Air Transport World

APALS at last. (Autonomous Precision Approach and Landing System)

Developed to provide Cold War precision bombing, new approach system will permit Cat. II landings without ground-based ILS

About a year from now, a Dornier 328 with a self-loading cargo of Texas skiers should make a bad-weather approach to the Aspen, Colo., airport, hemmed in by a box canyon. Its pilots will be cleared to land in Cat. II weather, following instrument landing system indications on their flight-director display. However, no ILS will be in use and the aircraft will be guided by a system that combines high-speed computers with GPS, inertial navigation, the 328's weather radar and techniques developed for dropping nuclear war-heads on theater-strategic targets in Europe.

Hatched in obscurity in a San Diego special-projects office and developed by a small team of engineers, Lockheed Martin's Autonomous Precision Approach and Landing System (APALS) may be one of the 1990s' biggest single advances in aircraft navigation. By the time this article appears, Lockheed Martin expects to have announced MOUs with three launch customers: Texas regional Lone Star Airlines and two carriers in Europe. The company will be committed to certificating APALS for Cat. II approaches in early 1997 and for Cat. III, with autoland, later that year.

The introduction truly is timely. Operators in Europe are facing near-term problems. Cat. III runways at London Heathrow and Amsterdam Schiphol have been downgraded because of multipath interference, caused by new buildings. The only option is to fit MLS receivers. But few runways are fitted with MLS. This was expected to happen 20 years ago, when European authorities agreed that in 1998, other FM wave-band users would be allowed to increase their power. Unfortunately, airlines---still dependent on ILS---will have to fit improved "FM-immune" receivers to block out the more powerful signals.

In the U.S., the MLS program is deader than the Betamax and the improvement of ILS came to a virtual halt years ago. The growth of air traffic and regional airlines, however, means that more flights are using, nonprecision approaches.

Eventually, landing guidance should be provided by GPS, using a small, ground-based differential transmitter to pro: vide the necessary accuracy. However, the Achilles' heel of GPS still is integrity. A GPS error can take minutes to report and correct. This is the primary justification for FAA's Wide Area Augmentation System (WAAS), under development by Wilcox, TRW and Hughes. A satellite-linked network of ground receivers and master stations, WAAS monitors the performance of GPS and communicates corrections via satellite in near-real time. WAAS is a prerequisite to using GPS for unaided navigation or for precision approaches (ATW, 9/95). If the schedule holds, WAAS will begin to become operational in the early 2000s but people familiar with the FAA's past record in deploying new hardware are not holding their breath. …

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