With satellite navigation vulnerable to terrorism, the USA is to keep its ground-based systems as back-up. Should others do the same?

Like it or not, where the USA leads in aviation, the rest of the world tends to follow. With the largest concentration of aircraft, civil and military, the USA tends to encounter and react to aviation opportunities and challenges in ways other regions ignore at their peril.

The question is: should the rest of the world follow the USA in its reaction to the exposure of the limitations of the global positioning system? After decades of evangelism on behalf of satellite-based navigation, the USA has been forced to accept that most of its ground-based navigation infrastructure will have to remain to act as a back-up in case GPS is jammed - accidentally or deliberately.

The Federal Aviation Administration still remains committed to GPS as the primary means of navigation in US airspace, but it looks unlikely to ever become the only means. Basic GPS, now that the US military has stopped deliberately degrading its accuracy, is already good enough for en route navigation and non-precision approaches. The much-maligned wide-area augmentation system (WAAS), to become operational at the end of next year, will provide a near-precision approach capability, while the local-area augmentation system (LAAS) will provide Category I landing capability from 2004.

More improvements are in the pipeline: extra geostationary satellites to increase the capability of WAAS and next-generation GPS satellites with more power and additional civil signals. All will help reduce the vulnerability of GPS to unintentional interference. But they will not eliminate the threat of deliberate jamming.

In recognition of that reality, the FAA plans to leave in place sufficient ground-based navigation aids to ensure that air transport operations can continue undisrupted if GPS is interfered with. With a robust back-up in place, the attraction of GPS as a target for terrorism is reduced. Why jam it if aircraft can still fly safely?

The price is that most of today's navaids must stay, must be maintained and eventually must be replaced - and aircraft must continue to be equipped to use them. The promised cost savings from the removal of ground and airborne equipment with the transition to satellite-based navigation will not be realised.

Under the FAA's plan, about half the USA's VHF omni-range (VOR) ground stations would be removed, all distance measuring equipment (DME) stations and Cat II/III instrument landing systems (ILSs) would be retained. Cat I ILSs would be reduced to one per airport, serving the runway best suited to be a back-up. Most aircraft would be required to continue carrying at least one VOR, one DME and one ILS and pilots will have to continue to be trained to use them. The FAA's concept is that aircraft would use GPS, WAAS and LAAS for en route navigation and non-precision and precision approach down to Cat I when the signals are available. So aircraft will need to be equipped with GPS, WAAS and LAAS.

This has led some sceptics to question the need ever to equip to use the GPS augmentations developed at such great expense. Basic GPS, VOR/DME and ILS - already standard equipment on many aircraft - is enough to get the job done. Should the rest of the world, now embracing satellite navigation at an accelerating pace, follow the USA's example? In Europe, where Galileo is to be developed as an adjunct and alternative to GPS, how does the USA's back-up plan affect the business case for charging users for some of the system's services? If GPS can be jammed, so can Galileo. Will users demand a refund every time there is an outage, or simply use the ground-based navigation infrastructure? What about countries which lack such an infrastructure and see satellite-based navigation as an affordable route to a word-class airspace system?

The answer lies in the assessment of the threat. In the USA, after 11 September, critical infrastructure such as GPS is seen as vulnerable to terrorist attack jamming. GPS jammers can interfere over a fairly large area. For the USA, with the world's largest air transport system and an economy heavily dependent on air travel, a costly back-up system makes sense.

Each region must make its own assessment of the threat and its own decision on what, if any, back-up system must be left or put in place. Ignoring the possibility of a threat is not an option. There are technologies the USA is considering to mitigate the threat of GPS disruption. Other countries would be well advised to take a look at them.

The transition to satellite-based navigation has not been stopped, but it has been slowed and it may never be complete. The future still remains in the future air navigation system.

Source: Flight International