## 1 Introduction  
Inter-Satellite Links (ISLs) allow for data exchange and provide precise pseudorange measurements between satellites in a specific constellation. ISLs intend to improve the positioning accuracy and orbit determination in Global Navigation Satellite Systems (GNSS), also time transfer and autonomous orbit determination performed on board the navigation satellites [1; 2; 3; 4; 5; 6; 7; 8; 9]. Connections between satellites can be used to transfer information, which might shorten the ephemeris update interval and enhance navigation [10]. The ISL system usually establishes links outside of the atmosphere, eliminating atmospheric effects, and it is also less impacted by multipath and interference than GNSS measurements [11]. Introducing ISLs to orbit determination could reduce the number of the ground stations required for orbit determination. The stations should be positioned around the world as optimally as possible; however, this is not always possible because of geographic or political reasons. Several studies into the impact of station distribution while ISLs are working have been conducted [12; 13; 14; 15; 16; 17; 18; 19]. The main findings of these works are that ISLs can considerably improve orbit determination, can help to reduce the number of ground stations exploited in the processing, and help in better clock error estimation. ISLs might be an essential part of other constellations, e.g., consisting of Low Earth Orbits (LEO) satellites. At present, the BeiDou Navigation Satellite System is the most advanced navigation system to integrate ISLs [10; 20; 21; 22; 23; 24]. The ISL payload enables observation of other satellites and ground stations with Ka-band single frequency pseudorange measurements [25; 22; 26; 27; 28]. Each satellite operates within a 1.5 s timeslot with another satellite, creating a link pair [5; 27]. The BeiDou constellation orbits and clock offsets are estimated simultaneously [29]. This generation of satellites can realize autonomous navigation (Auto-Nav). However, even with additional ISL measurements, the constellation is still affected by external environmental and technological effects [5; 25]. The ISL payload allows observation of other satellites and ground stations with Ka-band single-frequency pseudocode ranging measurements [5; 25; 26; 27]. In the BeiDou constellation, autonomous orbit determination with the on-board ISLs payload is conducted with additional anchor stations to suppress any relative motion of the constellation. Measurements between satellites and anchor stations, described as ground-satellite links (GSLs), use the same communication and measurement system as ISLs [25]. ISLs have been only thoroughly tested in orbit in the BeiDou constellation, but they are considered to be a fundamental component of the future Galileo constellation. According to a statement published on the European Commission webpage, a new generation of Galileo satellites is under development [30]. Among the many new capabilities relying on original technologies (e.g., new atomic clock technologies and use of full electric propulsion systems), the integration of ISLs is also planned. A demonstration flight and verification of the ISLs on board a Galileo 2 nd Generation spacecraft is planned for 2024 [30]. Simulation studies have been performed to assess the impacts of including ISLs in Galileo, focusing on the use of the ISL in orbit determination and clock estimation, as well as for time transfer purposes and possible extension with LEO satellites [31; 32; 33; 34; 35]. The main findings of these works are that ISL enhance the orbit and clock prediction accuracy and reduce the dependency from ground stations. In the simulation studies also the advantages of optical two-way inter-satellite links for orbit determination is discussed, showing potential improvements in accuracy compared to microwave links. Such connections will also help in time transfer and clock synchronization.