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Sodar 101

This section is designed to provide a brief introduction to Sonic Detection And Ranging (SoDAR) technology. Please expect it to be revised as part of a continuous improvement process.

A SoDAR is an acoustic instrument for monitoring the atmospheric environment similar to SONAR which is used for monitoring the ocean environment. The SoDAR operates by emitting a short pulse and listening to the atmospheric echo from that pulse. The echo is a result of the continuous interaction between the outward propagating acoustic energy and atmospheric turbulence. The echo when received by the SoDAR microphone array is processed for its frequency content. A change in frequency is associated with movement of the atmospheric turbulence by the local wind field. Using the Doppler shift principles, the echo frequency will be greater than the pulse frequency if this movement is toward the microphone array. A lower frequency with respect to the pulse frequency means that the movement is away from the microphone array.

The outgoing pulse is not affected in frequency until it is echoed by the interaction with atmospheric turbulence back to the microphones. This observed fact enables a frequency coherent SoDAR to associate a particular frequency shift with a particular distance from the microphone array. Using information from three different directions (and decoding the data in terms of its first order geometry (i.e. neglecting refraction effects), the altitude gated frequencies enable a full wind profile to be measured.

Processing of the SoDAR echo signal is complicated by the temporal variation of the background noise levels. These variations may affect the ability of the SoDAR to detect the echo. Furthermore, the echo intensity is influenced by the atmospheric attenuation of the sound intensity which is a function of atmospheric temperature, humidity and to a lesser extent pressure. These factors are accounted for in the SoDAR equation which will be thoroughly discussed at a later time.

ASC Sodars

The design philosophy for the ASC SoDAR system incorporates two components: (1) a signal processor unit that covers the observed range of atmospheric echo intensities and (2) a speaker (and microphone) detector that is optimized to the design pulse frequencies.

The signal processor unit is called the Acoustic Signal Processor (ASP). There have been a number of variations of the ASP including a bank of analog filters coupled with a microprocessor used in the early 1980s to the advanced digital ASP currently used in the ASC SoDAR systems. As the computers have become more powerful, additional function have been assigned to the ASP such as networking, diagnostic tasks, satellite communication and controller functions such as starting and stopping generators.

Using this design philosophy, different speaker emitters and detectors are employed to achieve a monitoring task that is most often related to the maximum effective sampling altitude. In addition, the interface control and display software packages are used by all of the ASC units. And the post processing database software will support any of the systems.

There are three main variations to the ASC SoDAR systems:

4000 series This is a high frequency SoDAR system. It operates around 4500 Hz and is designed to measure the wind field to a maximum of 200 meters. It utilizes the paradigm changing reflector enclosure that was introduced as part of this product in the early 1990s. The functionality and endurance of this enclosure is demonstrated to the continuous operation of a network of these units since the late 1990s. The 4000 series has been tested and retested more than any similar commercial product. And it is recognized as the most accurate SoDAR commercially available. The 4000 series is used for applications such as wind energy, emergency response, airport wind profiling and air pollution. It is ASC’s most popular SoDAR unit.

3000 series This is a mid-range frequency SoDAR system that is designed to measure to a maximum altitude of 400 meters. Like the 4000 series, it utilizes a reflector enclosure as an effective design to reduce background noise and protect the speaker from environmental degradation. This unit is most often used when intermediate altitude measurements are needed with a compact trailer mounted speaker array.

2000 series This is a low frequency SoDAR system that includes three parabolic dishes with enclosures to measure to altitudes of 750 meters. The 2000 series is the most flexible low frequency unit available commercially. It is capable of monitoring atmospheric wind fields to heights of 600 meters as frequently as many tower based instrument sample winds at 60 meters. This unique configuration is suitable for turbulence studies as well as measuring the mean wind profile.