GPS Integrity Architecture Opportunities

C.S. Miles, K. Kovach, J. Dobyne, K. Van Dyke, M. Weiss

Abstract:	The Global Positioning System (GPS) integrity architecture forms the basis for several GPS integrity augmentation systems, most notably, the Wide Area Augmentation System (WAAS), Receiver-based Autonomous Integrity Monitoring (RAIM) and its proposed successor the Advanced RAIM (ARAIM). This paper discusses several opportunities to increase the integrity of GPS. These opportunities are accompanied by the motivation to use the increased integrity to increase the performance of ARAIM and WAAS. Some of these opportunities already exist today, like taking credit for the GPS a priori failure rates determined by the Integrity Failure Modes and Effects Analysis (IFMEA). [citation] Some are part of the current GPS program, while others could be incorporated into the later stages of the GPS III and OCX programs. The current level of GPS constellation integrity performance is 1 e -4 per hour with the assumption that there are 10 satellites in view. This is an integrity performance of 1 e -5 per hour per satellite signal-in-space [3.5.1 GPS SPS PS]. The objective level of integrity specified in the GPS 800 series of specifications is 1 e -7 per hour per constellation or 1 e -8 per hour per satellite signal-in-space. Our last paper explored this GPS III Integrity Concept. [ION GNSS 2008] This objective level is equivalent to the level of integrity provided by WAAS or ARAIM. This level of core GPS integrity comes at a substantial cost in technical and programmatic difficulty and requires the full GPS constellation to be repopulated with high integrity satellites. This paper suggests that it may be beneficial to examine the viability of an incremental approach to improving GPS integrity in conjunction with corresponding improvements in ARAIM and WAAS performance. Three categories of GPS integrity architecture opportunities are identified. These opportunities are assessed for how WAAS and ARAIM can take advantage of them for both military and civilian users. First, those opportunities that exist today; they simply require engineering and verification to assess. Second, those that are part of the current GPS IIF, GPS III, and OCX programs; they require engineering and verification but will take time to realize the benefits. Third, a number of candidate GPS integrity architecture opportunities have been proposed as parts of the GPS III and OCX later stages. They were proposed as part of the effort to meet the 1 e -7 core GPS integrity requirement. This paper suggests that these opportunities be considered individually, in light of what effect they can have on ARAIM and WAAS for both civil and military users. Opportunities that exist today fall in two categories: 1) Take credit for the a priori failure rates in the IFMEA. WAAS assigns the full constellation failure rate of 1 e -4 to each GPS failure mode. Reducing this rate to the IFMEA derived rate would improve WAAS performance, but it would require analysis to substantiate the change in safety case and cost to make the change, so it is recommended that the potential performance improvement be assessed. ARAIM a priori failure rates have not yet been assigned for GPS, but it is recommended that the IFMEA derived rates be used. 2) Develop Interagency Forum for Operational Requirements (IFOR) requirements for ARAIM. IFOR requirements are derived from current GPS operations and form the basis assumptions that RAIM, GPS/Inertial, WAAS and GBAS depend upon for their safety cases. For example, RAIM depends upon IFOR-1, which states that “The URA shall be a conservative representation of the RMS of the URE.” ARAIM is concerned about common constellation wide failures like erroneous earth orientation parameters (EOP), so an IFOR requirement could be proposed to reduce the likelihood to near zero. Three improvements are planned within the GPS program that will improve the integrity of the service. 1) User Range Error (URE) performance will continue to improve. 2) Clock reliability is expected to continue to improve, and 3) ground-based input parameters are required to be validated by OCX. URE performance improvements are needed for ARAIM. Clock reliability improvements help both WAAS and ARAIM, and ground-based input parameter validation is one of the safeguards to reduce the likelihood of EOP faults. Five additional integrity architecture opportunities would provide additional integrity protection. 1) OCX command monitoring, 2) satellite-based clock monitoring and/or clock ensemble, 3) satellite-based signal monitoring, 4) satellite-based monitoring of satellite pointing and ephemeris errors, and 5) next generation atomic clock. Each of the five opportunities need to be analyzed as incremental improvements, to determine the most cost effective and technically viable set to implement.
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:	2592 - 2604
Cite this article:	Miles, C.S., Kovach, K., Dobyne, J., Van Dyke, K., Weiss, M., "GPS Integrity Architecture Opportunities," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2592-2604.
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