FAQ - VDL Mode 4

What is VDL Mode 4?
VHF Digital Link (VDL) Mode 4 is a VHF data link technology standardized by ICAO and ETSI, providing digital communication between mobile stations (aircraft and airport surface vehicles) and between mobile units and fixed ground stations. The data link is designed to support CNS/ATM digital communication services, including time- and safety-critical broadcast applications as well as point-to-point communication. It is characterized by very high delivery probability from the unique self-organizing TDMA (SOTDMA) function.

The data link transmits digital data in a standard 25 kHz VHF communications channel and divides the communication channel into a large number of time slots. The start of each slot is an opportunity for a station to transmit. As a result of this ‘self-organizing’ protocol, VDL Mode 4 is capable of operating outside the coverage of a ground infrastructure and can therefore support air-air as well as ground-air data communications and applications.

What is ADS-B?
Automatic Dependent Surveillance-Broadcast (ADS-B), is the core service of the CNS Network.
This service enables all units in operation to automatically send periodic reports with information about their identity, 3-dimensional position, velocity, intent and other pertinent data. Messages also include parameters required for the data link synchronization and slot allocation.

ADS-B messages are received and processed by all VDL Mode 4 units within radio range:
  - A Ground Station receives ADS-B messages and translates them into ASTERIX Category 21 format for transmission to the ground network. This data provides surveillance information about all VDL Mode 4-equipped mobile units within radio range of the Ground Station.
  - A mobile unit receives ADS-B messages from all VDL Mode 4-equipped mobile units within radio range. By connecting a CDTI/display to the data link, the pilot/driver can display ADS-B surveillance information.

What is GNS-B?
The GNSS augmentation Service – Broadcast (GNS-B) – is an uplink service intended to support a range of applications such as airport surface surveillance, terminal area, and en-route operations. Each ground station provides uplink broadcast of GNS-B messages generated in the ground station.

GNS-B provides increased position accuracy complying with the A-SMGCS concept and supports increased position accuracy of ADS-B reports.  
A mobile VDL Mode 4 unit that receives GNS-B data will use it to enhance its own position accuracy. The GNS-B data is also available for external use in the aircraft/vehicle.

What is FIS-B?
Flight Information Service-Broadcast (FIS-B) is an uplink service that provides local terminal area information to the mobile user. This data includes meteorological and operational flight information as well as runway visual range at a particular airport.

The ground station receives FIS-B data from dedicated equipment in the CNS network and transmits this data uplink broadcast over the VDL Mode 4 radio link. FIS-B messages are received by VDL Mode 4-equipped mobile units (aircraft and ground vehicles) and display FIS-B data to the pilot/driver.

What is TIS-B?
Traffic Information Service-Broadcast (TIS-B) is an uplink service that provides surveillance information from other surveillance sources to VDL Mode 4-equipped aircraft and ground vehicles.

The TIS-B service includes Management messages – information about the TIS-B service and the traffic information volume and Target messages – surveillance information about aircraft and ground vehicle targets.

The ground station receives TIS-B data from dedicated equipment in the CNS network and transmits this data uplink broadcast over the VDL Mode 4 radio link. TIS-B messages are received by VDL Mode 4-equipped mobile units (aircraft and ground vehicles) and display surveillance data additional to the inherent ADS-B information to the pilot/driver.

What is Point-to-point?
Point-to-point is the functionality required for addressed wireless communication services, e.g. the addressed guidance and control applications included in the A-SMGCS concept and other ATC functionality. Point-to-point messages can be exchanged between mobile users (air-to-air) and  between a mobile user and a ground-based IP address (air-to-ground)

Point-to-point communication provides communication with efficient data transfer, rapid and reliable link negotiation control functions, support for gate-to-gate functionality via the network, is ppplicable to A-SMGCS, routing, guidance and control and Airline Operational Communications (AOC).  

A-SMGCS
Airport Organiser serves as an integrated part of A-SMGCS. Its modern technology, certified for civil aviation, results in system architecture with an interface to existing surveillance infrastructure such as surface movement radar and multilateration. With point-to-point communication between ATC, vehicles and coordinators, Airport Organiser brings the airport system to the highest level of A-SMGCS defined by Eurocontrol and ICAO.

Extremely reliable and accurate ADS-B information combined with point-to-point communication enables the following features to be developed for ATC:
• ADS-B provides position, ID and intention for a complete surveillance picture of all ground movements.
• Predictability and robustness in the operation.
• Conflict detection and warnings for preventing runway incursions.
• Point-to-point communication allowing routing, guidance and control of vehicle movements.
• Restricted area allocation for safe and efficient runway operations.
• Confirmation and control of ground movement activities.