A method for synchronizing the transmission of messages in a communication system including a user terminal, a main communication network to the user terminal having a first station, and a secondary communication network to the user terminal having a second station, the first station, the second station and the user terminal each having a temporal reference, said method being characterized in that it comprises the steps of: • emitting, by the first station, a first synchronization signal to the second station and the user terminal, • measuring, by the second station and the user terminal, the instants of reception of the first synchronization signal (252 and 253), • adjusting the temporal references of the second station and of the user terminal, on the basis of the instants of reception of the first synchronization signal, • emitting, by the user terminal, a second synchronization signal to the first station, • emitting, by the second station, a third synchronization signal to the first station, • measuring, by the first station, the instant of reception of the second synchronization signal and the instant of reception of the third synchronization signal, and calculating a first time interval on the basis of these instants of reception, • transmitting, by the first station, the first calculated time interval to the second station, • emitting, by the first station, a fourth synchronization signal to the second station, • emitting, by the user terminal, a fifth synchronization signal to the second station, • measuring, by the second station, the instant of reception of the fourth synchronization signal and the instant of reception of the fifth synchronization signal, and calculating a second time interval on the basis of these instants of reception, • calculating, by the second station, a third time interval on the basis of the first time interval and of the second time interval, and • upon the emission of data by the second station to the user terminal, shifting of the instants of emission by a duration equal to the third time interval. • emitting, by the first station, a first synchronization signal to the second station and the user terminal, • measuring, by the second station and the user terminal, the instants of reception of the first synchronization signal (252 and 253), • adjusting the temporal references of the second station and of the user terminal, on the basis of the instants of reception of the first synchronization signal, • emitting, by the user terminal, a second synchronization signal to the first station, • emitting, by the second station, a third synchronization signal to the first station, • measuring, by the first station, the instant of reception of the second synchronization signal and the instant of reception of the third synchronization signal, and calculating a first time interval on the basis of these instants of reception, • transmitting, by the first station, the first calculated time interval to the second station, • emitting, by the first station, a fourth synchronization signal to the second station, • emitting, by the user terminal, a fifth synchronization signal to the second station, • measuring, by the second station, the instant of reception of the fourth synchronization signal and the instant of reception of the fifth synchronization signal, and calculating a second time interval on the basis of these instants of reception, • calculating, by the second station, a third time interval on the basis of the first time interval and of the second time interval, and • upon the emission of data by the second station to the user terminal, shifting of the instants of emission by a duration equal to the third time interval.