An Analysis of Inter-Satellite Link Topologies in Future GNSS Constellations: Operational Constraints and Figures of Merit

Future Global Navigation Satellite Systems (GNSSs) will integrate advanced technologies and methodologies to enhance the accuracy, robustness and resilience of navigation services. A key and promising innovation in this context is the integration of Inter-Satellite Links (ISLs), which facilitate low-latency, high-rate data transmission and accurate time and range measurements between satellites. One of the crucial aspects in the integration of ISLs is the determination of their connectivity strategies. In this study, we focus on the scheduling and planning of ISLs within a constellation modeled after the Galileo system, consisting of 24 Medium-Earth-Orbit (MEO) satellites, equipped with 2 terminals each. Our approach investigates the conditions under which a required topology (or contact plan) can be achieved, given a number of physical and operational constraints. To delineate the requirements for establishing an effective contact plan, we first define the limitations derived from mechanical and physical characteristics of the ISL terminals used. These constraints include the terminals’ field-of-regard (FOR), their transmission power levels limiting the maximum ISL range attainable, and potentially additional restrictions on the physical layer. The core of the scheduler consists in creating a graph which encompasses all the feasible links under the constraints outlined above. With two terminals per satellite, it becomes theoretically possible to establish a path connecting all satellites in the constellation in a closed chain. The close path topology allows the possibility to relay data with low latency enabling distributed clock solutions, tight system synchronization, high density of ISL measurements, and the implementation of closed-loop checks for time dissemination. In this paper, we present a scheduling strategy that ensures a systematic and full exploration of the initial graph and its potential topologies. The combination of constraints under which suitable closed topologies are found has been thoroughly analysed and the operational boundaries (in particular in terms of physical limitations on terminals) is outlined and discussed in detail.