Graham Warwick/WASHINGTON DC
B beset with funding and other issues surrounding its transition to satellite navigation, the last thing the US Federal Aviation Administration needs is a problem with its keystone Wide Area Augmentation System (WAAS) programme.
But a problem is precisely what the FAA is facing, at a time when the agency is trying to persuade Congress to restore funding for both WAAS and the Local Area Augmentation System (LAAS) and keep the user community behind its satnav transition plan.
In January, the agency stopped a key WAAS acceptance test earlier than planned. The 60-day test, begun in December, was intended to demonstrate the system's stability. Instead, it suffered a 100min loss of signal and excessive false alarms in the integrity monitor
WAAS is intended to increase the accuracy, availability and integrity of the global positioning system to allow its use by civil aviation as a sole means of navigation. The system broadcasts error corrections, integrity messages and additional navigation signals via geostationary satellites.
While accuracy is exceeding expectations, averaging 3.5m (11ft) against a target 7.6m, "we are not yet there on the safety side", admits Carl McCollough, the FAA's director of communication, navigation and surveillance systems.
The FAA met WAAS prime contractor Raytheon in mid-February to determine the impact of the problem on initial deployment of the system, which was planned for September. They decided not to reduce the integrity specification (a 1 in 10,000 chance of broadcasting misleading data), but the FAA admits achieving the required integrity "will hit schedule and availability". How much will depend on what level of service users are willing to accept.
Any delay is likely to make negotiations with a sceptical Congress on restoring funding more difficult and could upset the FAA's careful calculations of the benefits of satnav versus its costs.
Last year, Congress reduced funding for WAAS and eliminated spending on LAAS, and directed the FAA to take a "time out" to reassess its satnav transition plan. Lawmakers indicated they would consider restoring funding after the FAA had provided an updated satellite navigation investment analysis report.
Delaying WAAS initial operational capability into 2001 and reducing the level of service offered could play havoc with the FAA's calculations. The cost-benefit analysis in the investment report (completed last September) supports the FAA's plan for the phased introduction of satnav and gradual decommissioning of ground navigation aids, but reveals the delicate balance that must be maintained if the plan is ever to be realised.
The FAA's preferred transition plan is based on deploying a "robust WAAS" by 2007 to provide Category I approach capability across the USA, accelerating LAAS to supplement WAAS and provide Cat II/III capability by 2010, and decommissioning ground navaids in two phases: first (by 2008) to a minimum operational network providing coverage at, and along routes between, hub-and-spoke airports; and second (after 2015) to a basic back-up network providing coverage across the USA above 6,000ft and at strategic airports.
The FAA's problem in justifying the costs of its satnav transition plan is that the benefits are dependent on users equipping their aircraft - while the benefits to users are dependent on the FAA investing in the satnav infrastructure. The equation is different for the airlines than it is for general aviation (GA).
Phased approach
While both communities are in favour of the phased approach, the investment report says, the airlines "are much less enthusiastic about WAAS than LAAS", while "LAAS support is marginal" within GA.
The report admits WAAS does not provide much benefit for airliners already equipped with flight management systems, while low-end GA users have no need for LAAS's precision approach capability.
The FAA's transition plan tries to keep both communities on board, by accelerating LAAS, which the airlines want, and providing robust WAAS Cat I capability, which the GA community wants. Because of the possibility that GA users might resist the decommissioning of existing navaids, the FAA says there is a possible role for Loran-C to supplement the basic back-up network, which would otherwise provide only poor low-level en route coverage.
Now it looks like WAAS will not provide even limited Cat I capability in its initial form. The best users can expect, sources say, is vertical navigation guidance down to 350ft (110m). This would still provide benefit to GA users and to regional airliners, as it would give a lower minimum than a non-precision approach.
A "WAAS users summit" has been called for mid-March, at which the FAA and Raytheon will seek input from the aviation community on what level of service it will accept initially. "We know that it's important to get service to the community," McCollough says, "and its important to remember we are only talking about the initial capability. The end-state requirement does not change."
The problem for the FAA will be in getting to that end state, which requires substantial investment in additional ground stations and leased geostationary satellite capacity to provide robust Cat I capability across the USA. But Congress has blocked all spending on WAAS beyond the initial phase until the FAA can justified its satnav transition plan.
WAAS has been burdened by "excessive expectations", the agency admits, including "unrealistically optimistic schedules [and] inadequate cost estimates". The investment report says "the FAA's failure to deliver on these expectations has created a continuous crisis atmosphere". It says "there is little internal FAA support for WAAS or LAAS", a sign that the programme's problems run deeper than those which led to January's premature termination of the stability test.
Source: Flight International
