Preprints
https://doi.org/10.5194/wes-2025-191
https://doi.org/10.5194/wes-2025-191
10 Oct 2025
 | 10 Oct 2025
Status: this preprint is currently under review for the journal WES.

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

Kira Gramitzky, Florian Jäger, Doron Callies, Tabea Hildebrand, Julie K. Lundquist, and Lukas Pauscher

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.

Competing interests: One of the co-authors is a member of the editorial board of Wind Energy Science.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Kira Gramitzky, Florian Jäger, Doron Callies, Tabea Hildebrand, Julie K. Lundquist, and Lukas Pauscher

Status: open (until 07 Nov 2025)

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Kira Gramitzky, Florian Jäger, Doron Callies, Tabea Hildebrand, Julie K. Lundquist, and Lukas Pauscher
Kira Gramitzky, Florian Jäger, Doron Callies, Tabea Hildebrand, Julie K. Lundquist, and Lukas Pauscher
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Latest update: 10 Oct 2025
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Short summary
This study introduces an extended sea surface levelling method for the accurate offshore calibration of scanning lidars. This method can determine the alignment of the laser beam, including any vertical shift, and is independent of the scan pattern. Tests using real measurement data and a detailed uncertainty study confirm its reliability. The study offers a versatile calibration approach and improves confidence in offshore wind measurements with scanning lidars.
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