Beyond Global Navigation Satellite Systems–Based Georeferencing: Recent Trends in Positioning and Navigation

Positioning, navigation, and timing (PNT) information is critical for a range of critical civil, industrial, and defense applications. PNT relies predominantly on global navigation satellite systems (GNSSs), but GNSS signals are becoming increasingly vulnerable to natural and man-made disruptions, which motivated the development of alternative PNT technologies that can complement or substitute GNSS in denied environments. These approaches fall into three categories: opportunistic, infrastructure-based, and environment-based PNT. While numerous studies have examined individual techniques, the literature remains fragmented and lacks a comprehensive review. This paper reviews and synthesizes recent advances across all three categories, highlighting application scenarios, achieved accuracy, and limitations. Opportunistic PNT, i.e., using signals of opportunity, has evolved rapidly over the past decade with the introduction of 5G cellular systems and the large-scale deployment of low Earth orbit constellations, although their accuracy remains at the meter level. Infrastructure-based methods can achieve meter- to subcentimeter-level accuracy, but with trade-offs between accuracy, coverage, and reliability. Environment-based PNT requires no external infrastructure and continues to advance through improvements in visual and lidar simultaneous localization and mapping (SLAM) algorithms, while radar-SLAM offers a more robust alternative in environments where visual- or lidar-based systems may struggle. Despite all these alternatives, GNSS remains unmatched due to decades of infrastructure investment and operational evolution. Future resilient PNT in GNSS-denied environments will rely on adaptive multi-sensor integration, combining ambient signals, dedicated infrastructure, and SLAM to meet application-specific needs.