# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 3 Validation and Characterisation
### PTB Bistatic Lidar/Cup Anemometer (200 m Wind Met Mast)
With an intercept of 3.736\({}^{\circ}\) and a slope of 0.996, both instruments are in excellent agreement regarding the measured wind direction. Generally, a directional dependence of wind velocity measurements with respect to met mast booms has been extensively covered in the literature and is known as the _mast and boom effect_. Firstly, this effect causes large deviations in the wind velocity within a narrow angular range, when the anemometer is located in the lee of the mast. Secondly, an additional and more or less sinusoidal deviation in the range of \(\pm\)1% to \(\pm\)2% (for lattice towers) is induced over the entire angular range [6; 28; 29]. In order to mitigate the mast and boom effect specifically, this comparative measurement was conducted with respect to the top anemometer located at 200 m, even though there were many periods of fog at this measurement height. Overall, the directional effects due to the wakes of the nearby wind turbines and the PTB bistatic lidar's sensitivity to periods of fog necessitated the additional filtering of the data obtained in this particular comparative measurement campaign. Nevertheless, both the PTB bistatic lidar and the wind met mast's top anemometer show good agreement with mean deviations below 0.5% over a large range of wind speeds. Furthermore, the wind directions as measured by the PTB bistatic lidar and the wind vane mounted on the met mast are in excellent agreement. Figure 13: (**a**) Measurement site including two wind energy turbines located at 146\({}^{\circ}\) and 307\({}^{\circ}\) angles (a third wind turbine located south-east is not shown). Around the met mast, the areas corresponding to the wakes of the nearby wind turbines are highlighted between 100\({}^{\circ}\) to 165\({}^{\circ}\) and 285\({}^{\circ}\) to 330\({}^{\circ}\) angles. (**b**) Slightly larger view of the wind met mast and its orientation with respect to the measurement site. (**c**) Met mast with multiple booms mounted at different heights and at 145\({}^{\circ}\) and 307\({}^{\circ}\) angles. Figure 14: Results of the 2020 comparative measurement between the PTB bistatic lidar and a cup anemometer mounted on top of a 200 m met mast, showing the directional dependence of the mean deviation between the PTB bistatic lidar and the top anemometer for _unfiltered_ 10 min averages. Large perturbation effects due to the wakes of the nearby wind turbines are visible around angles 146\({}^{\circ}\) and 307\({}^{\circ}\). Additionally, fog occasionally causes very large, non-systematic deviations which are clearly visible between 90\({}^{\circ}\) and 270\({}^{\circ}\) angles. Figure 15: Results of the 2020 comparative measurement between the PTB bistatic lidar and a cup anemometer mounted on top of a 200 m met mast, showing the directional dependence of mean deviation between the PTB bistatic lidar and the top anemometer for _filtered_ 10 min averages. Perturbation effects due to the wakes of the nearby wind turbines have been reduced by directional filtering. Furthermore, periods of fog were excluded from the data.