Future Flight: Headwinds for WAAS

Part 5 of 12

Remember the FAA's Wide Area Augmentation System (WAAS), designed to supplement the Department of Defense's constellation of Global Positioning System satellites and allow GPS-based precision instrument approaches? WAAS is intended to increase the accuracy of GPS navigation by transmitting additional position and integrity data from ground stations to aircraft—and other users—via geostationary satellites.

When we last visited the subject (" Hello, WAAS," October 1999 Pilot), WAAS was promising to provide approach capability down to the same 200-foot ceiling and half-mile visibility minimums offered today by many Category I instrument landing system approaches. WAAS was scheduled to become operational this September.

Well, there's some good news and some bad news about WAAS.

The good news is that the accuracy being achieved with WAAS is almost four times better than expected. The aviation industry reaffirmed the need for WAAS at a March 15 summit called by AOPA and the Air Transport Association (ATA), which represents major airlines. The summit called for the FAA to fix the problems it has encountered with WAAS and to deploy an operational system as soon as possible.

The bad news is that achieving, or at least proving, the necessary integrity?the ability to provide timely warning of erroneous information that should not be used for navigation?has been more difficult than anticipated. As a result, the initial implementation of WAAS will be delayed at least a year. When it becomes available, WAAS will provide lateral and vertical navigation (LNAV/VNAV) approach capability to minimums of 350 feet and 1 mile. This won?t be quite as low as the previously promised 200 and one-half?but the approaches won?t require expensive approach lighting systems and other airport improvements needed for a Category I ILS, either. Lower minimums are expected in the future, but more development work and possibly operational experience will be required first.

"AOPA surveys show that most GA pilots? personal minimums are 300 feet or higher, and 350 and 1 are very close to that mark," said Doug Helton, AOPA?s vice president of air traffic services and technology.

Other industry groups participating in the summit also endorsed the initial 350-and-1 minimums. "We?d take 350 feet and 1 mile in a heartbeat," said Bob Blouin of the National Business Aviation Association. ATA, Regional Airline Association, and Cargo Airline Association representatives also supported WAAS.

What happened to WAAS on the way to implementation?

Problems have been identified in the system?s stability and integrity that will preclude the FAA?s commissioning any level of WAAS service this year. A 60-day stability test that required an accuracy of 7.6 meters was begun on December 13, 1999. The test was demonstrating an accuracy of 2 to 3 meters until a signal loss stopped the test. Most of the fixes had been incorporated by late March, with the remainder to be done by June.

In addition, a critical WAAS component?the integrity monitor?was not able to detect misleading information in December 1999. The current system is based on an earlier design, and analysis indicates that it must be changed. Achieving the high level of integrity required for certification is the biggest challenge and continues to drive the WAAS schedule, according to the FAA.

"The bottom line is this: There is little doubt that a WAAS system can achieve ILS-like performance, but the current design will have to be modified to do so. Furthermore, the FAA?s certification and acquisition offices, in collaboration with [contractor] Raytheon, must agree on a realistic means for proving system integrity?or, in other words, show that there?s virtually no chance of an undetected failure in the system," Helton said. "This will take an appropriate balance of mathematical analysis and operational experience with the system. The work should have been done more than a year ago and is one of the primary causes for the latest schedule slip."

The FAA is in the process of doing this, with some outside help. In January it established a WAAS integrity performance panel comprised of experts from Mitre, Stanford University, Ohio University, the Jet Propulsion Laboratory, and the FAA. The panel has already identified WAAS integrity solutions for LNAV/VNAV approaches. WAAS initial operational capability is now projected for December 2001.

LAAS takes shape

The Local Area Augmentation System (LAAS), which will transmit GPS correction signals from ground stations directly to aircraft, is planned to complement WAAS. LAAS will provide greater positional accuracy, but only in a small area?a LAAS ground station will generally serve only one airport, compared to the large service area provided by WAAS satellites.

In February the minimum operation performance specifications (MOPS) were published for LAAS, paving the way for development of LAAS-compatible GPS receivers. WAAS summit participants supported pushing ahead with LAAS as planned. Under the current schedule, LAAS could be commissioned for Category I approaches beginning in early 2002, but Cat III approaches will face the same integrity challenges as WAAS.

Transition time frame

The 1999 Federal Radionavigation Plan (FRP), released in February 2000, outlines a timetable for the transition to satellite-based navigation and the decommissioning of ground-based navaids. The plan was issued before the WAAS delays were quantified, so these decommissioning dates are expected to slip. The plan calls for a 30-percent reduction in the current federal network of 1,012 VOR/DMEs, 1,062 ILSs, and 750 NDBs between 2008 and 2010. Phase II would see further reductions to about 600 VOR/DMEs, 500 ILSs, and 280 NDBs in 2011 and 2012.

Final reductions beginning in 2013 would lead to a "basic backup network" of several hundred VOR/DMEs to provide en route navigation, nonprecision approaches, and precision approaches at the busiest airports in the event of disrupted satellite navigation. Some Category I ILSs (and all Category II/III ILSs) would be retained. The long-term future of loran is undecided, but it will be maintained for at least two more years.

Because WAAS is central to so many other aviation initiatives, including the planned overhaul of the air traffic control system, there is little question in the aviation industry that it should be implemented. "Even with all the problems, WAAS combined with LAAS still represents the best alternative for meeting all aviation requirements," Helton said. The question of timing and Capitol Hill support is a bit less clear, but as soon as there is an answer, we will share it with you.

Links to additional information about the Wide Area Augmentation System and other GPS issues can be found on AOPA Online ( www.aopa.org/pilot/links/links0005.shtml ). Next month?s installment of "Future Flight" will explore tomorrow?s engine controls. E-mail the author at [email protected] .