# Generic Methodology for Field Calibration of Nacelle-Based Wind Lidars
## 7 Discussion  
### On Lidar \(V_{los}\) Uncertainties  
An analysis of the measurement uncertainty results is performed in order to identify the prevailing uncertainty sources. Figure 14 illustrates the uncertainty assessment procedure in the form of a tree structure. From bottom to top, the approximate contribution of each uncertainty source to the next level of combined uncertainty is given. For example, the contribution of \(u_{mast}\) to \(u_{c},v_{hor}\) is computed as \(u_{mast}^{2}/u_{c,V_{hor}}^{2}\). This analysis demonstrates that:  
1. the uncertainty of the reference quantity \(u_{c,V_{ref}}\) accounts for 99% of the combined LOS velocity uncertainty \(u_{c,y}\);
2. \(>\)90% of the reference speed uncertainty \(u_{c,V_{ref}}\) is related to the combined reference HWS uncertainty \(u_{c,V_{hor}}\);
3. \(\sim\)94% of \(u_{c,V_{hor}}\) is due to the cup anemometer's calibration, operational and mast uncertainties. The LOS velocity calibration process accounts only for the remaining 6% with \(u_{inc}\) and \(u_{pos}\).  
Consequently, a **great majority of the total uncertainty on the LOS velocity is due to the calibration, operational and mast-mounting uncertainty components on the reference quantity**.  
This conclusion emphasises the need to improve the cup anemometer uncertainty assessment methodology. In particular, the spread of 1% between _Measnet_ accredited wind tunnels is unacceptably large: wind speed measurements from a cup anemometer should not be significantly dependent on the wind tunnel in which it is calibrated. Additionally, measurement errors due to the cup anemometers' sensitivities--to temperature, turbulence intensity, inflow angles, etc.--is incorporated through the classification uncertainty. If such systematic errors can be estimated, they are biases (see definition in VIM, [11]) and should be corrected for instead of considering them as an extra uncertainty.  
Currently, improvements in the generic calibration procedure cannot be easily identified since the uncertainty components related to the calibration process account for a negligible proportion of the total LOS velocity uncertainty.  
The main goal of this study is to propose and demonstrate a generic calibration procedure in order to make lidar measurements traceable to the International System of Units. Although a single sonic anemometer may conveniently replace the dual-instrument setup, no procedure to assess sonic anemometers' measurement uncertainties is given in the IEC 61400-12-1:2005 norm (but will be in the next revision, see [14]). Thus, the constrain was to use a cup anemometer for the HWS reference instrument. In addition, the wind industry shows a conservative attitude towards wind measurements: for decades, only cup anemometers had been considered.  
Figure 14: The ‘tree’ structure of the uncertainty assessment methodology including relative contributions of individual uncertainty sources.  
Alternatively, in the future, lidar-to-lidar calibrations may be performed, using a calibrated lidar as a reference. The principles of the 'white box' calibration will remain valid--replacing \(V_{ref}\) by the reference lidar \(V_{LOS}\) measurements and directly using uncertainties provided by the calibration certificate. Only slight adjustments of the procedure are anticipated. Lidar-to-lidar calibrations would reduce the calibration duration, however not the uncertainty.