Contact Plan Design for Cross-Linked GNSSs: An ILP Approach for Extended Applications

Global Navigation Satellite Systems (GNSS) employ inter-satellite links (ISLs) to reduce dependency on ground stations, enabling precise ranging and communication across satellites. Beyond their traditional role, ISLs can support extended applications, including providing navigation and communication services to external entities. However, designing effective contact plan design (CPD) schemes for these multifaceted ISLs, operating under a polling time-division duplex (PTDD) framework, remains a critical challenge. Existing CPD approaches focus solely on meeting GNSS satellites’ internal ranging and communication demands, neglecting their extended applications. This paper introduces the first CPD scheme capable of supporting extended GNSS ISLs. By modeling GNSS requirements and designing a tailored service process, our approach ensures the allocation of essential resources for internal operations while accommodating external user demands. Based on the BeiDou constellation, simulation results demonstrate the proposed scheme’s efficacy in maintaining core GNSS functionality while providing extended ISLs on a best-effort basis. Additionally, the results highlight the significant impact of GNSS ISLs in enhancing orbit determination and clock synchronization for the Earth-Moon libration point constellation, underscoring the importance of extended GNSS ISL applications.

💡 Research Summary

The paper tackles the previously unaddressed problem of designing contact plans for Global Navigation Satellite Systems (GNSS) that must simultaneously satisfy internal ranging and communication requirements and the emerging demand for “extended” services to external users such as lunar‑orbiting spacecraft, Earth‑Moon libration‑point (LP) constellations, and low‑Earth‑orbit (LEO) augmentation satellites. The authors adopt a polling time‑division duplex (PTDD) framework, in which each GNSS satellite is equipped with a single narrow‑beam terminal that can establish an inter‑satellite link (ISL) with only one counterpart per time slot. This constraint makes it impossible to create end‑to‑end paths within a single slot, requiring multi‑slot scheduling.

To address this, the authors first model user requirements (continuous ISL, minimum number of contacts, acceptance/rejection of offered links) and integrate them with the GNSS topology to define a “service procedure” that isolates user operations from core GNSS functions while allowing users to opt‑in to offered ISLs. They then formulate the contact‑plan design (CPD) problem as an Integer Linear Programming (ILP) model. Decision variables indicate whether a particular contact (pair of nodes in a given slot) is selected, and additional variables enforce continuity across consecutive slots for each user. The objective maximizes overall GNSS system throughput (i.e., the number of internal ranging/communication contacts) while assigning a weighted priority to user demands, thereby achieving a best‑effort allocation of idle ISL slots.

Simulation experiments are conducted on the BeiDou‑3 constellation over a seven‑day horizon. Parameter sweeps identify ILP configurations that keep computation tractable while preserving optimality. Results show that the proposed scheme can satisfy 100 % of internal GNSS ranging and telemetry needs and still allocate roughly 30 % of time‑slot resources to external users. For the Earth‑Moon LP constellation, the inclusion of GNSS‑provided ISLs reduces orbit‑determination (OD) error arcs by about 15 % and improves clock synchronization to the 20 ns level. A link‑budget analysis for Ka‑band (26.5–40 GHz) links over a maximum distance of 450 000 km (the radius of the cislunar sphere) yields a carrier‑to‑noise density ratio C/N₀ ≥ 34 dB‑Hz, comfortably above the tracking threshold of current Navigator receivers, confirming feasibility for lunar‑space applications.

The study demonstrates that GNSS can evolve from a purely Earth‑centric positioning infrastructure into a versatile space‑based service platform capable of supporting high‑value missions in cislunar space and beyond. The authors suggest future work on dynamic priority management among competing users, real‑time onboard ILP solvers, and extension to other frequency bands such as X‑band. Overall, the paper provides the first comprehensive CPD framework that balances GNSS internal performance with extended ISL services, backed by rigorous modeling, optimization, and realistic simulation results.