Global Navigation Satellite Systems Report of a Joint Workshop of the National Academy of Engineering and the Chinese Academy of Engineering (2012) / Chapter Skim
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Integrity Lessons from the WAAS Integrity Performance Panel--Todd Walter, Per Enge, and Bruce DeCleene
Integrity Lessons from the WAAS Integrity Performance Panel--Todd Walter, Per Enge, and Bruce DeCleene
Pages 207-228
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From page 207... ...
In particular, the integrity requirement, that the confidence bound fails to contain the true error in fewer than 1 in 10 million approaches, must apply to all users under all foresee 207
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WAAS uti lizes a network of precisely surveyed reference receivers located throughout the United States. The information gathered from these WAAS Reference Stations (WRSs)
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From page 209... ...
Thus, HMI arises if the user has been told that the error in position is small enough to support the operation, but in fact, it is not. The threat models must describe all known conditions that could cause the true errors to exceed the predicted confidence bounds.
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Unfortunately, true error sources usually have multiple dependencies, and these dependencies are different between the various error sources. Thus, the ensembles may need to be formed over narrow ranges of numerous parameters.
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From page 211... ...
Components will largely be divided into noise-like contributions, with some spread in their values, and bias-like contribu tions whose values are seen as fixed although unknown. Although many of these error sources may be deterministic, practically they may need to be described in stochastic terms.
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Each error source must be carefully analyzed, both individually and in relation to the other sources. Only then can the appropriate confidence bounds be determined.
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Thus, just by looking at the graph it is difficult to determine the actual distribution. The central-limit-theorem causes error distributions to approach gaussian as several independent sources are combined.
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Examples include tropospheric data from many different climates, ionospheric data from the worst times in the 11-year solar cycle, multipath data from the most clut tered environments, etc. Rare events are unlikely to be captured in small data sets.
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Together, the various threat models must be comprehensive in describing all reasonable conditions under which the system might have difficulty protecting the user. Ultimately they form a major part of the basis for determining if the system design meets its integrity requirement.
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, a term designed to describe residual satellite errors, must be sufficient to overbound the residual errors in the corrected satellite clock and ephemeris. Signal Deformations The International Civil Aviation Organization (ICAO)
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, a term designed to describe residual ionospheric errors, must account for inadequacies of the assumed iono spheric model, restrictions of the grid, and limitations of observability. The GIVE must be sufficient to protect against the worst possible ionospheric disturbance 1 WAAS Minimum Operational Performance Specification (MOPS)
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From page 218... ...
terms must protect against any changes in the ionosphere that can occur over that time scale.2 Because the physics of the ionosphere are incompletely understood, the most practical ionospheric threat models are heavily data driven and contain a large amount of conservatism. Tropospheric Errors Tropospheric errors are typically small compared to ionospheric errors or satellite faults.
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From page 219... ...
In one case, several half cycle slips were observed in the same direction each several minutes apart resulting in a several meter error. Cycle slip detection must be able to reliably catch unfortunate combinations of L1 and L2 half and full integer cycle slips in order to achieve an unbiased result.
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The UDREs and GIVEs must bound the uncertainty that may result from such estimation errors. Receiver Clock Estimate Errors Similarly, the satellite correction algorithm must estimate and remove the time offsets between the reference station receivers.
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From page 221... ...
Table 1 also shows the PRN, elevation, azimuth, and TABLE 1 Satellite Elevation and Azimuth Angles, Confidence Bounds, and Projection Matrix Values Both for the All-In-View Solution and Without PRN 8 si PRN s3i without PRN 8 EL AZ s3i 2 45.8° –32.3° 2.34 m 0.595 0.451 5 11.2° –76.8° 10.10 m 0.258 0.437 6 36.6° 48.4° 2.32 m 0.162 2.005 8 9.98° 73.0° 3.74 m 1.000 -- 9 61.4° 28.5° 2.03 m –1.928 –3.087 15 32.8° 151.0° 6.89 m –0.015 0.174 21 42.3° –136.0° 4.83 m 0.066 –0.003 122 40.6° 120.1° 6.19 m –0.139 0.022 FIGURE 2 Satellite elevation and azimuth values for a standard skyplot. PRN 8 is a low Walter-etal_Fig2.eps elevation satellite that if not included in the solution dramatically changes the influence of PRN 6.
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From page 222... ...
More surprisingly, the influences of PRNs 15, 21, and 122 change sign; therefore, what was a positive error for the all-in-view solution becomes a negative error for this particular subset. The changes in the s3i values with subset or superset position solutions limit the ability to verify performance exclusively in the position domain.
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. The broadcast WAAS confidence bounds are approximately three times larger than the nominal no-fault values (this inflation is necessary to protect against fault modes)
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The most effective method is to combine position domain monitoring with range domain monitoring. SMALL NUMBERS AND INTUITION The integrity requirement of 10–7 is an incredibly small number.
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Chief among these was the use of threat models. Threat models define our fault modes, how they manifest themselves, and how likely they are to occur.
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From page 226... ...
Pp. 2481–2498 in Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007)
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From page 227... ...
Pp. 3115–3122 in Proceedings of the 23rd International Technical Meeting of the Satellite Division of the Institute of Naviga tion (ION GNSS 2010)
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From page 228... ...
Pp. 2042–2050 in Proceedings of the 23rd International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GNSS 2010)
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