Graham Warwick/WASHINGTON DC

Raytheon has determined the sources of false alarms plaguing the wide-area augmentation system (WAAS). The impact of the problems on certification of the system has not been announced, as the US Federal Aviation Administration has yet to select from several schedule options on offer, says the WAAS prime contractor.

WAAS acceptance testing was halted in January due to excessive false alarms in the integrity monitor, which ensures that the position error never exceeds safe limits. Around 65% of false alarms are caused by multipath problems at five of the 25 reference stations which receive signals from global positioning system (GPS) satellites. Stations correct ionospheric and other errors, says Raytheon programme manager John Britigan.

Multipath problems will be overcome in the near term by resiting antennas at the affected reference stations, says Britigan. "We couldn't just locate them where we wanted to," he says, noting that most have to be placed at FAA air traffic control centres to ensure adequate security.

The other major source of false alarms is an unexpected timing change occurring when new Block IIR GPS satellites receive routine software updates, he says. It is unclear how this will be overcome.

Acceptance testing was halted because "cascading" false alarms reduced availability of the WAAS signal. Three or more false alarms in a row cause WAAS airborne receivers to "time out" for a few seconds, until they receive a position correction. This is unacceptable during a precision approach, which requires at least 150s continuous availability of the WAAS signal, Britigan says.

Because of the problems, the FAA has decreed that, to meet certification requirements, the system must perform additional integrity checks. This will require substantial re-engineering of the WAAS software, Britigan says.

To minimise delays, Raytheon has proposed that the FAA field the system initially with en route capability only, upgrading it later to provide non-precision vertical guidance. The ultimate goal, to provide Category 1 precision approach capability across the USA, remains unchanged.

Source: Flight International