Field data analysis for a range-based local airport monitor for WAAS

The Local Airport Monitor (LAM) concept has been proposed as an inexpensive and rapidly deployable implementation of LAAS. The LAM combines WAAS corrections with local monitoring to provide an error bound tight enough to enable Category I precision approach and landing. Two different strategies for LAM have been proposed - a range-based method and a position-domain method [1,2]. Both methods monitor WAAS by computing a discrepancy between WAAS corrected pseudoranges and locally measured pseudoranges. The discrepancy impacts the navigation error bound, and accordingly the integrity, continuity, and availability of the method. This paper analyzes field data in order to validate one of the proposed LAM implementations, the range-based method. The discrepancy statistic is computed for nine nominal days at Atlantic City. These data are exploited to validate performance simulations for LAM and to aid in selecting variable LAM parameters. Specifically, this study focuses on four aspects of the data which impact LAM operations. First, the distribution of the discrepancy statistic is considered. This investigation verifies the theoretical distribution model used in previous LAM availability studies. Second, time correlation of the discrepancy signal is considered. Data analysis shows that low-frequency components of the discrepancy signal, which persist for about 1000 seconds or more, dominate the discrepancy model and strongly impact system continuity, which is computed over an approach window of only 150 seconds. Third, the combined impact of discrepancies for all satellites in view is analyzed in the position domain. This position-domain analysis validates LAM performance and enables selection of variable LAM parameters. Fourth, biases in the position-domain discrepancy statistic are considered. These biases, which are observed to vary for each 24-hour data set, may result in an availability and continuity penalty not considered in previous research. Together, these effects indicate the need for refinement of LAM performance simulations, but otherwise support the feasibility of the LAM concept.