Kieran Daly, ATI

Top-ranking US officials are telling Asian and European nations that the USA remains committed to developing an augmented Global Positioning System (GPS) as the basis for a worldwide navigation network.

In an interview at Asian Aerospace '98 yesterday, they conceded that back-up navigation systems will be essential for many years to come but insisted that GPS, augmented by the US Wide Area Augmentation System (WAAS) or its European and Asian equivalents, is the only way of providing affordable and reliable navigation for the whole of the globe.

And they say other nations can rest assured that a solution will be found to the difficult question of selecting a second civil GPS frequency to overcome what they consider the two key weaknesses of GPS - vulnerability to jamming, and disruption during the 11-year sunspot cycle.


The reassurances come as Japan presses ahead with its own, extremely similar, version of WAAS - the MT-SAT network - but a fierce debate rages in Europe over the wisdom of continuing with the expense of the European Geostationary Navigation Overlay System (EGNOS).

George Donohue, US FAA associate administrator for research and acquisition, laid out the administration's position in an exclusive interview with the Flight Daily News sister organisation, the electronic news service ATI, shortly after an FAA report to Congress raised renewed criticism in the USA over its alleged backing away from the earlier commitment to providing a "sole means" navigation system.

Donohue strongly implies that many in the industry, and even some people in the FAA, have had an unrealistic view of what the concept of "sole means" would mean in practice.

"We know that during the sun-spot cycle the upper atmosphere can cause signal drop-out - which is one of the reasons why the military has always had two frequencies."

Donohue says there is no question that, in the areas 20í north and south of the equator and 5-10í from the North Pole, a second civil GPS frequency is needed if a worldwide service is to be provided. The second military frequency - L2 - cannot, for military operational reasons, be used by the civil community.

He also says it is clear that a solution has to be found that will at least mitigate against the risk, however remote, of either intentional or unintentional GPS interference.

Donohue says the accuracy and "ubiquity" of GPS mean it is here to stay, so the result is that more conventional navigation systems will have to stay as well, and work will have to be done on protecting GPS.

He points to examples of its great advantages in assisting oceanic navigation to levels of accuracy that will allow desired reductions in separation; allowing area navigation even in congested airspace; and bringing precision approaches to airfields that otherwise have no hope of implementing such procedures.

A second civil frequency would go a long way towards protecting GPS and a favoured candidate in today's crowded radio spectrum is emerging in the form of the 1.205GHz frequency, despite the military's resistance.


The problem is that the services' JTIDS state-of-the-art tactical data radios interfere with that frequency, but the Pentagon has already turned down other possibilities on similar grounds and there is a plausible argument that, in peacetime, the problem can be got around.

Two frequencies could be jammed more or less as easily as one, however, and the challenge of dealing with intentional interference is more difficult.

That back-up is a combination of conventional aids and the increased robustness that WAAS will bring when it is first introduced, if all goes to plan, within a year throughout most of the USA.

Source: Flight Daily News