International projects like Canada's QEYSSat project [24] and the UK's QUARC [25], as well as Singapore missions like SpooQy [26] and QUICK3 [27], are also reinforcing the global drive to advance quantum communication technologies. More specifically, the QEYSSat mission stands out for its unique approach in employing an uplink method to test the BB84 and BBM92 protocols, diverging from the common practice in other missions. The UK's QUARC mission focuses on providing secure satellite QKD service to the UK via a constellation of CubeSats, while Singaporean space mission projects are also considered among the most mature with seminal demonstrations from the SpooQy-1 CubeSat mission, which demonstrated polarization-based quantum entanglement correlations. Additionally, the ongoing QUICK3 mission aims to design a compact, true, and single-photon source that can enhance the secure data rates in satellite-based QKD scenarios when compared to conventional laser-based light sources. In parallel to these important milestones for satellite-QKD networks, the quantum engineering community is advancing space-based QKD technologies to deliver improvements in Key Performance Indicators (KPIs) for higher Secure Key Rates (SKR), innovations in space-QKD components for new system designs and architectures, and preparations for the future quantum space Internet [28]. The initial efforts to optimize the performance of satellite QKD downlinks are focusing on the design and development of state-of-the-artGHz on-board sources [29; 30; 31; 32], with improved KPIs and Size, Weight, and Power (SWAP) parameters. Alongside hardware advancements, there is significant theoretical work on the precise modeling of atmospheric channel [33; 34; 35; 36; 37; 38] network optimization [39; 40] and security considerations, including finite key size analysis [41] for satellite-to-ground QKD communications. Efforts are also underway to upgrade Optical Ground Stations (OGS) to be quantum-ready [42; 43], with technologies like Adaptive Optics (AO) systems and advanced free-space-to-fiber coupling techniques [44] being developed to optimize the integration of space- and ground-based infrastructures. Additionally, high-detection efficiency systems are being investigated to augment the OGS [45]; however, their deployment is constrained by factors such as bulkiness, the availability of cooling technologies, and cost considerations. These kinds of technologies would optimize the coupling performance, thus enabling the connection between the space segment and the terrestrial, fiber-based infrastructures. Amidst the advances in satellite-based QKD and the establishment of quantum communication infrastructures, there remains a need for studies and system designs that will bridge the terrestrial and space segments of QCI. This necessity underscores the importance of developing networks that seamlessly combine space and fiber technologies. This is where this research contribution stands. The contribution of this study lies in its methodology for determining the essential requirements for a given set of assumptions to achieve satellite QKD, where the aim is to bridge the fiber infrastructure and space-QCI segment, thus addressing a critical gap in the current research and development ecosystem. Our research also aims to open the door to the design phase of converged space/terrestrial QKD networks, thus leveraging the mature architecture design options for terrestrial distribution topologies and the KPIs provided by the future highly efficient, GHz-scale, and robust satellite P&M QKD downlinks. The remainder of this paper is structured as follows: Section 2 introduces the envisioned architecture, providing the foundational concepts and systems assumptions that underpin this work. Section 3 delves into the specifics of the software tool developed and utilized for this research, where the details of the satellite-to-ground QKD link modeling are presented. In Section 4, the results are presented, where a top-down approach is adopted.