The paper deals with the methodology of internal calibration of the satellite radar with synthesized aperture (SAR) of the EOSSAR project, built based on a hybrid multibeam offset mirror antenna with a low-element horn irradiation array. The purpose of the internal calibration procedure of the X-band multichannel satellite radar of the EOSSAR project is to achieve the required instrumental accuracy of radar images, which requires obtaining a highly accurate copy of the probing pulse on board and completing the identification of characteristics of the radar’s transmit and receive paths during the shooting. The paper theoretically substantiates and analyzes the complex routes of signals in the receiving and transmitting paths during the internal calibration procedure of radar equipment, in particular, to obtain a copy of the probing pulse and improve the correlation function during the digital processing of shooting data of a satellite SAR based on multibeam mirror antennas.