A partnership among NOAA/ARL/ATDD, Harvard University, and Aurora Flight Sciences Corporation aims to measure fluxes of CO2 and CH4 from an aircraft over the North Slope of Alaska. Concentrations of CO2 and CH4 will be measured using instruments developed at Harvard University, and atmospheric turbulence will be measured using the Best Aircraft Turbulence (BAT) probe developed at NOAA/ARL/ATDD. The BAT probe will be mounted on a Diamond DA-42 aircraft to measure wind speed and direction relative to the aircraft. Operationally, measurements of the motion and attitude of the aircraft will be combined with measurements from the BAT probe to calculate turbulent winds with respect to Earth, but this study will only evaluate the quality of the probe's measurement of wind speed and direction relative to the aircraft.

The BAT probe senses turbulence by measuring the pressure distribution around a hemisphere. A potential-flow model of the pressure field is then used to calculate the incidence angles (angles of attack and sideslip) of the airstream relative to the probe. Static air pressure is also measured.

The probe was tested in the Wright Brothers Wind Tunnel at M.I.T. in Cambridge, MA. The wind tunnel provided a large test section, airspeeds equal to those of the DA-42 aircraft, and very low flow distortion. The probe was mounted on a support that allowed its orientation (pitch and yaw) to be varied systematically. Measurements of the probe's orientation angles were made to an accuracy of 0.1º. These orientation angles were then compared to the incidence angles measured by the probe. Since the airflow in the tunnel is parallel to the tunnel centerline, a direct comparison between these angles is valid.

Results show good agreement between the incidence angles measured by the probe and the orientation angles measured with respect to the wind tunnel's centerline. The static air pressure measured by the BAT probe also agrees well compared with the wind tunnel standard. The results yielded new insight into how the probe functions and allowed us to validate and verify the operation of the probe under flight conditions. These results confirm the utility of the BAT probe for eddy-correlation measurement of turbulence.