The September 11 terrorist attacks on the US have focused attention on whether or not hijacked airliners could be taken over by ground control and remotely brought into land.

To the layman, such a scenario might seem possible, given that the amount of automation and IT on aircraft - or avionics - has been steadily increasing. But it is not quite so simple.

Autopilots have been around as long as jetliners; the AutoLand system, first developed in the 1960s, allows aircraft to land in bad weather, and mid-air collision avoidance transponders are compulsory on all modern aircraft. Direct control between the ground and aircraft has, however, been slow in coming, although there have been developments.

Not surprisingly, it is the military who are at the forefront. The US Navy retrofits its 1960s vintage F4 Phantom fighters as drones for target practice. These fly partly by autopilot and partly from the ground by remote control.

For aerial reconnaissance, the US Air Force has the Global Hawk, an unmanned high altitude aircraft made by Northrop Grumman. Four are flying, of which some have been in operation above Afghanistan. Remote- controlled fighters are more complicated, but both Boeing and Northrop are developing such aircraft.

However, if remote-controlled civilian aircraft are to be developed, communications will need to be improved, and at present, the priority is to provide real-time monitoring from the ground or broadband facilities to passengers, rather than control.

Iridium Satellite, operator of a satellite telephone network, announced in October that it could provide real-time monitoring of voice and data transmitted from airliner cockpits. Not to be outdone, Qualcomm, the mobile telephone operator said that using its Globalstar satellite network, it could not only provide data feeds, but real-time video as well.

"In this day and age it is a little incongruous to dig around in a smoking black hole to find out what happened," says Dan Colussy, Iridium's chief executive, referring to the search for the flight data recorder or black box after an airline crash.

If the data were to be transmitted from the aircraft directly to the ground, they would be available to investigators instantly. Iridium, which comprises some 66 satellites in low earth orbit, says its product, AirSat is the only one as yet to have been approved by the US Federal Aviation Authority for installation on aircraft. It is currently used to provide telephones in corporate jets.

This same system could be used to transmit real-time flight data to the ground. But the expected data rates for this are only 2.4 kilobits per second, Mr Colussy foresees that data will be transmitted to the ground only on the occurrence of some unusual event, as when an aircraft veers off course, is hijacked or is engaged in some unusual manoeuvre.

Expensive

Qualcomm says it can offer data rates of up to 128 kbps, which would be just enough for real-time video. But to offer such rates would mean many network resources, or "transponders" would have to be devoted to that one data stream.

"It will be horrendously expensive," says Brownlee Thomas, senior analyst with Giga Information Group, the analysts. To get even higher data rates on fast-moving aircraft is difficult, but Boeing, the aircraft manufacturer, thinks it can. Connexion by Boeing is its project to bring true broadband data communications to aircraft. The aircraft may be able to receive data at up to 20 megabits per second, but the transmission of data from the aircraft to the ground would be far lower.

Boeing expects Lufthansa to start testing Connexion next year, and to install it on long-haul flights towards the end of 2002 or the beginning of 2003.

Another system will be provided by Tenzing Communications, which signed an agreement with Scandinavian Airlines System on November 4, to start testing its on-board internet access system.

Tenzing does not currently plan to provide connection directly to the internet. Instead, it will store certain data on a server on the airliner which is updated every few minutes. Passengers will have access to their e-mail and other limited web information from the server.

However, both approaches have disadvantages. "The promoters of the various satellite systems ...have made optimistic assumptions about the cost of getting systems into aircraft," says Philip Clinch, director of aircraft operations and navigation at Sita, the provider of IT and telecommunications to the aircraft industry.

The other problem - at least from the perspective of remote control from the ground - is that the systems are asymmetric: ways to get large quantities of information onto an aircraft have been identified, but it has not yet been possible to transmit quickly enough in the other direction.

Sita has been providing technology for data communication between aircraft and the ground since the 1980s, using not only Inmarsat satellites, but also bog-standard VHF radio. It will soon be upgrading its networks to provide data rates of 64kbs, which might be enough to allow a ground-based operator to prevent an aircraft from being flown into the ground.

The problem, however, is security. "The challenge will be to identify how the system can block unauthorised use that would put the aircraft in danger," says Mr Clinch - because, could a terrorist not hack into the aircraft?