Necessity

The standard method of communication between an air traffic controller and a pilot is voice radio, using either VHF bands for line-of-sight communication or HF bands for long-distance communication (such as that provided by Shanwick Oceanic Control).

One of the major problems with voice radio communications used in this manner is that all pilots being handled by a particular controller are tuned to the same frequency. As the number of flights air traffic controllers must handle is steadily increasing (for instance, Shanwick handled 391,273 flights in 2006, an increase of 5.4% - or 20,000 flights - from 2005[1]), the number of pilots tuned to a particular station also increases. This increases the chances that one pilot will accidentally override another, thus requiring the transmission to be repeated. In addition, each exchange between a controller and pilot requires a certain amount of time to complete; eventually, as the number of flights being controlled reaches a saturation point, the controller will not be able to handle any further aircraft.

Traditionally, this problem has been countered by dividing a saturated Air Traffic Control sector into two smaller sectors, each with its own controller and each using a different voice communications channel. However, this strategy suffers from two problems:

• Each sector division increases the amount of "handover traffic". That is the overhead involved in transferring a flight between sectors, which requires a voice exchange between the pilot and both controllers, plus co-ordination between the controllers.

• The number of available voice channels is finite, and, in high density airspace, such as central Europe or the Eastern Seaboard of the USA, there may not be a new channel available.

In some cases it may not be possible or feasible to further divide down a section.

A new strategy is needed to cope with increased demands on Air Traffic Control, and data link based communications offers a possible strategy by increasing the effective capacity of the communications channel.