Method for calibrating a radio-based keyless access system of a motor vehicle, access system, and motor vehicle

A radio-based keyless access system for a transportation vehicle, the system comprising:

• a portable signal transmitter;
• a first transmitting device for the transportation vehicle for emitting an LF field;
• a second transmitting device for the transportation vehicle for emitting a calibration signal having a higher frequency relative to the LF field;
• and
• a signal processing device,
• wherein the access system is configured to automatically carry out a method for calibrating a radio-based keyless access system of the transportation vehicle,
• wherein the first transmitting device emits the LF field into an environment of the transportation vehicle,
• wherein based on a field strength of the LF field measured by a signal transmitter separate from the transportation vehicle, a determination is made whether the signal transmitter is situated within a predefined distance from the transportation vehicle;
• wherein the calibration signal having the higher frequency relative to the LF field is emitted by the second transmitting device in response to a predefined condition of the signal transmitter and/or to operation of the transportation vehicle being fulfilled,
• wherein the signal transmitter responds to the calibration signal by transmitting back a response signal having a higher frequency relative to the LF field,
• wherein a reference distance between the signal transmitter and the transportation vehicle is determined automatically based on a signal time-of-flight between the emission of the calibration signal and reception of the response signal by the transportation vehicle, and
• wherein the process of determining whether the signal transmitter is situated within the predefined distance from the transportation vehicle is calibrated automatically based on the reference distance determined based on the signal time-of-flight.