The state of the art of wind measurements on wind turbines is by the use of standard wind sensors (cup anemometers, wind vanes or sonics) mounted on the turbine nacelle (so called "nacelle anemometry"). Often the sensors are made redundant by mounting two sets side by side. Nacelle anemometry gives input to the control system for yaw and pitch control, and for start up and shut down, as well as for performance verification.
Meanwhile, nacelle anemometry is hampered by the fact that it is positioned behind the rotor, and therefore measures wind that is influenced by the wakes of blade roots, the blade root vortices, the flow over the nacelle, and the mounting arrangement for the nacelle anemometry. Furthermore, the wind sensors are dependant on accurate mounting, adjustments and calibrations. These uncertainties causes wind turbines to suffer from yaw errors and this is visible in wind farms, where the turbines will be facing different directions.
The photo shows a view of a wind farm with yaw errors.