Tropospheric Delay Threats for the Ground Based Augmentation System

GPS signal propagation is delayed by the troposphere compared to free-space propagation. Unlike the ionosphere, tropospheric delay at the GPS frequencies is not considered dispersive and does not cause code-carrier divergence. However, in the Ground-Based Augmentation System (GBAS), it may introduce different amounts of propagation delay between the GBAS ground facility and the aircraft. As a ranging error source that is not common between the GBAS ground facility and the user, it must be bounded. The purpose of this paper is to discuss tropospheric errors and the corresponding threat model for GBAS. There are two components in the tropospheric threat model: nominal vertical decorrelation and non-nominal weather differentials. The refractivity of air changes with height in the troposphere. The total delay of a radio signal through the troposphere is a function of the refractivity as it changes along the propagation path. At different altitudes, the GBAS ground facility and the aircraft observe different tropospheric delays on signals from the same satellite due to vertical decorrelation, even when the troposphere is quiet. The vertical decorrelation is mitigated by the user through modeling of the delay differences between the GBAS ground facility and the aircraft. The GBAS ground facility provides reference parameters that are incorporated into the model. Disturbances in the troposphere, such as heavy rain could create a weather differential, or a gradient between the GBAS ground facility and the user. The weather front is often modeled with a "weather wall," which is detailed in the paper.