Satellite communication is envisioned as a next generation communications due to its global coverage without relying on pre-deployed base station. There are many network simulators nowadays e.g., OPNET, Mininet and NS2. However, few simulators can simulate the exact trace of constellation while testing network performance. Satellite Tool Kit (STK) is an analysis software which is capable of generating orbit/ballistic ephemeris. In this paper, we present a co-simulation framework that integrates STK with NS2. We discuss the key components of the framework, including data exchanging and the synchronization of simulation time. We describe the process of modeling satellite constellations and configuring network nodes in NS2 using the data imported from STK. Additionally, we demonstrate the utility of the framework through a case study on the performance evaluation of a satellite communication system. The results of our experiments show that the co-simulation provides a more comprehensive understanding of satellite communication systems. It allows researchers to analyze the impact of various factors, such as satellite orbit parameters, network topologies, and communication protocols on the overall communication performance.