# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 3 Validation and Characterisation
### PTB Bistatic Lidar/Cup Anemometers (135 m Wind Met Mast)/Monostatic Lidar (WindCube)  
In 2015 and 2016, two comparative measurements between the PTB bistatic lidar and a 135 m met mast equipped with several cup anemometers were conducted at the  
Figure 6: The electrical signals from each balanced photodetector are digitised using one dedicated ADC sampling at 300 MHz. The signal is then mixed with a complex valued 75 MHz reference signal, low-pass filtered and subsampled at a rate of 50 MHz. Finally, the signal is transformed into the spectral domain using an FFT with a block length of 32,768 samples.  
Deutsche WindGuard testing ground in Aurich, Germany [21; 22]. In 2015, a cup anemometer mounted on a boom at a height of 100 m was used for reference, and the PTB bistatic lidar was positioned nearby at a distance of about 1 m. In this situation, both devices were exposed to the _disturbed wake flow_ of a nearby wind turbine. In 2016, the cup anemometer at the top of the met mast (135 m) was used for reference and the PTB bistatic lidar was positioned next to the met mast at a distance of about 3 m. For this second campaign, monostatic lidar (WindCube) data were also available and were incorporated into the comparative measurements. 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\({}^{-1}\). Generally, large error bars indicate that few measurement samples were available in the corresponding wind speed ranges.  
Figure 7 shows the deviation between the PTB bistatic lidar and the boom-mounted anemometer in _disturbed flow_ and for 1 averaging intervals. Here, the PTB bistatic lidar shows deviations below 1% over a large range of wind speeds from 6 m \(\cdot\) s\({}^{-1}\) to 12 m \(\cdot\) s\({}^{-1}\) when averaged over 1 s intervals. Figures 8 and 9 show the deviations between the PTB bistatic lidar and the top anemometer and between the monostatic lidar (WindCube) and the top anemometer in _undisturbed flow_ for 10 min and 1 min averaging intervals, respectively. In _undisturbed flow_, for 10 min averaging intervals and within bins in which at least 10 wind speeds were sampled, the PTB bistatic lidar's deviation with respect to the top anemometer reference amounts to less than 0.5% (cf. Figure 8), which is well within the cup anemometer's measurement uncertainty (roughly between \(\pm\)1% at 4 m \(\cdot\) s\({}^{-1}\) and \(\pm\)0.7% at 13 m \(\cdot\) s\({}^{-1}\)). Under the same conditions, the monostatic lidar shows deviations up to \(-\)1.5% (cf. Figure 8). In _undisturbed flow_ and for 1 s averaging intervals, the monostatic lidar shows large deviations (cf. Figure 9). This is because it resorts to interpolation for intervals shorter than its native sampling interval, which comprises multiple seconds for a single scan. Contrastingly, under the _same conditions_, the PTB bistatic lidar's deviation only increases significantly beyond 1% at very low or very high wind speeds. Extensive investigations and mathematical simulations showed that this is due to the different dynamic responses of the used sensing devices employed, and also due to the limited correlation of wind velocities sampled at slightly different locations [21].  
Overall, owing to its high spatial and temporal resolution, the PTB bistatic lidar has been proven to achieve lower measurement uncertainty than conventional lidar systems, independent of the averaging time.