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FORECASTS OF extraordinary growth in civil air traffic have become commonplace. The details vary, but a projected doubling of traffic by 2010 and a tripling by 2020 are widely accepted. There is just one problem - those numbers are not feasible, given the existing operational infrastructure. The problem is worst in Europe, where air traffic growth is already constrained by operational capacity. If the economic development of air transport is not to grind to a halt, a radical improvement in operational techniques has to emerge. The accepted solution, in some shape or form, is "free flight". Implementing free flight, however, requires major technical developments in avionics and air traffic control (ATC). The primary technology required is known as automatic dependent surveillance-broadcast (ADS-B). That is the technique that will provide not only controllers, but pilots too, with a highly accurate display of all nearby traffic complete with "intent" data. The good news is that pioneering European trials of just such a system are yielding robust reliability and demonstrating rapid progress towards a useable ADS-B architecture.

On 14 December, 1996, Flight International was on board when Scandinavian flag carrier SAS made a little bit of history with the first flight of a certifiable ADS-B display in a commercial airliner (see pictures). That display, christened the MMI5000 (as in "man-machine interface") effectively constitutes the first example of an emerging breed of avionics known as the cockpit displav of traffic information (CDTI) - in this case complete with moving map and extensive navigation capability. The CDTIs are expected to be the fundamental cockpit interface in the free-flight regime.

The MMI5000, with hardware by Hectronic (using a Philips colour liquid-crystal-display screen) and software by Carmenta, both of Sweden, replaces a cruder unit used for earlier trials. In SAS' Fokker F28 it is mounted in the centre pedestal and its size is severely constrained as a result - in glass-cockpit aircraft the navigation display screen would be used. Swedish civil-aviation-authority certification officials were also on board the flight from Stockholm Arlanda to approve the MMI5000 on a "no-hazard" basis.

In the F28, MMI5000 is on the centre pedestal

Swedish approval lays the ground for the MMI5000 to be incorporated into the 50% European Commission (EC)-funded North European ADS-B Network (NEAN) programme (Flight International, 7 February, 1996, P37). That is based on the use of the self-organising time-division multiple-access (STDMA) datalink on VHF frequencies to pass the position and intent data.

The unique feature of the MMI5000 is its traffic display functionality, but it is also a navigation display with limited flight-management system-capability. The result is that, on the one display, a pilot has a moving map, overlaid with navigation symbology and radar-like traffic data, with access, via "pop-up" menus, to flight plans, real-time meteorological reports, navigation aids, airfield data, a track deviation indicator and so on.

The air-to-air ADS-B functionality operates independently of -ATC's surveillance of the aircraft but, by adding VHF/STDMA ground stations to the equation, the same data can be used instead of radar to feed controllers. The concept is similar to the way that ADS is being introduced to bring surveillance to oceanic, airspace via satellite datalinks. The difference is that, whereas in oceanic operations the ADS-positions are sent at ATC-requested intervals, typically of several minutes. ADS-B reports are virtually continuous - typically being broadcast at 1s intervals. That ATC concept is incorporated in NEAN, and is being independently developed by the Swedish CAA.

A major feature of the Swedish and NEAN work is that it uses the global-positioning system (GPS) as its fundamental navigation and timing aid. This arises in two ways. Firstly, the aircraft involved are fitted with a Swedish developed GPS Transponder which uses GPS as its navigation source and, crucially, as the timing reference for the STDMA datalink. It is GPS' extremely accurate timing mechanism accurate to 340ns - which lies at the heart of the STDMA concept.

Secondly, because the ground stations by definition use STDMA equipment, they can also be used to transmit differential GPS (DGPS) corrections over their area of coverage. The result is that the Swedish/NEAN programme effectively provides a regional-area DGPS service - and much more economically than currently proposed satellite-based wide-area systems. Furthermore, the GPS Transponder is cheap enough to be practical for surface surveillance. At Arlanda, snow ploughs and runway friction-testers are already equipped as part of the trials. With the vehicle positions known, aircraft can be cleared to taxi sooner after snowclearing than has normally been the case.

The SAS team says that it is hard to know where to begin in listing the potential operating benefits of ADS-B. In the long term, it sees ADS-B as the basis for free flight, but before then it sees it as a valuable situational-awareness tool in approach operations to maximise runway throughput. SAS flight standards and development technical plot, Lars Lindberg, says: "I see ADS-B as a way of dividing responsibility between controllers and pilots. Today, on the approach, controllers ask me if I can see the runway and the aircraft ahead and then give me a visual clearance. But I could be looking at the wrong aircraft, and, anyway, it is hard to judge his speed, whereas with ADS-B I can be sure and should be able to get some kind of reduced separation." There are safety benefits too. "Just by having ADS-B you have better situational awareness", says Lindberg. He notes that it particularly helps solve the often raised issue of the loss of the valuable "party-line" effect when voice communication is replaced by datalinking.

Lindberg says: "We [SAS] are spending money on this because we see the benefits of this system as being very important for us to improve safety and capacity. We are up to the limit of capacity in some areas of Europe - and we are also acquiring new aircraft. Most other carriers are doing that, too. We have to create capacity to have growth in the system. This system addresses the crucial areas."

The airborne ADS-B functionality really does work. On the Stockholm flight one of the other GPS Transponder-equipped aircraft was observed on the display, complete with a vector arrow showing that it was climbing. That was no surprise to the Swedish team which has seen the system working successfully for most of the past year. At Arlanda, the researchers have watched aircraft climb out of Karlstad, 255km (140nm) west, as they pass about 4,000ft (1,220m). Lindberg says that "at normal altitude" the system has around the 460km range expected from a VHF-based architecture.

Early in 1997, the true performance of the NEAN architecture will become clearer as the ground network is completed. The Swedish network is already fully functional; the Danish element is complete and now being connected, and Germany is close behind. An additional station at Maastricht in the Netherlands is to be used for Eurocontrol upper-airspace trials under the PETAL 2 datalink communications trial, and an STDMA station is being implemented at Brussels as a standalone project. Other stations are coming to Eurocontrol's experimental site at Bretigny, France and at Rome as part of the ECs FARAWAY project and Madrid for its SUPRA programme.

By the end of 1997, 12 aircraft will be equipped with GPS Transponders, including the two SAS F28s; two Golden Air of Sweden Saab 340s (one is already equipped); a Maersk Helicopter of Denmark aircraft; at least one Lufthansa aircraft; a German business jet sponsored by the Aeroplane Owners and Pilots Association; another general-aviation aircraft as part of SUPRA, two Fairchild Metros of German commuter carrier OLT, and a Danish Nord 262 flight check aircraft. The FARAWAY team hopes to have three Alitalia McDonnell Douglas MD-80s equipped with at least the GPS Transponder and, possibly, the CDTI. Some 30 airport vehicles are to be equipped in Denmark, Germany and Sweden. SAS and Lufthansa are focused on ADS-B, but the Swedish work is also producing valuable data about the overall performance of the STDMA datalink and the navigation capability. SAS declares itself "pro-ADS-B", as opposed to pro-STDMA", but it is clear that the trials have anyway overwhelmingly supported the key claims of STDMA supporters in International Civil Aviation Organisation forums and elsewhere - particularly against proponents of alternative datalinks such as Mode S. Lindberg comments: "We were reluctant to do STDMA for a long time. Mode-S has benefits compared with STDMA, but we think the disadvantages are more than the advantages." That comes as good news to Lockheed Martin, which is still in deep discussions with the Swedish Space Corporation over the commercial exploitation of STDMA and the GPS Transponder in aviation. Lindberg, himself a denizen of the European Organisation for Civil Aviation Equipment working groups, concedes the "political" difficulties that STDMA raises, but robustly declares: "As an airline representative, I find it totally unacceptable to dismiss, on the basis of anything other than pure facts, a system which could fulfil our requirements."

The navigation possibilities are also of great importance. SAS and the Swedish CAA plan during 1997 to implement four GPS routes and also to construct a GPS non-precision approach to runway 32 at Angleholm. The procedure would replace the current complex NDB approach. Lindberg says: "It is not a good approach and it is one of the ones we want to do something about."

SAS is understandably excited about the possibilities now arising, but stresses repeatedly that the rationale for the work is fundamentally economic. Lindberg says: "Most airlines' operational goal is to offer safe, punctual service at a competitive price. So their requirement for a future CNS/ATM (communications navigation surveillance/air-traffic management) system will not only be that it increases safety and capacity, but that it allows full use of the actual performance of our aircraft - that is free flight."