OVER SEA
Scatterometry
Satellites
Research Programs

OVER LAND
Doppler Radar
U.S. Radar Coverage
Research Programs

ON SITE
Mini- SODAR
Research Programs

WIND POWER
Growth in Wind Energy
Finding Strong Winds
Wind Atlases
Site Studies

Web site created by
Paul Kelly
Graduate Student
Geo Information Science
Department of Geography
Salem State College
Salem, Massachusetts, USA.

E-mail comments and suggestions to pmkelly@prodigy.net.

Last updated 4/24/2002.

ON SITE

Mini-SODAR
SODAR (SOund Detecting And Ranging or SOund Doppler Accoustic Radar) uses short bursts of high frequency sound waves in a manner analogous to Doppler radar.  Small-scale air turbulence associated with small temperature differentials (at the size of the signal wavelength) will return weak but detectable backscatter.  Movement is detectable by Dopper processing, allowing determination of the wind speed in the radial direction of the receiving antenna.  In order to estimate the wind speed in three dimensions, three antennae are used, one of which is oriented vertically, the other two of which are offset sufficiently to solve for movement in the two horizontal dimensions.

SODAR is limited by several factors: the attenuation of sound over distance, environmental noise at the received frequencies, and precipitation, which yields a higher backscatter signal.  It can return estimates of wind speed and direction for every 5 m of height over its vertical range.

Mini-SODAR deploys the SODAR technology in a small, transportable system, approximately 1.2 x 1.2 m.  Its vertical range is lower than larger SODAR systems, usually 20 to 200 m rather than 50 to 500 m.  the published specifications on AeroVironment's Model 4000 mini-SODAR are,

Maximum Altitude: 200 meters
Minimum Altitude: 15 meters
Height Resolution: 5 meters
Transmit Frequency (approximate): 4500 Hz
Averaging Interval: 1 to 60 minutes (variable)
Wind Speed Range: 0 to 45 meters/second
Wind Speed Accuracy: < 0.5 meters/second
Wind Direction Accuracy: +/- 5 degrees
Weight: 255 lbs (116 kg)
Antenna Height: 4 ft (1.2 m)
Antenna Width: 4 ft (1.2 m)
Antenna Length: 5 ft (1.5 m)
 

A list of mini-SODAR manufacturers is available on the ISARS web site.

Research Programs
SODAR experimentation and development goes back about 25 years.  SODAR is currently used to detect shear winds at airports, conduct noise propagation studies, and monitor fire, air pollution, and the passage of frontal systems.  It is being evaluated for application to wind turbine siting.  ISARS (International Society for Acoustic Remote Sensing of the Atmosphere and Oceans), an informal association of scientists working on SODAR, organizes an annual conference to share research results.

In 1999, Ecofys Energy and Environment conducted a 3-month study to determine the suitability of mini-SODAR for wind energy siting .  Conducted at ECN’s wind turbine test site in the Netherlands, the study found that SODAR-estimated wind speeds were approximately 4 percent less than anemometer data (after filtering of data contaminated by rain noise and the operation of a nearby wind turbine).  The study identified a need to establish appropriate settings for signal-to-noise filtering and noted that, while mini-SODAR could be obtained “off-the-shelf,” data analysis and interpretation required significant time and understanding of the technology.

A paper published earlier this year compared mini-SODAR to anemometer data from KNMI’s 213 m mast in Cabauw, Netherlands.  It found that mini-SODAR-reported wind speeds varied from the mast’s anemometer by +1 to –1.5 m/s.  The same paper projects comparative costs for mini-SODAR and construction of a mast for anemometer readings.  It estimates that mini-SODAR becomes more cost-effective once mast heights reach 80 m or more.  It is noted that three years ago, wind turbines were typically 60 m high.  Today they are often 120 m.  The breakeven point will be even less if the evaluation site is off-shore, where mast construction costs are significantly higher.  “SODAR: een doorbraak in kosten en vergunningprocedure?” is available in Dutch.

While accuracy is a key concern for site evaluation, the cost comparison may make mini-SODAR the technology of choice as turbine heights increase—especially for off-shore locations.

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