Atmospheric Research SODAR Page

What is a SODAR?

A SODAR (SOund Detection And Ranging) system is an instrument for the measurement of wind velocity, remotely, from the ground. It operates by transmiting a short pulse of sound which is refracted by the small scale turbulence in the atmosphere. This turbulence is transported by the wind, and the radial velocity of the air can be determined by measuring the Doppler shift of the sound being refracted from the turbulence. The range of the turbulence is determined from the delay between the transmission of the acoustic pulse, and the reception of the refracted signal. By repeating this process in three different directions, each direction having a large component being orthogonal to the other two directions, the three dimensional wind field can be calculated.

The SODAR sends out a sound pulse every 6 seconds in one of the three directions, switching sequentially between these directions. The signal processing algorithm includes extensive filtering and averaging, to ensure a good signal to noise measurement. The frequency of the sound pulse is chosen to provide a compromise between attenuation (which increases with frequency) and environmental noise. A frequency of 1875 Hz is used by the ARPL SODAR. The wind velocity (in X, Y, Z) versus altitude, is displayed on the display electronics approximately every 3 minutes. computer, and is written to file for long term data logging.

The SODAR consists of three main subsystems;

• Antenna Subsystem;
• Control Electronics Subsystem;
• Display Computer Subsystem.

A block diagram of the SODAR instrument is shown below.

Block Diagram of SODAR

What Options are Available from ARPL?

Phased Array Antenna Option. As an option, the three fixed antennae may be replaced by a phased antenna array. This provides a compact format, which may be advantageous where space is limited. Phased array systems have the following disadvantages;

• More complex electronics and software, and thus a higher price;
• More failure modes, due to greater complexity;
• Greater susceptibility to environmental noise, due to problems with sidebands from the antenna gain pattern.

Simultaneous Sounding Option. As an option, the SODAR can be configured for simultaneous sounding on all three axes. This provides a three-fold increase in data rate, and hence an improvement in signal to noise and/or in temporal response.

Ground Sensor Suite A complete ground-level meteorological sensor suite (temperature, relative humidity, atmospheric pressure, wind velocity and direction, or a subset of these) can be provided, with data logging via the instrument computer.

Integration of the SODAR and a RASS. The baseline SODAR is not supplied as an add-on to a RASS. This is not recommended and gives a performance compromise. If the customer needs a combined RASS/SODAR, however, a combined unit can be delivered.

Modifications to the configuration of the hardware, or to the software (eg to the user display), may be undertaken by ARPL, as options within a contract, or as subsequent upgrade contracts.

What are the Advantages of SODARS?

SODARs provide;

• Continuous operation and measurement;
• No intrusion into the space being sampled;
• Low labour costs for measurements;

The state-of-the-art performances of the ARPL SODAR are summarised in the table. SODARS can be used for;

• Industrial process and thereby pollution control by real-time monitoring of the local atmospheric environment;
• Characterisation of the local micro-climate;
• Wind monitoring at airports and parachute drop zones;
• Wind Power assessment, including averages, distributions and wind shear data;
• Blast Noise Monitoring, Prediction and Minimisation.

What Are The Limitations of SODAR Operation?

• Range. Sound is attenuated in the atmosphere, with higher frequency sound attenuated much more than lower frequencies. With increasing temperatures, and/or lower relative humidity, the attenuation of sound increases. The height performance of a SODAR in a hot, dry, desert may only be 60% of the same instrument in a cool, damp, location.
• Audible Sound. The use of sound can limit its use in built-up areas.
• Background Noise. The background noise where a SODAR is operating can also limit aa SODAR's performance. In general SODARs should not be operated in areas where the noise level (at the frequency of operation of the SODAR) is high.
• Local Structures. SODARs should not be installed near structures (or vegetation) which can produce fixed echoes.

SODAR Performance

Overhead Presentations