ASTM D7145-05(2010)e1
(Guide)Standard Guide for Measurement of Atmospheric Wind and Turbulence Profiles by Acoustic Means
Standard Guide for Measurement of Atmospheric Wind and Turbulence Profiles by Acoustic Means
SIGNIFICANCE AND USE
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.
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.
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Designation:D7145 −05(Reapproved 2010)
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 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-
ment in the open atmosphere, although many of the definitions 3.2.5 acoustic scattering Cross-section Per Unit Volume (σ,
–1
and principles are also applicable to bistatic configurations. m ), n—fraction of incident power at the transmit frequency
This guide is not directly applicable to radio-acoustic sounding that is backscattered per unit distance into a unit solid angle.
systems (RASS), or tomographic methods.
3.2.6 acoustic attenuation (φ, dB/100m ), n—loss of acous-
1.3 The values stated in SI units are to be regarded as
tic power (acoustic wave amplitude) by beam spreading,
standard. No other units of measurement are included in this
scattering, and absorption as the transmitted wavefront propa-
guide.
gates through the atmosphere.
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
3. Terminology
level is half (3 decibels below) centerline beam power.
3.1 Definitions—Refer to Terminology D1356 for general
3.2.9 bistatic, adj—sodar configuration that uses spatially
terms and their definitions.
separated antennas for signal transmission and reception.
3.2 Definitions of Terms Specific to This Standard:
3.2.10 clutter, n—undesirable returns, particularly from
Note: The definitions below are presented in simplified,
sidelobes, that increase background noise and obscure desired
common, qualitative terms. Refer to noted references for more
signals.
detailed information.
3.2.11 decibel (dB), n—logarithmic (base 10) ratio of power
to a reference power, usually one-tenth bell; for power P1 and
This guide is under the jurisdiction of ASTM Committee D22 on Air Quality
reference power P2, the ratio is given by 10log (P1/P2).
and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved April 1, 2010. Published July 2010. Originally
3.2.12 directivity, n—concentration of transmitted power
approved in 2005. Last previous edition approved in 2005 as D7145 - 05. DOI:
(dB) within a narrow beam by an antenna, measured as a ratio
10.1520/D7145-05R10E01.
of power in the main beam to power radiated in all directions.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.2.13 Doppler frequency (f , Hz), n—shifted frequency
Standards volume information, refer to the standard’s Document Summary page on D
the ASTM website. measured at the receiver from the scattered acoustic signal.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D7145−05 (2010)
2 2
3.2.14 effective antenna aperture (A,m),n—product of 3.2.31 temperature structure parameter (C , K),
e T
antenna area with antenna efficiency. n—structure constant for measurement of fast-response tem-
perature differences over small spatial separations that ac-
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
C = temperature structure parameter, K m ,
frequencies.
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
G = antenna gain,
prior to the latest transmitted pulse sequence, usually from a
P = received electrical power, W,
scattering surface located beyond the maximum unambiguous
r
P = transmitted electrical power, W,
t
range.
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,
T = temperature in Kelvins, K,
3.2.25 range resolution (D , m), n—length of a segment of K
r
t = IPP, the maximum listening time between transmitted
the scattering volume along the axis of beam propagation. max
pulses or pulse sequences, s,
3.2.25.1 Discussion—Rangeresolutionequalshalftheprod-
–1
V = target velocity, ms ,
t
uct of speed of sound and pulse length (∆r=cτ⁄2).
∆r = range resolution, m,
3.2.26 received power (P , W), n—electrical power received
φ = combined viscous and molecular attenuation factor,
r
m
at an antenna during listening mode; the product of received
φ = excess attenuation factor,
x
λ = acoustic wavelength, m,
acousticpowerwithreceiverconversionefficiencyfromacous-
–1
σ = acoustic scattering crossection per unit volume, m ,
tic to electrical power.
3 and
3.2.27 scattering volume (m ), n—volume of a conical
τ = pulse length, s.
section in the atmosphere centered on the radial along which
the acoustic beam propagates.
4. Summary of Guide
3.2.27.1 Discussion—This is commonly calculated from the
4.1 The principles of atmospheric wind and turbulence
3 dB beamwidth.
profiling using the sound direction and ranging technique are
3.2.28 sidelobes, n—acoustic energy transmitted in a direc-
described.
tion other than the main beam (or lobe).
4.2 Considerations for sodar equipment, site selection, and
3.2.28.1 Discussion—Sidelobes vary inversely with antenna
equipment installation procedures are presented.
size and transmitted frequency.
4.3 Data acquisition and quality assurance procedures are
3.2.29 signal-to-noise-ratio, n—ratio of the calculated re-
described.
ceived signal power to the calculated noise power, frequently
abbreviated as SNR. 5. Significance and Use
3.2.30 sound detection and ranging (sodar), adj—remote 5.1 Sodars have found wide applications for the remote
sensing technique that generates acoustic pulses that propagate measurement of wind and turbulence profiles in the
through the atmosphere, and subsequently samples the scat- atmosphere, particularly in the gap between meteorological
tered atmospheric returns. towers and the lower range gates of wind profiling radars. The
n—instrument that performs these functions. sodar’sfarfieldacousticpowerisalsousedforrefractiveindex
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D7145−05 (2010)
calculations and to estimate atmospheric stability, heat flux, betweenlapsedtime(t),speedofsound(c),andradialrange(r)
and mixed layer depth (1-5) . Sodars are useful for these to the scattering surface is given by:
purposes because of strong interaction between sound waves
r 5 ct/2 (1)
and the atmosphere’s thermal and velocity micro-structure that
where the factor of 2 accounts for travel along outward
produce acoustic returns with substantial signal-to-noise ratios
propagating and return paths. Wind profiling sodars that
(SNR). The returned echoes are Doppler-shifted in frequency.
transmit a minimum of three radial beams resolve horizontal
This frequency shift, proportional to the radial velocity of the
and vertical wind components. Assuming homogeneity in the
scattering surface, provides the basis for wind measurement.
wind field above the sodar, trigonometry is used to resolve
Advantages offered by sodar wind sounding technology in-
distance along each radial, which is then converted to height
cludereasonablylowprocurement,operating,andmaintenance
above the sodar antenna. The user is then presented with a
costs, no emissions of eye-damaging light beams or electro-
vertical profile of wind, turbulence, and signal strength infor-
magnetic radiation requiring frequency clearances, and adjust-
mation.Heightranging,rangeresolution,andsignalqualityare
able frequencies and pulse lengths that can be used to optimize
functions of sodar performance and its operating environment,
data quality at desired ranges and range resolutions. When
as described below.
properly sited and used with adequate sampling methods,
sodars can provide continuous wind and turbulence profile
6.3 The Sodar Equation. The power received (P)bya
r
information at height ranges from a few tens of meters to over
sodar’s acoustic antenna is a product of sodar performance and
a kilometre for typical averaging periods of 1 to 60 minutes.
atmospheric attenuation factors. Sodar performance factors
include effective transmitted power (P) at its transmitted
t
6. Monostatic Sound Direction and Ranging
frequency(ies), effective antenna aperture (A ), transmitter and
e
6.1 Sodar Design Types. Most commercially available so- receiver efficiency factors (ε and ε ), and pulse length (τ).
T R
dars operate using a monostatic phased array antenna design Atmospheric scattering factors include the acoustic scattering
composed of a planar array of acoustic transmitters that form crossection (σ) and attenuation factors φ and φ . Attenuation
m x
the emitted beam and steer it towards the desired direction. factor φ represents “classical” viscous losses plus the com-
m
Other designs, to include non-phased antennas for each beam bination of molecular rotational and vibrational absorption.
and bi-static configurations, are also available. An advantage
The second factor (φ ) represents excess attenuation due to
x
offered by bi-static sodars is that they also utilize signals complex interactions of the acoustic beam with larger scale
scattered from small scale velocity fluctuations that are not
atmospheric features. The sodar performance and atmospheric
available in monostatic configurations. Except for beam factors are combined in a simplified monostatic sodar equation
forming, steering, and the simplified monostatic sodar
for received power:
equation, the information provided below is generally appli-
P 5 sodar performance atmospheric factors
$ % $ %
r
cable to those designs as well.
5 $~P A !~ε ε !~cτ/2!% $σφ φ %. (2)
t e T R m x
6.2 Description of Operation. A phased array monostatic
6.4 Sodar Performance. Sodar performance characteristics
sodar emits acoustic pulses (adiabatic compression waves) at a
includethesodartransmittedacousticpower,andtheefficiency
transmit frequency or frequencies. Pulses from each antenna
with which power is transmitted and received. P A is the
t e
are formed into a conical beam or wavefront with its vertex at
power-aperture product. A =AG⁄r is the solid angle sub-
e
the antenna. Individual transducer pulse timing or phase
tended by an antenna of aperture (A, m ) multiplied by the
shiftingmethods,indicatedbyΦinFig.1,areusedtoshapethe
effective aperture factor (G, the antenna’s gain), as viewed at
beam and steer it in the desired direction. As it travels along a
range (r) from the scattering volume. Range resolution (∆r=
radial direction through the atmosphere at speed of sound (c),
cτ/2) is the length (m), along the radial axis of signal
this acoustic wave experiences attenuation by spreading,
propagation, of the instantaneous scattering volume and de-
absorption, and scattering as described below. Temperature
finesthevolumefromwhichabackscatteredsignalisresolved.
inhomogeneitiesandsharpgradientsencounteredbythepropa-
Note that range resolution determines range gate thickness.
gating beam deform and scatter the beam. Wind velocity
Scattering surfaces that produce useful acoustic returns often
components along the axis of propagation also Doppler- shift
occupy only a fraction of the scattering volume in the real
the acoustic frequency of backscattered signals. A schematic
atmosphere (see Fig. 1 and 6.6).The magnitude of the returned
drawingofacousticwavefrontgenerationandbackscatterfrom
signals is directly p
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