A method for authorizing a user device to send a request to a vehicle , the vehicle comprising an acoustic transducer and an RF transceiver , the user device comprising an acoustic transducer and an RF transceiver; the method comprising: establishing a secure communication channel between the vehicle and the user device; activating the vehicle acoustic transducer to transmit an acoustic signal comprising a first unique ID, ID 1 , and creating a time stamp t 0 ; receiving , in the vehicle, at least one reflected acoustic signal having been reflected in a corresponding at least one object and creating a time stamp t 1 upon reception; for said at least one received reflected acoustic signal, determining a first time-of-flight ToF 1 as ToF 1 = t 1 -t 0 ; in the user device: - receiving and verifying the acoustic signal identity ID 1 transmitted by the vehicle and creating a time stamp t 2 upon reception; - generating a response signal using a previously agreed upon method for creating a second unique ID, ID 2 , based on the received signal identity ID 1 ; - transmitting the response signal as an acoustic signal comprising ID 2 and creating a time stamp t 3 for the transmission event; - determining a response delay time t D as t D = t 3 -t 2 ; and - transmitting an RF signal comprising t D to the vehicle over the secure communication channel; in the vehicle: - receiving the response signal, verifying ID 2 , and creating a time stamp t 4 , - receiving the RF signal comprising t D , - determining a second time-of-flight ToF 2 as ToF 2 = t 4 - t 0 - t D ; and if a difference between ToF 1 and ToF 2 is below a predetermined threshold value T max , authorizing the user device. establishing a secure communication channel between the vehicle and the user device; activating the vehicle acoustic transducer to transmit an acoustic signal comprising a first unique ID, ID 1 , and creating a time stamp t 0 ; receiving , in the vehicle, at least one reflected acoustic signal having been reflected in a corresponding at least one object and creating a time stamp t 1 upon reception; for said at least one received reflected acoustic signal, determining a first time-of-flight ToF 1 as ToF 1 = t 1 -t 0 ; in the user device: - receiving and verifying the acoustic signal identity ID 1 transmitted by the vehicle and creating a time stamp t 2 upon reception; - generating a response signal using a previously agreed upon method for creating a second unique ID, ID 2 , based on the received signal identity ID 1 ; - transmitting the response signal as an acoustic signal comprising ID 2 and creating a time stamp t 3 for the transmission event; - determining a response delay time t D as t D = t 3 -t 2 ; and - transmitting an RF signal comprising t D to the vehicle over the secure communication channel; - receiving and verifying the acoustic signal identity ID 1 transmitted by the vehicle and creating a time stamp t 2 upon reception; - generating a response signal using a previously agreed upon method for creating a second unique ID, ID 2 , based on the received signal identity ID 1 ; - transmitting the response signal as an acoustic signal comprising ID 2 and creating a time stamp t 3 for the transmission event; - determining a response delay time t D as t D = t 3 -t 2 ; and - transmitting an RF signal comprising t D to the vehicle over the secure communication channel; in the vehicle: - receiving the response signal, verifying ID 2 , and creating a time stamp t 4 , - receiving the RF signal comprising t D , - determining a second time-of-flight ToF 2 as ToF 2 = t 4 - t 0 - t D ; and - receiving the response signal, verifying ID 2 , and creating a time stamp t 4 , - receiving the RF signal comprising t D , - determining a second time-of-flight ToF 2 as ToF 2 = t 4 - t 0 - t D ; and if a difference between ToF 1 and ToF 2 is below a predetermined threshold value T max , authorizing the user device. <EOS>