Preventing blade tower strike of a wind turbine

A method of estimating a position of at least a part of a rotor blade of a rotor of a wind turbine during operation of the wind turbine, the part of the rotor blade being a first part, wherein the rotor blade is deflected due to a deflection motion of the rotor blade towards or away from a tower of the wind turbine and wherein the position is indicative of said the deflection, wherein the method comprises

• measuring a first parameter by a first measuring unit, the first measuring unit comprising an inertial measurement unit, wherein the measured first parameter is indicative of a relative position of at least a second part of the rotor blade or of a further rotor blade of the rotor,
• measuring a second parameter by a second measuring unit, the second measuring unit comprising at least a receiver for receiving global positioning data, wherein the measured second parameter is indicative of an absolute position of at least a third part of the rotor blade or of the further rotor blade of the rotor, and
• estimating the position of the at least first part of the rotor blade, wherein the estimating comprises employing a state estimator that estimates a state of the rotor blade based on at least the measured first parameter and the measured second parameter and deriving the position from the estimated state of the rotor blade.