A BDS–eLoran fusion positioning method for resilient PNT under reduced satellite availability

Satellite navigation and positioning systems have become an essential component of modern infrastructure, supporting a wide range of civil and industrial applications. However, satellite-based positioning systems are vulnerable to interference, spoofing, and equipment failures, making reliance on a single system increasingly risky. To enhance system availability and continuity, the concept of resilient Positioning, Navigation, and Timing (PNT) has therefore been proposed. This paper presents a unified BDS (BeiDou Navigation Satellite System)–eLoran (enhanced Loran) fusion framework for positioning under reduced satellite observability. The framework systematically integrates BDS and eLoran measurements across different observability conditions, ranging from full satellite availability (≥ 4 satellites) to degraded scenarios with three, two, or a single satellite. Experimental results show that incorporating eLoran measurements significantly improves the robustness and availability of BDS positioning under limited satellite visibility. In particular, when the number of available BDS satellites is insufficient for conventional GNSS positioning, eLoran-assisted fusion enables continuous positioning with meter-level horizontal accuracy under favorable geometric conditions. Overall, the proposed approach demonstrates a practical fusion strategy that supports graceful performance degradation and rapid recovery under satellite-degraded conditions, especially in maritime environments.