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WAAS/GPS

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Required — or SAAR — approval to fly RNP procedures . • LPV: Lateral precision with vertical guidance is the highest level of WAAS approach not requiring special aircrew training or certification . LPV provides the greatest precision outside the RNP environment — greater than the LNAV or LNAV/VNAV approaches . Essentially, the WAAS/GPS provides for precise lateral and vertical guidance with the WAAS navigator showing the aircraft’s position in reference to the published approach stored in its memory . With the WAAS/GPS tracking and comparing the aircraft to the defined centerline and glide path, the pilot sees indications akin to the VHF indicator on an ILS approach: the course deviation indicator shows offset from the runway centerline and a glide-path position indicator shows the plane’s position relative to the pre-programmed-but-imaginary glideslope leading to a touchdown point beyond the runway threshold . Nearly 2,100 LPV approaches existed as of April 2010 . More than half of them were at airports lacking an ILS; many others went to ILS-served airports, but for runway ends not so equipped . LPV approaches also are the most common type of approach now, exceeding even CAT I ILS approaches and the approach most general aviation pilots are apt to employ with their new WAAS navigators . • LNAV: This sort of approach also can provide curved guidance, but the vertical guidance is dependent on actual barometrically corrected local altimeter settings for flying the correct altitude at designated points on the approach . The FAA has commissioned about 4,600 LNAV approaches as of April 2010 . • LNAV/VNAV: An LNAV approach with the vertical descent path defined by a GPS-generated glide path displays a continuous target altitude for the path and leads to a DH comparable to the non-precision GPS approach . A certified altimeter input for the GPS must be part of the package . About 2,000 LNAV/VNAV approaches existed as of April 2010 . • GPS approach: This form of non-precision approach was first deployed in the early 90s, when the first approach-capable navigators began appearing in aircraft cockpits . Most of these approaches were drawn as overlays over existing non-precision approaches — NDB, DME arc and VOR approaches . The FAA created others as standalones, using arrival procedures more akin to an ILS or LOC approach, but solely dependent on local altimeter settings and an accurate altimeter . GPS approaches generally ended at a lower DH than any existing NPA . As more WAAS approaches have come into existence, the number of GPS approaches supported has declined . The FAA expects to continue to shrink the number of GPS approaches from approximately 540 in April 2010 .

WAAS Today: A Slightly Wounded System On April 3, 2010, Intelsat, the FAA contractor controlling the WAAS satellites, lost control of the Pacific WAAS satellite and the bird began drifting out of orbit and out of use . That’s one of two WAAS birds the system was designed to use for maximum service and coverage . But there’s no need to panic . According to the FAA and the Department of Transportation, a fully reliable WAAS/ GPS remains available, working and accurate over about 95 percent of the North American coverage area with northwest Alaska the lone exception . The agencies also cautioned three to five periods of unusable WAAS signals could occur during the year as the remaining satellite periodically shifts between ground links . But these periods are detectable and should be short in duration — about five minutes — according to the agency . The FAA was working a fix for this problem, focusing primarily on accelerating testing and activation of a replacement satellite already in orbit . Another WAAS bird is due for launch later this year, but getting it into service will take about 18 months . The FAA expects to restore full WAAS coverage soon . In the meantime, the system is operating under a single-point failure mode, with only the one WAAS satellite supporting the service . Thus, a renewed debate about the wisdom of shutting down Loran C stirred — actually, a debate about the need for an independent alternative system capable of picking up the jobs of satellites should they become unusable for any reason . WAAS Tomorrow: More Utility for NextGen, ADS-B While the wonders of LPV approaches and RNP procedures come into play today, in the here-and-now world of aviators flying appropriately equipped aircraft, another larger role looms for WAAS in NextGen . With the FAA moving toward a system that uses aircraft-GPS-generated position reporting through the automatic dependent surveillance–broadcast, the accuracy of WAAS/GPS makes possible the reinvention of traffic monitoring and management, which both the agency and users are seeking . WAAS-level accuracy can combine with ground hardware at airports to provide multiple simultaneous instrument approaches in the worst weather, with controllers confident of the position information before them, enhancing separation and traffic flow . WAAS and ADS-B can help separate ground traffic and taxiing aircraft . And, as radar systems leave service, Air Traffic Control can operate using ADS-B position relaying with WAAS accuracy, allowing tighter spacing of aircraft and more direct operations — all based on the knowledge these position reports are the most precise ever made available for air-traffic management . q

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Pilot's Guide to Avionics 2011-2012

Table of Contents for the Digital Edition of Pilot's Guide to Avionics 2011-2012