An ascertainment method for determining a final angle of vanes that are adjustable between an initial angle and the final angle and are part of a row of leading vanes which are arranged upstream of a row of trailing vanes of a compressor in a gas turbine, in order to prevent ice from forming in the compressor. During operation of the gas turbine, a current value of at least one process parameter is determined in a parameter ascertainment step, whereupon the final angle is defined in accordance with the value in a final-angle defining step, an aerodynamic speed being determined as the process parameter.