How accurate is the beam position of scanning lidars? An intercomparison

Abstract. Scanning wind lidar provide a flexible measurement solution to accurately quantify near-shore and offshore wind resource and site conditions to help reduce risk and costs for prospective offshore wind developments. Typically these devices are set up using a near-horizontal beam position (elevation angle) in order to obtain wind speed measurements at distances of 3-10 km at heights in the atmospheric boundary layer relevant to wind turbines. Consequently, precise knowledge of the beam position and any associated positioning errors of the lidar scan head is required to minimise the uncertainty in the measurement height and thus the resulting wind speed measurements. A number of Hard Target Testing (HTT) methods are typically used to determine and correct for beam position errors, and include the use of static hard targets (SHT; e.g. buildings), dynamic hard targets (DHT; i.e. drone) and the sea surface itself (Sea Surface Levelling; SSL). However, to-date, these methods have not been consistently compared. This paper therefore systematically compares the performance of the SHT, DHT and SSL methods in determining the static elevation angle error (elevation offset) for scanning lidar wind measurement campaigns. A comparison experiment is conducted using two independent pulsed scanning lidar devices (type Vaisala WindCube 400S) at a near-shore wind measurement test site in the UK, using an offshore meteorological mast as the static reference hard target across all methodologies. Differences in the mean elevation offset relative to the reference target range from -0.03^◦ to -0.20^◦ for SHT, -0.002^◦ for DHT, and from -0.05^◦ to +0.07^◦ for SSL. The uncertainties found from the methods range from approximately ±0.03^◦ for DHT to up to ±0.20^◦ for SSL. Overall, the results show that in the absence of offshore static hard targets, the DHT method can reliably determine the elevation offset with lowest uncertainty. While practically easier to execute, the SSL method showed more varying results between the two devices tested and exhibited larger uncertainties due to a number of factors including the determination of the sea surface distance, sea state and device tilt. When performed thoroughly, the elevation offset can be obtained from all methods to within the specified angular positioning error (±0.1^◦) of the scanning lidar model. While the SSL method exhibits larger uncertainties, it offers an alternative approach when resources (e.g. suitable SHT candidates) are limited and can be implemented regularly to quantify time-dependent angular drifts for both near-shore and offshore applications.