ASTM D7145-05
(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
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
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 3.2.3 acoustic refractive index, n—ratio of reference (at a
standard temperature of 293.15 K and 1013.25 hPa pressure)
1.1 This guide describes the application of acoustic remote
speed of sound value to its actual value.
sensing for measuring atmospheric wind and turbulence pro-
3.2.4 acoustic scatter, n—the dispersal by reflection, refrac-
files.Itincludesasummaryofthefundamentalsofatmospheric
tion, or diffraction of acoustic energy in the atmosphere.
sound detection and ranging (sodar), a description of the
3.2.5 acoustic scattering Cross-section Per Unit Volume (s,
methodology and equipment used for sodar applications, fac-
–1
m ), n—fraction of incident power at the transmit frequency
tors to consider during site selection and equipment installa-
that is backscattered per unit distance into a unit solid angle.
tion, and recommended procedures for acquiring valid and
3.2.6 acoustic attenuation (f, dB/100m ), n—loss of acous-
relevant data.
tic power (acoustic wave amplitude) by beam spreading,
1.2 This guide applies principally to pulsed monostatic
scattering, and absorption as the transmitted wavefront propa-
sodar techniques as applied to wind and turbulence measure-
gates through the atmosphere.
ment in the open atmosphere, although many of the definitions
3.2.7 backscatter, n—power returned towards a receiving
and principles are also applicable to bistatic configurations.
antenna.
This guide is not directly applicable to radio-acoustic sounding
3.2.8 beamwidth (degrees), n—one way angular width (half
systems (RASS), or tomographic methods.
angle at –3dB) of an acoustic beam from its centerline
2. Referenced Documents
maximum to the point at the beam periphery where the power
level is half (3 decibels below) centerline beam power.
2.1 ASTM Standards:
3.2.9 bistatic, adj—sodar configuration that uses spatially
D1356 Terminology Relating to Sampling and Analysis of
separated antennas for signal transmission and reception.
Atmospheres
3.2.10 clutter, n—undesirable returns, particularly from
3. Terminology
sidelobes, that increase background noise and obscure desired
signals.
3.1 Definitions—Refer to Terminology D1356 for general
3.2.11 decibel (dB), n—logarithmic(base10)ratioofpower
terms and their definitions.
to a reference power, usually one-tenth bell; for power P1 and
3.2 Definitions of Terms Specific to This Standard:
reference power P2, the ratio is given by 10log (P1/P2).
Note: The definitions below are presented in simplified, com- 10
3.2.12 directivity, n—concentration of transmitted power
mon, qualitative terms. Refer to noted references for more
(dB) within a narrow beam by an antenna, measured as a ratio
detailed information.
of power in the main beam to power radiated in all directions.
3.2.1 acoustic beam, n—focused or directed acoustic pulse
3.2.13 Doppler frequency (f , Hz), n—shifted frequency
(compression wave) propagating in a radial direction from its D
measured at the receiver from the scattered acoustic signal.
point of origin.
3.2.14 effective antenna aperture (A,m ), n—product of
3.2.2 acoustic power, n—relative amplitude or intensity e
antenna area with antenna efficiency.
(dB) of an atmospheric compression wave.
3.2.15 gain (G), n—increase in power (dB) per unit area
arising from the product of antenna directivity with efficiency.
This guide is under the jurisdiction of ASTM Committee D22 on Air Quality
n—non-dimensional effective aperture amplification factor
and is the direct responsibility of Subcommittee D22.11 on Meteorology.
arising from an antenna’s directivity.
Current edition approved March 1, 2005. Published May 2005. DOI: 10.1520/
D7145-05.
3.2.16 inter pulse period (t , s), n—time between the start
max
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
of successive transmitted pulses or pulse sequences.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.2.16.1 Discussion—The inter pulse period (IPP) is the
Standards volume information, refer to the standard’s Document Summary page on
inverse of the pulse repetition frequency (PRF) in Hertz (Hz).
the ASTM website.
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D7145 – 05
3.2.17 monostatic, adj—sodar configuration that uses the
same antenna for transmission and reception.
â = viscous and molecular sound absorption coefficient,
–1
3.2.18 Neper, n—natural logarithm of the ratio of reflected
Nepers per wavelength, m ,
A = effective antenna aperture, m ,
to incident sound energy flux density at a given range.
e
–1
c = speed of sound, ms ,
3.2.19 pulse, n—finite burst of transmitted energy.
2 –2/3
C = temperature structure parameter, K m ,
T
3.2.20 pulse length (t,s), n—duration of a single pulse.
e = receiver electromechanical efficiency,
R
3.2.21 pulse sequence, n—train of pulses, often at different
e = transmitter electromechanical efficiency,
T
frequencies.
f = central acoustic frequency transmitted by the sodar,
3.2.22 range (r, m), n—distance from the antenna surface to
Hz,
the scattering surface.
f = Doppler frequency, Hz,
D
3.2.23 range aliasing, n—sampling ambiguity that arises
G = antenna gain,
when returns are received from a transmission that was made P = received electrical power, W,
r
P = transmitted electrical power, W,
prior to the latest transmitted pulse sequence, usually from a
t
r = range from transmitter to a range gate, m,
scattering surface located beyond the maximum unambiguous
r = maximum unambiguous range, m,
range. max
t = time between transmission of an acoustic pulse and
3.2.24 range gate, n—conical section of the atmosphere
reception of returning echoes, s,
containing the scattering volume from which acoustic returns
T = temperature in Kelvins, K,
K
can be resolved.
t = IPP, the maximum listening time between transmit-
max
3.2.25 range resolution (D , m), n—length of a segment of
r
ted pulses or pulse sequences, s,
the scattering volume along the axis of beam propagation. –1
V = target velocity, ms ,
t
3.2.25.1 Discussion—Range resolution equals half the
Dr = range resolution, m,
product of speed of sound and pulse length (Dr=ct/2).
f = combined viscous and molecular attenuation factor,
m
3.2.26 received power (P ,W), n—electricalpowerreceived f = excess attenuation factor,
r x
l = acoustic wavelength, m,
at an antenna during listening mode; the product of received
s = acoustic scattering crossection per unit volume,
acousticpowerwithreceiverconversionefficiencyfromacous-
–1
m , and
tic to electrical power.
t = pulse length, s.
3.2.27 scattering volume (m ), n—volume of a conical
section in the atmosphere centered on the radial along which
4. Summary of Guide
the acoustic beam propagates.
3.2.27.1 Discussion—Thisiscommonlycalculatedfromthe 4.1 The principles of atmospheric wind and turbulence
3 dB beamwidth.
profiling using the sound direction and ranging technique are
described.
3.2.28 sidelobes, n—acoustic energy transmitted in a direc-
tion other than the main beam (or lobe). 4.2 Considerations for sodar equipment, site selection, and
equipment installation procedures are presented.
3.2.28.1 Discussion—Sidelobesvaryinverselywithantenna
size and transmitted frequency. 4.3 Data acquisition and quality assurance procedures are
described.
3.2.29 signal-to-noise-ratio, n—ratio of the calculated re-
ceived signal power to the calculated noise power, frequently
5. Significance and Use
abbreviated as SNR.
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 atmo-
through the atmosphere, and subsequently samples the scat-
sphere, particularly in the gap between meteorological towers
tered atmospheric returns.
and the lower range gates of wind profiling radars. The sodar’s
n—instrument that performs these functions. far field acoustic power is also used for refractive index
calculations and to estimate atmospheric stability, heat flux,
3.2.31 temperature structure parameter (C , K),
T
and mixed layer depth (1-5) . Sodars are useful for these
n—structure constant for measurement of fast-response tem-
purposes because of strong interaction between sound waves
perature differences over small spatial separations that ac-
and the atmosphere’s thermal and velocity micro-structure that
counts for the effects of molecular diffusion and turbulent
produce acoustic returns with substantial signal-to-noise ratios
energy dissipation into heat.
(SNR). The returned echoes are Doppler-shifted in frequency.
3.2.32 transmit frequency (f, Hz), n—selected frequency or
This frequency shift, proportional to the radial velocity of the
frequencies at which an acoustic transmitter’s output is
scattering surface, provides the basis for wind measurement.
achieved.
Advantages offered by sodar wind sounding technology in-
3.2.33 transmitted power (P, W), n—electrical power in
t
cludereasonablylowprocurement,operating,andmaintenance
watts measured at the antenna input; acoustic power radiated
by an antenna is the product of transmitted electrical power
with the conversion efficiency from electrical to acoustic
power.
The boldface numbers in parentheses refer to the list of references at the end of
3.3 Symbols: this standard.
D7145 – 05
costs, no emissions of eye-damaging light beams or electro- 6.3 The Sodar Equation. The power received (P)bya
r
magnetic radiation requiring frequency clearances, and adjust- sodar’s acoustic antenna is a product of sodar performance and
able frequencies and pulse lengths that can be used to optimize atmospheric attenuation factors. Sodar performance factors
data quality at desired ranges and range resolutions. When include effective transmitted power (P) at its transmitted
t
properly sited and used with adequate sampling methods,
frequency(ies), effective antenna aperture (A ), transmitter and
e
sodars can provide continuous wind and turbulence profile receiver efficiency factors (e and e ), and pulse length (t).
T R
information at height ranges from a few tens of meters to over
Atmospheric scattering factors include the acoustic scattering
a kilometre for typical averaging periods of 1 to 60 minutes. crossection (s) and attenuation factors f and f .Attenuation
m x
factor f represents “classical” viscous losses plus the com-
m
6. Monostatic Sound Direction and Ranging
bination of molecular rotational and vibrational absorption.
The second factor (f ) represents excess attenuation due to
6.1 Sodar Design Types. Most commercially available so- x
complex interactions of the acoustic beam with larger scale
dars operate using a monostatic phased array antenna design
atmospheric features. The sodar performance and atmospheric
composed of a planar array of acoustic transmitters that form
factors are combined in a simplified monostatic sodar equation
the emitted beam and steer it towards the desired direction.
for received power:
Other designs, to include non-phased antennas for each beam
and bi-static configurations, are also available. An advantage
P 5 $sodar performance%$atmospheric factors%
r
offered by bi-static sodars is that they also utilize signals
5 P A ! e e ! ct/2! sf f . (2)
$~ ~ ~ %$ %
t e T R m x
scattered from small scale velocity fluctuations that are not
6.4 Sodar Performance. Sodar performance characteristics
available in monostatic configurations. Except for beam form-
includethesodartransmittedacousticpower,andtheefficiency
ing, steering, and the simplified monostatic sodar equation, the
with which power is transmitted and received. P A is the
t e
information provided below is generally applicable to those
power-aperture product. A = AG/r is the solid angle sub-
e
designs as well.
tended by an antenna of aperture (A, m ) multiplied by the
6.2 Description of Operation. A phased array monostatic
effective aperture factor (G, the antenna’s gain), as viewed at
sodar emits acoustic pulses (adiabatic compression waves) at a
range (r) from the scattering volume. Range resolution (Dr=
transmit frequency or frequencies. Pulses from each antenna
ct/2) is the length (m), along the radial axis of signal
are formed into a conical beam or wavefront with its vertex at
propagation, of the instantaneous scattering volume and de-
the antenna. Individual transducer pulse timing or phase
finesthevolumefromwhichabackscatteredsignalisresolved.
shifting methods, indicated by F in Fig. 1, are used to shape
Note that range resolution determines range gate thickness.
thebeamandsteeritinthedesireddirection.Asittravelsalong
Scattering surfaces that produce useful acoustic returns often
a radial direction through the atmosphere at speed of sound (c),
occupy only a fraction of the scattering volume in the real
this acoustic wave experiences attenuation by spreading, ab-
atmosphere (see Fig. 1 and 6.6).The magnitude of the returned
sorption, and scattering as described below. Temperature inho-
signals is directly proportional to the percentage of the scat-
mogeneities and sharp gradients encountered by the propagat-
tering volume occupied by scattering surfaces and the intensity
ing beam deform and scatter the beam. Wind velocity
of the turbulence (C ) producing the return.
T
components along the axis of propagation also Doppler- shift
6.5 Pulse Length and Inter Pulse Period (IPP). Pulse length
the acoustic frequency of backscattered signals. A schematic
and IPP (t ) define height and velocity limits for valid sodar
drawingofacousticwavefrontgenerationandbackscatterfrom max
signals.Pulselengthandsystemsettlingtime(timeofrecovery
a reflecting surface is presented in Fig. 1.After its transmission
from the state of excitation during pulse transmission) deter-
of an acoustic pulse train, the sodar switches to listening mode
mine the minimum height (first range gate) from which
for backscattered acoustic signals. Returning signals are char-
backscattered signals can be received. IPP determines the
acterized by their intensity (amplitude), spectral width,
maximum range from which unambiguous backscattered re-
Doppler-shifted frequency, and lapsed time (t) from initial
turns are received. If a
...
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