Session A3b: Next-Generation Satellite Navigation Technology

An Analysis of Inter-Satellite Link Topologies in Future GNSS Constellations: Operational Constraints and Figures of Merit
Giulia Schievano, Gabriele Giorgi and Grzegorz Michalak, Institute of Communications and Navigation German Aerospace Center (DLR)
Location: Beacon A

Future Global Navigation Satellite Systems (GNSSs) are expected to incorporate advanced technologies and methodologies to enhance the accuracy, robustness and resilience of navigation services for both terrestrial and Low-Earth Orbit (LEO) users. A key and promising innovation in this context is the integration of Inter-Satellite Links (ISLs), which facilitate both high-rate data transmission and accurate time and range measurements between satellites. This advancement has the potential to significantly reduce reliance on ground infrastructure for tasks such as the Orbit Determination and Time Synchronization (ODTS), data dissemination, and integrity monitoring.
One of the crucial aspects in the integration of ISLs in GNSS is the determination of ISL connectivity topologies. Specifically, ISLs should be scheduled to optimize a given criteria (e.g. full connectivity among all satellites over pre-determined time intervals) while accounting for physical and operational constraints, such as the limitations imposed by the link terminals architecture and orbital environment.
In this study, we focus on the planning, scheduling and organizing of ISLs within a constellation modeled after the Galileo system, consisting of 24 Medium-Earth-Orbit (MEO) satellites. Our approach investigates the conditions under which desirable ISL topologies 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 number of terminals available on each satellite, its transmission power levels limiting the maximum ISL range attainable, and potential restrictions on the link between terminals, due to factors such as signal polarization or frequency separation. The visibility constraints are specified in a general manner, applicable to both optical and radio frequency (RF) terminals. Specifically, the Field-of-Regard (FoR) is characterized by maximum elevation (El) and azimuth (Az) angles, defined with respect to the satellite’s body reference frame, which describe the terminal’s angular operational envelope. In the case of optical terminals, which produce narrow beams, these angles represent the capability of a mechanical gimbal platform to scan a three-dimensional space, offering two degrees of freedom. For RF systems, which typically generate broader beams, the directional and shape characteristics of the antenna pattern are often achieved using an antenna array (with or without mechanical rotations). This antenna array consists in multiple spatially arranged elements and it produces a focused radiation pattern using phase control. The description of the visibility cone's aperture in terms of elevation and azimuth is similarly applicable to this context.
Another desirable option is the implementation of bidirectional two-way links, which offer advantages especially in terms of time synchronization (enabling simultaneous two-way time transfer). To establish such links, it is necessary to utilize terminals capable of emitting polarized signals or operating at distinct wavelengths. However, this approach restricts communication to complementary terminals pairs that transmit and receive signals with matching polarization or wavelength on the transmit and receive lines, respectively. The impact of this constraint on the construction of close ISL topologies is also analyzed.
Finally, we also integrate in the ISL scheduling a minimum required duration for the persistence of the link. This accounts for the time needed for the ISL acquisition and tracking, before useful data can be exchanged, ensuring also that the link can be maintained for a sufficient duration to support stable communication and data exchange.
All constraints described above have been coded into an ISL scheduler developed in Python. This algorithm specifically covers the cases where the satellites are equipped with two terminals. The core of the scheduler consists in creating a graph which encompasses all the possible and feasible links under all the constraints outlined above.
With two terminals per satellite, it becomes theoretically feasible to establish a path connecting all satellites in the constellation, in either a closed or open chain. The preference lies with the closed option, allowing implementation of closed-loop checks for data and time dissemination. However, achieving a closed cycle is often more challenging than attaining an open path, due to the additional constraint that the path must return to its starting vertex.
The proposed scheduler algorithm identifies one or more Hamiltonian paths (i.e. that visits all satellites/nodes once), while considering Earth masking and all aforementioned constraints. The closed topology search is carried out without using any metrics or heuristics to guide the process. During search, priority can be given to paths formed by longest or shortest ISLs. This method ensures a systematic and thorough exploration of the graph and its possible topologies.
The combination of constraints under which closed topologies occurs has been thoroughly analyzed and the operational boundaries (in particular in terms of physical limitations on terminals) is outlined and discussed in detail.
The preliminary findings, derived from the analysis of both real orbit data and simulated datasets, indicate a significant reduction in the number of Hamiltonian cycles and/or feasible topologies when terminals are forced by complementarity or maximum range constraints. However, when the field-of-regard approaches a hemispherical view, the likelihood of achieving closed topologies remains consistently high, irrespective of the minimum required links persistence, i.e. the links could be theoretically established permanently (saving full rotational constraints of the optical heads preventing continuous azimuthal rotations), or the presence of complementarity constraints.
In this study we highlight the benefits of a dual-terminal approach, enabling open/close chain topologies, with respect to single-terminal scenarios such as the one envisioned for the Galileo 2nd Generation. The latter (single terminal) relies on minimizing the number of switches required to connect the entire system, while the former (dual terminal) would enhance data relay latency, reaching complete data dissemination within one topology. Such dual-link systems would improve navigation capabilities, through the development of more sophisticated synchronization approaches. For example, clock ensembles could be formed by using all pair-wise clock offset measurements relayed at low latency across the entire constellation. Also, fully inter-connected GNSSs in a dual-terminal approach enables conceiving multi-role GNSS constellations in support of satellite communication networks by offering an own intra-system data layer at low latency.