Calibration and verification of high resolution wind speed measurements with quadcopter UAS in a wind tunnel with active grid

The SWUF-3D fleet of unmanned aerial systems (UAS) is utilized for in situmeasurements of turbulence as a contribution to closing observational gaps inthe atmospheric boundary layer (ABL). The wind measurement algorithm usedhas only been calibrated in the free field up to this point. Therefore, we presentthe calibration and verification in a wind tunnel.Calibration is performed in x- and y-coordinate directions of the UAS bodycoordinate frame and in wind speeds of 2 . . . 18ms1. We investigate the mea-surement accuracy under different angles of sideslip (AoS) and wind speeds aswell as the portability of the calibration coefficients to other UAS of the fleet.The wind tunnel is equipped with an active grid which is capable of generat-ing measurement scenarios like gusts, velocity steps and statistical turbulence.This allows systematic verification of the measurement capabilities and identifi-cation of limitations. As a reference for the UAS measurements we use constanttemperature anemometers (CTAs).With the derived calibration coefficients the uncertainty depends on the windspeed magnitude and increases with higher wind speeds, resulting in an overallroot-mean-square error (RMSE) of less than 0.2ms1. Applying the calibra-tion coefficients from one UAS to others within the fleet results in comparableaccuracies, showing omission of wind tunnel calibration for the remaining UAS.Furthermore, the wind measurement is susceptible to high AoS at high windspeeds. The RMSE for measurements in different gusts is up to 0.6ms1. Inthe most extreme velocity steps (i.e. a lower speed of 5ms1 and an amplitudeof 10ms1) the maximum RMSE occurs and exceeds 1.3ms1. For variancesbelow approx. 0.5m2s2 and 0.3m2s2, the maximum resolvable frequen-cies of the turbulence are about 2Hz and 1Hz, respectively. The verificationin the wind tunnel and the determination of uncertainties helps the analysesof atmospheric measurements in complex terrain and in wind parks where theSWUF-3D fleet is primarily deployed.