A Swedish-led technology could provide a key element of the Future Air Navigation System.

Kieran Daly/LONDON

THE GLOBAL-NAVIGATION satellite-system-synchronised, self-organising, time-division, multiple-access (STDMA) data- link really needs a much better name. It is one thing for the dedicated souls serving on the International Civil Aviation Organisation's (ICAO) technical committees to cope with this sort of linguistic horror, but there is now good reason to think that a much wider aviation community will have to incorporate "STDMA" into its vocabulary. Indeed, it may become one of the most important technical elements of the Future Air Navigation System (FANS).

In the rarefied world of the FANS planners, data-links are the number one technical concern. Data-linking pervades the entire philosophy behind the FANS and, even now, new applications for it are emerging: as a replacement for voice communications; to pass position reports via satellites; to convey intentions; to enable satellite-aided approaches; and to assist with airport surface-surveillance. The list goes on, but one major potential result is a proliferation of types of data-link, each with its own infrastructural and avionics requirements.

The problem has been recognised for some time, but there is a desperate need for the FANS and, since many of these data-links are technically reasonably mature, air-traffic-management (ATM) planners have been tempted to press ahead with their introduction. Throughout all this, there has always been the niggling knowledge that the STDMA data-link appeared to offer a robust, all-encompassing, alternative. At least three issues have kept it in the background, however: its aviation application was less mature, it was poorly understood and its primary champion - Sweden - was a relatively small player in a FANS world dominated by the USA, UK and France.


This was not for want of effort by the Swedish civil-aviation authority. The director of international projects of its Swedavia arm, Johnny Nilsson, has become one of the most enduring fixtures at the host of conferences and committees where FANS issues are thrashed out. Voluminous test-data under one arm, lap top computer under the other, Nilsson has briefed, and rebriefed anyone who will listen. His reception has varied from bafflement to respect, but, above all, there has been a reluctance to come to terms with the implications of what he is saying.

That may be about to change. Not only has the frustratingly slow pace of FANS implementation let the Swedish technology catch up, but renewed interest in another FANS concept - automatic dependent surveillance-broadcast (ADS-B) - has focused new attention on STDMA work. ADS-B could fundamentally transform ATM - and it is accordingly controversial, particularly when linked with so-called "free-flight" concepts. In some circles, however, it is seen as perhaps the only way to win substantial ATM efficiency-gains in the type of high-density airspace, where it is proving difficult-to-extract benefits from the FANS - notably Europe. Crucially, the STDMA data-link is well suited to the ADS-B application.

ADS-B-proponents, with Swedavia prominent among them, envisage an ATM system in which the positions and intentions of aircraft in the air and on the ground are constantly broadcast - by data-link - to each other and to air-traffic control. Everyone in the system has real-time access to precisely the same data, via similar displays, with a consequently vast improvement in situational awareness. The STDMA data-link is a prime candidate for the resulting robust, time-critical, communication requirement. Nilsson, understandably, is pleased to see the progress being made, declaring: "I think that the message is starting to come across: it is very, very difficult to have situational awareness throughout all phases of flight without STDMA [datalinking]. I also think that there is now a fairly mature understanding that the data-link issue is one of the most critical ones for these new technologies. But most people in aviation have been so used to thinking of a dedicated system for each special type of application that they have not really thought about a system where you can get a multitude of applications."

An important quality of the STDMA data-link for aviation users is that its use of global-navigation-satellite systems (GNSS) - primarily the global-positioning system (GPS) for now - ensures extremely precise (sub-second) co-ordination between the communicating stations. The principle of the STDMA data-link is that access is divided into discrete time-slots - each equating to one message of up to 32 bytes at 9,600 bits/s (a basic ADS message for example). Longer messages, such as text, use more than one slot. The system is now capable of handling 9,000 32-byte messages/min and has a 25kHz bandwidth. Nilsson says, that not only is the concept superior to the carrier- sense multiple access data-link envisaged for communications, but it can also replace the Mode-S data-link for surveillance (and for traffic alert and collision avoidance, for that matter), and it is capable of handling differential GNSS data. for precision approach guidance or for surface-surveillance purposes.

He explains: "STDMA/GNSS is a toolbox and, from that, you can build almost any application. I think that message is starting to matter in the airline industry. So the question is what are the data-link options? I would say that no one disagrees that STDMA is a very, very, good option. Then people start to look at the other options, but this is the only one of the future technologies that works." He concedes that, for example, Mode-S also works (for surveillance at least), but he comments: "The question then is 'can VHF STDMA do the same thing?' and the answer is yes."


Nilsson accepts that he is, to some extent, in a race against time, but comments: "If the focus had been on STDMA, then there could probably be systems out there working already." In fact there are, but not in the air-transport world. As with the GNSS, air transport may be the most demanding customer for the technology, but it will ultimately be among the most numerically insignificant. STDMA technology is already in limited use in the maritime- and surface-transport communities. Furthermore, helicopter operators in Malaysia and Scandinavia are pursuing it as a way of passing operational data and bringing surveillance into offshore operations (Flight International, 12-18 April, 1995).

In the air-transport world, STDMA activity is about to be stepped up substantially. By now, a Scandinavian Airlines System (SAS) Fokker F28 should be flying in revenue service with Swedavia's "GNSS/STDMA transponder" fitted, together with a dedicated cockpit-screen for ADS-B use. The SAS programme is one element of the most extensive aviation-STDMA evaluation to date - the North European ADS-B Network (NEAN). The programme is 50% funded by the European Commission (EC) and falls under the ATM item in the EC's Trans-European Transport Network project. The work programme is to demonstrate the benefits of ADS-B systems to the user, to develop and validate the technology for ATM and to establish a cost database. Eventually, as many as 15 aircraft are due to be involved but, in the short term, they will include at least two SAS F28s, one now equipped and one to be ready later this month, a Golden Air of Sweden Saab 340, a Lufthansa Airbus type and possibly a Boeing 737. Lufthansa is a keen supporter of ADS-B as a possible part of future ATM. Talks are under way with a Danish commuter carrier as well.

The current ground stations are at Stockholm Arlanda, Stockholm Bromma, Norrkoping, Jonkoping, Gothenburg and Malmo Sturup in Sweden; Braunschweig, Bonn, Bremen, Berlin and Frankfurt in Germany; and Copenhagen, Aalborg and Billund in Denmark. Most of the Swedish and Danish stations, plus Braunschweig, are already operating. Eventually, it is intended to have ten to 15 airborne units and 20-25 ground stations under the NEAN effort. The Swedish, Danish and German civil-aviation authorities are the main partners - together with the airlines - plus the UK Civil Aviation Authority for certification and validation work. If the STDMA concept is to enter service, and particularly if ADS-B is to become reality, a great deal of work will be required to hone the appropriate displays and controls in the air and on the ground. Nilsson notes: "There will be a lot of issues on the human-machine interface [HMI] to determine exactly how the ATM workstation should be and how to address the HMI in the cockpit."

At SAS, F28 chief technical pilot Lars Lindberg is chairman of a project group looking at new navigation aids, and he is specifically responsible for the NEAN input. SAS is incorporating its NEAN activity into a wider programme, looking at GPS navigation and differential GPS (DGPS) approaches - using the STDMA data-link to pass the differential corrections. That comes about because the GNSS/ STDMA transponder includes a GPS navigation function, and the cockpit processor has a limited flight-management-system capability. Lindberg explains: "The primary target is to demonstrate ADS-B capability, but also to use some of the navigation information from the system." In the second phase of SAS' programme, there will be greater concentration on the navigation function, using the GPS data and, in the third phase, the airline will use DGPS to conduct approaches to certain Scandinavian airports where the only options now are non-directional-beacon-aided approaches or other non-precision procedures.


The F28's airborne equipment consists firstly of the GNSS/STDMA transponder, which incorporates the GPS; a communications element with STDMA processing; and a VHF transceiver. The GPS receiver is an off-the-shelf unit, which is incorporated into the transponder, but production transponders would not necessarily have the GPS integrated inside. Mounted in the cockpit centre-pedestal of the thoroughly analogue F28, is the "MMI" - a colour screen with a 486 processor and integral database. The initial transponders, are scheduled to be replaced, with updated units, which will comply with draft ICAO standards and recommended procedures.

At SAS, the programme could have radical consequences for operating procedures. Lindberg says: "We have been following this and other things for several years, and it is our view that ADS-B could be a crucial part of the CNS/ATM environment." He adds: "In Scandinavia, we have quite a few airports with a precision approach at one end and not at the other. We know that controlled flight into terrain is a big issue and a big problem - so we are very interested in this." The data-link debate has a long way to go yet, but there is no doubt that Johnny Nilsson and his colleagues at Swedavia have changed the agenda.

Source: Flight International