GPS World - January 2008 - (Page 64)

INNOVATION | Avionics & Transportation TABLE 1. (a) Lateral Navigation (LNAV) and (b) Localizer Performance with Vertical Guidance (LPV) requirements (a) LNAV Performance Availability HAL VAL Horiz. 95% accuracy Probability of HMI IOC Req. 99.9% 556 m N/A 100 m Full LPV Req. 99.99% 556 m N/A 36 m 10-7 per hour 10-7 per hour 10 s Over 100% of CONUS Full LPV Req. 99% 40 m 50 m Vert . 2.0 m Horiz. 1.5 m 10-7 per approach 6.2 s 100% + of CONUS Time to alarm 10 s Coverage (b) LPV Performance Availability HAL VAL 95% Accuracy Probability of HMI Over 100% of CONUS IOC Req. 95% 556 m 50 m Vert. 7.6 m Horiz. 7.6 m 10-7 per approach Time to alarm 6.2 s Coverage 75% + of CONUS TABLE 2. WAAS full Localizer Performance with Vertical Guidance (LPV) Measured Requirement Performance LPV CONUS 99% availability LPV Alaska 95% availability 100% of CONUS 75% of Alaska 100% of CONUS 88% of Alaska LNAV CONUS 99.99% avail- 100% of ability CONUS Vertical accuracy Horizontal accuracy Time to alarm 2.0 m 95% bound 1.5 m 95% bound 6.2 s 100% of CONUS 0.95 m 95% bound 0.60 m 95% bound 6.2 s 0 Probability of HMI 10-7 horizontal and vertical uncertainty needed to perform an approach. The tables also state the requirements on the probability of WAAS providing Hazardously Misleading Information (HMI). WAAS must meet its integrity requirements anywhere a receiver can perform a WAAS-user position solution. Integrity is 64 GPS World | January 2008 ensured by users computing Horizontal and Vertical Protection Limits (HPL and VPL). The HPL and VPL are overbounded estimates of the potential errors in positions computed using WAAS. To ensure integrity, the HPL and VPL must bound user position errors with a probability confidence level of 99.99999%. HPL and VPL are computed by using the overbounding variances of the residual errors, including the User Differential Range Error (UDRE) and the Grid Ionospheric Vertical Error (GIVE), after applying WAAS corrections. Users take and adjust these variances with their local position and combine them with the receiver bounding variances to compute the VPL and the HPL. The HPL and VPL have to be below set (HAL and VAL) thresholds to perform a particular type of WAAS-assisted approach. This includes operations in regions outside U.S. national airspace. The LPV service volume is naturally limited by the coverage of the ionospheric corrections (because LPV users must apply these corrections). The WAAS integrity requirements are met everywhere within the GEO footprints. New Satellites. In July 2007, two GEO satellites replaced the legacy IOC Inmarsat Atlantic Ocean Region West (AOR-W) and Pacific Ocean Region (POR) GEO satellites. Pseudorandom noise code (PRN) 135 is assigned to the Galaxy 15 satellite (FAA designator Central Region West or CRW) operated by Intelsat and positioned at 133°W. PRN 138 is assigned to the Anik F1R satellite (FAA designator Central Region East or CRE) operated by Telesat (formerly Telesat Canada) and positioned at 107°W. The new satellites will provide superior ranging capabilities compared to the IOC GEO satellites and ensure that all WAAS users have dual GEO coverage (a significant reliability improvement). FIGURE 2 shows the footprints of the two GEO satellites. New Stations. In June 2006, an additional Master Station was integrated into the system. The additional Master Station ensures that WAAS will always have at least two Master Stations operational even when one Master Station is down for routine maintenance or upgrades. 133°W 107°W p FIGURE 2 WAAS GEO satellite footprints Legacy WRS L Full F LPV WRS p FIGURE 3 Legacy and 13 additional WAAS reference stations Added IGP Legacy IGP Deleted IGP p FIGURE 4 Updated ionospheric grid-point locations All reference stations in the system have been upgraded to use a new GPS receiver that provides detailed information about GPS signal quality for use in an enhanced signal-quality monitoring algorithm. Thirteen additional reference stations (see FIGURE 3) have been installed to improve availability in CONUS, Alaska, Mexico, and Canada. The four Alaskan stations were cut over into the system in August 2006 and the Mexican and Canadian stations in September 2007. To take advantage of the new reference stations, the ionospheric grid mask has been expanded to cover all of North America (see FIGURE 4). FIGURE 5 shows the availability improvement realized when the Alaskan reference stations were integrated into the system. The performance in Alaska is described by two plots. The first (left) graph shows a 95% bound on the VPL. A red color at a www.gpsworld.com 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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