# Generic Methodology for Field Calibration of Nacelle-Based Wind Lidars
## 8 Conclusions  
In the present paper, we developed and demonstrated a generic calibration methodology for wind nacelle lidars. Two different possible calibration concepts were first identified, and their strengths and weaknesses discussed. The retained approach is the so called 'white box' calibration. It consists in calibrating all the inputs of the reconstruction algorithms applied by lidars to estimate wind characteristics. In the 'white box' approach, the line-of-sight velocity calibration is central as all Doppler wind lidars first estimate the line-of-sight velocity and then use it for wind field reconstruction. Consequently, the white box calibration concept is generic since applicable to any lidar system with a similar procedure. In opposition, the 'black box' methodology is specific to each reconstructed parameter and requires expensive measurement setups.  
The methodology to calibrate and assess the uncertainties of line-of-sight velocity measurements was described and illustrated by the examples of the calibration of two commercially developed lidars: a pulsed multi-beam system developed by _Avent Lidar Technology_; a continuous wave circularly scanning system developed by _ZephIR Lidar_. Calibration results from both lidars have proven to be consistent: the difference between the lidar-measured line-of-sight velocity and reference quantity value lies in the 0.5%-0.9% range. The excellent agreement observed is evidence of the feasibility of the line-of-sight velocity calibration. Uncertainties can be expected to vary between \(\approx\)3.5% at 3 m\(\cdot\)s\({}^{-1}\) to \(\approx\)2% at 16 m\(\cdot\)s\({}^{-1}\).  
Using a simple reconstruction algorithm applied to a two-beam nacelle lidar, inputs uncertainties were propagated to the reconstructed wind speed and relative direction. The assumed degree of correlation between uncertainty components proved to be of critical importance. Results also showed that uncertainties: on wind speed are of same magnitude as line-of-sight velocity uncertainties; on the relative wind direction decrease with the wind speed; are insensitive to the wind direction for normal turbine operational conditions.  
Thanks to the generic calibration methodology, traceable lidar-estimated wind characteristics can be obtained and their uncertainties quantified.  
The calibration procedures were developed within the Unified Turbine Testing (www.UniTTe.dk) project lead by _DTU Wind Energy_ and funded by _Innovation Fund Denmark_. The two lidar systems tested and calibrated in this study were kindly provided by the manufacturers, _ZephIR Lidar_ and _Avent Lidar Technology_. The authors are thankful for the support, in particular to [PERSON] on Avent's side, and to [PERSON] and [PERSON] on Zephir's side. This work would not have been possible without the tremendous help of [PERSON] at the Havvose Test Station, whose skills in lidar field testing were truly appreciated. The authors would also like to thank [PERSON] and [PERSON] for their help handling the lidar systems, and [PERSON] for collecting and organising the data.  
[PERSON] was in charge of the research work and wrote the paper. [PERSON] and [PERSON] supervised the research work and the manuscript conception. All co-authors participated to the improvement of the manuscript.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.