# A NavCom Signal Authentication Scheme Based on Twice Two-Way Satellite Time Transfer
## 1 Introduction  
In recent years, the development of Low Earth Orbit (LEO) communication systems has progressed rapidly, with a specific emphasis on integrating navigation enhancement functions. Through the integration of communication and navigation (NavCom) signals [1; 2], it would be able to achieve multiple functionalities from a single satellite, leading to resource savings in terms of both orbit and frequency allocation. Moreover, this integration serves to reduce hardware costs while simultaneously enhancing the performance of both communication and navigation performance.  
The mainstream navigation satellite system signals are broadcasted with a detailed structure open to the public and processed passively in receivers. While this feature makes satellite navigation an open service with unlimited user capacity, it also brings the threat of spoofing attacks by allowing the construction of counterfeit signals [3]. Since the power grid, financial industries, vehicle autopilots, and other civilian infrastructures rely on the credible position and timing information, these spoofing attacks could severely threaten their security and robustness. Hence, protecting receivers from spoofing attacks is a significant measure to improve the robustness and security of navigation services. Several attempts have been made to address security authentication for navigation signals: (1) Open Service Navigation Message Authentication (OSNMA), a navigation message authentication method adopted by Galileo [4], and (2) Chip-Message Robust Authentication (Chimera), a spreading code encryption method adopted by GPS [5]. However, they still encounter challenges in independently authenticating and authorizing a massive number of users. In addition, they cannot recognize repeater spoofing interference. Repeater spoofing is a method of deceiving attackers by receiving real satellite navigation signals through a repeater and sending spoofing signals to the target after a certain delay. The signal generated by this spoofing method carries exactly the same information, including navigationmessages and spreading codes, as the real signal. Thus, traditional security authentication schemes, such as OSNMA and Chimera, cannot recognize repeater spoofing interference.  
Actually, researchers primarily focus on achieving navigation functionality within communication signals in the NavCom signals study [6; 7; 8; 9; 10; 11; 12; 13; 14]. So far, however, very little attention has been paid to treating communication and navigation as a whole, resulting in the full integration of communication and navigation functions having not been thoroughly explored. Furthermore, the potential of leveraging the two-way broadcasting capability of communication signals to enhance the security of navigation signals remains largely untapped.  
To address these gaps, this paper proposes a novel NavCom signal authentication scheme for LEO satellite systems based on twice TWSTT. By utilizing the temporal and spatial physical attributes of the signal, this scheme effectively mitigates the risks posed by repeater spoofing interference and significantly enhances the security of the NavCom signal.  
This paper is structured as follows: Section 2 presents a twice TWSTT NavCom signal authentication scheme, providing a comprehensive explanation of its fundamental principles and the detailed model of authentication detection. Section 3 analyzes the security performance of this authentication scheme. Finally, Section 4 concludes the paper.