A Review of Sodar Accuracy

What accuracy and reliability can today be expected from SODAR wind measurements? Is there traceable evidence for performance? Environmental factors, turbulent fluctuations and non-uniform terrain all affect the wind speed uncertainty. So site-to-site variations for SODAR-mast comparisons can be large.

On a uniform terrain site, differences between a SODAR and a mastmounted cup anemometer will arise due to turbulent fluctuations and wind components being measured in different spaces, as well as to variable background noise. We develop theories for turbulence-related random fluctuations due to finite sampling rates and to sampling from spatially distributed volumes.

Effects can be minimized by selecting the environment and selectively filtering the data for periods of low fluctuations. But there is still real difficulty in answering the question: How good is a SODAR? Most field use, away from an idealized test environment, appears to produce SODAR-mast rms differences greater than the 0.1 m s-1 or less typically quoted by SODAR manufacturers.

However, in these real environments it is likely that much of the difference arises from the mast sensors and the SODAR actually measuring in different spaces. We show some field results which reinforce this view. Both the turbulencerelated random fluctuations and systematic errors in complex terrain (where systematic wind shears arise) can potentially be removed by use of a vertical column geometry.

Field results from a new bistatic receiver shed some light on the differences between such ‘common volume’ sampling and the usual monostatic sampling.