Performance of Advanced RAIM When an Earth Orientation Parameter Fault Affects the Satellites in View One at a Time

Y.C. Lee

Abstract:	RNAV instrument approaches with LPV lines of minima down to 200 ft (LPV-200) above the runway threshold allow airport access equivalent to a category I instrument landing system. Recently there has been much interest in providing a robust LPV-200 service worldwide through the 2020s relying on Receiver Autonomous Integrity Monitoring (RAIM). RAIM that is currently widely used world-wide to ensure integrity for en route through nonprecision approach (NPA) phases of flight is based on the assumption that only one satellite fault occurs at any one time. However, in the RAIM design for LPV-200 instrument approaches, the possible occurrence of simultaneous multiple satellite faults needs to be considered. None of the conventional RAIM methods, which assume one satellite fault at a time, can guarantee detection of such simultaneous multiple faults with high enough probability to meet the integrity requirement. A constellation-wide, consistent fault is one affecting all satellites in a constellation in a mutually consistent manner such that the fault is completely undetectable by RAIM, unless another independent constellation unaffected by the fault is used in the position solution. A possible cause of a constellation-wide, consistent fault is one that occurs in the generation of Earth Orientation Parameters (EOPs) and EOP Predictions (EOPPs). These faults are of particular concern as they have been identified as a potential integrity-failure mode in the current GPS Standard Positioning Service (SPS) Performance Standard [1]. EOPs are used to transform between ground-based coordinates (terrestrial reference frame) and “inertial” coordinates (celestial reference frame). Several different Advanced RAIM (ARAIM) methods have been published proposing ways to handle constellation-wide satellite faults such as an EOP fault. One of them is the Optimally Weighted Average Solution (OWAS) method [2], which, using two independent constellations, detects the presence of a fault or faults affecting only one of the two constellations at a time by comparing the position estimates from the two respective constellations. However, OWAS and other inter-constellation comparison methods of similar kind lead to a very significant degradation of RAIM availability for LPV-200. A more recent one is the Modified Chi-Square method [3], which is a special purpose ARAIM that can detect the presence of an EOP fault affecting one of the two constellations with minimal degradation of RAIM availability for LPV-200. Another method, called Airborne Detection Monitor, proposed in [4] estimates the earth rotation angle by computing the satellite earth-centered-earth-fixed (ECEF) position vectors using a set of currently broadcast ephemeris data and the latest ground-validated set loaded onto aircraft prior to takeoff. In the Modified Chi-Square method and the Airborne Detection Monitor, it is assumed that that the EOP fault affects the whole constellation (or one of the two constellations if two constellations are used). However, the GPS ephemeris data, including the EOPs and EOPPs, are currently uploaded to the satellites one satellite at a time. Until recently it was not known how often EOP faults occur and how large a position error they may cause. An EOP fault occurred in June, 2012, causing a satellite position error on the order of a few kilometers. Fortunately it affected only one satellite before it was detected and there was no compromise of safety. However, if the faulty ephemeris data had been uploaded to all the satellites, a user position error would have been as large as hundreds of meters in the East-West direction. The GPS ephemeris data is currently uploaded to the satellites, one at a time at approximately 45-min intervals. This paper shows how an EOP fault affects the ARAIM performance when the ephemeris data is uploaded one at a time with some minimum period of time between the uploads. The purpose of this paper is to evaluate how an ordinary ARAIM algorithm assuming a single fault at a time would perform in the presence of the EOP fault when the ephemeris data is uploaded to the satellites one at a time. The analysis determines under what conditions the EOP fault can be detected with a probability that meets the probability of Hazardous Misleading Information (HMI). The performance depends on a few important factors: minimum time interval between updates, the maximum delay (delay until the fault is detected and corrected by some means other than RAIM such as GPS Master Control Station), rate at which the fault-induced range bias error increases, and the sequence with which the ephemeris data is uploaded to the satellites in view of the user. The analysis develops criteria to determine under what conditions in terms of satellite update intervals, maximum delays, and number of satellites in view, EOP faults can be detected by ARAIM before integrity is compromised. The criteria, in turn, are used to evaluate the availability of integrity in the presence of EOP fault for different constellation sizes. References [1] GPS Standard Positioning Service Performance Standard, U.S. Department of Defense, 4th edition, September 2008. [2] Lee, Y., “Two New RAIM Methods Based on the Optimally Weighted Average Solution (OWAS) Concept,” NAVIGATION, Journal of the Institute of Navigation, Vol. 54, No. 4, Winter 2007. [3] Lee, Y., “New Advanced ARAIM with Improved Availability for Detecting Constellation-wide Faults, Using two Independent Constellations,” ION International Technical Meeting, January 30 to February 1, 2012, Newport Beach, California [4] B. Pervan, et al., “EOP Threat Model and Airborne Detection Concept,” Briefing Presented at the EU-US Cooperative WG-C ARAIM Subgroup Meeting, Washington D.C., June 25, 2011. The contents of this material reflect the views of the author. Neither the Federal Aviation Administration nor the Department of Transportation makes any warranty or guarantee, or promise, expressed or implied, concerning the content or accuracy of the views expressed herein. Approved for Public Release: 13-1263. Distribution Unlimited. © 2013 The MITRE Corporation. All rights reserved.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2690 - 2701
Cite this article:	Lee, Y.C., "Performance of Advanced RAIM When an Earth Orientation Parameter Fault Affects the Satellites in View One at a Time," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2690-2701.
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