Dual-Frequency Smoothing for CAT III LAAS: Performance Assessment Considering Ionosphere Anomalies

Strong ionosphere storms are the most-threatening potential fault mode for the Local Area Augmentation System (LAAS). Multi-frequency GPS techniques are known to be an effective means of reducing or removing ionosphereinduced problems. Among those techniques, of great interest are Divergence-Free smoothing (denoted here as DFree) and Ionosphere-Free smoothing (denoted here as IFree). This paper discusses the practical benefit of these methods in the context of Category III LAAS. First, this paper discusses a dual-frequency LAAS that implements DFree. Although DFree significantly mitigates the threat of ionosphere anomalies as compared to single-frequency carrier smoothing that is employed in conventional LAAS, the remaining threat must be addressed to meet the stringent integrity requirement of CAT III landing. This paper uses an ionosphere monitoring method to detect conditions in which the residual risk is unacceptable and evaluates the long-term system availability of the resulting DFree-plus-monitoring method. Simulations show that this approach achieves more than 99.9% availability at more than 50% of the Conterminous United States (CONUS). Next, this paper discusses a dual-frequency LAAS that implements both DFree and IFree. By definition, IFree is completely immune to all ionosphere-related problems; however, it cannot be nominally used in LAAS because of the large error size of the smoothed signals. This paper discusses the use of IFree as a backup in case of ionosphere anomaly under which DFree LAAS would otherwise be unavailable. In particular, the trigger conditions at which the system should switch from DFree to IFree are studied from the view point of maximizing system availability.