SoDAR) - Model 101 - Sonic Detection and Ranging Technology
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.