Ramon Lopez/WASHINGTON DC

Estimated life-cycle costs of the wide-area augmentation system (WAAS) have increased by 21% and "major concerns" have surfaced over signal interference, according to a US Federal Aviation Administration report to Congress obtained exclusively by Flight International.

The report warns that "technical uncertainties" relating to intentional jamming, unintentional interference and solar activity could require an independent backup to the global positioning system (GPS) and the WAAS.

The cost of retaining "some network of ground-based navigation systems indefinitely as a backup to GPS/WAAS" has not been factored into the revised life-cycle cost estimate, the report says.

The FAA emphasises that Raytheon is "on schedule and within budget" on its $483 million contract to develop the WAAS, but estimates that life-cycle costs have risen by $638 million, to $3.05 billion, because of rises associated with communications satellites.

The original estimate was based on leasing transponders on two Inmarsats, but the FAA says that two to four extra "microsats" will be needed. Lease costs over the lifetime of the WAAS have been more than doubled, to $1.28 billion.

A report on GPS backup options, such as Loran C and VOR/DME (VHF navigation beacon/distance measuring equipment), will be provided to the FAA's Joint Resources Council by early April, says the report. One solution is to provide civil access to a second GPS signal, but the Departments of Defense and Transportation "-have been unable to agree on a second coded frequency", the report says.

The agencies agreed to select a second civil frequency by March 1998, for use on the next generation of GPS satellites (Block IIF), but "have not yet identified a second frequency option which is mutually acceptable". The FAA wants access to the military L2 coded signal. "If the [second civil] frequency is other than L2, the costs increase and benefits are delayed," the study warns.

In its report to Congress, the FAA acknowledges that the low-power GPS and WAAS signals are susceptible to interference. It adds: "The potential exists for an individual or organisation to jam GPS or WAAS signals and cause a loss of GPS and WAAS service over a large area."

Source: Flight International