Controller- Pilot Data Link Communication (CPDLC)

The standard method of communication between an air traffic controller and a pilot is voice radio, using either VHF bands for line-of-sight communication or HF bands for long-distance communication.

One of the major problems with voice radio communications used in this manner is that all pilots being handled by a particular controller are tuned to the same frequency. As the number of flights air traffic controllers must handle is steadily increasing, the number of pilots tuned to a particular station also increases. This increases the chances that one pilot will accidentally override another, thus requiring the transmission to be repeated. In addition, each exchange between a controller and pilot requires a certain amount of time to complete; eventually, as the number of flights being controlled reaches a saturation point, the controller will not be able to handle any further aircraft.

Traditionally, this problem has been countered by dividing a saturated air traffic control sector into two smaller sectors, each with its own controller and each using a different voice communications channel. However, this strategy suffers from two problems:

• Each sector division increases the amount of "handover traffic". That is the overhead involved in transferring a flight between sectors, which requires a voice exchange between the pilot and both controllers, plus co-ordination between the controllers.
• The number of available voice channels is finite, and, in high density airspace, such as central Europe or the Eastern US Seaboard, there may not be a new channel available.

In some cases it may not be possible or feasible to further divide down a section.

A new strategy is needed to cope with increased demands on air traffic control, and data link based communications offers a possible strategy by increasing the effective capacity of the communications channel.

Controller–pilot data link communication (CPDLC) is a means of communication between controller and pilot, using data link for ATC communication. At the highest level, the concept is simple, with the emphasis on the continued involvement of the human at either end and the flexibility of use.

The CPDLC application provides air-ground data communication for the ATC service. This includes a set of clearance/information/request message elements which correspond to voice phraseology employed by air traffic control procedures. The controller is provided with the capability to issue level assignments, crossing constraints, lateral deviations, route changes and clearances, speed assignments, radio frequency assignments, and various requests for information. The pilot is provided with the capability to respond to messages, to request clearances and information, to report information, and to declare/rescind an emergency. The pilot is, in addition, provided with the capability to request conditional clearances (downstream) and information from a downstream air traffic service unit (ATSU). A “free text” capability is also provided to exchange information not conforming to defined formats. An auxiliary capability is provided to allow a ground system to use data link to forward a CPDLC message to another ground system.

The sequence of messages between the controller and a pilot relating to a particular transaction (for example request and receipt of a clearance) is termed a ‘dialogue’. There can be several sequences of messages in the dialogue, each of which is closed by means of appropriate messages, usually of acknowledgement or acceptance. Closure of the dialogue does not necessarily terminate the link, since there can be several dialogues between controller and pilot while an aircraft transits the ATSU airspace.

All exchanges of CPDLC messages between pilot and controller can be viewed as dialogues.

The CPDLC application has three primary functions:

• the exchange of controller/pilot messages with the current data authority,
• the transfer of data authority involving current and next data authority, and
• downstream clearance delivery with a downstream data authority.

Simulations carried out at the Federal Aviation Administration's William J. Hughes Technical Center have shown that the use of CPDLC meant that "the voice channel occupancy was decreased by 75 percent during realistic operations in busy en route airspace. The net result of this decrease in voice channel occupancy is increased flight safety and efficiency through more effective communications."

Today, there are two main implementations of CPDLC:

• The FANS-1 system originally developed by Boeing, and by Airbus as FANS-A, is now commonly referred to as FANS-1/A, and is primarily used in oceanic routes by widebodied long haul aircraft. It was originally deployed in the South Pacific in the late 1990s and was later extended to the North Atlantic. FANS-1/A is an ACARS based service and, given its oceanic use, mainly uses satellite communications provided by the Inmarsat Data-2 (Classic Aero) service.
• The ICAO Doc 9705 compliant ATN/CPDLC system, which is operational at Eurocontrol's Maastricht Upper Airspace Control Centre and has now been extended by Eurocontrol's Link 2000+ Programme to many other European Flight Information Regions (FIRs). The VDL Mode 2 networks operated by ARINC and SITA are used to support the European ATN/CPDLC service.

The Data Link Services Implementing Rule (DLS-IR) was adopted on 16 January 2009 by the European Commission and published as Regulation 29/2009 - Data link services for the Single European Sky. The DLS-IR is legally binding and applies directly to Air Navigation Service Providers (ANSPs) and to Aircraft Operators.

Air navigation service providers and operators have reported technical issues when implementing Regulation (EC) No 29/2009, particularly disconnections, known as Provider Aborts (‘PAs’), of existing air-ground data communications enabling the operations of data link services (‘DLS’) and which are beyond acceptable performance levels. For that reason certain air navigation service providers have already taken mitigation measures, consisting of the restriction of DLS operations to aircraft equipped with specific avionics through so-called ‘white lists’, so as to address potential safety impacts of those PAs in the operations of data link service.

At the request of the Commission, the European Aviation Safety Agency (‘EASA’) conducted an investigation to identify the root cause or causes of those technical issues and to recommend measures to address them. The investigation revealed that the random PA occurrences could not be attributed to a single, predictable cause but rather to a combination of factors related to the radio frequency environment and to the current single frequency implementation of the data link infrastructure. It was found that this excessive rate of random PAs causes a degradation in the network performance potentially presenting aviation safety risks by increasing the pilots and controllers' workload and creating confusion leading to a loss of situational awareness.

EASA concluded in its investigation report that acceptable data link performance levels can only be established by deploying a multi-frequency infrastructure, which is also to be optimized for radio frequency interference prevention. It recommended establishing and implementing a plan of actions to further investigate the identified technical issues and to validate the necessary technical solutions. However, these measures require time and EASA therefore recommended to reconsider the date of application of Regulation (EC) No 29/2009 and the timeframes of that Regulation.

For North Atlantic Crossings the latest deadline for CPDLC is January 2020.

Expected Benefits of CPDLC

·        Less communication on the ATC frequency;

·        Increased sector capacities;

·        More pilot requests can be dealt with simultaneously;

·        Reduced probability of miscommunication (e.g. due call sign confusion);

·        Safer frequency changes, hence fewer loss of communication events.