Quantification of Motion-Induced Measurement Error on Floating Lidar Systems

Abstract. Floating lidar systems (FLS) are widely used for offshore wind site assessment and their measurements show good agreement when compared to trusted reference sources. Though, some influence of motion on mean wind speed data from FLS has to be assumed but could not have been quantified with experimental methods yet because the involved uncertainties are larger than the expected impact of motion. This study describes the motion-induced bias on horizontal mean wind speed estimates from FLS with the help of simulations of the lidar sampling pattern of a continuous-wave (CW) velocity-azimuth display (VAD) scanning wind lidar. Analytic modelling is used to validate the simulations. It is found that the error depends on amplitude and frequency of tilt motion, the relative angle between wind direction and tilt motion, and the strength of wind shear. The results are used to quantify the measurement deviation that is caused by motion for the example of the Fugro SEAWATCH Wind LiDAR Buoy (SWLB) carrying a ZX 300M lidar. For the test case of "normal" wave conditions, the bias is as low as 0.04 % and for "strong" waves the estimated error is -0.14 % of the measurement value. The reason for these low errors lies in a fortunate combination of the frequencies of lidar prism rotation and tilt motion.