Accurate and robust positioning is a critical enabler for Unmanned Aerial Vehicle (UAV) applications, ranging from mapping and inspection to emerging Urban Air Mobility (UAM). While Global Navigation Satellite Systems (GNSS) remain the backbone of absolute positioning, their performance is severely constrained by UAV platform characteristics and complex low-altitude environments. This paper presents a system-level review of GNSS-based UAV positioning. Instead of treating GNSS in isolation, we first link mission requirements and platform constraints, such as aggressive dynamics and Size, Weight, and Power (SWaP) limitations, to specific positioning challenges. We then critically evaluate the spectrum of GNSS techniques, from standalone and Satellite-Based Augmentation System (SBAS) modes to high-precision carrier-phase methods including Real-Time Kinematic (RTK), Post-Processed Kinematic (PPK), Precise Point Positioning (PPP), and PPP-RTK. Furthermore, we discuss multi-sensor fusion with inertial, visual, and Light Detection and Ranging (LiDAR) sensors to mitigate vulnerabilities in urban canyons and GNSS-denied conditions. Finally, we outline key challenges and future directions, highlighting integrity-aware architectures, Artificial Intelligence (AI)-enhanced signal processing, and multi-layer Positioning, Navigation, and Timing (PNT) concepts. The review provides a structured framework and system-level insights to guide resilient navigation for UAV operations in low-altitude airspace.

Laser Inter-Satellite Links (LISL) are increasingly pivotal for next generation GNSS, offering high precision ranging with low power and high directionality, while reducing dependence on global ground tracking distribution. This study presents a simulation based Precise Orbit Determination (POD) analysis for the BDS-3 constellation augmented by LISL constraints. We focus on a practical link scheduling strategy under visibility constraints and evaluate how LISL observations strengthen orbit and clock estimation, particularly when ground tracking is limited.A feasible LISL topology is designed for the BDS-3 MEO constellation based on precise ephemerides. On same orbit plane links are organized into a closed-loop ("hand-in-hand") geometry to stabilize the configuration. The links on different orbit plane employ a dynamic time varying allocation strategy, reserving capacity to flexibly connect with IGSO satellites and adjacent MEO planes, thereby enhancing cross-plane connectivity. Link feasibility is validated using an Earth-occlusion and atmospheric tangency model. To mitigate relative clock errors, dual-way ranging is applied to separate geometric distance from clock offsets. Based on this configuration, LISL observations are simulated to generate adjacency matrices and distance time series for assessing geometric stability.POD experiments were conducted for DOY 197–227, 2023, under two scenarios: (1) a global network using MEGX stations, and (2) a regional network utilizing eight iGMAS stations in China. The dynamic model employs ionosphere-free combinations and standard CODE dynamic strategies, including solar radiation pressure, Earth albedo , and antenna thrust models. LISL ranges are introduced as constraints with 1 mm a priori precision.Results demonstrate that LISL constraints significantly enhance both orbit and clock estimation, with the most substantial gains observed in the regional tracking scenario. Validated against CODE precise products, clock accuracy improves from 0.086 ns to 0.068 ns, with smoother overlapping Allan deviation. For the global network, the 3D orbit RMS decreases from 5.0 cm to 4.2 cm. For the regional network, where GNSS-only solutions are limited to decimeter-level accuracy, adding LISL reduces the along-track, cross-track, and radial RMS by 80.8%, 76.5%, and 74.0%, respectively (82.5% improvement in 3D RMS). Independent Satellite Laser Ranging (SLR) residuals confirm these improvements, highlighting the potential of LISL to ensure robust, high-precision orbit products for future autonomous navigation.