# Coastal Wind Measurements Using a Single Scanning LiDAR
## 3 Results
### Data Availability and Accuracy Indicators
#### 3.3.1 CNR Variation on the Measurement Range  
As shown in previous studies [30], the CNR can serve as a good indicator for evaluating the reliability of LiDAR-generated measurement data. Generally, the fraction of the unreliable contaminated data increased as the CNR value decreased. Before estimating the variation in data availability and accuracy associated with the measurement range, the relationship between the CNR value and the accuracy of the 100s wind speed were investigated. Here, the relative absolute errors (RAEs) for the 10-minute means were employed as an index of accuracy. The definition of RAE is given by  
\[\text{RAE}=\frac{|u_{100s}-u_{\text{v1}}|}{u_{\text{v1}}}\times 100. \tag{9}\]  
Figure 14 shows a plot of the RAEs for the 100s as a function of the CNR values for Case 8. The red circles and error bars indicate the bin-averaged and the standard deviation values, respectively. As expected, the CNR values were obviously correlated with the RAEs. The accuracy of the 100s tended to gradually decline, as the CNR took on values below \(-23\) dB. The accuracy worsened significantly when the CNR value fell below \(-30\) dB.  
Figure 13: Ratio of 100s to V1 wind speeds for Case 8, as a function of the wind direction obtained from V1 at a height of 47 m ASL.  
Figure 15 shows the frequency distribution of the CNR values for measurement ranges from 500 to 3000 m as determined from the Case 8 data collected over a three-month period. As shown, the peak frequency in the CNR distribution at a 500 m range occurred around \(-\)15 dB. As the distance increased, the peak frequency shifted toward lower CNR values. The CNR distribution for the 3000 m distance had one peak around \(-\)25 dB and the other smaller peak around \(-\)33 dB. While it is preferable to set a higher CNR threshold to increase the quality of data used for actual assessment, a higher CNR threshold will reduce the data availability, especially further away from the LiDAR position. For example, if a CNR value of \(-\)30 dB is used as the data reliability threshold, 33% of the raw measurements at a distance of 3000 m should be excluded from the analysis. The greatest number of usable observations are available at the 500 m distance.  
Figure 16 shows the data availability for various CNR thresholds as a function of the distance from the 100s for Case 8. As illustrated in the figure, the CNR threshold of \(-\)30 dB was set, and we can expect data availability of 85%, 80%, and 70% at distances of 2000, 2500, and 3000 m, respectively. According to the report from the Carbon Trust on the acceptance criteria for floating LiDAR technologies [10], a data availability of 85% is required in order to qualify as an acceptable method to tradition measurement with met masts. Given this requirement, a distance of 2000 m would appear to be the maximum effective range for the application of offshore wind measurements using the current scanning LiDAR. The CNR variation along the measurement range is likely influenced by factors including the specific device, height, and atmospheric conditions.  
Figure 14: Scatter plot of relative absolute errors (RAE) as a function of the sweep averaged CNR for Case 8. The red circles and error bars indicate the bin-averaged values and the standard deviations, respectively, at a bin width of 1 dB for each CNR range.  
Figure 16: Data availability for various CNR thresholds (\(-32\), \(-30\), \(-28\), \(-26\), \(-24\), and \(-22\) dB) as a function of the distance from 100s for Case 8.  
Figure 15: Occurrence frequency of the CNR values at distances of (**a**) 500 m, (**b**) 1000 m, (**c**) 2000 m and (**d**) 3000 m from the LiDAR position. Analyzed for Case 8.