Range-Domain Fault Monitoring for Terrestrial Ranging Systems

Gianluca Zampieri, Brandon Weaver, Okuary Osechas, Gerhard Berz, Michael Meurer

Abstract:	Introduction & Motivation: In this paper we propose a set of range-domain checks for DME fault monitoring. The proposed methods take advantage from previous measurements in order to build suitable statistical self-consistency tests. The tests are applied in batch to measurements coming from each visible DME station. The detection of ranging faults in radio-navigation system is one of the crucial features in order to enable reliable Safety-of-Life (SoL) services, which are mostly required in aviation operations. For NavAids-based operations providing integrity is still an open challenge as witnessed by the activities carried out in SESAR WP10 APNT and EUROCAE WG-107. Unlike GNSS, where integrity is hard to monitor at satellite level, for NavAids (like DME) it is more practical to implement integrity checks on the ground. Executive monitoring is implemented directly at the transponder, on the ground, and allows to directly shut-down a ground station if a fault is detected in the transmission stage. This level of monitoring is useful in preventing signal-in-space (SiS) faults, but it does not prevent ranging errors that stem from propagation phenomena. Among the possible error sources affecting the signal propagation, that of multipath is surely one of the biggest challenges for aviation SoL applications (Schneckenburger, 2018). Previous work suggests several approaches in order to provide integrity services with DME. Some of them requires IRS to be available on-board (Crespillo, 2017) while others are enabled by system modifications for ground and/or aircraft (i.e. eDME)(Li, 2013). Despite, the promising performance achieved by these works they require significant hardware modification or the presence of specific sensors on-board which might not be te case for all aircrafts. The development of range-domain monitor is a crucial step in the development of navigation services with integrity, based on NavAids. In particular the detection of multipath-related faults is of interest. Moreover, the implementation details must be accommodated in the overall integrity budget. In light of these facts we propose two range self-consistency checks, both of which are simple enough to be implemented in any FMS. Methodology: The proposed methods are aimed to the identification of step and ramp types of fault characteristics of multipath propagation. It is assumed that faults will bias the measurements while the variance will remain unchanged. Under nominal behavior DME range errors are normal distributed with zero mean and standard deviation 0.05 NM (one sigma) (ICAO, 2008). The large variability of the range measurements, compared to the relatively smooth airplane motion, impose some constrains in the definition of suitable test statistics. Consequently, performing a test directly using previous samples versus the new measure might not be a consistent check due to large variability of the data. Range Domain Tests: We propose using a smoothed range prediction as a refence to be tested against the new measurement. The idea behind is to remove the noise component from the sequence of previous measurements in order to reduce the variability on the reference term. One is a snapshot method, while the other is based on a batch of measurements. Snapshot Test: Generate a prediction exploiting the previous ranges (smoothing). Then the test is defined as the normalize difference between prediction minus measurements. The test follows a normal distribution with unitary variance. Under null hypothesis (H0, no fault) the test has zero mean, while in case of faulty measurement (H1 hypothesis) the test is biased. An appropriate threshold will then define the two decision regions. Sequential Test: For a sequential test, we use the sum of individual normalized squared difference between prediction minus measurements over a time window. Since the residuals are Gaussian distributed the test will follow a Chi-square distribution with degree of freedom equal to the size of the time window N. Under null hypothesis (H0, no fault) the test follows a central Chi-square, while in case of faulty measurement (H1 hypothesis) the test follows a non-central Chi-square. An appropriate threshold will then define the two decision regions. Analysis: In this paper we are going to consider two kind of faults, steps and rumps. Different values for the amplitude and the duration of the faults are will be presented. In the analysis we will consider the nominal DME accuracy as well as the measured accuracy reported by (Lo, 2013; Harris, 2012). We will also present an analysis using different smoothing techniques with different smoothing parameters (e.g. weights, smoothing time, etc..). Preliminary Results and Conclusions: The preliminary results using range domain tests and simulated data show the ability of the snapshot method to detect anomalous steps in the measurements (in this simulation we injected a step fault with bias equal to 1000 [m]). Considering a probability of false alarm (Pfa) of 10^-5 and a probability of miss detection (Pmd) of 10^-5 the simulation suggests a minimum detectable bias of 810 [m]. The sequential test is currently under evaluation. We expect the results to confirm the ability of test in detecting ramp-type faults. The proposed monitors represent fundamental building blocks in the development of integrity operations based on NavAids. The ability of detect steps and ramps is particularly useful for the detection of multipath. In addition, the implementation details will be presented and can be easily integrated in an integrity budget. References: Berz, G. (2013). Can Current DME Support PBN Operations with Integrity? In Proceedings of the 26th International Technical Meeting of the ION Satellite Division, ION GNSS+. Crespillo, O. G. (2017). Detection of DME ranging faults with INS coupling. Integrated Communications, Navigation and Surveillance Conference (ICNS). Harris, M. (2012). Performance of Current Distance Measuring Equipment and Implications on Alternative Position Navigation and Timing for Aviation,. Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN. ICAO. (2008). Performance-Based Navigation (PBN) Manual ICAO DOC 9613. Li, K. (2013). Robust DME Carrier Phase Tracking under Flight Dynamics. Proceedings of the 2013 International Technical Meeting of The Institute of Navigation. Lo, S. (2013). Distance Measuring Equipment Accuracy Performance Today and for Future Alternative Position Navigation and Timing (APNT). Nashville, TN: Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013). Schneckenburger, N. (2018). Characterization and Mitigation of Multipath for Terrestrial based Aviation Radionavigation. J Inst Navig.
Published in:	Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021) September 20 - 24, 2021 Union Station Hotel St. Louis, Missouri
Pages:	2242 - 2252
Cite this article:	Zampieri, Gianluca, Weaver, Brandon, Osechas, Okuary, Berz, Gerhard, Meurer, Michael, "Range-Domain Fault Monitoring for Terrestrial Ranging Systems," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 2242-2252. https://doi.org/10.33012/2021.18133
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