WAAS Availability Over the Solar Maximum

T. Schempp, B. Stimmler, Raytheon

Abstract:	WAAS is a critical system upon which the FAA is developing advanced navigation and surveillance systems under their Next Generation Air Transportation System (NextGen) initiative. The system is performing at or above the safety of life performance level, and has been continuously available for nearly a decade. WAAS augments the GPS standard positioning service by providing a positioning, navigation and timing service that permits pilots to safely rely on satellite navigation for even the most advanced operations. The system provides robust position, navigation and timing suitable for use in all phases of flight, from en-route through non-precision approach (LNAV), Lateral NAVigation / Vertical NAVigation (LNAV/VNAV), and Localizer Performance with Vertical guidance (LPV) approach capabilities everywhere in North America. Although not designated as a Category 1 approach, the LPV minimums are equivalent to Category 1 for all operators, allowing for a 200 foot decision height. At present (February 2013) there are 3,055 LPV approaches at 1,533 airports in the United States, with more approaches being created every month. Unlike other LNAV/VNAV approaches, the WAAS based approaches are not limited or suspended when cold weather impacts barometric based VNAV systems. Airports can affordably upgrade to ILS like approach capabilities, because WAAS provides a precision approach landing capability without requiring additional navigation infrastructure on the ground. Expansion of LPV approaches within the reference station footprint is a simple matter of constructing new approaches. No additional infrastructure (other than meeting the airport design standards, obstacle clearance and lighting requirements) is required. The WAAS system also serves a foundational role in the transition to dependant surveillances as well. The FAA’s mandate for Automatic Dependant Surveillance-Broadcast (ADS-B) requires aircraft operators to equip with high-precision PNT capabilities too. The more precise the PNT, the more benefits available from ADS-B. While unaugmented GPS solutions may be able to meet the positioning requirements, the WAAS system is generally viewed as the most affordable equipage option, and is not dependant on a GPS constellation beyond the minimum constellation assured by the Department of Defense. Non-aviation applications are just as prevalent. Most commercially available GPS receivers bought today are WAAS enabled, allowing everyone from hikers and bikers to surveyors, farmers and rescue workers to realize the benefits of the system. Over the last four years WAAS has gone through several significant upgrades including the integration of a third GEO Satellite, enhancements to the satellite orbit and clock estimation algorithms and fielding a new ionospheric correction algorithm. Together with these performance enhancements WAAS achieves 100% availability over most of CONUS and Alaska. The purpose of this paper is to show the WAAS LPV availability for all CONUS and Alaska airports with defined LPV approaches, and to examine, analyze and characterize the types of events that affect availability of service. A detailed methodology for characterizing the performance of WAAS will be presented. Two years of data analyzed month by month will show how ionosphere activity, weakness in the GPS constellation, switching uplink stations and other specific WAAS events affect the availability and continuity at various airports. The paper includes a discussion of various upgrades that have been implemented in the WAAS system over the years, specifically focusing on the optimized Kriging parameters and storm detector thresholds that were implemented to maximize availability during solar maximum periods. Analysis of the data will show enhanced performance during ionospheric storms resulted in a drastic improvement to availability at CONUS and Alaska airports. Today, SBAS systems utilize dual frequency L1-CA/L2 semi-codeless measurements to provide precise corrections and integrity bounds to aircraft for use in both enroute and precision approach operations. In contrast to the ground system, the aircraft are limited to the sole use of L1-CA measurements, since the L2 frequency does not lie in a protected Aeronautical Radio Navigation Service (ARNS) frequency band. The single biggest source of error in an SBAS position solution, and the limiting factor in system performance, is the ionospheric corrections. Using two frequencies the ground system is capable of creating corrections for ionospheric delays, although the uncertainty bounds associated with these corrections must be inflated to account for the unpredictable effects of ionospheric storms. During ionospheric storms the ionospheric delay may change rapidly over a short distance; WAAS responds to such storms by inflating the GIVE to a very large value which is not useful for an LPV approach. This paper will quantify the effect of ionospheric storms on WAAS over the last two years of solar cycle 24. In addition to providing corrections for GPS satellites, WAAS provides orbit and clock information for the GEO satellite signal. User equipment is able to utilize GEO satellites just like GPS satellites, lowering the DOP especially during GPS satellite outages. GEO ranging is more challenging than GPS ranging for several reasons. Since the satellite location appears stationary from earth (minimal orbital movement based on inclination and eccentricity means the GEO is ‘nearly fixed’ in space), it is difficult to separate orbit error from clock error without a network of reference stations that provide observability for the GEO satellite on all perspectives. Further, some multipath interference may vary very slowly with time given the limited geometry change in the station/GEO geometry, thus increasing the likelihood of ‘standing’ multipath which degrades performance. The WAAS reference stations currently only utilize L1 GEO measurements, which are affected by ionospheric correction errors. The uncertainty introduced by these factors limit the minimum broadcast error bounds. For GEO satellites, the minimum value of the GEO satellite correction error bound is 2.28 meters. For GPS satellites which are not constrained by these shortcomings a lower bound of 0.91 meters is achieved. WAAS performance could be improved by decreasing these lower bounds in the future. The benefits of GEO ranging are quantified in this study in two ways. The two years of data are analyzed without GEO Satellite ranging to quantify how GPS satellite maintenance and Dilution of Precision (DOP) holes affect system availability without GEO ranging. The analysis is then repeated with a lower GEO UDRE floor to quantify the benefits of further improving the GEO satellite ranging algorithms.
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:	902 - 911
Cite this article:	Schempp, T., Stimmler, B., Raytheon,, "WAAS Availability Over the Solar Maximum," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 902-911.
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