ASTM D7145-05(2015)

Standard Guide for Measurement of Atmospheric Wind and Turbulence Profiles by Acoustic Means

SIGNIFICANCE AND USE
5.1 Sodars have found wide applications for the remote measurement of wind and turbulence profiles in the atmosphere, particularly in the gap between meteorological towers and the lower range gates of wind profiling radars. The sodar’s far field acoustic power is also used for refractive index calculations and to estimate atmospheric stability, heat flux, and mixed layer depth (1-5).3 Sodars are useful for these purposes because of strong interaction between sound waves and the atmosphere’s thermal and velocity micro-structure that produce acoustic returns with substantial signal-to-noise ratios (SNR). The returned echoes are Doppler-shifted in frequency. This frequency shift, proportional to the radial velocity of the scattering surface, provides the basis for wind measurement. Advantages offered by sodar wind sounding technology include reasonably low procurement, operating, and maintenance costs, no emissions of eye-damaging light beams or electromagnetic radiation requiring frequency clearances, and adjustable frequencies and pulse lengths that can be used to optimize data quality at desired ranges and range resolutions. When properly sited and used with adequate sampling methods, sodars can provide continuous wind and turbulence profile information at height ranges from a few tens of meters to over a kilometre for typical averaging periods of 1 to 60 minutes.
SCOPE
1.1 This guide describes the application of acoustic remote sensing for measuring atmospheric wind and turbulence profiles. It includes a summary of the fundamentals of atmospheric sound detection and ranging (sodar), a description of the methodology and equipment used for sodar applications, factors to consider during site selection and equipment installation, and recommended procedures for acquiring valid and relevant data.  
1.2 This guide applies principally to pulsed monostatic sodar techniques as applied to wind and turbulence measurement in the open atmosphere, although many of the definitions and principles are also applicable to bistatic configurations. This guide is not directly applicable to radio-acoustic sounding systems (RASS), or tomographic methods.  
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this guide.

General Information

• Status: Historical
• Publication Date: 31-Mar-2015
• ICS: 17.140.99 - Other standards related to acoustics
• Technical Committee: D22 - Air Quality
• Drafting Committee: D22.11 - Meteorology

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D7145 −05 (Reapproved 2015)
Standard Guide for
Measurement of Atmospheric Wind and Turbulence Profiles
by Acoustic Means
This standard is issued under the fixed designation D7145; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.1 acoustic beam, n—focused or directed acoustic pulse
(compression wave) propagating in a radial direction from its
1.1 This guide describes the application of acoustic remote
point of origin.
sensing for measuring atmospheric wind and turbulence pro-
files.Itincludesasummaryofthefundamentalsofatmospheric 3.2.2 acoustic power, n—relative amplitude or intensity
sound detection and ranging (sodar), a description of the (dB) of an atmospheric compression wave.
methodology and equipment used for sodar applications, fac-
3.2.3 acoustic refractive index, n—ratio of reference (at a
tors to consider during site selection and equipment
standard temperature of 293.15 K and 1013.25 hPa pressure)
installation, and recommended procedures for acquiring valid
speed of sound value to its actual value.
and relevant data.
3.2.4 acoustic scatter, n—the dispersal by reflection,
1.2 This guide applies principally to pulsed monostatic
refraction, or diffraction of acoustic energy in the atmosphere.
sodar techniques as applied to wind and turbulence measure-
3.2.5 acoustic scattering Cross-section Per Unit Volume (σ,
ment in the open atmosphere, although many of the definitions
–1
m ), n—fraction of incident power at the transmit frequency
and principles are also applicable to bistatic configurations.
that is backscattered per unit distance into a unit solid angle.
This guide is not directly applicable to radio-acoustic sounding
systems (RASS), or tomographic methods.
3.2.6 acoustic attenuation (φ, dB/100m ), n—loss of acous-
tic power (acoustic wave amplitude) by beam spreading,
1.3 The values stated in SI units are to be regarded as
scattering, and absorption as the transmitted wavefront propa-
standard. No other units of measurement are included in this
gates through the atmosphere.
guide.
3.2.7 backscatter, n—power returned towards a receiving
2. Referenced Documents
antenna.
2.1 ASTM Standards:
3.2.8 beamwidth (degrees), n—one way angular width (half
D1356 Terminology Relating to Sampling and Analysis of
angle at –3dB) of an acoustic beam from its centerline
Atmospheres
maximum to the point at the beam periphery where the power
level is half (3 decibels below) centerline beam power.
3. Terminology
3.2.9 bistatic, adj—sodar configuration that uses spatially
3.1 Definitions—Refer to Terminology D1356 for general
separated antennas for signal transmission and reception.
terms and their definitions.
3.2.10 clutter, n—undesirable returns, particularly from
3.2 Definitions of Terms Specific to This Standard:
sidelobes, that increase background noise and obscure desired
Note: The definitions below are presented in simplified,
signals.
common, qualitative terms. Refer to noted references for more
3.2.11 decibel (dB), n—logarithmic (base 10) ratio of power
detailed information.
to a reference power, usually one-tenth bell; for power P1 and
reference power P2, the ratio is given by 10log (P1/P2).
3.2.12 directivity, n—concentration of transmitted power
This guide is under the jurisdiction of ASTM Committee D22 on Air Quality
(dB) within a narrow beam by an antenna, measured as a ratio
and is the direct responsibility of Subcommittee D22.11 on Meteorology.
of power in the main beam to power radiated in all directions.
Current edition approved April 1, 2015. Published April 2015. Originally
ε1
approved in 2005. Last previous edition approved in 2010 as D7145 – 05 (2010) .
3.2.13 Doppler frequency (f , Hz), n—shifted frequency
DOI: 10.1520/D7145-05R15. D
For referenced ASTM standards, visit the ASTM website, www.astm.org, or measured at the receiver from the scattered acoustic signal.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
3.2.14 effective antenna aperture (A,m ), n—product of
Standards volume information, refer to the standard’s Document Summary page on e
the ASTM website. antenna area with antenna efficiency.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7145−05 (2015)
3.2.15 gain (G), n—increase in power (dB) per unit area counts for the effects of molecular diffusion and turbulent
arising from the product of antenna directivity with efficiency. energy dissipation into heat.
n—non-dimensional effective aperture amplification factor
3.2.32 transmit frequency (f, Hz), n—selected frequency or
arising from an antenna’s directivity.
frequencies at which an acoustic transmitter’s output is
3.2.16 inter pulse period (t , s), n—time between the start
max achieved.
of successive transmitted pulses or pulse sequences.
3.2.33 transmitted power (P, W), n—electrical power in
t
3.2.16.1 Discussion—The inter pulse period (IPP) is the
watts measured at the antenna input; acoustic power radiated
inverse of the pulse repetition frequency (PRF) in Hertz (Hz).
by an antenna is the product of transmitted electrical power
3.2.17 monostatic, adj—sodar configuration that uses the
with the conversion efficiency from electrical to acoustic
same antenna for transmission and reception.
power.
3.2.18 Neper, n—natural logarithm of the ratio of reflected
3.3 Symbols:
to incident sound energy flux density at a given range.
â = viscous and molecular sound absorption coefficient,
3.2.19 pulse, n—finite burst of transmitted energy.
–1
Nepers per wavelength, m ,
3.2.20 pulse length (τ, s), n—duration of a single pulse. 2
A = effective antenna aperture, m ,
e
–1
3.2.21 pulse sequence, n—train of pulses, often at different
c = speed of sound, ms ,
2 –2/3
frequencies. C = temperature structure parameter, K m ,
T
ε = receiver electromechanical efficiency,
R
3.2.22 range (r, m), n—distance from the antenna surface to
ε = transmitter electromechanical efficiency,
T
the scattering surface.
f = central acoustic frequency transmitted by the sodar,
3.2.23 range aliasing, n—sampling ambiguity that arises
Hz,
when returns are received from a transmission that was made
f = Doppler frequency, Hz,
D
prior to the latest transmitted pulse sequence, usually from a
G = antenna gain,
scattering surface located beyond the maximum unambiguous P = received electrical power, W,
r
range. P = transmitted electrical power, W,
t
r = range from transmitter to a range gate, m,
3.2.24 range gate, n—conical section of the atmosphere
r = maximum unambiguous range, m,
max
containing the scattering volume from which acoustic returns
t = time between transmission of an acoustic pulse and
can be resolved.
reception of returning echoes, s,
3.2.25 range resolution (D , m), n—length of a segment of
T = temperature in Kelvins, K,
r
K
the scattering volume along the axis of beam propagation.
t = IPP, the maximum listening time between transmitted
max
3.2.25.1 Discussion—Rangeresolutionequalshalftheprod- pulses or pulse sequences, s,
–1
V = target velocity, ms ,
uct of speed of sound and pulse length (∆r=cτ⁄2).
t
∆r = range resolution, m,
3.2.26 received power (P , W), n—electrical power received
r
φ = combined viscous and molecular attenuation factor,
m
at an antenna during listening mode; the product of received
φ = excess attenuation factor,
x
acousticpowerwithreceiverconversionefficiencyfromacous-
λ = acoustic wavelength, m,
–1
tic to electrical power.
σ = acoustic scattering crossection per unit volume, m ,
3.2.27 scattering volume (m ), n—volume of a conical and
section in the atmosphere centered on the radial along which τ = pulse length, s.
the acoustic beam propagates.
3.2.27.1 Discussion—This is commonly calculated from the
4. Summary of Guide
3 dB beamwidth.
4.1 The principles of atmospheric wind and turbulence
3.2.28 sidelobes, n—acoustic energy transmitted in a direc-
profiling using the sound direction and ranging technique are
tion other than the main beam (or lobe).
described.
3.2.28.1 Discussion—Sidelobes vary inversely with antenna
4.2 Considerations for sodar equipment, site selection, and
size and transmitted frequency.
equipment installation procedures are presented.
3.2.29 signal-to-noise-ratio, n—ratio of the calculated re-
ceived signal power to the calculated noise power, frequently 4.3 Data acquisition and quality assurance procedures are
abbreviated as SNR.
described.
3.2.30 sound detection and ranging (sodar), adj—remote
5. Significance and Use
sensing technique that generates acoustic pulses that propagate
through the atmosphere, and subsequently samples the scat-
5.1 Sodars have found wide applications for the remote
tered atmospheric returns.
measurement of wind and turbulence profiles in the
n—instrument that performs these functions.
atmosphere, particularly in the gap between meteorological
3.2.31 temperature structure parameter (C , K), towers and the lower range gates of wind profiling radars. The
T
n—structure constant for measurement of fast-response tem- sodar’sfarfieldacousticpowerisalsousedforrefractiveindex
perature differences over small spatial separations that ac- calculations and to estimate atmospheric stability, heat flux,
D7145−05 (2015)
and mixed layer depth (1-5). Sodars are useful for these r 5 ct/2 (1)
purposes because of strong interaction between sound waves
where the factor of 2 accounts for travel along outward
and the atmosphere’s thermal and velocity micro-structure that
propagating and return paths. Wind profiling sodars that
produce acoustic returns with substantial signal-to-noise ratios
transmit a minimum of three radial beams resolve horizontal
(SNR). The returned echoes are Doppler-shifted in frequency.
and vertical wind components. Assuming homogeneity in the
This frequency shift, proportional to the radial velocity of the
wind field above the sodar, trigonometry is used to resolve
scattering surface, provides the basis for wind measurement.
distance along each radial, which is then converted to height
Advantages offered by sodar wind sounding technology in-
above the sodar antenna. The user is then presented with a
cludereasonablylowprocurement,operating,andmaintenance
vertical profile of wind, turbulence, and signal strength infor-
costs, no emissions of eye-damaging light beams or electro-
mation.Heightranging,rangeresolution,andsignalqualityare
magnetic radiation requiring frequency clearances, and adjust-
functions of sodar performance and its operating environment,
able frequencies and pulse lengths that can be used to optimize
as described below.
data quality at desired ranges and range resolutions. When
6.3 The Sodar Equation. The power received (P)bya
r
properly sited and used with adequate sampling methods,
sodar’s acoustic antenna is a product of sodar performance and
sodars can provide continuous wind and turbulence profile
atmospheric attenuation factors. Sodar performance factors
information at height ranges from a few tens of meters to over
include effective transmitted power (P) at its transmitted
t
a kilometre for typical averaging periods of 1 to 60 minutes.
frequency(ies), effective antenna aperture (A ), transmitter and
e
6. Monostatic Sound Direction and Ranging receiver efficiency factors (ε and ε ), and pulse length (τ).
T R
Atmospheric scattering factors include the acoustic scattering
6.1 Sodar Design Types. Most commercially available so-
crossection (σ) and attenuation factors φ and φ . Attenuation
m x
dars operate using a monostatic phased array antenna design
factor φ represents “classical” viscous losses plus the com-
m
composed of a planar array of acoustic transmitters that form
bination of molecular rotational and vibrational absorption.
the emitted beam and steer it towards the desired direction.
The second factor (φ ) represents excess attenuation due to
x
Other designs, to include non-phased antennas for each beam
complex interactions of the acoustic beam with larger scale
and bi-static configurations, are also available. An advantage
atmospheric features. The sodar performance and atmospheric
offered by bi-static sodars is that they also utilize signals
factors are combined in a simplified monostatic sodar equation
scattered from small scale velocity fluctuations that are not
for received power:
available in monostatic configurations. Except for beam
P 5 sodar performance atmospheric factors
forming, steering, and the simplified monostatic sodar $ % $ %
r
equation, the information provided below is generally appli-
5 P A ε ε cτ/2 σφ φ (2)
$~ !~ !~ !% $ %
t e T R m x
cable to those designs as well.
6.4 Sodar Performance. Sodar performance characteristics
6.2 Description of Operation. A phased array monostatic
includethesodartransmittedacousticpower,andtheefficiency
sodar emits acoustic pulses (adiabatic compression waves) at a
with which power is transmitted and received. P A is the
t e
transmit frequency or frequencies. Pulses from each antenna
power-aperture product. A =AG⁄r is the solid angle sub-
e
are formed into a conical beam or wavefront with its vertex at
tended by an antenna of aperture (A, m ) multiplied by the
the antenna. Individual transducer pulse timing or phase
effective aperture factor (G, the antenna’s gain), as viewed at
shiftingmethods,indicatedbyΦinFig.1,areusedtoshapethe
range (r) from the scattering volume. Range resolution (∆r=
beam and steer it in the desired direction. As it travels along a
cτ/2) is the length (m), along the radial axis of signal
radial direction through the atmosphere at speed of sound (c),
propagation, of the instantaneous scattering volume and de-
this acoustic wave experiences attenuation by spreading,
finesthevolumefromwhichabackscatteredsignalisresolved.
absorption, and scattering as described below. Temperature Note that range resolution determines range gate thickness.
inhomogeneitiesandsharpgradientsencounteredbythepropa-
Scattering surfaces that produce useful acoustic returns often
gating beam deform and scatter the beam. Wind velocity occupy only a fraction of the scattering volume in the real
components along the axis of propagation also Doppler- shift
atmosphere (see Fig. 1 and 6.6).The magnitude of the returned
the acoustic frequency of backscattered signals. A schematic signals is directly proportional to the percentage of the scat-
drawingofacousticwavefrontgenerationandbackscatterfrom
tering volume occupied by scattering surfaces and the intensity
a reflecting surface is presented in
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: D7145 − 05 (Reapproved 2010) D7145 − 05 (Reapproved 2015)
Standard Guide for
Measurement of Atmospheric Wind and Turbulence Profiles
by Acoustic Means
This standard is issued under the fixed designation D7145; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Editorially corrected Equation 6 in April 2010.
1. Scope
1.1 This guide describes the application of acoustic remote sensing for measuring atmospheric wind and turbulence profiles. It
includes a summary of the fundamentals of atmospheric sound detection and ranging (sodar), a description of the methodology and
equipment used for sodar applications, factors to consider during site selection and equipment installation, and recommended
procedures for acquiring valid and relevant data.
1.2 This guide applies principally to pulsed monostatic sodar techniques as applied to wind and turbulence measurement in the
open atmosphere, although many of the definitions and principles are also applicable to bistatic configurations. This guide is not
directly applicable to radio-acoustic sounding systems (RASS), or tomographic methods.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this guide.
2. Referenced Documents
2.1 ASTM Standards:
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
3. Terminology
3.1 Definitions—Refer to Terminology D1356 for general terms and their definitions.
3.2 Definitions of Terms Specific to This Standard:
Note: The definitions below are presented in simplified, common, qualitative terms. Refer to noted references for more detailed
information.
3.2.1 acoustic beam, n—focused or directed acoustic pulse (compression wave) propagating in a radial direction from its point
of origin.
3.2.2 acoustic power, n—relative amplitude or intensity (dB) of an atmospheric compression wave.
3.2.3 acoustic refractive index, n—ratio of reference (at a standard temperature of 293.15 K and 1013.25 hPa pressure) speed
of sound value to its actual value.
3.2.4 acoustic scatter, n—the dispersal by reflection, refraction, or diffraction of acoustic energy in the atmosphere.
–1
3.2.5 acoustic scattering Cross-section Per Unit Volume (σ, m ), n—fraction of incident power at the transmit frequency that
is backscattered per unit distance into a unit solid angle.
3.2.6 acoustic attenuation (φ, dB/100m ), n—loss of acoustic power (acoustic wave amplitude) by beam spreading, scattering,
and absorption as the transmitted wavefront propagates through the atmosphere.
3.2.7 backscatter, n—power returned towards a receiving antenna.
3.2.8 beamwidth (degrees), n—one way angular width (half angle at –3dB) of an acoustic beam from its centerline maximum
to the point at the beam periphery where the power level is half (3 decibels below) centerline beam power.
3.2.9 bistatic, adj—sodar configuration that uses spatially separated antennas for signal transmission and reception.
This guide is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved April 1, 2010April 1, 2015. Published July 2010April 2015. Originally approved in 2005. Last previous edition approved in 20052010 as
ε1
D7145 - 05.D7145 – 05 (2010) . DOI: 10.1520/D7145-05R10E01.10.1520/D7145-05R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7145 − 05 (2015)
3.2.10 clutter, n—undesirable returns, particularly from sidelobes, that increase background noise and obscure desired signals.
3.2.11 decibel (dB), n—logarithmic (base 10) ratio of power to a reference power, usually one-tenth bell; for power P1 and
reference power P2, the ratio is given by 10log (P1/P2).
3.2.12 directivity, n—concentration of transmitted power (dB) within a narrow beam by an antenna, measured as a ratio of power
in the main beam to power radiated in all directions.
3.2.13 Doppler frequency (f , Hz), n—shifted frequency measured at the receiver from the scattered acoustic signal.
D
3.2.14 effective antenna aperture (A , m ) , ), n—product of antenna area with antenna efficiency.
e
3.2.15 gain (G), n—increase in power (dB) per unit area arising from the product of antenna directivity with efficiency.
n—non-dimensional effective aperture amplification factor arising from an antenna’s directivity.
3.2.16 inter pulse period (t , s), n—time between the start of successive transmitted pulses or pulse sequences.
max
3.2.16.1 Discussion—
The inter pulse period (IPP) is the inverse of the pulse repetition frequency (PRF) in Hertz (Hz).
3.2.17 monostatic, adj—sodar configuration that uses the same antenna for transmission and reception.
3.2.18 Neper, n—natural logarithm of the ratio of reflected to incident sound energy flux density at a given range.
3.2.19 pulse, n—finite burst of transmitted energy.
3.2.20 pulse length (τ, s), n—duration of a single pulse.
3.2.21 pulse sequence, n—train of pulses, often at different frequencies.
3.2.22 range (r, m), n—distance from the antenna surface to the scattering surface.
3.2.23 range aliasing, n—sampling ambiguity that arises when returns are received from a transmission that was made prior to
the latest transmitted pulse sequence, usually from a scattering surface located beyond the maximum unambiguous range.
3.2.24 range gate, n—conical section of the atmosphere containing the scattering volume from which acoustic returns can be
resolved.
3.2.25 range resolution (D , m), n—length of a segment of the scattering volume along the axis of beam propagation.
r
3.2.25.1 Discussion—
Range resolution equals half the product of speed of sound and pulse length (Δr = cτ ⁄2).
3.2.26 received power (P , W), n—electrical power received at an antenna during listening mode; the product of received
r
acoustic power with receiver conversion efficiency from acoustic to electrical power.
3.2.27 scattering volume (m ), n—volume of a conical section in the atmosphere centered on the radial along which the acoustic
beam propagates.
3.2.27.1 Discussion—
This is commonly calculated from the 3 dB beamwidth.
3.2.28 sidelobes, n—acoustic energy transmitted in a direction other than the main beam (or lobe).
3.2.28.1 Discussion—
Sidelobes vary inversely with antenna size and transmitted frequency.
3.2.29 signal-to-noise-ratio, n—ratio of the calculated received signal power to the calculated noise power, frequently
abbreviated as SNR.
3.2.30 sound detection and ranging (sodar), adj—remote sensing technique that generates acoustic pulses that propagate
through the atmosphere, and subsequently samples the scattered atmospheric returns.
n—instrument that performs these functions.
3.2.31 temperature structure parameter (C , K), n—structure constant for measurement of fast-response temperature
T
differences over small spatial separations that accounts for the effects of molecular diffusion and turbulent energy dissipation into
heat.
3.2.32 transmit frequency (f, Hz), n—selected frequency or frequencies at which an acoustic transmitter’s output is achieved.
D7145 − 05 (2015)
3.2.33 transmitted power (P , W), n—electrical power in watts measured at the antenna input; acoustic power radiated by an
t
antenna is the product of transmitted electrical power with the conversion efficiency from electrical to acoustic power.
3.3 Symbols:
–1
â = viscous and molecular sound absorption coefficient, Nepers per wavelength, m ,
A = effective antenna aperture, m ,
e
–1
c = speed of sound, ms ,
2 –2/3
C = temperature structure parameter, K m ,
T
ε = receiver electromechanical efficiency,
R
ε = transmitter electromechanical efficiency,
T
f = central acoustic frequency transmitted by the sodar, Hz,
f = Doppler frequency, Hz,
D
G = antenna gain,
P = received electrical power, W,
r
P = transmitted electrical power, W,
t
r = range from transmitter to a range gate, m,
r = maximum unambiguous range, m,
max
t = time between transmission of an acoustic pulse and reception of returning echoes, s,
T = temperature in Kelvins, K,
K
t = IPP, the maximum listening time between transmitted pulses or pulse sequences, s,
max
–1
V = target velocity, ms ,
t
Δr = range resolution, m,
φ = combined viscous and molecular attenuation factor,
m
φ = excess attenuation factor,
x
λ = acoustic wavelength, m,
–1
σ = acoustic scattering crossection per unit volume, m , and
τ = pulse length, s.
4. Summary of Guide
4.1 The principles of atmospheric wind and turbulence profiling using the sound direction and ranging technique are described.
4.2 Considerations for sodar equipment, site selection, and equipment installation procedures are presented.
4.3 Data acquisition and quality assurance procedures are described.
5. Significance and Use
5.1 Sodars have found wide applications for the remote measurement of wind and turbulence profiles in the atmosphere,
particularly in the gap between meteorological towers and the lower range gates of wind profiling radars. The sodar’s far field
acoustic power is also used for refractive index calculations and to estimate atmospheric stability, heat flux, and mixed layer depth
(1-5)). . Sodars are useful for these purposes because of strong interaction between sound waves and the atmosphere’s thermal and
velocity micro-structure that produce acoustic returns with substantial signal-to-noise ratios (SNR). The returned echoes are
Doppler-shifted in frequency. This frequency shift, proportional to the radial velocity of the scattering surface, provides the basis
for wind measurement. Advantages offered by sodar wind sounding technology include reasonably low procurement, operating,
and maintenance costs, no emissions of eye-damaging light beams or electromagnetic radiation requiring frequency clearances, and
adjustable frequencies and pulse lengths that can be used to optimize data quality at desired ranges and range resolutions. When
properly sited and used with adequate sampling methods, sodars can provide continuous wind and turbulence profile information
at height ranges from a few tens of meters to over a kilometre for typical averaging periods of 1 to 60 minutes.
6. Monostatic Sound Direction and Ranging
6.1 Sodar Design Types. Most commercially available sodars operate using a monostatic phased array antenna design composed
of a planar array of acoustic transmitters that form the emitted beam and steer it towards the desired direction. Other designs, to
include non-phased antennas for each beam and bi-static configurations, are also available. An advantage offered by bi-static sodars
is that they also utilize signals scattered from small scale velocity fluctuations that are not available in monostatic configurations.
Except for beam forming, steering, and the simplified monostatic sodar equation, the information provided below is generally
applicable to those designs as well.
6.2 Description of Operation. A phased array monostatic sodar emits acoustic pulses (adiabatic compression waves) at a
transmit frequency or frequencies. Pulses from each antenna are formed into a conical beam or wavefront with its vertex at the
antenna. Individual transducer pulse timing or phase shifting methods, indicated by Φ in Fig. 1, are used to shape the beam and
The boldface numbers in parentheses refer to the list of references at the end of this standard.
D7145 − 05 (2015)
FIG. 1 Acoustic Wavefront Generation and Backscatter
D7145 − 05 (2015)
steer it in the desired direction. As it travels along a radial direction through the atmosphere at speed of sound (c), this acoustic
wave experiences attenuation by spreading, absorption, and scattering as described below. Temperature inhomogeneities and sharp
gradients encountered by the propagating beam deform and scatter the beam. Wind velocity components along the axis of
propagation also Doppler- shift the acoustic frequency of backscattered signals. A schematic drawing of acoustic wavefront
generation and backscatter from a reflecting surface is presented in Fig. 1. After its transmission of an acoustic pulse train, the sodar
switches to listening mode for backscattered acoustic signals. Returning signals are characterized by their intensity (amplitude),
spectral width, Doppler-shifted frequency, and lapsed time (t) from initial pulse transmission. Returns from lower heights are
received sooner than returns from greater heights. The relationship between lapsed time (t), speed of sound (c), and radial range
(r) to the scattering surface is given by:
r 5 ct/2 (1)
where the factor of 2 accounts for travel along outward propagating and return paths. Wind profiling sodars that transmit a
minimum of three radial beams resolve horizontal and vertical wind components. Assuming homogeneity in the wind field above
the sodar, trigonometry is used to resolve distance along each radial, which is then converted to height above the sodar antenna.
The user is then presented with a vertical profile of wind, turbulence, and signal strength information. Height ranging, range
resolution, and signal quality are functions of sodar performance and its operating environment, as described below.
6.3 The Sodar Equation. The power received (P ) by a sodar’s acoustic antenna is a product of sodar performance and
r
atmospheric attenuation factors. Sodar performance factors include effective transmitted power (P ) at its transmitted
t
frequency(ies), effective antenna aperture (A ), transmitter and receiver efficiency factors (ε and ε ), and pulse length (τ).
e T R
Atmospheric scattering factors include the acoustic scattering crossection (σ) and attenuation factors φ and φ . Attenuation factor
m x
φ represents “classical” viscous losses plus the combination of molecular rotational and vibrational absorption. The second factor
m
(φ ) represents excess
...