Science: Groping in fog for the right approach: While aviation authorities argue about new auto-landing systems, Britain could be losing vital air traffic to European airports, writes Steve Homer

AS YOU land at a fogged-in airport, you may not realise that the pilot has let go of the wheel. But with the right equipment, aircraft can make a safe landing even in the densest fog.

Auto-landings are not commonplace, but having the option to use them is important, because if a landing had to be delayed due to the weather conditions this would have a severe knock-on effect on other flights. However, the instrument- landing system (ILS) that is essential for such zero-visibility landings, and for bad weather landings generally, is under threat, because of indecision and foot-dragging by the world's civil aviation authorities.

ILS could become obsolete before a replacement is ready to take over. This could mean that British airports, which are prone to visibility problems, could lose out in competition with continental competitors.

Having invested millions in today's ILS technology, airports and airlines are able to deliver passengers safely to the runway without worry. That at least is the theory. But the radio frequencies used by the ILS system are covered by international treaty, and the International Civil Aviation Organisation (ICAO) has agreed to give up some of them by 1998.

Today's ILS is based on a system designed in the Thirties. A tightly focused beam of UHF and VHF radio frequencies is detected by the aircraft, which then 'follows the beam home'. But it is a narrow beam and can often be difficult to find, particularly in bad weather.

ICAO adopted a new, more flexible, system called Microwave Landing System (MLS) in 1970. MLS offers many advantages over today's ILS. The most important are that it can be picked up over a wider area, guiding pilots to the correct 'glidepath' for their descents to the runways, and that the system is less susceptible to interference. MLS also incorporates a data channel, so information can be broadcast to aircraft as they approach.

In trials conducted by the Civil Aviation Authority (CAA) and the Defence Research Establishment, this data channel has been used to transmit complicated approach paths to runways. This could extend operating hours, as they could use routes at night that avoid the built-up areas where noise is a problem.

The world was supposed to start using MLS in 1985. This did not happen and today only a few test MLS systems are operational.

The need for a replacement system for ILS is becoming urgent. The radio spectrum is very crowded. With broadcast television and radio, cellular phones, microwave transmission systems, satellite communications, emergency service radios and the like, the spectrum is divided into small chunks and carefully controlled. In order to be allowed to use the necessary MLS frequencies, the ICAO had to agree with international radio authorities to give up some of the 'guard' frequencies that had been left free to protect ILS equipment from interference. These changes are supposed to take place in five years' time but already there seem to be problems in some countries with transmitters affecting ILS frequencies.

While all the world's major civil aviation authorities officially support MLS, a row is breaking out because eventually satellite-based systems will be accurate enough to guide an aircraft to the ground. The problem is with the 'eventually'.

Using the American GPS and Russian Glonass satellite navigation systems, and communications satellites, aviation authorities are creating a satellite navigation, surveillance and communications system, part of which could be used for an aircraft- landing operation. While the systems in operation today only have an accuracy of around 100 metres - too inaccurate for a landing system - tests have shown that by adding a ground- based transmitter that sends out a correcting signal, results can be produced almost good enough for a basic approach. But there are still two problems - the reliability of the system and its operational accuracy.

Different ILS systems are designed to cope with different degrees of severe weather. They are divided into three categories, depending on the height from which the pilot can see the runway and decide if it is safe to land. Category I is for fairly straightforward landing with visibility of at least 200ft, while Category III is for landings with the least visibility and is further divided into a and b, with IIIb for 'zero decision height', when the pilot does not see the runway before the wheels touch down. There are only 17 airports fitted out to this standard in the world; all are in the UK.

There is no chance of satellite systems operating to Category III before the end of the century. With unknown interference and operational characteristics at airports, it will just not be possible for these satellite systems to be certified in time. Further, there is still much work to be done on the accuracy of these systems and their susceptibility to interference.

Maintaining Category IIIb operation in the UK is vital. Many flights leave for airports that are fogged in, and for which Category III conditions apply, but arrive to a clear airport - a common scenario on British winter mornings. If there is no replacement for ILS, these planes will have to wait at their departure airports until the rain or fog clears from their destination. That is not a recipe for happy holidaymakers.

There are some who argue for a third course. Keep ILS operational at those airports that need a Category II/III operation, and migrate the others over to a GPS system that would be good enough for Category I.

Proponents of this approach argue that there would be only a slight possibility of interference affecting ILS. They point out that the 'glideslope', providing the angle of descent, is supplied by UHF frequencies, which will not be changed; also, that if the VHF 'localiser' signal, which gives the centre line of the runway, was momentarily lost due to interference, the aircraft would simply have to perform a standard 'go-around' manoeuvre.

What is more, ILS equipment should be better able to 'reject' spurious VHF signals with improved hardware on the aircraft: this is on the way, as all new aircraft from 1 January 1995, and all existing aircraft from 1 January 1998, must have improved ILS systems fitted that should prove more resistant to interference.

It now appears that the Department of Trade and Industry, which controls the allocation of frequencies in the UK, will protect ILS frequencies by continuing to restrict VHF transmissions around airports, as pressure on the relevant frequencies is not particularly high. The international situation, however, is patchy.

Politically, both the ICAO and the CAA are now wedded to MLS and the relevant committees are staffed with members linked to established paths, that is, to the MLS approach.

However, at a meeting in Geneva in March, the American FAA made a statement, implying that the FAA believed it could go directly to a satellite system, at least for its reasonable visibility (Category I) needs. This is a blow to MLS's backers.

There is no clear right or wrong on what approach should be taken for future landing systems, but the question is of crucial importance to the UK. Not only does Britain have the most densely packed ILS map in the world, with a total of 74 ILS runways, but our major airports also need a fully operational ILS system to offer year-round viability. Without that, traffic will be lost to other airports in Europe; and once lost, it is unlikely to return.