GPS World - January 2008 - (Page 67)

Avionics & Transportation | INNOVATION 100 99 Percent of CONUS scription of the undersampled threat model along with its values is given in a research paper authored by JPL’s Lawrence Sparks and colleagues (see Further Reading). 98 97 96 95 94 93 92 91 90 7/22/07 Stormy (Space) Weather In October and November 2003, a series of intense ionospheric storms adversely affected single-frequency GPS users all over the world. WAAS was operational during these storms and, as expected, the ionospheric storm detector disabled LPV operations throughout CONUS. A thorough analysis of the storm concluded that these conservative measures were justified and WAAS users were protected from the effects of the storm. (See the paper by Habereder et al. listed in Further Reading for more details). The WAAS integrity requirement must be met in all regions where a user can perform an LPV approach. Along the edge of coverage, it is possible for ionospheric effects to trickle in that have not been well sampled by the reference stations. During extreme storms, these effects can be significantly worse than previously observed. To address this concern, WAAS has added an “extreme storm detector.” The extreme storm detector will detect extreme ionospheric storms and disable availability everywhere in the WAAS service region. The detector has been carefully tuned to only trip during significant ionospheric disturbances like those in October and November 2003 and July 2000. Such events are rare and do not seriously affect the overall availability and continuity of the system. Current Performance and Future The recent enhancements significantly improve WAAS availability everywhere in North America. A summary of the system performance is given in FIGURE 6 and in Table 2. The map graphics show a 99% bound on the VPL. A red color at a particular location indicates that the VPL is less than or equal to 50 meters during 99% of the day. Orange means that the VPL is less than 35 meters 99% of the day, which indicates that an LPV200 approach is available 99% of the day. The second graph shows the percent FURTHER READING 7/27/07 8/1/07 8/6/07 Date 8/11/07 8/16/07 8/21/07 p FIGURE 7 Thirty-one days of shadow test results showing the percent of CONUS achieving 100% LPV availability of the CONUS and Alaska region, which achieves various VPLs during 95, 99, and 100% of the day. Although the results indicated show the performance for a particular day (August 22, 2007 — measured from the shadow test system running Release 6/7) , extensive testing over the past several months has shown that WAAS will have outstanding LPV availability over all of CONUS and Alaska. An analysis of 31 continuous days (July 22 to August 22, 2007) of availability data collected from the shadow system showed that more than 98% of CONUS achieved 100% LPV availability each day (see FIGURE 7). Further availability improvements are planned for a release towards the last quarter of 2008. These enhancements include implementation of the enhanced signal quality monitor, expanded ionospheric grid-point coverage in the Alaska region, and several other software improvements that will further improve coverage in Mexico. Analysis to date shows WAAS will meet all of its performance goals (listed in Tables 1 and 2) with margin by the end of 2008 offering LPV service to most of North America and LPV200 service to most of CONUS. TIMOTHY SCHEMPP is an engineering fellow with Raytheon Company in Fullerton, California. He holds a master’s degree in applied mathematics from California State University, Fullerton, and is currently the technical director for the Wide Area Augmentation System project. Development of WAAS “The NSTB: A Stepping Stone to WAAS” by Andrew Hansen in GPS World, Vol. 9, No. 6, June 1998, pp. 73–77. Institute of Navigation, Salt Lake City, Utah, September 19–22, 2000, pp. 1180–1190. Solar Effects on GPS “GPS, the Ionosphere, and the Solar Maximum” by R.B. Langley in GPS World, Vol. 11, No. 7, July 2000, pp. 44–49. WAAS Standards Minimum Operational Performance Standards for Global Positioning/ Wide Area Augmentation System Airborne Equipment, RTCA/DO-229D, prepared by SC-159, RTCA Inc., Washington, D.C., December 13, 2006. Ionospheric Threat Model “Extreme Ionospheric Storms and Their Impact on WAAS” by L. Sparks, A. Komjathy, and A.J. Mannucci in Proceedings of IES2005, the 11th International Ionospheric Effects Symposium, Alexandria, Virginia, May 3–5, 2005, paper A105. FAA WAAS Monitoring Web Site William J. Hughes Technical Center WAAS Test Team: www.nstb. tc.faa.gov GPS Signal Waveform Distortion “Impact of Evil Waveforms on GBAS Performance” by C. Macabiau and E. Chatre in Proceedings of PLANS 2000, IEEE Position, Location and Navigation Symposium, San Diego, California, March 13–16, 2000, pp. 22–29. “Robust Signal Quality Monitoring and Detection of Evil Waveforms” by R.E. Phelts, D.M. Akos, and P. Enge in Proceedings of ION GPS-2000, the 13th International Technical Meeting of the Satellite Division of The www.gpsworld.com WAAS Performance During Ionospheric Storms “Current WAAS Performance and Expected Full Operational Capability Performance,” by H. Habereder, T. Schempp, and M. Bailey in Proceedings of European Navigation Conference GNSS 2004, Rotterdam, May 16–19, 2004. GPS Aircraft Navigation “Aircraft Landings: The GPS Approach” by G. Dewar in GPS World, Vol. 10, No. 6, June 1999, pp. 68–74. January 2008 | GPS World 67 http://www.nstb.tc.faa.gov http://www.nstb.tc.faa.gov http://www.gpsworld.com

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GPS - January 2008 Contents Out in Front Expert Advice The Money-Go-Round u-Nav Latest Acquisition Apples to Apples Global SBAS 2008 GPS Receiver Survey Advertisers Index & Company Directory The Manufacturer's Road Year of the Who Working Indoor Up and Down Good, Better, Best Marketplace Classifieds Seen + Heard

GPS World - January 2008

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