Potential back-ups to the global-positioning system (GPS) are being considered by the US Federal Aviation Administration, which is backing away from sole-means use of GPS for navigation because of concerns over signal interference.

Although the FAA says that "-no decision has been made", it is now likely to approve the GPS-based wide-area augmentation system (WAAS) for primary-means use only, which will require a back-up system.

A request for information on potential back-up systems was issued in January and the FAA's Joint Resources Council will be briefed in March on the alternatives. The FAA is examining Loran-C, inertial-navigation systems, "skeleton" VHF omni-range/distance-measuring equipment (VOR/DME) systems and an anti-jam GPS antenna developed by the military.

Use of the existing Loran-C radio-navigation system is being championed by the US general-aviation (GA) community, but opposed by the airlines, which would prefer VOR/DME as the back-up system. Loran is widely installed in GA aircraft, but not used in airliners.

US Loran ground stations are scheduled to be switched off at the end of 2000, shortly after the WAAS is planned to become operational, while VOR/DME stations are due to be phased out between 2006 and 2010.

The GA and airline communities want to continue using the systems with which their aircraft are already equipped, and the FAA says that, if a GPS back-up is required, "-it might be a combination" of the available systems.

One problem with using Loran is that there has been no significant investment in the system since it was flagged for termination in 1994. As part of its evaluation, the FAA plans to transmit WAAS correction signals over an experimental Loran transmitter, which could result in Loran stations becoming, in effect, ground-based satellites.

Source: Flight International