GPS-based aircraft landing systems with enhanced performance: Beyond accuracy

The Local Area Augmentation System (LAAS) is a differential GPS navigation system being developed to support aircraft precision approach and landing with guaranteed accuracy, integrity, continuity, and availability. To quantitatively appraise navigation integrity, an aircraft computes vertical and lateral protection levels using the standard deviations (sigma) of pseudorange correction errors broadcast by the LAAS Ground Facility (LGF). Thus, one significant integrity risk is that the true standard deviation of the pseudorange correction error distribution may grow to exceed the broadcast correction error sigma during LAAS operation. This event may occur due to unexpected anomalies of GPS measurements or the statistical uncertainty of the true error distribution.; This thesis presents two approaches to ensure that the error distribution based on the broadcast sigma overbounds the true error distribution for a LAAS Category I (CAT I) precision approach. First, real-time sigma monitoring is needed to detect violations due to unexpected anomalies with acceptable residual integrity risk. Both the statistical sigma estimation method and Cumulative Sum (CUSUM) method are useful in this respect. The thesis demonstrates that these two sigma-monitoring algorithms together are capable of detecting any size of sigma violations that is hazardous to users. Second, sigma inflation is necessary to account for imperfect knowledge of the true error distribution. A new and detailed method of sigma inflation factor determination was created and validated with test results using the Stanford LGF prototype and a "pseudo-user" receiver. This test demonstrated that sigma overbounding with the resulting inflation factor is sufficient to support LAAS CAT I operation.; Another concern related to sigma overbounding is that the conservatism applied to LAAS CAT I is no longer feasible if a navigation system requires higher performance. Thus LAAS CAT II/III precision approaches, which may need to meet tightened Vertical Alert Limit and higher availability requirements, cannot tolerate high levels of sigma inflation. This thesis describes how Position Domain Monitoring (PDM) may be used to improve system availability by reducing the inflation factor for the standard deviation of pseudorange correction errors. LAAS prototype testing verified the utility of PDM to enhance CAT II/III user availability.