Operational Envelope and Link Scheduling for Inter-Satellite Links in Next-Generation GNSSs

Future Global Navigation Satellite Systems (GNSSs) may implement Inter-Satellite Links (ISLs) enabling one-to-one connections between satellites of the constellation, making available an intra-system communication layer, supporting clock synchronization, and providing precise ranging. We analyze in this work the problem of scheduling ISLs for a number of scenarios in which satellites are equipped with a limited number of ISL terminals capable of point-to-point connections. Two baseline architectures are addressed. The first is based on the current Galileo system, in which two ISL terminals are assumed to be installed on each navigation satellite. The second architecture is an envisioned evolution of the Galileo system, with name Kepler, in which a Galileo-like Medium Earth Orbit (MEO) segment is complemented with a constellation of LEO satellites carrying ISL terminals and zenith-pointing antennas for GNSS signal monitoring in space. The MEO satellites are equipped with two terminals for MEO-to-MEO links and a third terminal for linking to Low Earth Orbit (LEO) satellites. The MEO-to-MEO links between neighboring satellites in the same orbital plane are assumed continuous. Different schedulers are proposed for the two scenarios, factoring multiple operational constraints such as the number of available ISL terminals on each satellite, the terminal's field-of-regard (azimuth and elevation span), the maximum link range, and the minimum required duration of each link. Within these boundaries, a link schedule is applied to determine which satellites are paired over time. The pairing criterion aims at establishing dynamically changing closed paths that maintain all the satellites of the constellation connected over time. In this paper, we analyze the link scheduling strategy devised on the two system architectures proposed. We provide a number of key performance parameters as function of the operational envelope of the ISL terminals and, for the Kepler scenario, for a number of different LEO constellations.