# The NEWA Ferry Lidar Experiment: Measuring Mesoscale Winds in the Southern Baltic Sea
## 2 Ship Lidar Technology  
The Ship Lidar System applied for the NEWA Ferry Lidar Experiment has been developed by Fraunhofer IWES since 2009, and is an integrated system comprising the following components:  
* a vertically profiling Doppler lidar device of the type Windcube v2 by the manufacturer Leosphere, (Orsay, France) which is the primary measurement sensor, capturing the wind velocity at up to 12 height levels above the instrument;
* a combination of an xSens MTi-G (Enschede, Netherlands) attitude and heading reference sensor (AHRS) and a Trimble SPS361 satellite compass (Sunnyvale, CA, USA), used to record high-resolution motion information that is required to correct the lidar wind data;
* a weather station collecting atmospheric data including air temperature, air pressure, and relative humidity and precipitation, complementing the dataset. All data are collected and synchronized on a measurement computer. The Windcube v2 lidar device has a sampling resolution of about 0.7 s per line-of-sight (LoS) measurement and measures successively at four azimuthal positions along a cone with a half-opening angle of 28\({}^{\circ}\) followed by a vertical beam. From these five measurements the wind velocity vector is reconstructed and updated after each new LoS measurement, resulting in a measurement frequency of about 1.4 Hz. Further information on the ship lidar technology and its components can be found in Reference [11]. Figure 2 shows a typical installation of the system on deck of a medium-size vessel. Figure 2: Installation of ship lidar System on deck of a medium-size vessel—© Fraunhofer IWES. An indispensable element of the ship lidar technology is the motion correction algorithm combining recorded lidar and motion data in order to provide motion-corrected wind data. The algorithm applies the basic principles outlined in Reference [12] and referred to in, e.g., Reference [13]; the recorded wind velocity vector is corrected for the translational velocity of the vessel and the involved platform rotations (including the heading of the ship as well as roll and pitch), respectively, according to  
\[V_{\text{true}}=TV_{\text{obs}}+\Omega\times TM+V_{\text{CM}} \tag{1}\]  
where \(V_{\text{true}}\) is the desired wind velocity vector in the reference coordinate system, \(V_{\text{obs}}\) is the measured wind velocity vector in the platform frame of reference, \(T\) is the coordinate transformation matrix for a rotation of the platform frame coordinate system to the reference coordinates, \(\Omega\) is the angular velocity vector of the platform coordinate system, \(M\) is the position vector of the platform coordinate system, and finally \(V_{\text{CM}}\) is the translational velocity vector at the center of motion of the platform with respect to a fixed coordinate system. For the present configuration of the Fraunhofer IWES Ship Lidar System, a simplified motion correction was applied, where the vessel tilting was essentially ignored due to a negligible impact on the results. This modification was justified in a dedicated verification campaign, in which the system was tested under representative conditions against the fixed reference measurements of the FINO1 met mast in the German North Sea (for further details, see Reference [11]). The results confirm that the ship motions indeed affect the high-frequency data of the lidar but that these effects are averaged out when deriving the 10-min mean values; 10-min averaged lidar and reference data agree within \(\pm\)0.5 m/s in a distance of up to about 1.5 km and show a reasonable correlation with a coefficient of determination (R\({}^{2}\)) for a linear fit of 0.99. How the motion correction acts on the data of the NEWA Ferry Lidar Experiment is shown in detail in Section 5. The ship lidar technology should be further seen in the context of the floating lidar technology, a technology that has gained some attention within the offshore wind industry in the past decade (cf. Reference [14]).