Sodar devices use the active emission of sound to measure winds in the atmosphere. In its simplest form, sodar measures the Doppler shift as a result of reflections from the density gradients associated with turbulent eddies of heat in the atmosphere.
The most common type of sodar in the wind energy industry and for general atmospheric boundary layer sensing is the monostatic system, wherein the emitter and receiver are co-located. As with DBS or VAD lidar, monostatic sodar devices measure the wind speed in multiple different azimuth angles to obtain the data required to measure wind speed and direction. This measurement can be done using multiple “horns” pointing in different directions, or a single speaker array that uses phased array techniques to steer the beam.
Sodar devices are relatively inexpensive compared to most Doppler lidar systems with a similar nominal range of a few hundred meters, and have generally lower power requirements for the same range. Several commercially available systems are sold to operate off grid, relying on a small solar panel and battery. Sodars have also been operated in snowy or freezing environments using inbuilt heaters to keep the antenna array free of snow or ice.
Like lidars, sodars are usually optimized around a specific measurement range that has implications for the minimum measurement range or the probe length; a sodar that can measure to more distant locations usually has a greater probe length and a first measurement point farther from the source.
Sodars have limitations that result from the measurement technique. They lose sensitivity and availability with height because of beam spreading and signal attenuation, and lose availability in neutral or stable conditions as the atmospheric turbulence that they use for measurements decreases. This response can result in systematic loss of data in stable atmospheric conditions at height. These and other issues are discussed in Bradley (2007) and Emeis (2010).
Sodar retrievals are also undermined by ambient noise, which can reduce the signal-to-noise ratio (SNR) of the signal from the atmosphere. Although this can be cancelled out by monitoring and removing ambient noise, such processing increases the complexity of the system. Similarly, sodar devices can be impacted by echoes from hard structures, such as wind turbines, meteorological towers, or forests. Although such effects can be mitigated by careful placement of the sodar, this approach can also have the effect that the sodar unit must be repositioned further away from a wind turbine than a vertically profiling lidar.
The international wind energy industry has taken steps to develop recommended practices for the use of sodars (and other ground-based remote-sensing devices) for wind energy applications. Recommended practices for resource assessment and a summary of other relevant standards can be found in Clifton et al. (2013a).
Monostatic Sodars , which are typically used for measuring winds in the lower 500 m of the atmospheric boundary layer, have a TRL of 9 and have been commercially available for the last 40 years or more.