Ionosphere Monitoring Methodology for Hybrid Dual-Frequency LAAS

Strong ionosphere storms are a potential threat for the Local Area Augmentation System (LAAS). During these storms, very large spatial gradients of ionosphere delays might cause significant errors in user position estimation. Therefore, LAAS needs to continuously monitor ionosphere behavior in order to ensure integrity against the ionosphere anomalies. This paper introduces a new ionosphere monitoring method using dual-frequency GPS signals. This method consists of two algorithms, each of which measures a different aspect of the ionosphere delay behavior. The first algorithm estimates the rate of change of the ionosphere delay in time by using dual-frequency carrier-phase measurements. Although the algorithm estimates the temporal gradients very precisely, it cannot observe ionosphere fronts which appear to be stationary. The second algorithm directly computes the ionosphere spatial differences between the LAAS Ground Facility (LGF) and the user, estimating the ionosphere delays with dual-frequency code measurements. By definition, this algorithm has no unobservable condition. However, the estimated differences are noisy. Combining these two algorithms, we develop an ionosphere monitoring method in which these algorithms work complementarily. We then formulate a theoretical Vertical Protection Level (VPL) customized to this method. Availability simulations show that the system with the new monitor provides better performance than conventional single-frequency LAAS.