SATERA will propose and validate an integrity estimator for space-based ADS-B systems based on crosschecking positions reported in ADS-B messages with position estimations provided by space-based MLAT systems. To reach this goal, SATERA will apply a stepped approach starting by the definition of an initial OSED providing a high-level operational description at system level, identifying all the involved stakeholders, and detailing the CNS equipage of the fleet and the available air traffic services. Then, SATERA will adapt the system architecture described in the ED-142A for a composite ground-based surveillance system providing ADS-B and WAM surveillance to cope with the particularities of a space-based system. Next, the research team will develop theoretical models to assess and benchmark the proposed system architecture leading to a full list of functional requirements and interfaces description that will be captured in a Functional Description Document. SATERA will also adapt for a space-based system the performance requirements it has to meet according to the ED-142A for ENR-low/medium/high traffic densities. Afterwards, SATERA will develop a system performance prediction tool to compute the theoretical performance of a MLAT system whose receiving stations are onboard of a constellation of LEO small satellites. This tool will take into consideration all the error sources that can affect the MLAT system performance and will be used to update the initial OSED (if needed) and system architecture, and to select the most suitable constellation for validating the SATERA solution at TRL2 using an end-to-end system evaluation tool that will be developed by the research team as an evolution of the system performance prediction tool. The TRL2 validation will consist of several validation exercises addressing the provision of composite space-based ADS-B+MLAT surveillance in the EURSAM and NAT corridors for realistic traffic.