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The network time protocol (NTP) has been used since the 80s to synchronize the time of various terminal devices in the network, and is one of the oldest network protocols still in use today. Thanks to its excellent design and implementation, it can show good synchronization performance and stability even in complex network environments (such as terrestrial Internet). In recent years, with the expansion of the scale of the satellite Internet, the satellite network structure has gradually changed from a simple point-to-point connection mode to a complex networking mode, and the frequently changing inter-orbital inter-satellite links have brought great complexity to the network. Therefore, it is important to perform the time synchronization in complex satellite networks, especially when the laser-based transmission technology applied. In this paper, we introduce the NTP system into the laser inter-satellite networks. To the best of our knowledge, this is the first time the two systems working together, enabling performing the data transmission in the simple, but effective way. We investigate two kinds of transmission scenarios: the time synchronization happened in the co-orbit and the crossorbit cases. To perform the investigation, we set up a 30-satellites simulation platform, where their virtual connections were followed by the rule of laser inter-satellite networks. The NTP details were discussed by the two transmission scenarios, including the implementation structure, the delay results and the factors that could impact the synchronization performance. By successful implementation in laser inter-satellite networks, the NTP suggests an alternative approach to perform the time synchronization in the simple and low-cost way, by dealing with the complex and free-access networks across the whole earth.
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