# Study of Fast and Reliable Time Transfer Methods Using Low Earth Orbit Enhancement
## 1 Introduction  
The technology of time transfer has resolved the problem of synchronizing clocks between two locations. Currently, time transfer can be executed through a variety of methods, including fiber optics, broadcast radio signals, telephones, networks, and more. The Global Navigation Satellite System (GNSS) time transfer method, which employs radio signals for time transfer, offers extensive coverage, high accuracy, and ease of operation [1]. In recent years, GNSS time transfer based on Precise Point Positioning (PPP) solutions has experienced rapid development [2]. The PPP method combines pseudorange and carrier-phase observation data, with precision products, such as precise orbits and clock deviations, to achieve high-precision time transfer services [3]. [PERSON] et al., achieved PPP time transfer with an accuracyof 0.1-0.2 ns and frequency stability of 1 \(\times\) 10\({}^{-15}\)-2 \(\times\) 10\({}^{-15}\) in half a day using International GNSS Service (IGS) precise satellite clock differential products [4]. However, attaining PPP time transfer accuracy at the sub-nanosecond level typically necessitates a substantial amount of convergence time, spanning several tens of minutes [5]. In practical applications of GNSS time transfer, the method's real-time reliability is affected by environmental interference on the receiving terminal when receiving satellite signals [6; 7]. Numerous scholars have engaged in research aimed at enhancing the performance of PPP time transfer. [PERSON] et al. [8] proposed a method to enhance PPP time transfer stability by constraining receiver clock offset, resulting in a significant short-term stability improvement. [PERSON] et al. [9] created a multi-frequency ionosphere elimination PPP time transfer model for the BeiDou Satellite Navigation System (BDS). The tri-frequency PPP time transfer achieved an accuracy of 0.61 ns and the dual-frequency accuracy was 0.75 ns, demonstrating the advantages and feasibility of BDS tri-frequency PPP in time transfer applications. [PERSON] et al. [10] performed PPP time transfer by incorporating multi-GNSS observations and ambiguity fixing techniques. The results demonstrated that integrating multi-system observations into PPP time transfer reduced the standard deviation by 38.1% compared to the traditional GPS-only approach. In addition to the aforementioned methods that enhance PPP time transfer performance through parameter constraints, multiple frequencies, and multi-system integration, the advent of low Earth orbit (LEO) satellites has introduced a novel avenue for performance enhancement. Since the 1990s, numerous organizations have launched constellations of low Earth orbit (LEO) communications satellites, with the LEO Iridium U.S. communications satellite constellation being the most prevalent example. [11]. Compared with GNSS satellites, LEO satellites offer an increased signal strength and a faster geometry change due to their lower orbital altitudes. This confers robust anti-spoofing and anti-jamming capabilities, ensuring normal continuous signal reception in signal-obscuring environments [6; 12]. The ongoing evolution of LEO satellites has led to new advancements in Global Navigation Satellite Systems. [PERSON] et al. [13; 14] established the viability of developing integrated positioning, navigation, and timing (PNT) systems with LEO satellites by analyzing signal strength, propagation times, and the geometric dynamics between LEO satellites. This confirms that LEO-augmented GNSS systems are achievable. [PERSON] et al. [15] formulated a PPP model for ionospheric removal using GPS and LEO satellites. They confirmed that GPS combined with LEO satellites allows for a shorter PPP convergence time than just processing sole GPS data. [PERSON] et al. [16] created simulation software to simulate GNSS and LEO observations from ground stations. Their simulation outcomes indicated that LEO-augmented GNSS could reduce the PPP convergence time to 5 min. [PERSON] et al.