## 1 Introduction
Looking forward, Russia's next-generation GLONASS-K2 satellites will feature enhanced capabilities through two-phased array antennas transmitting FDMA/CDMA signals. This upgrade integrates both laser and radio frequency ISL technologies while incorporating higher-precision atomic clocks, promising substantial improvements in system performance and navigation accuracy [15]. The Galileo navigation constellation currently does not employ ISLs, but the European Space Agency (ESA) is actively conducting exploratory research on ISL technology. In the realm of radio frequency ISLs, ESA has initiated two projects: GNSS+ and ADVISE. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Satellite Type** & **Launch Time** & **Number** &
\begin{tabular}{c} **Operating** \\ **Frequency** \\ \end{tabular} & **Duplex Mode** \\ \hline GPS-IIR & 1997–2004 & 6 & UHF & Half-duplex \\ GPS-IIR-M & 2005–2009 & 7 & UHF & Half-duplex \\ GPS-IIF & 2010–2016 & 12 & UHF & Half-duplex \\ GPS-III & 2018–present & 6 & UHF, Ka, V & Half-duplex \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Indicators of the operating characteristics of GPS satellites. In the GNSS+ project demonstration, the ISLs operate in the C-band, utilizing a time-division multiple access (TDMA) ranging and communication system. This system enables bidirectional dual-frequency pseudo-range measurements between satellites, achieving an accuracy of 1-2 cm [16]. Building on this, ESA proposed the ADVISE project, which significantly reduced payload mass and radio frequency power consumption by improving ISL design and algorithms. This solution employed a frequency-hopping time-division multiple access (FTDMA) system, achieving orbit determination accuracy of approximately 1 cm and time synchronization performance better than 0.4 ns [17]. Simultaneously, ESA is actively exploring optical quantum links (OQLs) and laser links (Kepler), aiming to investigate new directions for ISL development and enhance the performance of navigation systems [18]. Since the completion of the BeiDou-1 dual-satellite system at the end of 2000, China has adopted a \"three-step\" development strategy and officially completed the construction of the BeiDou-3 Global Navigation Satellite System (BDS-3) on 31 July 2020 [19]. Furthermore, China plans to establish a \"more ubiquitous, integrated, and intelligent\" national comprehensive Positioning, Navigation, and Timing (PNT) system by around 2035 [20; 21]. Figure 1 illustrates the development timeline of the BeiDou satellite navigation system. Between 2015 and 2016, China launched five experimental satellites (II-S, I2-S, M1-S, M2-S, and M3-S), equipped with Ka-band ISL payloads. These satellites conducted in-orbit tests on technologies such as topology structure, communication protocols, antenna design, and autonomous navigation algorithms, achieving orbit determination accuracy better than 2 m [22]. Starting in 2017, BDS-3 satellites began carrying ISL payloads to enable inter-satellite measurement and communication. By December 2018, the initial BDS-3 constellation was successfully established, consisting of 18 Medium Earth Orbit (MEO) satellites and 1 Geostationary Earth Orbit (GEO) satellite [23]. [PERSON] et al. processed 30 days of ISL measurement data from 18 MEO satellites using a distributed approach based on the extended Kalman filter, demonstrating that the three-dimensional orbital error could be less than 2.30 m over 30 days [24]. [PERSON] et al. analyzed the orbit determination performance of 18 MEO satellites and 1 GEO satellite using 43 days of ISL measurement data from 2019. The results showed that the radial orbit error of the MEO satellites was on the order of 2-4 cm, while that of the GEO satellite was on the order of 8-10 cm [25]. Currently, all BDS-3 satellites are equipped with Ka-band ISL payloads, enabling precise orbit determination and time synchronization through inter-satellite precision measurement and data transmission. [PERSON] et al.