A method for calibrating a radio-based keyless access system of a transportation vehicle, an access system, and a corresponding transportation vehicle. Based on a measured field strength of an LF field, it is determined whether the signal transmitter is located at most at a specified distance from the transportation vehicle. A second transmission device emits a higher-frequency calibration signal in response to which the signal transmitter transmits a response signal back to the transportation vehicle. A reference distance from the signal transmitter to the transportation vehicle is automatically determined from the propagation time of the calibration signal and the response signal. The measured LF field strength-based determination of whether the signal transmitter is located at most at a specified distance from the transportation vehicle is then calibrated automatically based on the determined reference distance.