RUNNING an orderly air-traffic-management (ATM) system using airways, by definition, confines aircraft to a fraction of the airspace available. At a time when the skies are becoming increasingly crowded - particularly in Europe - any ATM system which fails to use all available airspace is, therefore, giving up part of a valuable resource.

The concept of using all the available airspace, thus giving pilots the potential to choose their own direct routes, is generally known as "free flight". Eurocontrol, the European organisation for the safety of air navigation, says, however, that, even in the USA, which is free flight's prime exponent, the aerospace community is having problems defining what it will be in practice, and how it can be made to work safely.


The need for capacity

There is universal agreement in the air-transport industry that air traffic over Europe, compared with today's levels, will be double what it is today at some point between 2010 and 2015, and keep on growing. Air-traffic services have coped so far by using traditional flow patterns, largely through ATM efficiencies wrought under the European air-traffic-control harmonisation and integration programme, together with a traffic-growth slowdown in the early 1990s, and a little help from the increasing number of aircraft equipped to fly accurate area navigation (RNAV, or offset) tracks.

This cannot last indefinitely, however, even though aircraft suitably equipped to fly reduced vertical-separation minima (RVSM) above flight level 290 (29,000ft/8,800m) will slowly come into operation in great enough numbers to make the transition feasible five years from now. The potentially increased capacity which RVSM could create by reducing vertical separation requirements from 2,000ft to 1,000ft is considerable, because it doubles the available flight levels in precisely the height-band which airlines use en route.

As things stand today, adherence to an airways-based system, enhanced by RNAV, will continue until enabling technologies for something like a free-flight environment have been developed and decided, and their surveillance integrity certificated.

In the USA, free flight is an integral part of the nation's commitment to a global-positioning-system (GPS)-based communications-navigation-surveillance (CNS)/ATM concept for the future, not just for en route navigation, but down, eventually, to precision approaches. Mode-S is the surveillance/datalink medium planned, although using squitter on Mode S transponders (effectively a continuous transmission rather than only responding to secondary-radar interrogation) is accepted as essential for the provision of a free-flight regime.

David Watrous, president of the Radio Technical Commission for Aeronautics, observes that the air-traffic system in use today was designed in the 1950s, and that reduced separation standards through CNS/ATM will provide room for air-transport industry growth to "2012, or maybe 2025" (Air Navigation International, Volume 3, Number 6, 6-23 April, 1997). He has, however, bemoaned the lack of close co-operation between government and industry to provide the means. Speaking at the UK Royal Aeronautical Society in April, he said: "We need an architecture and plan to guide change so we know how to get from where we are to where we are going."

Lacking the USA's confidence in GPS as a sole primary-navigation base for CNS/ATM, and still sceptical about the integrity of any kind of automatic-dependent-surveillance system for surveillance use in high-density airspace, Europe is pursuing a multi-path route to managing the skies 20 years from now. Eurocontrol surveillance systems chief Phil Escrit says: "I'm not a card-carrying member of the fan-club of any of the datalink systems."

Air-traffic control by datalink is essential for the future, says Escrit, although the most important part of the surveillance process is an aircraft's known position relative to other aircraft. Transponder-based surveillance does not depend upon the aircraft's navigation systems to calculate and report its position, which might be reported wrongly, says Escrit. That is the argument for Mode S, he explains, describing it as a transponder-based system designed for surveillance which can also be operated as a datalink. That is why Mode S is seen as a sound candidate for surveillance.

Using terrestrial navigation systems such as distance-measuring equipment (DME) and onboard inertial-navigation systems (INS), Europe has the means for aircraft equipped with electronic flight-management systems to fly accurate four-dimensional (4-D - the fourth dimension is time) trajectories. Eurocontrol says that DME/DME fixes, plus INS, will give accurate enough positioning for some measure of separation reduction, provided that the surveillance system is good. Europe, therefore, has the choice of navigating accurately by terrestrial or global navigation-satellite systems or a combination of both.

Datalinking technology is extremely advanced, and so the communications part of CNS/ATM need not be a problem. The issue, then, comes down again to surveillance. At the moment, primary en route surveillance is carried out by secondary radar, using Mode-C transponders - a system which is reaching the limits of its effectiveness in the busiest sectors during the high season.


Enabling technologies

The technologies exist to provide two-way datalinking between air-traffic-control centres (ATCCs) and aircraft. Mode-S transponders with datalink may be Europe's front-runner, but there is also the very-high-frequency (VHF) Swedish-developed self-organising time-division multiple-access (STDMA) datalink. With either system, it is possible for pilots and air-trafÌc-control staff to uplink, downlink or modify tactically entire 4-D trajectories.

Europe, like the USA, has officially hitched its ATM wagon to Mode S, and that wagon is rolling. The pre-operational European Mode S Station (POEMS) development contract is "well into the design stage", says Eurocontrol. The UK and France have chosen official contractors - Raytheon Cossor and Thomson-CSF, respectively - and Germany is about to choose. Confusingly, however, the European Commission has funded research into an alternative system, the North European automatic dependent surveillance-broadcast (ADS-B) Network (NEAN), based on the Swedish STDMA system. Eurocontrol's Maastricht ATCC, meanwhile, has equipped itself with an experimental STDMA capability.

ADS-B is seen by some airlines in the USA and Europe - notably United Airlines, Lufthansa and SAS - as the answer to Europe's datalink and surveillance needs and the most viable enabler of free flight. The ADS function is the same as that used on certain oceanic routes for oceanic ATCCs to maintain surveillance of aircraft well out of radar range. Aircraft position and "intent" data are linked by satellite to the ATCCs, with an update every several minutes. ADS-B, however, provides an almost continuous broadcast of aircraft position, performance and intent, enabled by STDMA technology. Over land, it does not have to depend upon satellite communications, needing only a network of ground stations within VHF range. Escrit argues, however, that a good ADS system is all very well in uncrowded skies, but does not offer the guaranteed integrity needed for surveillance in high-density airspace.

ADS-B is not reliant on STDMA technology, and the USA plans to achieve it using extended-squitter Mode S. A highly desirable secondary characteristic of the STDMA system, however, is that it uses the GPS as its navigation and timing sources, while its VHF station network provides a regional differential GPS integrity-assurance system almost as a by-product. This, if the NEAN does not come up with unforeseen snags, would make satellite-based wide-area correction systems such as the European geostationary-navigation overlay system unnecessary.

US freighter operators, faced with a compulsory order to fit traffic-alert and collision-avoidance systems (TCAS) like their passenger- operator counterparts, are asking to be allowed to fit ADS-B instead, indicating a strong, independent, level of confidence in the system. On the anti-collision issue, however, there is a strong lobby for the TCAS, which uses Mode-S-band transponders.

It is Eurocontrol's plan to enhance surveillance, by using Mode S for downlinking data, rather than using a two-way link. The latter is demonstrably achievable in the long-run, the agency confirms. Transponder-interrogation rate would be the same as the antenna-rotation rate, which is between 4s and 10s, whereas STDMA's update rate could be virtually continuous. One immediate effect of implementation will be a much-improved short-term conflict-alert (STCA) system, says Eurocontrol.



In 1999, European states are expected to commit to production systems. Eurocontrol has set 2001 for compulsory Mode-S installation in new-build aircraft, and 2003 for retrofit.

The Mode-S data-downlink requirement published by Eurocontrol demands the following information from the aircraft (the first three are already required from Mode C):

- flight callsign;

- transponder capability report;

- altitude in 25ft intervals;

- magnetic heading;

- roll angle;

- track-angle rate (drift-angle rate of change);

- vertical rate of climb or descent;

- true track-angle/ground speed.


Later, upon "the expansion of Mode S services in Europe", the following additional information will be called for:

- selected (as opposed to actual) flight level/altitude;

- selected magnetic heading;

- selected course;

- selected IAS/Mach number.


European free-flight?

Eurocontrol will not put a date on the implementation of free-flight, saying that it has not even been defined yet. Enhanced surveillance, RVSM and RNAV together, however, might deliver something close to the free-flight concept by providing greater system capacity, more "intent" information to controllers and a far more reliable STCA system.

On present official plans, however, the kind of air-traffic situational awareness which STDMA could give to pilots as well as controllers is not on the cards, unless the airlines decide to purchase it in addition to Mode S equipment, or unless the STDMA lobby succeeds in changing the course of an already-rolling Mode-S system.

Source: Flight International