is generally aimed vertically, and the other two are tilted slightly from the vertical at an orthogonal angle. Each of the individual antennas may use a single transducer focused into a parabolic reflector
to form a parabolic loudspeaker
, or an array of speaker driver
s and horns (transducer
s) all transmitting in-phase
to form a single beam. Both the tilt angle from the vertical and the azimuth angle of each antenna are fixed when the system is set up.
Phased-array antenna systems use a single array of speaker drivers and horns (transducers), and the beams are electronically steered by phasing the transducers appropriately. To set up a phased-array antenna, the pointing direction of the array is either level, or oriented as specified by the manufacturer.
The horizontal components of the wind velocity are calculated from the radially measured Doppler shifts
and the specified tilt angle from the vertical. The tilt angle, or zenith angle, is generally 15 to 30 degrees, and the horizontal beams are typically oriented at right angles to one another. Since the Doppler shift of the radial components along the tilted beams includes the influence of both the horizontal and vertical components of the wind, a correction for the vertical velocity is needed in systems with zenith angles less than 20 degrees. In addition, if the system is located in a region where vertical velocities may be greater than about 0.2 m/s, corrections for the vertical velocity are needed, regardless of the beam's zenith angle.
The vertical range of sodars is approximately 0.2 to 2 kilometers (km) and is a function of frequency
, power output, atmospheric stability, turbulence
, and, most importantly, the noise environment
in which a sodar is operated. Operating frequencies range from less than 1000 Hz to over 4000 Hz, with power levels up to several hundred watts. Due to the attenuation characteristics of the atmosphere, high power, lower frequency sodars will generally produce greater height coverage. Some sodars can be operated in different modes to better match vertical resolution and range to the application. This is accomplished through a relaxation between pulse length
and maximum altitude.
Wind profiler
A wind profiler is a type of weather observing equipment that uses radar or sound waves to detect the wind speed and direction at various elevations above the ground. Readings are made at each kilometer above sea level, up to the extent of the troposphere...
to measure the scattering of sound waves by atmospheric turbulence. SODAR systems are used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
.
Sodar systems are like radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
(radio detection and ranging) systems except that sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
waves rather than radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
waves are used for detection. Other names used for sodar systems include sounder, echosounder and acoustic radar.
Doppler Sodar
Commercial sodars operated for the purpose of collecting upper-air wind measurements consist of antennas that transmit and receive acousticAcoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
signals. A mono-static system uses the same antenna for transmitting and receiving, while a bi-static system uses separate antennas. The difference between the two antenna systems determines whether atmospheric scattering is by temperature fluctuations (in mono-static systems), or by both temperature and wind velocity fluctuations (in bi-static systems).
Mono-static antenna systems can be divided into two categories: those using multiple axis, individual antennas and those using a single phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
antenna. The multiple-axis systems generally use three individual antennas aimed in specific directions to steer the acoustic beam. One antenna is generally aimed vertically, and the other two are tilted slightly from the vertical at an orthogonal angle. Each of the individual antennas may use a single transducer focused into a parabolic reflector
Parabolic reflector
A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
to form a parabolic loudspeaker
Parabolic loudspeaker
A parabolic loudspeaker is a loudspeaker which seeks to focus its sound in coherent plane waves either by reflecting sound output from a speaker driver to a parabolic reflector aimed at the target audience, or by arraying drivers on a parabolic surface...
, or an array of speaker driver
Speaker driver
A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...
s and horns (transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s) all transmitting in-phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
to form a single beam. Both the tilt angle from the vertical and the azimuth angle of each antenna are fixed when the system is set up.
Phased-array antenna systems use a single array of speaker drivers and horns (transducers), and the beams are electronically steered by phasing the transducers appropriately. To set up a phased-array antenna, the pointing direction of the array is either level, or oriented as specified by the manufacturer.
The horizontal components of the wind velocity are calculated from the radially measured Doppler shifts
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
and the specified tilt angle from the vertical. The tilt angle, or zenith angle, is generally 15 to 30 degrees, and the horizontal beams are typically oriented at right angles to one another. Since the Doppler shift of the radial components along the tilted beams includes the influence of both the horizontal and vertical components of the wind, a correction for the vertical velocity is needed in systems with zenith angles less than 20 degrees. In addition, if the system is located in a region where vertical velocities may be greater than about 0.2 m/s, corrections for the vertical velocity are needed, regardless of the beam's zenith angle.
The vertical range of sodars is approximately 0.2 to 2 kilometers (km) and is a function of frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, power output, atmospheric stability, turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
, and, most importantly, the noise environment
Noise pollution
Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...
in which a sodar is operated. Operating frequencies range from less than 1000 Hz to over 4000 Hz, with power levels up to several hundred watts. Due to the attenuation characteristics of the atmosphere, high power, lower frequency sodars will generally produce greater height coverage. Some sodars can be operated in different modes to better match vertical resolution and range to the application. This is accomplished through a relaxation between pulse length
Pulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
and maximum altitude.