Graham Warwick/ATLANTA

The US Federal Aviation Administration has decided on an architecture for the Local-Area Augmentation System (LAAS), planned to replace the instrument landing-system (ILS) beginning early next century.

The LAAS will increase the accuracy, availability and integrity of the global-positioning system (GPS) to be used for precision approaches and automatic landings.

The FAA has decided internally on an architecture for the LAAS and has begun working with manufacturers and users to build "broad-consensus" support for its solution, says Dick Arnold, head of the agency's GPS programme. The USA plans to present its chosen architecture to the International Civil Aviation Organisation in February 1997, for adoption as a worldwide standard.

Arnold says that the FAA has "-tried as best we can" to design an architecture which is "backward compatible", with differential-GPS (DGPS) landing systems now being certificated for "private-use" Special Category I (SCAT I) operations. The same DO-217 VHF datalink will be used to broadcast GPS error-corrections from the ground station to incoming aircraft, for example.

The LAAS architecture is based on more accurate carrier-phase-tracking GPS receivers, but the FAA hopes to move to cheaper narrow-correlator code-tracking receiver technology, if it can, to minimise the cost to aircraft operators, Arnold says. Other features of the architecture include the likely use of "pseudolites" - ground-based pseudo-satellites - to provide the GPS availability required for Cat III landings, he says.

He went on to say that the FAA plans to award a contract for a LAAS prototype ground-station to be delivered in 1998, with prototype Cat III systems to be deployed at several airports for operational evaluation by 2001. Plans call for USILS ground-stations to be replaced between 2005 and 2010.

Changes to the FAA's Wide-Area Augmentation System (WAAS) programme mean that Cat I capability will not now be provided until 2001. Arnold says that the new WAAS contract with Hughes calls for a fully operational en route system by 30 November, 1998, allowing primary-means use of GPS for non-precision approach. Cat I capability will then be added in two phases: addition of a maintenance-monitoring system; and doubling of the number of geostationary communications-satellites available to broadcast the WAAS accuracy and integrity messages.

The initial system is based on two Inmarsat satellites. Four are required for Cat I operation, Arnold says. Although negotiations with Europe on access to a third Inmarsat are progressing, he says, the FAA is also examining options, from installing WAAS payloads on other commercial spacecraft, to launching its own satellite.

Source: Flight International