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. INTRODUCTION The Local Airport Monitor (LAM) concept was proposed by the Federal Aviation Administration (FAA) to accelerate deployment of Category I (CAT I) Local Area Augmentation System (LAAS). Difficult problems including ionosphere gradients and signal deformation have arisen during development of LAAS. These hazards have made LAAS certification increasingly complex and expensive. By contrast, the Wide Area Augmentation System (WAAS) is already in operation. Although today’s WAAS cannot yet meet CAT I integrity standards, its accuracy, which is better than 2 m (95%), is sufficient to support CAT I approach operations under nominal conditions. The basic idea of LAM is to implement additional WAAS monitoring at an airport to detect off-nominal system behavior. This monitoring enables the LAM to rebroadcast WAAS corrections with a tightened error bound, based on local monitoring. The LAM requires no changes to existing hardware, because WAAS corrections are broadcast to LAAS-equipped aircraft in the standard LAAS VHF Data Broadcast (VDB) format. Figure 1 conceptually shows LAM operation for CAT I precision approach. The LAM could be significantly cheaper to install than conventional LAAS, because LAM will require fewer certified components. The LAM exploits already certified WAAS monitors, so the receiver hardware and software of the ground station would be much simpler than LAAS. However, LAM may not achieve the same CAT I availability as conventional LAAS, even though it achieves full CAT I integrity. Since LAM is designed for CAT I service only, LAAS is required for CAT II/III service in the future. Therefore, it is evident that the LAM is not a replacement for LAAS but a stepping-stone toward CAT II/III LAAS. More discussion about advantages and limitations of LAM is given in [3]. There are at least two proposed concepts for LAM implementation, which are a position-domain monitoring concept and a range-based concept. In the positiondomain monitoring concept, which was suggested by MITRE [2], LAM computes a position solution applying WAAS corrections. If the difference of this position solution and the surveyed antenna location is below a certain threshold, the satellite set is declared to be usable for CAT I precision approach. On the other hand, the range-based LAM, which was proposed by Stanford [1], is more similar to conventional LAAS. The LAM ground station broadcasts the discrepancy between locally generated corrections and received WAAS corrections as a B-value. Then, a LAAS-equipped aircraft computes its own protection levels. Although both approaches are appropriate to implement LAM, this paper focuses on the range-based approach. Since an aircraft calculates its protection levels in the range-based LAM relying on the broadcast discrepancy statistic, characteristics of the discrepancy signal need to be investigated to assess system performance parameters including availability and continuity. This paper evaluates statistics of the discrepancy signal using field data provided by the Federal Aviation Administration Technical Center (FAATC) to analyze the performance of the range-based LAM. DISCREPANCY STATISTIC In its role monitoring the WAAS broadcast, the key LAM measurement is the discrepancy between the WAAS pseudorange correction, i W , and a locally derived pseudorange correction, i L . The LAM evaluates this discrepancy, labeled i δ in (1), for each of the N satellites in view. The discrepancy statistic is then incorporated into the error bound for the LAM.