Method for calibrating a radio-based keyless access system of a motor vehicle, in which: - by means of a first transmitting device of the motor vehicle, an LF field is emitted into an environment of the motor vehicle, and - based on a field strength of the LF field measured by means of a signal generator separate from the motor vehicle, it is automatically ascertained whether the signal generator is located no further than a predetermined distance from the motor vehicle, wherein - if a predetermined condition coupled to the signal generator and/or to an operation of the motor vehicle is fulfilled, a calibration signal having a higher frequency than the LF field is transmitted by means of a second transmitting device of the motor vehicle, and in response, the signal generator then sends back to the motor vehicle a response signal having a frequency that is also higher than the LF field, - a reference distance of the signal generator from the motor vehicle is automatically ascertained from a signal delay between transmitting the calibration signal and receiving the response signal by the motor vehicle, and - the ascertainment, based on the measured field strength of the LF field, of whether the signal generator is located no further than the predetermined distance from the motor vehicle is automatically calibrated depending on the reference distance ascertained on the basis of the signal delay; wherein an LF reference signal having a predetermined transmitting power is transmitted by means of the first transmitting device, and a field strength of the LF reference signal is measured by means of an LF receiving device arranged on the motor vehicle at a fixed distance from the first transmitting device, - a correction factor is ascertained from the measured field strength of the LF reference signal, the predetermined transmitting power and the distance between the first transmitting device and the LF receiving device, which correction factor indicates an environmental deviation of the measured field strength of the LF reference signal from a predetermined reference field strength, and - the correction factor is also taken into account for calibrating the ascertainment of whether the signal generator is located no further than the predetermined distance from the motor vehicle, wherein the LF reference signal is transmitted in the direction of the ground and an LF reference signal reflected from the ground is received by means of the LF receiving device, and an influence of the ground is taken into account in the measured field strength of the LF reference signal. - by means of a first transmitting device of the motor vehicle, an LF field is emitted into an environment of the motor vehicle, and - based on a field strength of the LF field measured by means of a signal generator separate from the motor vehicle, it is automatically ascertained whether the signal generator is located no further than a predetermined distance from the motor vehicle, wherein - if a predetermined condition coupled to the signal generator and/or to an operation of the motor vehicle is fulfilled, a calibration signal having a higher frequency than the LF field is transmitted by means of a second transmitting device of the motor vehicle, and in response, the signal generator then sends back to the motor vehicle a response signal having a frequency that is also higher than the LF field, - a reference distance of the signal generator from the motor vehicle is automatically ascertained from a signal delay between transmitting the calibration signal and receiving the response signal by the motor vehicle, and - the ascertainment, based on the measured field strength of the LF field, of whether the signal generator is located no further than the predetermined distance from the motor vehicle is automatically calibrated depending on the reference distance ascertained on the basis of the signal delay; wherein an LF reference signal having a predetermined transmitting power is transmitted by means of the first transmitting device, and a field strength of the LF reference signal is measured by means of an LF receiving device arranged on the motor vehicle at a fixed distance from the first transmitting device, - a correction factor is ascertained from the measured field strength of the LF reference signal, the predetermined transmitting power and the distance between the first transmitting device and the LF receiving device, which correction factor indicates an environmental deviation of the measured field strength of the LF reference signal from a predetermined reference field strength, and - the correction factor is also taken into account for calibrating the ascertainment of whether the signal generator is located no further than the predetermined distance from the motor vehicle,