A dual-frequency ground based augmentation system prototype for GPS and BDS

To keep the protection level low and meet the integrity requirement at the same time, we developed a dual-constellation ground-based augmentation system (GBAS) prototype to integrate the BeiDou navigation satellite system (BDS) into the current GPS-based GBAS algorithm. The measurements from the satellites are affected by the ionosphere. Thus, the objective of this work is to resolve these ionosphere issues by utilizing a dual-frequency dual-constellation (DFDC) GBAS prototype. According to the work presented by Stanford GPS Laboratory, two types of dual-frequency carrier-smoothing methods were implemented in the DFDC GBAS prototype, namely the divergence-free (DFree) smoothing method and the ionosphere-free (IFree) smoothing method. Among them, the DFree smoothing method corrects for the effect of the ionosphere temporal gradient. The IFree smoothing method completely removes all ionosphere-related errors, and the residual ionosphere error after the correction is zero, but the noise level is increased. A DFDC GBAS prototype would achieve the required integrity by combining the DFree and IFree methods and switching between them based on the ionosphere conditions. There are three switching strategies. In the first strategy, the system only depends on the ionosphere monitor, detecting the faulty signals and changing the smoothing filter for these signals to the IFree method. The second strategy is computing both DFree-based VPL and IFree-based VPL and comparing them to determine which VPL is smaller. After that, the system chooses the better smoothing method for all signals simultaneously. In the last strategy, the system will switch all signals to the IFree smoothing method when the monitors detect any anomalous signals. The ionosphere monitors independently detect each measurement and exclude the signals affected by anomalous ionosphere. In accordance with the results of the monitors, aircraft can switch between the DFree and IFree methods. The field test results of the GBAS prototype including the single-frequency dual-constellation case, the dual-frequency single constellation case, and the dual-frequency dual-constellation case are presented in this paper.