Aircraft Communications

The term VHF Digital Link was adopted by the International Civil Aviation Organization (ICAO) Aeronautical Mobile Communications Panel (AMCP), at its first meeting in November 1991, to refer to digital communications carried on the Aeronautical VHF band. The Aeronautical VHF Band is the section of the Very High Frequency spectrum allocated to Aeronautical Service by the International Telecommunications Union. It is made up of the following two groupings: 108-118 MHz assigned to the purpose of radio-navigation and 118-137 MHz which is used for radio-communications.

The plan for VHF band to become a data carrier was proposed in the ICAO Future Air Navigation Systems (FANS) committee report issued in 1988. The airline community had recognized the benefits of aircraft data link communications 10 years before the FANS report and had implemented the VHF version of ACARS.

The ICAO reserved four VHF channels: 136.900, 136.925, 136.950, and 136.975 MHz for data communications worldwide. This later decision catered for the reservation of frequencies in order for such data service to be implemented in an environment when the existing aviation VHF spectrum was considered saturated and congested with existing VHF channels for air traffic analog voice.

VHF Digital Link (VDL) was put in motion by 1991 through the effort of the Aeronautical Mobile Communications Panel (AMCP) with a plan to increase the capacity of the VHF band; and develop a standard for an Aeronautical Telecommunications Network (ATN) data link service using VHF radio.

The AMCP has developed standards for VDL Modes 1-4 in which the modes provide different capabilities which currently have political divisions.

Mode 1 was created with the intent of using analog radios incorporating a device to install a coded signal on the existing carrier wave. This mode was never carried forward as analog radios were already viewed as dinosaurs. Further considerations brought about Mode 2 and 3 which present the position of the FAA wanting to use all new digital radios with a 25 kHz frequency spacing and the Eurocontol concept of going with 8.33 kHz frequency spacing.

On air transport aircraft, the communications, navigation, and surveillance functions will continue to be processed by separate devices:

• Communications: VHF data radio or a satellite data unit;
• Surveillance: Mode S transponder; and GNSS data link;
• Navigation: Global positioning systems and instrument landing systems multi mode receivers.

VHF Digital Link Mode 2 (VDL-2)

VHF Digital Link Mode 2 (VDL-2) was conceived in the early 1990s as a method of providing high-speed data communications to aircraft. From the outset VDL-2 was intended to support safety critical Air Traffic Control communications. In addition, airline operational data would also be supported by VDL-2; a service traditionally supplied using ACARS.

Global deployment of VDL-2 is now rapidly gaining acceptance by the airlines and Air Traffic Control agencies. In the United States, the FAA has been using VDL-2 in the Miami region with selected airlines prior to a nationwide rollout of the system. In Europe, Eurocontrol has strongly endorsed the operational benefits of VDL-2. Eurocontrol is offering financial assistance to help airlines install VDL-2 equipment, reduced route charges will follow for properly outfitted aircraft, and finally a mandate will be introduced to bring about full compliance, possibly, by the end of the decade.

The Swedish CAA and its commercial arm, Swedavia, promote the fact that the VDL Mode 4 system can simultaneously support communications, navigation, and surveillance applications. The combined functions used in the VDL Mode 4 system may be its greatest deterrent as each function has specific performance requirements that cannot be met by a combined system. Blending the CNS functions in a single Line Replaceable Unit (LRU) increases the probability that an aircraft will lose more than one of the functions, due to the failure of a single piece of airborne or ground-based equipment.

The FAA and Eurocontrol are in the process of creating a mutually agreeable “Future Communications” study that will address the requirements for a global future communications system to enable the implementation of enhanced ATM applications from today through 2030.

Who would have ever guessed that when Charles Taylor enabled the Wright Brothers to take to the air and then signaled them using hand gestures that he was in fact the first air traffic controller. Subsequent communications devices have been evolving as fast as aviation technology. I have always known aircraft technicians were great communicators in their own right.