A method for determining the distance between an authentication device carried by a user and a motor vehicle, each including a wireless communication module so as to exchange a data frame, the data frame being modulated by changing the phase of a reference signal. The method includes the following steps of: the vehicle receiving a modulated reference signal, sent by the device, demodulating the received signal in order to extract an in-phase component and a quadrature component therefrom, computing the power value of the signal on the basis of the maximum amplitude value of the in-phase component and of the maximum amplitude value of the quadrature component, and determining the distance between the device and the vehicle on the basis of the computed power value.