US airlines have tacitly accepted that global navigation satellite systems (GNSS) as a sole-means aid is not going to be approved, according to the US Air Transport Association (ATA).

The trade body has appealed to the US Congress not to reduce funding for the global positioning system Wide Area Augmentation System (GPS/WAAS), insisting that GPS/WAAS is an indispensable navigation system even if the product will never be operated as a sole-means device.

The principal fear of the airlines is that the multi-million dollar investments in GPS on-board equipment will have been wasted if Congress decides to reduce system funding.

The ATA's fears derive from language used in the transportation section of the Senate Appropriation Committee's appropriations Bill, which the ATA believes threatens to end the WAAS programme.

Europe has been warning the US Federal Aviation Administration since the programme began in the early 1990s that augmented GPS would never provide sole-means integrity levels, especially for precision approaches.

With the Europeans less dependent on sole means GPS, the UK National Air Traffic Services chief engineer (navigation services) Jim Lawson says that the long-term future is in augmented GNSS as a prime means, backed by advanced inertial navigation systems.

For the medium term, airlines should equip with multi-mode receivers capable of using signals from GPS, microwave landing systems and instrument landing systems, advises Lawson, adding: "DME [distance measuring equipment] will always be there."

Abandoning the GPS/WAAS programme now would be embarrassing for the USA, which had persuaded the International Civil Aviation Organisation and International Air Transport Association to back the GPS/ WAAS concept, with the intention of making it a sole-means system from en route navigation to Category 1 approaches.

The test system is in operation about three months ahead of schedule, and WAAS signals are being transmitted to test prototype GPS/WAAS receivers.

The ATA now urges Congress to enable WAAS development to continue uninterrupted by approving the full fiscal 1999 funding of $140 million.

Source: Flight International