GRAHAM WARWICK / WASHINGTON DC
Drawbacks of global positioning system requires the Federal Aviation Administration to ensure a back-up
Not one but two events last September changed the future of satellite-based navigation.
In the first, the US Department of Transportation's respected Volpe centre confirmed what everyone already knew: that the global positioning system (GPS) is vulnerable to interference, accidental or deliberate. In the other event, confirming everyone's fears, determined terrorists showed they would stop at nothing to inflict damage on the USA.
Last month, the Federal Aviation Administration brought together all users of the US airspace system to brief them on the results of the confluence of those two events - its plan for providing a back-up to GPS as the USA continues the transition to satellite-based navigation.
In a nutshell, the FAA plans to retain most of its ground-based navigation aids for at least the next 20 to 30 years. This calls into question the cost savings forecast for both the FAA and airspace users from the long-planned transition to satellite navigation.
At the heart of the FAA's plan remains the assumption that GPS, enhanced by the wide-area augmentation system (WAAS) and local-area augmentation system (LAAS), will become the primary means of en route navigation and precision approach in US airspace. But aircraft will have to carry some ground-based navigation systems as back-ups, and airlines could see little in the way of savings from reduced avionics equipage.
Experiments have shown that GPS interference, intentional or unintentional, can block the satellite signals over a large area. Aircraft caught inside the interference area in instrument conditions could request radar vectors from air traffic control to guide them out of the area or to a safe landing, but the FAA is concerned about controller and pilot workload.
The agency has developed back-up strategies for airspace users ranging from light aircraft to heavy airliners. The intention is to reduce the threat that GPS will be jammed deliberately by ensuring air transport operations can continue without disruption. "GPS will not be a convenient target," says Mike Harrison, head of the FAA's air traffic control architecture office.
Under the back-up plan, a general aviation pilot losing GPS will climb to 5,000ft (1,500m), at which altitude at least one VHF omni-range (VOR) station should be within line of sight. The pilot will then navigate from VOR to VOR until outside the interference, or fly to a VOR-equipped airport and land (see diagram).
This requires the FAA to maintain a minimum network of 450-550 VORs, down from over 1,000, to ensure no aircraft is ever more than 140km (75nm) from a ground station when a GPS outage occurs. The FAA plans to begin removing VORs in 2007, and to eliminate the present airways structure by 2012. Current VOR coverage will be maintained in mountainous regions, Alaska and Puerto Rico.
The air transport back-up plan assumes most airliners are equipped with flight management/ inertial navigation systems (FMS/INS) providing an area navigation (RNAV) capability with or without GPS by using ground-based distance measuring equipment (DME). With FMS/INS and signals from multiple DMEs, crews will be able to continue through areas of interference as planned using RNAV. This requires the FAA to retain all 930 of its DMEs.
Ensuring aircraft can take off and land inside interference areas is a key element of the plan to maintain normal operations during GPS outages. This requires the retention of instrument landing systems (ILSs) at all airports currently equipped. All Category II and III ILSs will be retained indefinitely, but the FAA plans to reduce Cat I ILSs to one per airport, serving the runway best suited as a back-up. Except at capacity-constrained airports, multiple-runway ILSs will be removed but lighting retained.
The plan to remove Cat I ILSs beginning in 2010 has been criticised as forcing operators to use WAAS and LAAS. Harrison sees no problem with this. Although WAAS will not provide the promised Cat I service, it will provide a near-precision approach capability soon after it finally becomes operational in December next year, he says. LAAS will provide Cat I capability beginning in late 2004, but not Cat II or III for the foreseeable future.
Another concern with the FAA's plan is the equipage requirements. The agency foresees three levels. Operational contingency allows aircraft to be equipped only with GPS or GPS/WAAS, but requires pilots to follow visual flight rules until clear of interference. Back-up capability requires aircraft to carry VOR and ILS receivers, but allows dispatch out of, through, and into areas of interference. Aircraft will have to navigate VOR to VOR and may not be able to land at the destination airport.
Redundant capability will allow aircraft to fly the planned route independent of interference, using RNAV, but requires an FMS/INS, updated by DME, and ILS. The FAA is proposing that all aircraft using airspace above 18,000ft, regardless of size, be required to carry back-up capability. Airlines are likely to need redundant capability, but Harrison admits there is room for discussion as to how redundant they need to be.
The FAA is continuing to look at Loran, which proponents see as a natural ground-based complement to GPS.
While it agrees the system is the best "theoretical" back-up for GA users, the FAA says Loran is five to seven years away from being usable for non-precision approaches.
Source: Flight International
