A method for detecting a message provided with a preamble with a number of pulses in a signal sent by an interrogator or a transponder in mode S, said method including a step for decomposition of said signal into an amplitude signal and into a complex phase signal, a step for detection of said preamble by correlating the amplitude signal with a replica signal of the expected pulses, and a phase control step executed by adding together the samples of the phase signal for the duration of the pulses of said preamble and by comparing the sum obtained with a threshold. The method may include a step for time synchronization by correlation of the complex phase signal with a reference sequence formed by one or more known bits before the decoding of the data. The method applies notably to the decoding of interrogation messages borne by low-power signals.