GRAHAM WARWICK / WASHINGTON DC
GPS augmentation system is 'significantly exceeding' requirements and could be operational by June next year
All safety issues with the wide-area augmentation system (WAAS) have been resolved, and handover to the US Federal Aviation Admini-stration is set for March next year, says prime contractor Raytheon.
While the FAA could take up to six months to commission the GPS augmentation system, WAAS may be operational as early as June next year, says Raytheon's John Birtigan. Integrity issues delaying the programme are resolved, and performance is "significantly exceeding" the requirements for initial operational capability (IOC), he says.
At IOC, WAAS will provide a lateral and vertical navigation (LNAV/VNAV) capability supporting GPS non-precision approaches down to a 300-350ft (90-105m) decision height at runway visibilities down to 1.4km (0.75nm). At the same time, or soon after, Raytheon expects the FAA to declare operational an enhanced service, called APV 1.5 or localiser precision approach (LPA). This will support approaches down to 250-300ft.
WAAS was originally intended to provide Category I precision approach capability and a 200ft decision height. "With APV 1.5 we are within 50ft of Cat I," says Birtigan. Further enhancements are planned to provide a capability called APV 2.0, which will support more approaches down to 250ft. "With APV 2.0 we will be close to Cat I," he says.
The enhancements include improved GPS error-correction algorithms, more ground reference stations and additional geostationary (GEO) satellites to broadcast the WAAS messages to users. No timetable has been set for the enhancements, as they are tied to FAA funding, but Raytheon expects a contract this year for the first phase, which includes a third GEO satellite.
Availability of the different WAAS service levels is determined by horizontal and vertical protection limits, which bound the maximum possible GPS position errors. If the continuously calculated protection limits exceed the alarm limits for a particular approach, the receiver will flag the pilot to abort. Reducing the limits allows the approach mimima to be lowered.
LNAV/VNAV and LPA correspond to a vertical protection limit of 50m. "The actual performance is 25m, and we need to get to 20m," says Birtigan. The 20m limit required for APV 2.0 is currently achieved with 95% availability only in the centre of the USA.
Source: Flight International
