Wide Area Augmentation System Performance Margin Characterization

Nathan Vary, Patricia Morrison, Emily McCord, Karl Shallberg, Swen Ericson, Eric Altshuler, Zachary Allen, Lucas Laske, Luke Bucklew

Abstract: The Wide Area Augmentation System (WAAS) will soon reach its 20-year milestone for providing aircraft with navigation services over the entirety of North America. The services WAAS provides to an aircraft are categorized as Non-Precision Approach (NPA) and Approach with Vertical guidance (APV). The user can determine what level of WAAS service is available at the user location by comparing the computed horizontal and vertical protection levels (HPL, VPL) against the horizontal and vertical alert limits (HAL, VAL) defined for each class of service. Based on the alert limits, APV services are further delineated as LNAV/VNAV, LPV and LPV200. For this paper, the services of most interest are LPV and LPV200 where LPV requires the HPL to be within a HAL of 40 m and the VPL to be within a VAL of 50 m, and LPV200 specifies the same horizontal limit but has a lower VAL of 35 m, which is consistent with aircraft precision approach operations. It is well known from monitoring WAAS performance over its service life that positioning accuracy typically is far better than the computed protection levels would suggest, with the protection levels many times greater than the actual position errors. This margin between the position errors and protection limits stems from the stringent safety-of-life integrity requirements where WAAS ensures that protection levels computed by the user bound the user position error at a probability of 99.99999% per approach operation, equivalent to underbounding only 1 in 10 million approach operations. To achieve such high confidence in the protection levels, the various system threats such as ionospheric disturbances, ground system equipment biases, satellite failures, etc., are accounted for in a conservative way that assures even unobserved threats are mitigated. In this way, the WAAS safety-of-life integrity requirement is always satisfied. However, should a user have non-safety-of-life applications or the ability to mitigate some of these threats on their own, the performance margin normally present in WAAS could be exploited in favor of higher availability of service for the given application or operation. The purpose of this paper is to rigorously characterize this performance margin and qualitatively describe the threats that cause the protection levels to be so inflated. The approach taken for demonstrating WAAS performance margin consists of identifying days of interest from past service history that stressed WAAS performance and developing an analysis capability to investigate every possible position solution from the network of WAAS reference stations. The stressed days are expected to show significantly higher position errors than usual and are primarily associated with ionospheric events, which is understandable given the challenges of estimating ionospheric delays and confidence bounds for a grid of points covering the WAAS service area. Some days with interesting ionospheric behavior were provided from monitoring and analysis conducted by WAAS 2nd level engineering at the Mike Monroney Aeronautical Center in Oklahoma City. Other stressed days were taken from daily WAAS and GPS performance monitoring conducted at the FAA William J. Hughes Technical Center (WJHTC). In addition to the 50-day stressed data set identified, a 30-day nominal data set was defined to contrast its level of performance. Therefore, in all, 80 days of data were used to conduct this WAAS performance characterization. The analysis capability was developed to evaluate every possible position solution with its associated protection level against LPV and LPV200 alert limits for all days of interest using 1 Hz data from all 38 WAAS reference stations. Note that Honolulu was not evaluated due to lack of LPV/LPV200 Coverage/Service. Also note the latitude of Tapachula is less than 15 degeres, and as such, never has monitored ionosphere and will never have a solution with GEO satellites in them Protection levels were computed in the same manner as a user with User Differential Range Error (UDRE) bounds for satellite clock and ephemeris and Grid Ionosphere Vertical Error (GIVE) bounds for ionosphere. The UDREs, GIVEs, range measurements, weighting and geometry matrices were output from the WAAS prototype for satellite subset analysis. This subset analysis computed and saved position errors and normalized errors by their protection levels for every satellite geometry meeting LPV and LPV200 HAL/VAL. For this analysis using the 80 days of data, this equates to approximately 1.9 trillion position solutions being evaluated. The results from this position domain analysis, along with a related range domain analysis, were quantified using various statistical measures as well as a Gaussian overbound methodology. The intent of this work is to develop a new WAAS performance model that users can potentially utilize for non-safety of life applications or where a user augments their solution with additional monitoring capabilities. The paper will also provide a qualitative review of threats the ground system considers in its integrity analysis to provide some insight into the monitoring a user needs to consider when leveraging a new WAAS performance model.
Published in: Proceedings of the 35th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2022)
September 19 - 23, 2022
Hyatt Regency Denver
Denver, Colorado
Pages: 312 - 333
Cite this article: Vary, Nathan, Morrison, Patricia, McCord, Emily, Shallberg, Karl, Ericson, Swen, Altshuler, Eric, Allen, Zachary, Laske, Lucas, Bucklew, Luke, "Wide Area Augmentation System Performance Margin Characterization," Proceedings of the 35th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2022), Denver, Colorado, September 2022, pp. 312-333.
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