“But now it works perfectly”, adds Michael Zaddach. “They adapted to this feature and it works perfectly. But the first time it was really heavy weather – 400?ms is not so much, but our apron controllers are very special.”

Base stations

The TETRA infrastructure at the airport is now very extensive. Five Motorola TETRA base stations linked to a central Dimetra IP switch cover the whole airfield, right out to the perimeter fence and to some distance beyond. Overlapping coverage between the base stations helps to ensure that communication will be maintained even if
one of them fails; users’ radios will simply roam to another station.

Each base station transmits five TETRA radio carriers, four for voice and one for data only. These provide 15 channels for voice traffic at each site. Michael Zaddach comments that this large capacity might seem somewhat luxurious in times of normal operation, but it becomes very important when the airport is under stress – for example, in adverse weather, when flights are delayed, or when there is an alarm condition.

Within the airport buildings, a comprehensive indoor distribution system incorporating radiating cables (‘leaky feeders’) ensures excellent radio coverage even on the underground levels of the terminals.

In addition, the TETRA network is interconnected with the telephone system and with control centre applications for the airport’s security and fire service. It also includes some dispatcher workstations – including two PC stations for dispatching the passenger shuttle buses – and interfaces for voice recording.

Plans exist to enlarge the system with a further TETRA base station to the north if the third runway is built. With the IP-based Dimetra system, installing it is likely to be a fairly simple task.

Service structures

Users across the airport campus buy their radio service on a subscription plan with three levels of service – economy, business and premium. They are offered a choice of Motorola TETRA terminals – a handportable and a vehicle-mounted radio, with or without GPS. On top of this, users can choose their grade of network access: basic radio communications only, radio plus internal telephone interconnect, external telephone calls, data services.

Prices on this menu begin at €52 per month for a basic ‘economy’ radio service – a rate which, as Michael Zaddach freely acknowledges, is noticeably more than an ordinary mobile phone would cost. “Many people ask us, ‘Why is it so expensive?’”, he says. “And that’s a big problem for us, especially for users that need no very high availability and reliability and so on. But we have set up an airport usage policy – and we have set up a clause in this policy that everyone who is on the apron needs a TETRA device, especially for emergency situations.”

In such an emergency, he says, a user carrying only a mobile phone might be out of contact, which could be dangerous.

Also for safety reasons, he continues, the department does not normally allow user bodies to bring private radio systems to the airport, but insists that they use the TETRA network instead. “If the customer wants to introduce his own radio system, he has to get an admission from the airport and of course we have control of all the frequencies used here. And we get major troubles if we allow anyone to bring their own radio system. We have many radio systems here – for air-to-ground communication, for special purposes, for the authorities and so on. So it’s
very restricted.”

He adds that the airport already has to deal with TV productions at the airport and celebrities arriving: they want to bring their satellite trucks and these need to communicate. A special office at the airport deals with such requests to ensure that they will not cause problems. “You have to check very carefully the frequencies they are using, otherwise you get interference with the important systems in the airport”, Zaddach comments.

For a premium services such as external telephone calls, users pay for access to the telephone interconnect, plus call charges at a rate comparable to normal telephony costs. But user groups can save money by choosing their service grades appropriately to match the speed of response they need in the event of a fault. Robert Wondra cites an example from among the ground handlers. “We have 50 radios – ten radios for the team leaders and 40 radios for the loader personnel”, he says. “The 40 radios are usually on the economy tariff because, if there are 39 other radios around, it doesn’t matter. The ten team leaders have radios on the business tariff – and if one radio goes broken, the team leader has a changed radio the next day.”

Comparing costs

Reviewing some of the lessons learned from the project, Michael¡ Zaddach highlights the surprising fact that the TETRA network is working out cheaper than the old analogue installation. “That’s not very easy to see, because the old system was introduced in 1992 and we had used it for such a long time”, he says. “So the depreciation of the old infrastructure was zero. And if you compare the price, the new system is more expensive, for sure. But if you compare it by the depreciation time, the new system is much cheaper.
(中国集群通信网 | 责任编辑：陈晓亮)