The VHF Data Link or VHF Digital Link (VDL) is a means of sending information between aircraft and ground stations (and in the case of VDL Mode 4, other aircraft). Aeronautical VHF data links use the band 117.975–137 MHz assigned by the

International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...

Aeronautical mobile (R) service Aeronautical mobile (R) service (short: AM(R)S; | also: ''aeronautical mobile radiocommunication (R) service'') is – according to ''Article 1.33'' of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defin ...

. There are ARINC standards for ACARS on VHF and other data links installed on approximately 14,000 aircraft and a range of

ICAO The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...

standards defined by the Aeronautical Mobile Communications Panel (AMCP) in the 1990s. Mode 2 is the only VDL mode being implemented operationally to support

Controller Pilot Data Link Communications Controller may refer to: Occupations * Controller or financial controller, or in government accounting comptroller, a senior accounting position * Controller, someone who performs agent handling in espionage * Air traffic controller, a person w ...

(CPDLC).

ICAO VDL Mode 1

The ICAO AMCP defined this Mode for validation purposes. It was the same as VDL Mode 2 except that it used the same VHF link as VHF ACARS so it could be implemented using analog radios before VHF Digital Radio implementation was completed. The ICAO AMCP completed validation of VDL Modes 1&2 in 1994, after which the Mode 1 was no longer needed and was deleted from the ICAO standards.

ICAO VDL Mode 2

The ICAO VDL Mode 2 is the main version of VDL. It has been implemented in a Eurocontrol Link 2000+ program and is specified as the primary link in the EU

Single European Sky The Single European Sky (SES) is a European Commission initiative that seeks to reform the European air traffic management system through a series of actions carried out in four different levels (institutional, operational, technological and contr ...

rule adopted in January 2009 requiring all new aircraft flying in Europe after January 1, 2014 to be equipped with CPDLC. In advance of CPDLC implementation, VDL Mode 2 has already been implemented in approximately 2,000 aircraft to transport ACARS messages simplifying the addition of CPDLC. Networks of ground stations providing VDL Mode 2 service have been deployed by

ARINC Aeronautical Radio, Incorporated (ARINC), established in 1929, was a major provider of transport communications and systems engineering solutions for eight industries: aviation, airports, defense, government, healthcare, networks, security, and t ...

and

SITA Sita (; ) also called as Janaki and Vaidehi is a Hindu goddess and the female protagonist of the Hindu epic, ''Ramayana''. She is the consort of Rama, the avatar of the god Vishnu, and is regarded as a form of Vishnu's consort, Lakshmi. She ...

with varying levels of coverage. The ICAO standard for the VDL Mode 2 specifies three layers: the ''Subnetwork'', ''Link'', and ''Physical Layer''. The Subnetwork Layer complies with the requirements of the

Aeronautical Telecommunication Network The Aeronautical Telecommunication Network media access control In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...

'' (MAC) sublayer. The Data Link protocol is based on the ISO standards used for dial-up

HDLC High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...

access to

X.25 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts ...

networks. It provides aircraft with a positive link establishment to a ground station, and defines an addressing scheme for ground stations. The MAC protocol is a version of

Carrier Sense Multiple Access Carrier-sense multiple access (CSMA) is a medium access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus or a band of the electromagnetic s ...

(CSMA). The VDL Mode 2 Physical Layer specifies the use in a 25 kHz wide VHF channel of a modulation scheme called Differential 8-

Phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...

with a symbol rate of 10,500 symbols per second. The raw (uncoded) physical layer bit rate is thus 31.5 kilobit/second. This required the implementation of VHF digital radios.

ICAO VDL Mode 3

The ICAO standard for VDL Mode 3 defines a protocol providing aircraft with both data and digitized voice communications that was defined by the US FAA with support from Mitre. The digitized voice support made the Mode 3 protocol much more complex than VDL Mode 2. The data and digitized voice packets go in Time Division Multiple Access (TDMA) slots assigned by ground stations. The FAA implemented a prototype system around 2003 but did not manage to convince airlines to install VDL Mode 3 avionics and in 2004 abandoned its implementation.

ICAO VDL Mode 4

The ICAO standard for VDL Mode 4 specifies a protocol enabling aircraft to exchange data with ground stations and other aircraft. VDL Mode 4 uses a protocol (Self-organized Time Division Multiple Access, STDMA, invented by Swede

Håkan Lans Anders Håkan Lans (born 2 November 1947 in Enskede) is a Swedish inventor. He holds two patents: * a memory controller for a framebuffer: "Data processing system and apparatus for color graphics display". Framebuffer with memory controllers had ...

in 1988) that allows it to be self-organizing, meaning no master ground station is required. This made it much simpler to implement than VDL Mode 3. In November 2001 this protocol was adopted by

as a global standard. Its primary function was to provide a VHF frequency physical layer for ADS-B transmissions. However it was overtaken as the link for ADS-B by the Mode S radar link operating in the 1,090 MHz band which was selected as the primary link by the ICAO Air Navigation Conference in 2003. The VDL Mode 4 medium can also be used for air-ground exchanges. It is best used for short message transmissions between a large number of users, e.g. providing situational awareness, Digital

Aeronautical Information Management Aeronautics is the science or art involved with the study, design, and manufacturing of air flight–capable machines, and the techniques of operating aircraft and rockets within the atmosphere. The British Royal Aeronautical Society identifie ...

D-AIM
, etc.. European Air Traffic Management modernization trials have implemented ADS-B and air-ground exchanges using VDL Mode 4 systems. However, on air transport aircraft the operational implementations of ADS-B will use the

Mode S The aviation transponder interrogation modes are the standard formats of pulsed sequences from an interrogating Secondary Surveillance Radar (SSR) or similar Automatic Dependent Surveillance-Broadcast (ADS-B) system. The reply format is usually refe ...

link and of CPDLC will use VDL Mode 2.

References

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External links

Mode 2:
Eurocontrol Link 2000+ ProgrammeEurocontrol Single European Sky
Mode 4:
GP&C Systems International AB

Airbands Avionics Aircraft instruments