# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 4 Conclusions and Outlook  
This paper has outlined the design details and theory of operation of PTB's high-resolution bistatic lidar. It has also provided an overview of selected comparative measurements. The comparative measurements between the PTB bistatic lidar and the cup anemometers mounted on the met masts show that the measurement uncertainty of PTB's bistatic lidar is well within the measurement uncertainty of traditional cup anemometers (roughly between \(\pm 1\%\) at 4 m s\({}^{-1}\) and \(\pm 0.7\%\) at 13 m s\({}^{-1}\)) while being fully independent of its site. The comparative measurement between the PTB bistatic lidar and an industry-standard ultrasonic anemometer (CSAT3B) in turbulent flow shows that the PTB  
Figure 16: Results of the 2020 comparative measurement between the PTB bistatic lidar and a cup anemometer mounted on top of a 200 m met mast, evaluated according to IEC 61400-12-1 using filtered 10 min averages. Both instruments show good agreement with mean deviations between \(+0.25\%\) and \(-0.5\%\) over a large range of wind speeds. Figure 17: 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 linear orthogonal regression of the filtered wind directions measured by the PTB bistatic lidar and the wind vane mounted on the met mast. bistatic lidar provides distortion-free measurement capability which is free of any angular dependence and is in excellent agreement with the theoretical \(-7/3\) power law in the inertial subrange. To summarise, the high-resolution bistatic lidar developed at PTB has been proven to provide spatially and temporally highly resolved remote measurements of the wind velocity at heights ranging from 5 m to 200 m over any terrain. Furthermore, it provides measurement results with exceptionally low measurement uncertainty that are traceable to the SI units. Thus, it is planned to use the PTB high-resolution bistatic lidar as a transfer standard for other wind velocity remote sensing devices in the future. A full measurement uncertainty budget is currently in preparation and it is planned to be addressed in a separate paper. Overall, the PTB bistatic lidar shows great potential to enhance the capability and accuracy of measurements in applications such as wind potential analysis, the power curve evaluation of wind turbines and atmospheric turbulence analysis. Nevertheless, there is still room for improvement. Adjusting the measurement height is currently slow, requiring multiple scans over an interval of several minutes. Therefore, the optomechanical section is being completely reworked. This is moreover being combined with an improved signal processing unit using the latest radio-frequency system-on-chip (RFSoC) technology. In the future, this will enable the fast and autonomous selection of measurement heights, allowing the PTB bistatic lidar to perform wind profile measurements. The improved signal processing will take the performance closer to real-time, increasing both the spectral resolution and the number of aerosols detectable per unit of time, which is also a requirement for fast beam alignment. Several desirable functionalities and improvements are still missing from the PTB bistatic lidar at its current stage. Firstly, because of the PTB bistatic lidar's optical measurement principle, its measurement capability is degraded during weather conditions such as precipitation and fog. It is thus desirable to incorporate the automatic detection and mitigation of these conditions based upon the SNR and other spectral characteristics of the received Doppler spectra. Secondly, a thorough investigation of the measurement uncertainty as a function of the SNR and the characteristic distributions of correlated Doppler peaks for varying meteorological conditions is needed in order to improve upon the achieved measurement uncertainty by fine-tuning the digital signal processing chain.