# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 3 Validation and Characterisation
### PTB Bistatic Lidar/Cup Anemometer (200 m Wind Met Mast)  
In 2020, a comparative measurement between the PTB bistatic lidar and a Thies Clima First Class Advanced cup anemometer mounted on top of a 200 m met mast was conducted near the summit of the Rodeser Berg near Kassel, Germany. Both the PTB bistatic lidar and the top cup anemometer were measuring at a height of 200 m for about three weeks with air temperatures ranging from \(-\)2.1 \({}^{\circ}\)C to 21.1 \({}^{\circ}\)C at a height of 10 m and from \(-\)2.7 \({}^{\circ}\)C to 17.8 \({}^{\circ}\)C at a height of 187 m. Additionally, wind direction data were obtained from a Thies Clima First Class wind vane mounted on a boom of the wind met mast at a height of 187 m. In order to enable a reasonable comparison, this particular measurement campaign required the use of filtering operations, removing periods of fog during which the lidar measurements were significantly impaired. Furthermore, to enable an evaluation according to IEC 61400-12-1, effects of turbulence due to the wakes of the nearby wind turbines were filtered out. Because of the low detection rate of particles at the measurement height of 200 m, 10 min averages are used throughout the following evaluation. Figure 13 shows the overall measurement site, which includes two wind turbines (a third wind turbine located south-east is not shown). The PTB bistatic lidar was positioned next to the met mast at a distance of 4.6 m. All angles stated in this section are referenced to the 0\({}^{\circ}\) north direction, counting in a clockwise direction (meteorological wind direction). Multiple pairs of oppositely mounted booms are located at different heights on the wind met mast at 145\({}^{\circ}\) and 325\({}^{\circ}\) angles. The wind turbines are located at 146\({}^{\circ}\) and 307\({}^{\circ}\) angles. Figure 14 shows the deviation of the PTB bistatic lidar to the top anemometer for _unfiltered_ 10 min averaged wind speeds larger than 4 m\(\cdot\)s\({}^{-1}\). However, these unfiltered samples are problematic in two ways. Firstly, large perturbation effects due to the wakes of the nearby wind turbines are visible around 146\({}^{\circ}\) and 307\({}^{\circ}\) angles, and secondly, due to the PTB bistatic lidar's optical measurement principle, fog occasionally causes large, non-systematic deviations which are clearly visible between 90\({}^{\circ}\) and 270\({}^{\circ}\) angles. Thus, to enable a more reasonable comparison, the data were filtered around the angles corresponding to the wakes of the nearby wind turbines, and periods of fog were filtered using an upper limit on the backscattered signal amplitude. Figure 15 shows the deviation between the PTB bistatic lidar and the top anemometer for _filtered_ 10 min averages, indicating good agreement between both instruments with absolute deviations below 2.5%. Figure 12: Results of the 2019 comparative measurement between the PTB bistatic lidar and a CSAT3B ultrasonic anemometer mounted on top of a 30 m mast in turbulent flow, showing the ensemble cospectra \(C_{out}\) between the wind velocity components \(u\) and \(w\) (absolute values) of the PTB bistatic lidar and the CSAT3B sonic anemometer. The dashed line indicates the theoretical \(-\)7/3 power law in the inertial subrange. Figure 16 shows the deviation between the PTB bistatic lidar and the top anemometer for filtered wind speeds ranging from 4 m\(\cdot\)s\({}^{-1}\) to 19.5 m\(\cdot\)s\({}^{-1}\). Here, wind velocity data were evaluated according to IEC 61400-12-1 [5], sorting horizontal wind speeds into discrete bins with a resolution of 0.5 m\(\cdot\)s\({}^{-1}\). Large error bars indicate that few measurement samples were available in the corresponding wind speed ranges. Again, both the PTB bistatic lidar and the top anemometer show good agreement, with mean deviations below 0.5% over a large range of wind speeds. Figure 17 shows the linear orthogonal regression of the wind directions measured by the PTB bistatic lidar and the wind vane mounted on the met mast.