A “collateral” secondary benefit of ATTI mode is the possibility to use this mode to measure real wind velocity at a determinate altitude. If you take off in GPS mode and rise vertically up to 100 ft (or any other altitude), your Phantom will try to maintain its position with GPS satellites’ signal, thus indicating a horizontal speed 0.0 mph (or m/s or km/h). After checking that there are no obstacles in its path, you can change mode from P to ATTI: the aircraft will drift with the existing wind and you can see how the numbers in the horizontal speed indicator start to increase from the initial zero value. In a few seconds, such number will stabilise, indicating then the wind speed at that altitude. Do not let the drone drifting alone for a long time, specially if the wind is strong. After checking that wind speed at your scheduled flight altitude is under limits, you can fly with the confidence that it will be possible to go back home against that wind.