Space systems exploiting modularity, formation flying, payload distribution and fractionation concepts can be used for a variety of applications, including Earth observation and on-orbit servicing. Concerning Earth observation, in a Formation Flying SAR (FF-SAR) the Synthetic Aperture Radar (SAR) payload is distributed among several smaller platforms to enhance performance of a monolithic SAR, by exploiting the physical separation among the receivers. In this paper, a FF-SAR concept is described that exploits a cluster of three receiving-only platforms flying with separations of a few hundred meters. The cluster is then separated of about 100 km from an illuminator of opportunity, i.e. a transmitting-receiving SAR. The system is completely passive in the meaning that it exploits the signal transmitted by an illuminator of opportunity already in orbit. The paper presents an overview of the mission concept as well as system and platform design. Each platform is 12 U and has a modular architecture, being realized by assembling subsystems or modules, each composed by a number of Cubesat Units. In the paper, the antenna, propulsion and relative navigation modules are described. In addition, FF-SAR performance are preliminary evaluated within a realistic simulation environment, including orbit and attitude propagation, scene simulation, and image synthesis
