BRIEFLY, FOR THE UNINITIATED...
Sodar (sonic detection and ranging) systems are used to
remotely measure the vertical turbulence structure and the wind profile 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. A more familiar related term may be sonar,
which stands for sound navigation ranging. Sonar systems detect the
presence and location of objects submerged in water (e.g., submarines) by means of
sonic waves reflected back to the source. Sodar systems are similar except
the medium is air instead of water and reflection is due to the scattering of
sound by atmospheric turbulence.
Most sodar systems
operate by issuing an acoustic pulse and then listen for the return signal for
a short period of time. Generally, both the intensity and the
Doppler (frequency) shift of the return signal are analyzed to determine the
wind speed, wind direction and turbulent character of the atmosphere. A
profile of the atmosphere as a function of height can be obtained by analyzing
the return signal at a series of times following the transmission of each
pulse. The return signal recorded at any particular delay time provides
atmospheric data for a height that can be calculated based on the speed of
sound. Sodar systems typically have maximum ranges varying from a few
hundred meters up to several hundred meters or higher. Maximum range is
typically achieved at locations that have low ambient noise and moderate to high
relative humidity. At desert locations, sodar systems tend to have reduced
altitude performance because sound attenuates more rapidly in dry air.
Sodar systems can be used in any application where the winds aloft or the
atmospheric stability must be determined, particularly in cases where time and
cost are of the essence. Some typical applications include: atmospheric
dispersion studies, wind energy siting, wind shear warning, emergency response
wind monitoring, sound transmission analyses, microwave communications
assessments and aircraft vortex monitoring.
Some of the advantages of sodar systems are obvious compared to erecting tall
towers with in-situ wind and temperature sensors. First, a sodar
system can generally be installed in a small fraction of the time it takes to
erect a tall tower. And when all of the costs are considered, a sodar
system will generally offer a very attractive alternative. Also, the practical height limit for
meteorological towers is about 150 m (500 ft). Most sodar systems will obtain reliable
data well beyond this altitude. Using a sodar system instead of a tall
tower will also avoid many liability issues. Sodar systems do have some drawbacks compared to
tall towers fitted with in-situ wind sensors. Perhaps the most significant is the
fact that sodar systems generally do not report valid data during periods of heavy
precipitation. Another consideration is that sodar systems primarily
provide measurements of mean wind. Other wind parameters, such as wind
speed standard deviation, wind direction standard deviation and wind gust, are
usually either not available or not reliable. This is because to obtain a
wind measurement sodar systems sample over a volume and at multiple points in
space and time, whereas an in-situ wind sensor on a tall tower samples
instantaneously at a point in space and time.
SOME SODAR HISTORY...
Sound propagation in the atmosphere has been studied for at least 200 years,
but it has only been in the last 50 years that acoustic scattering has been
used as a means to study the structure of the lower atmosphere. In the
United States during World War II, acoustic backscatter in the atmosphere was
used to examine low-level temperature inversions as they affected propagation in
microwave communication links. During the late 1950's, acoustic scattering
from the atmosphere was investigated both experimentally and theoretically in
the Soviet Union, and researchers in Australia showed that atmospheric echoes
could reliably be obtained to heights of several hundred meters. Beginning
in the late 1960's and early 1970's, scientists at the U.S. National Oceanic and
Atmospheric Administration (NOAA) demonstrated the practical feasibility of
using acoustic sounders to measure winds in the atmosphere by means of the Doppler shift and
to monitor the structure of temperature inversions.
During the 1970's, the
engineering design of acoustic sounders was seriously pursued by
several groups of researchers in the United States. One of the earliest
commercial systems was the Model 300 developed by AeroVironment, Inc. in California. This
system was designed primarily to measure the turbulent
structure of the atmosphere and reached heights up to several hundred meters. In
1974, NOAA developed the Mark VII which was a portable system that was called an
acoustic echosounder. Both the Model 300 and the Mark VII were designed
around a single 1.2-meter (4-foot) diameter parabolic dish, and a facsimile
recorder was used to provide an analog record of backscatter data.
1975, researchers at the University of Nevada at Reno and at Scientific Engineering
System, Inc. (SES) developed the first digital-based acoustic sounder by
incorporating a microcomputer into the system. Subsequent work at
SES and at NOAA led to the development of a three-axis digital-based
acoustic sounder or sodar system capable of measuring both the Doppler shift and
backscatter intensities in real time. The three-axis system provided the
means of determining the vertical profile of the horizontal wind speed and
direction. By the late 1970's, SES had
developed a commercial Doppler sodar system called an Echosonde®.
During the early 1980's, Radian Corporation used the SES Echosonde as the basis for developing a microcomputer-based three-axis Doppler sodar
During the 1980's, there were also parallel developments of Doppler
sodar systems by other U.S. companies. Xonics, Inc. developed the Xondar
sodar system which was capable of making wind profile and turbulence
measurements. AeroVironment, Inc. developed their pulsed Doppler sodar
system, the AeroVironment
Invisible Tower (AVIT). This was a
three-axis system based on three adjacent parabolic dishes which were operated
sequentially. One was pointed vertically and the other two were tilted 30
degrees from the vertical in horizontally orthogonal directions.
Organizations in Australia, Japan, Germany and France have also developed commercial
systems. Perhaps the most notable is a French company, Remtech.
Remtech was one of the earliest to commercialize phased-array sodar systems
capable of measuring Doppler shifts as well as turbulence parameters up to a height of
1000 meters or more. Remtech was also one of the first to apply multiple-frequency
coding to sodar technology, a method used to extend altitude
performance. Other companies that have developed commercial sodar systems
include Metek and Scintec
Corporation in Japan, and Atmospheric Research
Pty Ltd in Australia.
Phased-array sodar systems were developed in the United States during the late 1980's and early 1990's
by Xonics, Radian Corporation and
AeroVironment, among others.
Radian no longer manufactures sodar systems. In 2005, the sodar group at
AeroVironment purchased the rights to the AeroVironment sodar system and became
Atmospheric Systems Corp.
Model VT-1 sodar system was developed during the late 1990's.
The Model VT-1 is a phased-array Doppler sodar system that utilizes a laptop
computer for much of the system control and operation. Due to this
innovation, the Model VT-1 is greatly simplified compared to other earlier
systems which required extensive external electronics and larger computer
systems. It is also battery powered and completely self-contained within a
portable, modular cabinet, making it suitable for use at virtually any
location. It's high-frequency operation and low side-lobe noise make it
nearly immune to interference from ambient noise and ground clutter, both of which
have been limiting factors for many sodar systems past and present.
SODAR THEORY OF OPERATION
The motion of the atmosphere is the result of general wind flow and turbulence
(the irregular fluctuations of small-scale horizontal and vertical wind
currents). Atmospheric turbulence
is generated by both thermal and mechanical forces.
Thermal turbulence results from temperature differences, or gradients, in
the atmosphere. Mechanical
turbulence is caused by air movement over the natural or man-made obstacles that
produce the “roughness” of the earth's surface.
Turbulence from either source results in turbulent air parcels or eddies
of varying sizes.
acoustic (sound) pulse transmitted through the atmosphere meets an eddy, its
energy is scattered in all directions. Although
different scattering patterns result from thermal and mechanical turbulence,
some of the acoustic energy is always reflected back towards the sound source.
That backscattered energy (atmospheric echo) can be measured using
a monostatic sodar system. A monostatic sodar system is one in
which the transmitting and receiving antennas are collocated, and thus the
scattering angle between the target eddies and the sodar antenna is 180
degrees. The backscattered energy is caused by thermally-induced
In a bistatic
sodar system, the transmitting and receiving antennas are at different
locations, and hence scattering angles other than 180 degrees are relevant. At a scattering angle other than 180 degrees, both thermal and
mechanical turbulence come into play. In principle, this provides for a
stronger and more continuous signal, but nearly all commercial sodar systems are
monostatic because their design is simpler and more practical.
Much information about the atmosphere can be derived from monostatic sodar
systems. The intensity or
amplitude of the returned energy is proportional to the CT2
function, which, in turn, is related to the thermal structure and stability
of the atmosphere. CT2
has characteristic patterns during ground-based radiation
inversions, within elevated inversion layers, at the periphery of convective
columns or thermals, in sea breeze/land breeze frontal boundaries, and at any
interface between air masses of different temperatures.
Due to the Doppler effect, measuring the shift in the frequency
of the returned signal relative to the frequency of the transmitted signal
provides a measure of air movement at the position of the scattering eddy.
When the target (a reflecting turbulent eddy) is moving toward the
sodar antenna, the frequency of the backscattered return signal will be higher
than the frequency of the transmitted signal.
Conversely, when the target is moving away from the antenna, the
frequency of the returned signal will be lower.
This is the physical characteristic that is used by Doppler sodar systems
to measure atmospheric winds and turbulence.
By measuring the intensity and the frequency of the returned
signal as a function of time after the transmitted pulse, the thermal structure
and radial velocity of the atmosphere at varying distances from the transmission
antenna can be determined. Additional
information can be obtained by transmitting consecutive pulses in the vertical
direction and in two or more orthogonal directions tilted slightly from the
vertical. Geometric calculations
can then be used to obtain vertical profiles of the horizontal wind direction
and both horizontal and vertical wind speeds.
A sodar system transmits and receives acoustic signals within a specific
frequency band. Any background
noise within this frequency band can affect signal reception.
Since the return signal strength usually varies inversely with target
height, the weaker signals from greater heights are more readily lost in the
background noise. Thus high levels
of background noise may reduce the maximum reporting height to a level below
that obtainable in the absence of noise. Certain noise sources can also
bias the sodar data. Thus, it is
important to identify potential noise sources and estimate the background noise
level when evaluating a candidate site for a sodar system.
One of the other principle problems with sodar systems is ground clutter.
Interference from ground clutter occurs when side-lobe energy radiating
from a sodar antenna on transmit is reflected back to the antenna by nearby
objects such as buildings, trees, smokestacks or towers. This reflected
side-lobe energy can overwhelm the atmospheric return signal and cause the
component wind speeds reported by a sodar system to be zero-biased.
Thus, sodar systems must either be located in areas with wide-open wind fetches
(i.e., areas with no reflecting objects), or they must be designed to
substantially eliminate side-lobe energy.
SODAR SYSTEMS IN GENERAL
Nearly all commercial sodar systems on the market today are of the monostatic variety. That
is, either the transmit and receive antennas are collocated or they are
one-and-the-same. Also, most sodar systems now on the market are multi-axes
sodar systems, meaning they have the ability to detect signal frequency shift
in three or more radial directions and use that data to derive both the profile of wind speed and direction and the
vertical intensity structure of the atmosphere.
Probably the most
fundamental component of a Doppler sodar system is the antenna, and this
is where the various commercial sodar systems may differ the most. One of the challenges of designing sodar systems is to make the antenna
weatherproof. Several approaches are used to accomplish this. The
earliest approach was to use a parabolic dish, typically about 1.2 meters
(4 feet) in diameter, with the focal point directed upward. A speaker is
then located at the focal point with the horn pointed downward toward the dish,
which achieves the requirement of keeping precipitation out of the speaker
driver. Generally, an enclosure is used around the parabolic dish to
reduce side-lobe interference and to shield the antenna from wind noise and
general background noise. In a multi-axes system, typically three
parabolic-dish antennas are used with one pointing vertically and the other two
tilted slightly from the vertical (usually 20 to 30 degrees) and pointing in
horizontally orthogonal directions. In operation, the three antennas can
be used either sequentially or simultaneously. In simultaneous operation,
the three antennas operate at different frequencies so the return signals do not
interfere with each other.
A more recent approach to designing sodar antennas has been to use an array
of many smaller elements, perhaps as few as 16 or as many as 100 or more,
consisting of piezoelectric tweeter drivers and horns. Although more
complicated in principle compared to the parabolic dish approach, the antenna
array approach offers some advantages. Unlike the parabolic dish antennas
which are limited in power in some respects by the availability of high-power
speaker drivers, the power of an antenna array can be increased simply by adding
more elements. However, the real driving force in developing sodar systems
with antenna arrays has been, perhaps, the use of phased-array technology.
Phased-array technology provides the capability to electronically steer a
sound beam in any direction. Thus, in a phased-array system, a single
antenna array can be used to obtain data along multiple axes.
One of the fundamental problems in designing phased-array antennas is to keep
precipitation out of the array elements. There are two general
approaches for accomplishing this: 1) use specially-designed folded horns
attached to each of the array elements or 2) use a reflector board so
that the array does not have to point upward. Each approach has its
advantages and disadvantages. When a folded-horn is used, the array can be
positioned horizontally, and, depending on the efficiency of the design,
relatively little shielding around the array may be required. The folded
horns must be designed, of course, to have high efficiency. In the
reflector board approach, the array is usually positioned vertically and the
sound beams are reflected upward by the reflector board. This prevents
precipitation from entering the array speaker drivers and makes it possible to
use off-the-shelf speakers as the array elements. Perhaps the greatest
drawback to this approach is that the reflector board and enclosure result in a
physically larger system. In cold climates, the folded-horn arrays must be
heated to melt snow, whereas in a reflector board system, the reflector board is
heated instead, which may offer some practical advantages.
Another fundamental difference in sodar systems is the use of
single-frequency versus frequency-coded pulses. In single-frequency
systems, only a single frequency is transmitted. Single-frequency sodar
systems make a distinctive pinging
noise when in operation. Single-frequency operation provides for simplicity and accuracy and
data at the lowest levels (as low as 15 to 20 m) can be collected due to the
short transmit pulse length. Single-frequency systems can also be tested
in the field using an independent testing device (a sodar
transponder). In a system using a frequency-coded pulse, the transmit pulse is
comprised of several different frequencies which are emitted serially, causing the system
to make a singing noise when in operation.
Frequency coding of the transmit pulse is done to gain maximum altitude
without losing altitude resolution. Although frequency coding
may enhance altitude performance, it may offer some drawbacks as well. Depending on how it is implemented, frequency coding may
unwanted smoothing of the data in a phased-array system. And when a sodar
system is operated for maximum altitude, data quality at the lower levels may be
degraded because of the long delay between samples. Frequency coding also makes the field
testing of sodar systems more difficult, probably precluding the use of a sodar
Signal processing is another area where sodar systems may differ
substantially. Most commercial sodar systems currently on the market use a
Fast Fourier Transform (FFT) to derive the signal Doppler shift, but a
variety of techniques may be used both before and after FFT processing,
primarily to improve signal detection. One technique is to average the
signal. Signal averaging may be used either in the time domain or the frequency
domain in an attempt to reduce noise and improve the signal-to-noise ratio,
which is usually the primary criterion for data acceptance. Perhaps the
two major approaches to signal processing are to either: 1) average the spectra
for all pulse sequences and then locate the region of maximum spectral energy or
2) locate the region of maximum spectral energy in each pulse sequence and then
average the results. In the latter approach, the number of valid samples
for an averaging period may be used as an additional data acceptance
criterion. In either case, a technique known as "bin averaging"
is often used to locate the frequency region of the signal within the working
frequency bandwidth. This also helps to improve the spectral resolution of
Data storage and presentation capabilities also vary significantly
amongst the various commercial sodar systems. Most systems will provide
both text and plotted data showing the profiles of the horizontal and vertical
wind and a facsimile display showing intensity data. The individual wind
component data may also be provided, which can be very useful for quality
control purposes. A display showing the spectra data is also very useful
in ascertaining system operation, but not all systems provide this. Data
pertaining to signal quality are also usually displayed and recorded. At a
minimum, the signal-to-noise ratio is normally provided, but there is no common
definition of this amongst sodar manufacturers. Due to the large volume of
data generated by a sodar system, typically only the data averages are recorded
and not the raw input signal. Every sodar system has it's own unique
format for recording data. Even if only the data averages are recorded, special
software will generally be required to process, validate, report and archive
THE ART MODEL VT-1 IN PARTICULAR
The Model VT-1 sodar system consists of an antenna cabinet and an electronics
module. (See figure below.)
The antenna cabinet includes:
a base constructed from stainless steel tubing.
a reflector board and support hinged to the base, constructed from
PVC paneling and stainless steel tubing.
several PVC panels lined with acoustical foam that form an
enclosure around the antenna.
a battery compartment underneath the reflector board.
The electronics module
a lower compartment with a phased-array antenna (containing 48
piezoelectric transducers), an audio transmit amplifier, and the antenna
electronics. The antenna
electronics include five printed circuit boards (two distribution circuit
boards, a transmit circuit board, a receive circuit board, and an interface
an upper compartment with a laptop computer and associated
Optional accessories include a modem and cellular telephone, a
data acquisition system (interfaced with the laptop computer) and in situ
sensors for wind, temperature, relative humidity, solar radiation and
The phased-array antenna is tilted 20º from the vertical and
the reflector board is set at an angle of 35º from the horizontal.
The complete system has a footprint that is approximately 1.2 m
(48") by 1.2 m (48"). Due to the angled sides of the cabinet and the electronics
module attached to the back of the cabinet, the Model VT-1 occupies a space
of about 1.5 m by 1.8 m (60" by 72").
The maximum height of the unit is approximately 1.5 m (60").
The foam-lined antenna cabinet substantially reduces side-lobe
interference and shields the antenna array from external noise sources and wind
The reflector board allows the antenna array to be mounted at an angle
from the horizontal, which keeps water and debris out of the array and enables
the unit to perform better during adverse weather conditions.
The reflector board directs one beam vertically and two beams18º from the
vertical in orthogonal horizontal directions.
The antenna array and cabinet can be leveled by means of the four
adjustable feet on the base of the cabinet.
For cold climates, an optional electric-heating system is available to heat the reflector board to melt snow and ice.
This includes a snowfall sensor to activate the heater only during snowfall
events and thus minimize power requirements.
The battery compartment can house sufficient batteries for
approximately five days of operation. For
extended or continuous use, an external power supply such as a battery charger, AC/DC power
supply or solar-electric power system must be provided.
The Model VT-1 phased-array Doppler sodar system generates three transmit beams.
A pulse of acoustic energy with a transmit frequency near 4500 Hz is
generated digitally by the laptop computer for each beam.
Although the pulse duration is adjustable, normally each pulse has a
duration of 100 ms (the default setting) and a corresponding physical pulse
length of approximately 34 m. Because
the backscattered pulse signal is folded over onto itself, the effective length
of the pulse return signal is one-half the physical length or 17 m in the case
of a 100 ms pulse. The beam
half-width is approximately 5º.
beam is pointed vertically while the other two beams are directed in orthogonal
directions in the horizontal 18º from the vertical.
Pulses are transmitted consecutively along each of the three beam axes.
The figure to the right is a top view of the Model VT-1 and shows a
schematic of the pulse geometry. The
pulsing sequence is W, V
and then U.
is the vertical pulse. V is a tilted
pulse transmitted with its horizontal component parallel to panel A of the Model
VT-1 cabinet. U is also a tilted pulse transmitted
orthogonal to the V
pulse in the horizontal plane and with its horizontal component perpendicular to
After each pulse is transmitted, it propagates through the
atmosphere at the speed of sound (approximately 340 m/s).
Depending on the range height selected, the system will pause between
each transmitted pulse to allow sufficient time for the system to receive the
return echo signals from that maximum height.
Data can be processed by the Model VT-1 for various heights for each
pulse transmitted, up to the maximum range specified.
For a 300-m range setting, the round-trip distance is 600 m and the delay time between pulses is approximately
2 seconds. Due to the three-pulse sequence (W, V and U), the sampling interval along each beam is
about 6 seconds when the range setting is 300 m.
Hence, a maximum of approximately 150 samples will be obtained along each
beam during a 15-minute (900-second) averaging interval.
The actual number of samples is somewhat lower due to overhead processing
time and will vary somewhat depending on the processor speed of the system’s
computer. For a 150-m range
setting, the delay time between pulses is about one-half as long, and approximately twice the number of transmit pulses are transmitted
along each beam for the same averaging interval.
Each transmitted pulse is electronically-steered toward the
reflector board by the phased-array antenna.
This is accomplished using two complimentary outputs (sine and cosine)
supplied by the audio power amplifier. These
outputs are input to the opto-isolated triac switching (transmit) circuit board
located in the lower compartment of the electronics module.
Three transmit-axis logic signals are used to turn on the corresponding
set of triacs at the appropriate phase angles for each transducer group.
There are a total of eight transducer groups with six transducers in each
group. All 48 transducers in the
phased array are used to transmit the pulses for each of the three beams.
Each transmitted pulse is directed at the reflector board at an
angle that will cause the pulse to be reflected in the desired direction.
The reflector board is set at a 35º angle from the horizontal and the
plane of the array is set at an angle of 70º from the horizontal (20º from the
vertical). The angle of reflection
is equal to the angle of incidence. Therefore,
the pulse for the vertical beam, which is emitted perpendicular to the plane of
the array, is directed at the reflector board at an angle of incidence of 55º. This occurs with all transducers in the antenna array
operating in phase. The pulses for
the two beams that are tilted 18º from the vertical are directed at the
reflector board at appropriate angles of incidence by phase shifting adjacent
transducers by 90º in both axes of the array.
After transmitting a pulse, the antenna shifts to receive mode.
The return signal is produced when the transmitted acoustic pulse
interacts with small-scale atmospheric turbulence and a portion of its energy is
backscattered. This backscattered
energy is received by the phased- array antenna via the antenna reflector board
and is processed by the receiver electronics.
In receive mode, all of the individual transducer signals within each of
the eight groups of six transducers are summed and then appropriately phase
shifted using simple op-amp integrator circuits.
The result is applied to a final differential input amplifier for each
beam axis. All three receive
signals, corresponding to the returned W,
V, and U signals,
are thus generated simultaneously during all pulse sequences, but only the one
appropriate signal is fed to the laptop computer via a set of three relay
After the received signal has been processed by the receiver
electronics and fed to the laptop computer, its energy and frequency are
analyzed using a highly-optimized Fast Fourier Transform (FFT).
The energy in the received signal is related to the strength of the
atmospheric discontinuities encountered. The
shift in the frequency of the received signal relative to the transmitted
frequency is directly proportional to the radial motion of the atmosphere (i.e.,
the wind) relative to the antenna. After determining the radial wind speeds along each of the
three beams, geometric relationships are used to derive the horizontal
components and ultimately the magnitude and direction of the resultant wind at
each height for the configured averaging time.
The final averaged data, as well as the data from individual pulse
sequences, can be displayed by the laptop computer.
Optional corrections can be made for the effect of the vertical
wind speed on the radial speeds measured along the tilted beams.
Wind speed standard deviations for each component are also calculated.
The data reported at each height pertain to the effective sampling depth.
Since the effective sampling depth is a function of the number of points
in the sample (the FFT size), the sampling rate (the number of measurements per
second), and the transmit pulse length, it is dependent on the system
For each output interval, a Reliability Value is assigned to the
data at each height. Initially, a value of 9
is assigned if the data meet the user-defined data acceptance criteria; a value
of 0 is assigned if it does not. (Intermediate
values are assigned later during data processing and validation.)
SODAR GLOSSARY OF TERMS