The failure of the recent conference in Montreal on landing systems to come out in favour of a single solution will have been a great disappointment to the proponents of individual systems - but it will have been greeted with sighs of relief just about everywhere else. Not only does it remove the spectre of a perhaps-inappropriate system being foisted on an unwilling user, but it also removes the danger of over-hasty development of new approach systems.

Ever since it became apparent that the voracious appetite of commercial-radio broadcasting was incompatible with the future integrity of the instrument-landing system (ILS) in some parts of the world, there has been a distinct possibility that the search for a universal replacement would end in tears.

The need for a universal replacement was itself never a universal need. In many places, the only thing wrong with the ILS is the age of some of the installations. The airlines, as users, are more than happy with it; the airports, as service providers, are happy with it. Their only problem was that they wanted the reassurance that any short-term replacement for their geriatric equipment would not be rendered obsolescent by the adoption of a new standard in 1996.

The most obvious candidate to replace the ILS - the microwave-landing system (MLS) - is not seen in many places as obviously the best candidate. The MLS is less prone to signal corruption, but it is really little more than a more flexible (and more expensive) ILS.

Two countries which had been most exercised by the apparent early demise of the ILS - the USA and Canada - opted some years ago for a wholesale replacement of the ILS by the MLS. Subsequently, both have reconsidered, and both have abandoned the idea, (although the USA will persist with a small number of MLS installations in key locations).

What has swayed the USA more than anything else, is the promise of something, which is much more than just a better ILS, the differential global-positioning system (DGPS). The DGPS holds the promise of at least two major advances over other systems, it is centred more on the aircraft than on the airport; and it offers the prospect of a seamless union of primary-navigation system and landing system.

The trouble is that, despite the obvious promise, the DGPS is not yet capable of replacing the ILS in the most demanding of approaches. To demonstrate precision approaches in Category 1 conditions is a useful demonstration of future potential, but not of immediate value. No matter how rapid the technical advances which are made, there will not be a wholesale swing to DGPS approaches until the system offers at least the same level of performance and integrity as that offered by the ILS or MLS. That it will, eventually, seems beyond doubt - but "eventually" can be a long time in aviation. The developers of the DGPS need that time to develop a reliable system.

None of that is either surprising or new, so was the protracted consultation and debate over ILS replacement, a waste of time and resources? No. Without the debate, there might not have been the compromise of the status quo - those countries which wish to stick with the ILS can; those which wish to move to the MLS can; those which wish to press on with development of the DGPS can.

Without the compromise, there would not be a future for the one universal solution, which has come out of all this, the multi-mode receiver (MMR). With the MMR, the world does not have a universal system, but it does have a universal key, not only to disparate systems but to the peaceful co-existence of those systems.

As long as the MMR, is as good as it should be, an airline will not be faced with choosing one system or several systems to match those in use, at different destinations. Selecting the appropriate landing system on approach should be little more complicated than selecting the appropriate approach frequency on the radio.

The compromise reached in Montreal was the correct one, that it was reached is a credit to the industry and it is rare indeed that that can be said of fudge.

Source: Flight International