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 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 ICAO 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 1090 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 (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 link and of CPDLC will use VDL Mode 2.
Read more about this topic: VHF Data Link
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