Category I WAAS coming at last?

 - September 21, 2006, 5:41 AM

The FAA’s announcement last month that its GPS wide area augmentation system (WAAS) will support ILS-like 200-foot Category I approaches marks the agency’s third swing at this, the system’s Holy Grail. When WAAS was launched in 1995, Category I was promised by 1997, but the system’s rocky progress over the following years consigned Category I to the benches.

In 2004, the FAA relaunched Category I, but, being predicated on using accuracy-enhancing dual frequency transmissions from the DOD’s future GPS III, its introduction slipped to 2013. Since 2004, however, WAAS performance has dramatically improved and the system no longer needs GPS III. At its third time at bat, WAAS appears set for a home run.

“This is a welcome initiative,” said Bob Lamond, NBAA director of air traffic services. “The sooner we can move away from the limitations of ground-based navigation aids, the better it will be for the entire aviation community.”

Yet while WAAS lateral accuracy was always within ICAO and FAA ILS localizer standards, the critical test for WAAS was whether it could meet Category I’s vertical requirements, where GPS is typically much less precise. To evaluate this, the FAA, USAF and Eurocontrol flew 429 WAAS approaches at airports in the continental U.S. and the UK, using Category A, B, C and D aircraft.

The agencies also performed extensive simulations and analyzed more than 2.1 billion data points, recorded every second of each day since WAAS was commissioned in 2003. The results showed that, compared with the permissible ILS vertical error of 4.4 meters on 95 percent of occasions, WAAS achieved better than 3.3 meters.

This is significantly better performance, and much less expensive to attain, than the airlines’ preferred 250-foot-DH RNP technique. The airlines have always rejected WAAS, opting to wait for the long-delayed, but more costly, local area augmentation system (LAAS) to provide the Category I complement to RNP. Now, industry sources suggest that successful WAAS Category I implementation might prompt the FAA to shelve LAAS indefinitely.

Much still needs to be done before the first public WAAS Category I procedures appear in the middle of next year. The FAA is reviewing ILS Category I criteria to determine which ILS-specific limitations can be waived for WAAS operations.

Correspondingly, airports wanting WAAS Category I approaches must survey their non-ILS runway environments for criteria compliance, including approach obstructions, lighting and runway markings. Following that, FAA approach plate production and flight checks–typically a six- to nine-month process–can begin. Initial WAAS Category I approaches will be overlaid on existing ILS procedures to develop operator experience.

Only Garmin offers TSO C-146 “Gamma-3” panel-mount units for Category I WAAS, although other manufacturers are expected to announce equipment, including FMS-compatible C-145 “Beta-3” remote units, within 12 months. The -3 nomenclature recognizes the higher data rates required for LPV (lateral precision with vertical guidance) and Category I operations. Lower data rate -1 and -2 units support Lnav and Lnav/Vnav. The FAA has invited potential suppliers to apply for shared-cost development programs.