Introduction

What is ATC Data Link?

Data link is a technology that allows for the exchange of text-based messages between suitably equipped Air Traffic Control (ATC) ground systems and aircraft (CPDLC).

Data link also permits the automatic exchange of information between the ground system and the aircraft’s avionics (ADS-C).

Data link is used by the AOC (Airline Operational Control) areas of various airlines to send and receive messages relating to operational information affecting a particular flight, such as the passing of updated weather or flight plan information.

This data link system operates over ACARS (Aircraft Communications Addressing and Reporting System) and each ACARS equipped aircraft has a unique address.

CNS/ATM

In 1983, as a strategy to counter global increases in air traffic and an aging worldwide infrastructure, the ICAO Council established the Special Committee on Future Air Navigation Systems (FANS).

The role of the Committee was to study, identify and assess new technologies, including satellite technology, and to make recommendations for the future development of navigation systems for global civil aviation.

The proposal developed by the FANS committee came to be known as the CNS/ATM concept.

The CNS/ATM system is based on global communications systems, global navigation systems, and Automatic Dependent Surveillance (ADS). Air Traffic Management (ATM) is a result of  these integrated systems being used to provide a range of Air Traffic services.

The CPDLC and ADS data link applications were designed for transportation across the Aeronautical Telecommunications Network (ATN).

Until the ATN became available, Boeing and Honeywell built a FANS application to run on the existing ACARS system. This avionics package became known as FANS-1 and was certified on a Qantas B747-400 in June 1995. The Airbus Industrie equivalent system is known FANS-A, and these systems are known collectively as FANS-1/A.

The ACARS networks are designed for the transmission of character-based messages to and from the aircraft, whereas the CPDLC and ADS applications intended for the ATN were defined using a binary (bit-oriented) message set.

The binary messages produced by these applications cannot be transmitted over the ACARS networks without being packaged as character messages.

The FANS-1/A system achieved the packaging of binary messages as character-based messages in accordance with the ARINC 622 specification.

FANS-1/A and ATN Implementations

FANS-1/A technology is being implemented in oceanic airspace around the world and is being used in domestic airspace in Australia and Maastricht's Upper Airspace Centre (MUAC). Maastricht uses a gateway that allows communication with both FANS-1/A aircraft and those fitted with ATN data link avionics.

The US Federal Aviation Administration (FAA) uses FANS-1/A technology in oceanic airspace in both the Pacific and Atlantic Oceans, and although a short trial of ATN data link was run in the airspace around Miami, Florida, any plans to expand domestic data link operations in the Continental USA was shelved in 2004. The FAA is now embarking upon its NextGen system, which will see an initial rollout of the delivery of Pre-Departure Clearances (PDC) via data link from domestic control towers in 2013 and will expand to a dual-stacked (FANS and ATN) CPDLC service in US domestic airspace from 2017. The ATN side of the dual-stacked system will be based on the current work (2010) of a joint RTCA/EUROCAE working group - SC214/WG78 - that is currently redefining some of the CPDLC and ADS-C messages originally specified in the ICAO SARPS. The intention is to converge the FANS and ATN worlds to allow as many aircraft as possible to participate in data link services across oceanic and domestic airspace. The ICAO Operational Data Link Panel (OPLINKP) has also been re-established and held its first meeting in October 2010, following a five-year hiatus. The Panel will be working on a number of topics, including revising SARPS, where required, and working to endorse relevant industry data link standards.

Meanwhile, the LINK 2000+ programme in Europe has been working aggressively to expand the use of domestic ATN-based data link capability across adjacent ATS Providers (ATSP). A series of mandates, commencing in February 2013, have been defined for equippage of ANSPs and aircraft operating above FL285. Existing FANS-1/A+ aircraft are exempt from the mandate - http://www.eurocontrol.int/link2000/public/standard_page/implementing_rule.html

A number of pioneer European Airlines have equipped their domestic fleets with ATN equipment, although FANS-1/A currently remains the only data link option for oceanic traffic (there is currently no ATN version of ADS-C). However, the initial implementation had a limited message set, had no integration of the CPDLC application and the aircraft's FMC, and was only certified to Level D, which required a voice-readback of any profile changing CPDLC uplink message and ruled out the exchange of clearance requests and route clearances. The FANS-1/A package is integrated with the FMC and is certified to Level C.

The implementation of FANS-1/A activity across the South Pacific Region is monitored and administered by the South Pacific FANS Interoperability Team (FIT) on behalf of the member States of the Informal South Pacific ATS Coordinating Group (ISPACG). As FANS-1/A implementation has gathered momentum a number of similar groups have now been established to cover the various regions.

FANS-1/A System Components

The data link system is made up of a number of components. These components consist of hardware, software and service providers.

ATS Providers supply some of the system infrastructure, but the current data link system relies on the networks of Communications Service Providers, such as SITA and ARINC, for the delivery of data link messages. Other CSPs are Avicom Japan and Aerothai.

The Communications Service Providers are commercial entities that offer similar services, but run their networks in different configurations.

Uplink messages are processed through a FANS router, which determines the addressee (customer) of the message and delivers the message as appropriate. Messages for different customers are passed between the major communications service providers via internetworking agreements.

Communications Media

Data link messages can be sent either via a VHF or satellite network, or by HF. FANS-1/A services can also be provided by VDL Mode 2, an advanced digital VHF data network. This provides two "flavours" of VHF networks for FANS services, POA (Plain Old ACARS) and AOA (ACARS Over AVLC (VDL Mode2)). Software within the Central Processing System automatically decides the most efficient (and cheapest) path for delivery of the message, depending on the location of the aircraft. HFDL is a service offered by ARINC. While the transit times of messages sent by HFDL do not meet the requirements for current reduced separation standards (i.e. 30/30NM), HFDL provides communications in polar regions where neither VHF nor satellite networks are available.

ATC messages are delivered by the same path as other messages, such as AOC flight plan uplinks. There is currently no priority assigned to ATC messages, although longer messages are split and delivered in defined chunks or blocks.

When in VHF range, it is possible for a data link-equipped aircraft to downlink information simultaneously via the satellite and the VHF networks if one of the channels is receiving a large data block.

Inter-Networking

Some ATS providers and airlines contract data link services through competing Communications Service Providers. As an example, SITA provides the data  link services for the Australian system TAAATS, whereas United Airlines contracts its services from ARINC.

To ensure that messages from one network are delivered to customers of another network, SITA and ARINC operate an internetworking agreement and associated connections to exchange messages.

Diagram

This diagram is a simplified representation of internetworking between the SITA and ARINC networks showing an ATS Unit that is a SITA customer exchanging messages with an ARINC airline customer.

ATC Data Link News Copyright © 1999, Craig. J. Roberts - Page last modified: November 3, 2010