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About Sodar
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.
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.
In 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 system.
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 Doppler sodar 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 in Germany, Kaijo 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.
The ART 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.
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.
When an 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 turbulence only.
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.
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 Doppler 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 result in 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 transponder.
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 the FFT.
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 sodar data.
THE ART MODEL VT-1 IN PARTICULAR
Physical Description
The Model VT-1 sodar system consists of an antenna cabinet and an electronics module. (See figure below.)
The antenna cabinet includes:
Ÿ a base constructed from stainless steel tubing.
Ÿ a reflector board and support hinged to the base, constructed from PVC paneling and stainless steel tubing.
Ÿ several PVC panels lined with acoustical foam that form an enclosure around the antenna.
Ÿ a battery compartment underneath the reflector board.
The electronics module includes:
Ÿ 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 circuit board).
Ÿ an upper compartment with a laptop computer and associated accessories.
Ÿ interconnecting cables.
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 precipitation.
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 noise. 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.
Functional Description
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.
One
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.
W
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
panel A.
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
switches.
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
configuration.
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.)
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