This paper proposes a real-time capable global tropospheric duct threat monitor to predict the additional tropospheric delays resulting from anomalous atmospheric conditions like ducting. These atmospheric effects are not seldom with a probability of occurrence in the order of 90% in continental and coastal regions globally. Mismodeling these effects can result in tens of meters in slant-range error and adequately accounting them translates in improved accuracy and tighter integrity bounds. The proposed monitor uses the ERA-5 numerical weather model (NWM) data to predict the occurrences of ducting and the resulting ranging error. Additionally, an operational analysis of the improved achieved using the proposed threat monitor is performed by analysis the ability of the monitor to predict these ducting induced ranging error at Singapore and Chennai international airports where we have noticed significant variability in atmospheric water vapor and refractivity. The proposed monitor provides corrections for the ducting induced ranging error accurate to within ± 50 cm 95% of the time, which is a significant improvement that will enable terrestrial systems to support increasingly demanding performance-based navigation systems.