As the demand for cost-effective and scalable Earth Observation (EO) missions continues to grow, OHB seeks to develop a standardised satellite platform capable of supporting various payloads while maintaining flexibility across different mission requirements. This thesis contributes to that effort by identifying key engineering drivers and applying Model-Based Systems Engineering (MBSE) methods to analyse and optimise the platform design, with a particular focus on the Payload Data Handling Unit (PDHU) for an optical payload. The study follows a structured approach, beginning with the identification of engineering drivers that guide the design and sizing of the Envoy platform. By selecting requirements that encompass all potential mission needs - prioritising the highest values to ensure broad applicability - the platform is designed to remain consistent across multiple missions, minimising redesign efforts and reducing long-term costs.