A wind
turbine (1) is provided, having a wind
turbine tower (2) and at least one rotatable blade (5), and further comprising a
system for measuring rotor blade vibration of said wind
turbine. The
system comprises at least one Doppler
radar unit (7) operatively configured to emit and receive
radar signals, the
radar unit being mounted on the wind turbine
tower at a position above the lowest position of the at least one blade, the radar unit being positioned so as to measure reflections of an emitted radar
signal from the
turbine blade. A
processing unit is configured to receive measurement data from the radar unit and to determine, by analysis of Doppler shift in received
radar signals relative to transmitted signals due to movement of the blade towards or away from the turbine
tower, the velocity of the blade in the direction towards or away from the turbine tower. Using a radar unit to measure blade velocity allows a determination to be made of the vibrations occurring in the blade without needing an internal sensor in the blade. This reduces manufacturing and maintenance costs of the blades since sensors in the blades will not need to be replaced, and sensors positioned on the tower are easier to replace in the field.