Alignment of Scanning Lidars in Offshore Campaigns – an Extension of the Sea Surface Levelling Method

Abstract. The expansion of offshore wind energy creates an increasing potential for the use of scanning lidars for wind resource assessment, power curve verification, and wake monitoring. These applications require accurate measurements, which require precise alignment calibration. However, performing such calibration offshore is challenging due to the absence of fixed hard targets. The Sea Surface Levelling (SSL) method, which uses the sea surface as a reference, has emerged as a promising alternative but so far could not determine the static elevation offset and lacked a rigorous uncertainty evaluation. This study presents a generalisation of the SSL method that enables simultaneous determination of pitch, roll, and elevation offset by incorporating scans at multiple elevation and azimuth angles, either with RHI or PPI scans. An analysis of uncertainties is performed, taking into account incorrect distance measurements of the lidar's point of entry into the water, the influence of waves and statistical noise. The extended SSL method is applied to data from a scanning lidar (Vaisala WindCube WLS400S) installed on the transition piece of an offshore wind turbine in the German North Sea. Results show that pitch and roll can be determined with high confidence and reproducibility, even with incomplete azimuth coverage. For the first time, we demonstrate that elevation offset can be derived directly from SSL, though its accuracy depends strongly on distance determination. Further, correcting the lidar pulse length improves the agreement with an independent validation. Wave effects were negligible under calm conditions but are expected to increase in rougher seas. Overall, the extended SSL method enables the alignment calibration with typical uncertainties of 0.03°-0.04° at suitable elevation angles, in this example from -1.5° to -0.3° at a lidar height of around 20 m. The extended SSL method provides a robust, transferable alternative to hard-target calibration for scanning lidars offshore.