# Dynamic Data Filtering of Long-Range Doppler LiDAR Wind Speed Measurements
## 3 Measurement Setups
### Offshore Ground-Based Comparative Measurement Campaign
#### 3.1.1 LiDAR Measurements  
The used data was captured from 21.12.2013 15:35 h (UTC) till 19.01.2014 7:55 h (UTC). During this time period, the LiDAR was operated in a so called staring-mode with a fixed azimuth angle \(\varphi\) and a low elevation angle of \(\theta\) = 0.2\({}^{\circ}\), aiming at the ultrasonic anemometer at 41.5 m height at FINO1. The measurement frequency was set to \(f_{\mathrm{stare}}=2\) Hz with a pulse repetition frequency of 20 kHz, while capturing 82 equidistant range gates from 361 m to 2811 m with a range step of 30 m and 100 equidistant range gates from 2811 m to 2911 m with a 1 m range step. The pulse length was set to 200 ns or 59.96 m (FWHM).  
Within the measurement duration of 28 days 16 h and 20 min, we were forced to interrupt the measurements for a total of 18 days 8 h and 30 min. The resulting comparable time intervals are comprised of 10 days 7 h and 50 min.  
The positioning of the measurement near the anemometer on the FINO1 platform was ensured by an iterative hard-target method. First, we tracked the meteorological mast via horizontal PPI measurements (Plan-Position-Indicator scan) followed by vertical RHI measurements (Range-Height-Indicator scan) to identify the boom with the anemometer. We adjusted the final positioning of the measurement volume with the accuracy of the LiDAR system of 0.1\({}^{\circ}\) in azimuth and elevation. When the wind induced movements of the mast-boom-system are neglected, the maximum possible deviation of height of the anemometer and the centre of the range gate can be calculated as  
\[\Delta h_{max}=\pm\sin(0.1^{\circ})\cdot 2864\ \mathrm{m}=\pm 5.00\ \mathrm{m} \tag{14}\]  
The inclined measurement of 0.2\({}^{\circ}\) in combination with a pulse length of 59.96 m leaded to a negligible height difference within a range gate of 0.21 m. We verified the positioning of the LiDAR device by long term GPS measurements in combination with the geometrical dimensions of the substation. This resulted in an azimuthal orientation referred to the ultrasonic anemometer of \(\varphi\) = 306.47\({}^{\circ}\).  
Figure 6: Layout of the wind farm “alpha ventus” with measurement geometry of staring mode LiDAR with an azimuthal orientation of 306.47\({}^{\circ}\) and an elevation of 0.2\({}^{\circ}\) (red). Crosses represent wind turbines, the circle the platform FINO1 and the square the substation AV0. The measurement positions are indicated by the red line.  
In this data set, wind directions have been measured at FINO1 within a range of 110\({}^{\circ}\) and 285\({}^{\circ}\). Due to the fixed measuring geometry of the staring LiDAR, this could only measure the in-beam wind speed component. The result is a cosine relation between the wind speed in the wind direction frame of reference, \(u_{mfr}\), and the projected wind speed, \(u_{lfr}\) (Equation (17)). For an incoming wind direction of 216.47\({}^{\circ}\), the LiDAR measured perpendicular to the wind direction. Thus, the lateral wind speed component tends to become zero in average, which is why the turbulence intensity converges to infinity (Figure 7).