This master thesis is about the study of an Active Corner Reflector (ACR) to be used in C-band for Sentinel-1 satellite. The purpose is to design and implement a prototype to test to extract the first outcomes and suggestion for next improved version. Because of passive corner reflectors drawbacks (they are large, heavy, has an expensive installation, needs for maintenance because of weather or thermal conditions), this project proposes an active transceiver with a compact design and main requirements to guarantee a very low coupling transmitting and receiving antennas. The goal is to achieve a first prove of an active transponder which needs to be low cost, portable and reliable. In order to aims a low mutual impedance between transmitting and receiving antennas, the purpose is to design circularly polarized array antennas. The report content follows the study and the procedure to aim the active transponder with circularly polarized antennas. The introductory chapter offer a brief description of Sentinel-1 satellite. The next chapter defines the ACR requirements and scheme components with a first test in order to obtain indicative parameters. The project finally focuses on the design and realization of the circularly polarized patch array antenna.