Method for activating a vehicle function and associated activation device

A method for starting a motor vehicle by an activation device, from a user's portable device. The method includes: Detecting closure of the opening elements of the vehicle; Transmitting ultra-wideband waves into the passenger compartment; Receiving the waves and, for each transmitter module: i) Comparing a time-of-flight value with predetermined time-of-flight values, ii) Determining the presence of the portable device in the passenger compartment, iii) If the portable device is situated in the passenger compartment, then iv) Activating the vehicle function, and v) Comparing a received power value with predetermined received power values, vi) Determining the presence of the portable device in the passenger compartment: 1) If the portable device is not detected in the passenger compartment, then calculating a correction value between the received power and the predetermined received power, 2) The activation device storing the correction value for subsequent presence detection of the device in the passenger compartment.