To correct the wind vector measurements for attitude and payload motion all Euler angles and the lateral velocity vector of ACTOS have to be determined. Therefore, a navigation unit based on a two-antenna differential GPS (Global Positioning Systems) and an inertial navigation system is included in ACTOS. A CSI Wireless Vector- GPS is used to measure three dimensional position, heading, and three dimensional lateral velocity. The position and velocity is derived from standard GPS running in the differential mode to increase the absolute accuracy. The heading is estimated from phase-shift measurements of the carrier signal between the two antennas separated by about 1 m.

The inertial navigation system is based on optical-fiber gyros, accelerometers, and a navigation computer made by iMAR GmbH/St. Ingbert, Germany. In the current setup only roll and pitch angle of ACTOS are derived from the optical-fiber gyros. All parameters are stored with a temporal resolution of 0.1 s and interpolated to a resolution of 0.01 s to correct the 100 Hz sonic data. With this interpolation no information is lost since the highest frequencies of ACTOS motion was observed around 1 Hz.

Figure 1 and 2 show a measurement example of a time series from the Winningen'05 campaign with the helicopter-borne ACTOS. The three velocity vector components of ACTOS as derived from dGPS are shown in the upper panel of Fig. 1. The pendulum motion can be clearly seen. The lower panel shows the attitude angles, the yaw angle is divided by a factor of 10 for better presentation. Figure 2 shows the wind velocity vector components (upper panel) as measured by the sonic (not corrected for attitude and payload motion). The lower panel shows the corrected wind vector components.

Between t = 38330 and 38355 s ACTOS was flown through a cumulus cloud. For this time the roll angle shows a 1 Hz oscillation due to the increased turbulence. The corrected vertical velocity component shows a subsidence at the cloud edges and strong vertical updrafts in the middle of the clouds.