We introduce UPSim (UxNB Propagation Simulator), an open-source, Ray Tracing-calibrated solution for spatially consistent FR3 (16.95 GHz) air-to-ground propagation modeling in Uncrewed Aerial Vehicle (UAV) networks. Instead of executing point-to-point ray launches for every ground position, UPSim derives deterministic visibility regions directly from global 3D building geometry (3D-GloBFP) via fast shadow projection. It then augments these regions with Line-of-Sight (LOS) state-specific, UAV height- and distance-dependent path loss, spatially correlated large-scale fading, and log-logistic small-scale fading parameterized by over 136 million Ray Tracing (RT) channels. Validation against reference RT data demonstrates that UPSim accurately reproduces empirical attenuation maps, outperforming the 3GPP baseline by reducing the aggregate Kolmogorov-Smirnov distance from 0.33 to 0.04. The resulting maps enable route-based analysis of channel evolution in complex urban layouts, capturing critical trajectory-level metrics such as spatial outage persistence. Consequently, UPSim provides a highly scalable, open-source middle ground between computationally expensive full RT and purely stochastic models for mobility-aware 6G aerial network planning.

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