In Intelligent Transport Systems (ITS), the unavailable Road Side Unit (RSU) becomes an increasing serious safety-related problem because of its important role in ITS. However, there is no existing method to solve this problem effectively and stably nowadays. To solve the unavailable RSU problem, a novel 2-tier integrated communication system is proposed in this thesis to address the issue of unavailable RSU in ITS. Compared to some other solutions proposed in the previous research works, which mostly focus on improving the system performance by adjusting parameters of vehicular ad-hoc network among vehicles, the proposed 2-tier communication network, called ITS-LEO Integrated System (ILIS), is composed of conventional ITS system and Low Earth Orbit (LEO) mobile satellite system (MSS), where the LEO MSS is utilized as the complementary network when the RSU is unavailable. Since the LEO MSS primary message will get affected when overflowing messages from ITS to LEO MSS, we prioritize LEO MSS primary message over the overflowed message to minimize the effect, which is based on that the emergency message (EMsg) is given higher priority over routine message (RMsg) to get access to the channel in ITS. To optimize the utility of network resource, two different overflowing mechanisms are proposed in ILIS to improve system efficiency under different traffic density. Furthermore, we propose a bandwidth reservation protection mechanism for ILIS to increase the ITS network performance. A real-time simulation program in C++ is developed to evaluate the performance of ILIS in terms of Packet Loss Rate (PLR) and Delay, and simulation results show that adding LEO MSS as a complementary network to ITS is an effective way to solve the problem of an unavailable RSU.