The key remaining step is a comprehensive test of the entire system at sea, which is much more difficult than test flights over land.
剩下的关键步骤是一项整个系统在海面上的综合测试，难度远远大于在陆地上的飞行测试。
2
Modern communication system is a land-sea-space three-dimensional communication network whose realization is mainly restrained by underwater communication.
现代通信系统是陆-海-空三维通信网，三维通信网的组成主要制约于水下通信。
3
Shanghai has fostered a comprehensive transportation network that incorporates land, sea, and air travel, as well as a convenient urban transportation system.