Pilot's Guide to Avionics 2011-2012 - 21

PILOT’S GUIDE

approaches WAAS has made possible . And, the FAA promises several hundred more each year through the decade’s end . WAAS: The System If you already are familiar with the basics of GPS, you’re most of the way there . GPS, developed during the past 30 years by the Department of Defense, employs more than two dozen satellites in low-Earth orbit . Those satellites broadcast time-coded signals heard by GPS receivers; these receivers compare the data from three or more satellites and use the coded data to calculate their position on or over Earth . While the precision of the data broadcast and receivers’ capability is high, small technical issues can compound to reduce the accuracy of the receiver’s calculations . Among those issues are satellite movement or drift, tiny clock errors, and signal-travel variations caused by atmospheric conditions . When compounded, these typically small input flaws generate position errors in the receivers’ calculations; receivers’ accuracy abilities are only as good as the data they use to make the calculations . As a result, nominal GPS accuracy is between 50 and 100 feet laterally and vertically . The FAA and its contractors created the WAAS system to correct such errors and provide corrected information to receivers . The WAAS architecture involves more than 25 geographically diverse ground stations, each one precisely surveyed with its known position used to compare its own position to calculation of the regular GPS satellite signals . In the simplest terms, WAAS ground stations work like normal GPS receivers with a crib sheet — they actually know where they are and can compare this knowledge to the calculation that results from the satellite data . When these ground stations detect inaccuracies, they send a correction for their region to a set of master stations, which process the information and send it back to a pair of geostationary WAAS satellites parked above North America . Those satellites, in turn, broadcast corrected signals on a separate frequency, corrections tailored to each ground station’s service area . The WAAS channel in the GPS navigator receives the correction signal, which results in demonstrated accuracy of about 9 feet, laterally and vertically . The demonstrated accuracy far exceeds the goals of 50 feet laterally and 66 feet vertically; furthermore, the system reportedly never has shown errors in excess of about 40 feet . This vertical accuracy capability, in particular, underpins a family of new instrument approach procedures known as lateral precision with vertical guidance, or LPV, as well as other benefits, such as required navigation performance arrival, departure and newer RNAV approach procedures .

Starting after these years of culminated efforts, with WAAS commissioning in 2003, hundreds of runway ends either have or can get a precision-approach procedure capable of guiding airplanes down to as low as 300 feet above the ground at most airports, and as low as 200 feet at airports with the proper lighting and runway aids . This is as good as the ILS, but without the $1 .5 million to $2 million equipment, site-preparation, installation and testing costs . By contrast, the FAA pegs the cost of a new LPV approach at less than $50,000 — less when the agency tackles creating multiple approaches at one airport at the same time . The results of this new capability are striking . As of April 2010, more than 2,000 new LPV approaches were operational, with more than 4,600 of the others and more to come . These new approaches — LNAV, LNAV/VNAV and LPV — benefit nearly 500 non-Part 139 airports and more than 50 Part 139 airports . Clearly, the system is delivering on its promise of enhancing IMC access to smaller airports . And, this number doesn’t cover those RNP procedures, which helped expand arrival and departures at many airports . New Access in New Ways With WAAS/GPS capable of supporting a number of new ways for aircraft to get around when the weather goes down, the FAA has focused considerable resources on expanding the availability of these procedures, which are only possible with WAAS receivers — and, in some cases, special approval for aircraft and pilot training to use them . Here’s a snapshot of the benefits from WAAS and what they offer pilots: • RNP: Required navigation performance uses WAAS/GPS — alone or with other inputs, such as inertial reference and inertial navigation inputs — to provide precise defined navigation as the primary source of guidance . RNP standards define small route corridors that can be used in transitions, arrivals, departures and approaches where geographic features or traffic densities otherwise make the usual arrival types impractical or excessively risky . Curved approaches in mountainous areas or within the arrival routes of a different airport in busy airspace are among the signature advantages of meeting RNP standards . RNP capabilities are defined by the degree of accuracy they provide, with RNP 1 .0 at one mile, all the way down to RNP 0 .3 — yes, three-tenths of a mile for approaches and certain transitions . To obtain FAA use of RNP-based procedures and benefits, the aircraft’s navigation system must monitor its own accuracy and provide a means to alert the flight crew should accuracy become suspect . Both aircraft and crew also must obtain Special Aircraft and Aircrew Authorization
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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