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 situ
measurements of turbulence as a contribution to closing observational gaps in
the atmospheric boundary layer (ABL). The wind measurement algorithm used
has only been calibrated in the free field up to this point. Therefore, we present
the calibration and verification in a wind tunnel.
Calibration is performed in x- and y-coordinate directions of the UAS body
coordinate frame and in wind speeds of 2 . . . 18 m s⁻¹. We investigate the mea-
surement accuracy under different angles of sideslip (AoS) and wind speeds as
well 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 constant
temperature anemometers (CTAs).
With the derived calibration coefficients the uncertainty depends on the wind
speed magnitude and increases with higher wind speeds, resulting in an overall
root-mean-square error (RMSE) of less than 0.2 m s⁻¹. Applying the calibra-
tion coefficients from one UAS to others within the fleet results in comparable
accuracies, showing omission of wind tunnel calibration for the remaining UAS.
Furthermore, the wind measurement is susceptible to high AoS at high wind
speeds. The RMSE for measurements in different gusts is up to 0.6 m s⁻¹. In
the most extreme velocity steps (i.e. a lower speed of 5 m s⁻¹ and an amplitude
of 10 m s⁻¹) the maximum RMSE occurs and exceeds 1.3 m s⁻¹. For variances
below approx. 0.5 m⁻² s⁻² and 0.3 m⁻² s⁻², the maximum resolvable frequen-
cies of the turbulence are about 2 Hz and 1 Hz, respectively. The verification
in the wind tunnel and the determination of uncertainties helps the analyses
of atmospheric measurements in complex terrain and in wind parks where the
SWUF-3D fleet is primarily deployed.