An active transponder for synthetic aperture radar systems includes a receiving antenna for receiving a first radiofrequency signal modulated according to a first train of one or more first pulses, separating means comprising two outputs outputting the first radiofrequency signal, second processing means connected to a first output of the separating means to generate a code synchronized with the first pulses, signal generating means connected to the second output and to the second processing means generate a second radiofrequency signal modulated by the code, and a transmitting antenna means to transmit the second radiofrequency signal to generate, for each one of the first pulses, a sequence of one or more second pulses, the code being synchronized with the second pulses.