## 4 Remote Sensing Techniques
### Doppler Lidar  
Doppler lidar technology to measure wind is fairly new. The first commercial small-sized (<100 kg) Doppler lidars appeared on the market only about 15 years ago [76]. Today, they are widely applied for example in wind energy research [77; 78], and networks of continuous Doppler lidar measurements are being established (e.g., EU/ESSEM COST Action ES1303: \"Towards operational ground based profiling with ceilometers, Doppler lidars and microwave radiometers for improving weather forecasts\" (TOPROF)). However, the field is still undergoing fast developments, and new methods to apply the technique are introduced continuously. The basic principle of a Doppler lidar is that the instrument transmits a laser beam (either a continuous wave or a pulse) to the atmosphere and then receives a small part of the signal backscattered from aerosols and other particles flowing in the air along with the wind. From the Doppler shift between the transmitted and received signal, it is possible to derive the radial wind speed component (\(v_{r}\)) along the laser beam (Figure 4). The radial wind speed can be expressed in terms of the three orthogonal wind vector components (\(u_{1}\), \(u_{2}\), \(u_{3}\)) and the zenith (\(\varphi\)) and azimuth (\(\theta\)) angles of the lidar line of sight  
\[v_{r}(\theta,\varphi)=u_{1}\cos\theta\sin\varphi+u_{2}\sin\theta\sin\varphi+u_{ 3}\cos\varphi. \tag{22}\]  
When combining information from at least three different lines of sight (\(v_{r_{1}}\), \(v_{r_{2}}\), \(v_{r_{3}}\)), it is possible to obtain the three-dimensional wind vector (\(u_{1}\), \(u_{2}\), \(u_{3}\)). The majority of Doppler lidars today are operated as single-lidar systems, but also set-ups consisting of two or three lidars have been introduced [78]. A single Doppler lidar set-up can be used with various scanning strategies (e.g., [78; 79]):* Doppler Beam Swinging (DBS) scan consists of 3-5 beams, of which one can be vertical (three- and five-beam systems) and others inclined with a fixed elevation angle (\(\alpha\)) of about 60-70\({}^{\circ}\), and an azimuth angle (\(\theta\)) of 90\({}^{\circ}\) (or 270\({}^{\circ}\)) between the beams. An illustration of a DBS scanning pattern with five beams is shown in Figure 4.
* Velocity-Azimuth Display (VAD) scan has many inclined beams with a fixed azimuth angle between neighboring beams and a fixed elevation angle that can be chosen by application. With very low level (small elevation angle) scans, it is possible to observe the horizontal variability of the wind, and with a higher elevation angle, more accuracy can be achieved for the measurement of mean wind speed profiles [80]. * The Range Height Indicator (RHI) scanning technique applies a fixed azimuth angle, and the elevation angle changes between the beams. This type of scan, when applied to the direction of the incoming wind, can be used to detect anomalies in a turbulent wind field [81; 82; 83]. * Plan Position Indicator (PPI) scan varies the azimuth angle with a fixed elevation angle and therefore takes measurements on a conical surface. * Nacelle-based Doppler lidars deployed on top of a wind turbine hub can provide valuable information on the incoming wind [11; 76]. The scan patterns can be complicated with respect to providing the best possible coverage and availability in the scale of the rotor disc swept by the blades [79]. The Doppler lidar technique can provide various possibilities to investigate wind variability both in time and in space. The instrumentation is easy to apply, relatively inexpensive and measurements can reach high altitudes, even above the tallest meteorological mats. Therefore, for example in wind energy, Doppler lidar can be a good alternative to meteorological mast measurements, especially in environments where traditional meteorological mats cannot be built or their deployment is challenging or costly, for example offshore. Doppler lidar measures the mean wind speed with a high and known accuracy (e.g., [69; 85; 86]).