  The development of an integrated space-air-ground network (SAGIN) requires
sophisticated satellite Internet emulation tools that can handle complex,
dynamic topologies and offer in-depth analysis. Existing emulation platforms
struggle with challenges like the need for detailed implementation across all
network layers, real-time response, and scalability. This paper proposes a
digital twin system based on microservices for satellite Internet emulation,
namely Plotinus, which aims to solve these problems. Plotinus features a
modular design, allowing for easy replacement of the physical layer to emulate
different aerial vehicles and analyze channel interference. It also enables
replacing path computation methods to simplify testing and deploying
algorithms. In particular, Plotinus allows for real-time emulation with live
network traffic, enhancing practical network models. The evaluation result
shows Plotinus's effective emulation of dynamic satellite networks with
real-world devices. Its adaptability for various communication models and
algorithm testing highlights Plotinus's role as a vital tool for developing and
analyzing SAGIN systems, offering a cross-layer, real-time and scalable digital
twin system.
