GMV is currently leading, under ESA contract, an assessment study to define a demonstration mission for space-based space surveillance. The project team includes QinetiQ Space as responsible for the platform and RAL Space for the payload activities. During the first phase of the study a high-level definition of a future operational mission has been carried out including the definition of user requirements for a future Space Based Space Surveillance (SBSS) service. During the second phase of the study a precursor mission to demonstrate the SBSS operational needs has been defined. The present paper presents the results of both phases, including architectures definition and expected performances.