Datalink Access

Request GDC services via datalink through Honeywell’s Primus Epic® Communications Management Function (CMF) datalink platform. Datalink, or the Aircraft Communications Addressing and Reporting System (ACARS), is a robust two-way data communications system between aircraft and ground systems. A complete datalink communication, which may be generated either manually or automatically, is referred to as a datalink message. Messages from the aircraft to the ground are referred to as downlink messages and messages from the ground to the aircraft are referred to as uplink messages.

This page provides information specific to the Primus Epic CMF implementation for the Citation Sovereign, Gulfstream G350/G450/G500/G550, and Hawker Horizon aircraft. For detailed step-by-step procedures describing how to access GDC services from these aircraft, please refer to the Primus Epic CMF Reference Guide.

Datalink System
- Datalink Avionics

As part of the Primus Epic integrated avionics system, the CMF is a next-generation datalink platform designed for both software flexibility and hardware expandability. The CMF communicates primarily through a Very High Frequency (VHF) transceiver, although optional airborne equipment may include a Satellite Communications (SATCOM) system to provide datalink capability via Ultra High Frequency (UHF) transmissions to satellites. The Epic Multifunction Control Display Units (MCDUs) provide the interface between the flight crew and the CMF.

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- Datalink Infrastructure

By default, the Epic CMF communicates via the ground-based Aircraft Communications Addressing and Reporting System (ACARS) VHF network at a rate of 2400 bits per second (bps). The ACARS network includes the worldwide Société Internationale de Télécommunications Aéronautiques (SITA) subnetwork, China's Aviation Data Communications Corporation (ADCC) subnetwork, the Aeronautical Radio of Thailand Ltd. (AEROTHAI) subnetwork, the Avicom Japan Co. Ltd. (AVICOM) subnetwork, and Brazil's Departamento de Controle do Espaço Áereo (DECEÁ) subnetwork. Based on position information provided by the aircraft Flight Management Systems (FMSs), the CMF automatically tunes the VHF transceiver to the appropriate subnetwork. Efficient tuning requires that FMS position initialization be completed. In areas where VHF coverage is unavailable, the CMF may use the Inmarsat Aero-H, Aero-H+, or Aero-I satellite UHF networks at rates of up to 9600 bps. This provides both packet mode (datalink) and circuit mode (voice and data) capabilities on multiple channels. The CMF switches to and from the satellite UHF network based on the availability of ground-based VHF network coverage.

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- Datalink Service Provider

As a provider of flight support services, the GDC is at the hub of the system. In addition to performing host processing for CMF datalink message traffic, the GDC has telephone, fax, and network connections to domestic and international ATC facilities, Fixed Base Operators (FBOs), multiple weather providers, and customer flight departments. Customers who choose the GDC as the flight support services provider to their aircraft equipped with the Primus Epic CMF gain significant advantages because Honeywell provides the datalink hardware, software, and services.

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- Line of Sight

All Epic CMF transmissions require line of sight to a VHF ground station or Inmarsat satellite as appropriate. Establishing and maintaining line of sight is most often a concern when transmitting VHF on the ground due to the curvature of the Earth, high surrounding terrain, and manmade structures. VHF transmissions from many airports are simply not possible because the nearest VHF ground station is below the horizon or blocked by surrounding terrain. Even at an airport with a local VHF ground station, VHF transmissions from certain areas of the airport may not be successful due to manmade structures obstructing line of sight. In flight, VHF coverage is normally excellent, although coverage limitations may exist in remote areas of the world or at low altitudes.


Transmitting via satellite while on the ground is generally reliable, although line of sight issues may still arise due to surrounding terrain and manmade structures because the Inmarsat satellites are in equatorial geostationary orbits. In flight, the curvature of the Earth is a concern only at latitudes greater than 70° North or South. Except at these high latitudes, satellite coverage while in flight is seamless.

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- Epic CMF Configuration

Configuration of the Epic CMF is performed by modifying certain parameters stored in the Primus Epic Aircraft Personality Modules (APMs). Parameters stored in the APMs, which are also used by elements of the Primus Epic system other than the CMF, include the aircraft registration (or permanent callsign), ACARS airline identifier ("GS" for GDC services), ICAO airline identifier ("GDC" for GDC services), and ICAO aircraft type designator. Correct configuration of these parameters is required for proper CMF operation and ACARS routing. APM parameters are configured by creating or modifying an APM settings file using a PC-based software tool developed by Honeywell and provided by the aircraft manufacturer.

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- Epic CMF Software

The Epic CMF uses software developed by the GDC which is referred to as an Aircraft Modifiable Information (AMI) table. This allows the GDC to offer AMI tables with MCDU displays and CMF datalink messages that correspond to its evolving array of services. Use of the AMI table developed by the GDC is required in order to access full GDC services via datalink.

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Epic CMF Displays
- Display Access

To access the Epic CMF displays, press the DLK (datalink) function key on the MCDU. If a single new message is available, the new message is accessed directly, or if multiple new messages are available, the NEW MESSAGES display is accessed. Otherwise, the MAIN MENU display is accessed.

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- Display Groups

Datalink functions provided by the CMF are accessible through four groups of displays: Flight Management System (FMS) displays, Aeronautical Operational Communication (AOC) displays, Air Traffic Services (ATS) displays, and system displays.


Flight Management System (FMS) Displays


FMS displays provide access to FMS-related datalink functions, such as requesting flight plans and winds and temperatures aloft forecasts for the FMS wind model. These functions require FMS performance initialization to be completed and confirmed.


Aeronautical Operational Communication (AOC) Displays


AOC displays are defined by the GDC as part of the AMI table, which allows the GDC to add, modify, or delete displays as needed. Functions available through the AOC displays include free-text messages, terminal weather reports and forecasts, and D-ATIS reports. On the AOC displays, ‘action’ SEND and PRINT prompts are yellow, uplink text blocks and non-modifiable parameters are blue, and modifiable parameters are green. All other text is white. Additionally, the solicit character for a mandatory entry is a □ (box) and the solicit character for an optional entry is a - (dash). The solicit character for a display prompt is a < or > (caret) and the solicit character for an ‘action’ SEND or PRINT prompt is an * (asterisk).

SEND prompts on the AOC displays are available only when all mandatory entries have been completed. In addition, the current transmission mode is shown above the prompt so the user is aware how the downlink message will be sent. Possible values include GRD VHF (ground-based VHF network), SAT UHF (satellite-based UHF network), and NO COMM (no datalink communications available). SEND prompts also change to SENDING and then to either SENT or NOT SENT as appropriate when line selected. The CMF will attempt to send an AOC downlink message via all available transmission channels and modes for a period of five minutes before NOT SENT is displayed.


Air Traffic Services (ATS) Displays


The primary function currently available through the ATS displays is westbound transatlantic oceanic clearances. These displays support air traffic services provided by ATC without direct GDC involvement.


System Displays


The system displays provide access to various system functions, including datalink region and frequency management.

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- Display Logs

Because a complete datalink communication is referred to as a datalink message, regardless of whether it is a flight plan request downlink, D-ATIS report uplink, or free-text message downlink (e.g., ETA FBO 1135L NEED ICE), when any new AOC or ATS uplink message is received by the Epic CMF the message title is listed in the NEW MESSAGES display log and the appropriate scratchpad advisory is displayed. Message titles may be line selected from the NEW MESSAGES display log to view the complete message. All new uplink messages remain in the NEW MESSAGES display log until the message has been viewed.


Once a new uplink message has been viewed, the message is moved to the MESSAGE LOG, WEATHER LOG, GDC ATS LOG, REPORTS LOG, TEST LOG, or ATS LOG display as appropriate for later review. Message titles listed in a log may be line selected to view the complete message.


Additionally, after line selecting SEND, REQUEST, or ACKNOWLEDGE to transmit a downlink message, the display is ‘frozen’ for later review from the MESSAGE LOG, WEATHER LOG, GDC ATS LOG, REPORTS LOG, TEST LOG, or ATS LOG display as appropriate. Display titles listed in a log may be line selected to view the complete display.

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Pre-Departure Clearances (PDCs)

PDCs are departure clearances received via datalink and are available at many airports in the United States to Epic CMF-equipped aircraft. The aircraft, including any callsigns, must be registered through the GDC with the FAA. Use of PDCs at participating airports is expected by ATC once registered.


A PDC is based on a filed IFR flight plan, regardless of whether the flight plan was filed by the GDC, through an FSS, or via DUATS. Approximately 20 minutes prior to the filed time of departure of the flight plan, ATC will generate and then forward the PDC to the GDC for storage. With this in mind, request the PDC no earlier than 15 minutes prior to the filed time of departure. Because this short time is often insufficient to receive the clearance and depart as planned, the GDC recommends filing the flight plan with a time of departure 30 minutes earlier than the actual intended time of departure.


If ATC has forwarded the PDC to the GDC, the PDC is sent to the aircraft as a datalink message. If the GDC has not received the PDC from ATC, a datalink message is sent to the aircraft indicating that the PDC has not been received from ATC and that the PDC may be requested again in 5 minutes. Multiple PDC requests may be sent until 10 minutes prior to the filed time of departure, after which ATC should be contacted by voice to obtain the departure clearance.


Once the PDC is received, the flight crew is required to follow the clearance. Be sure to page forward through the clearance until END OF CLEARANCE is displayed. An aircraft may receive only one PDC per airport per day and a PDC will not be available if there is any change to the filed route and/or altitude or if the clearance needs to be negotiated. A PDC is valid for two hours beyond the filed time of departure.


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Oceanic Clearance Delivery (OCD)

Delivery of oceanic clearances via datalink for eastbound transatlantic flights for the Gander Oceanic Control Area (OCA) is available from Gander Area Control Centre (OACC) to Epic CMF-equipped aircraft. This service is referred to as Oceanic Clearance Delivery (OCD). The aircraft, including any callsigns, must be registered through the GDC with Gander OACC.


When flight planning, ensure that the phrase “AGCS” (an acronym for Air to Ground Communication System) is included in item 18 the filed ICAO flight plan. This remark informs Gander OACC that the flight crew desires to receive the oceanic clearance via datalink.


Gander OACC sends the clearance to the GDC 10 to 60 minutes prior to aircraft entry into oceanic airspace. For aircraft in flight, Gander OACC generally sends the clearance by 70 West longitude. For aircraft departing Gander (CYQX), Goose Bay (CYYR), and St. John’s (CYYT) airports, Gander OACC sends the oceanic clearance to the GDC at the same time it sends the departure clearance to the tower. Readback of the oceanic clearance is given to the tower, after which the tower issues the departure clearance.


With automatic position reports enabled, the GDC automatically sends the clearance to the aircraft as a datalink message as soon as it is received from Gander OACC. If automatic position reports are disabled, the flight crew must request the clearance. Begin requesting the clearance approaching 70° West longitude, but if the clearance is not received by 25 minutes prior to entry into oceanic airspace, contact Gander OACC on the appropriate voice frequency.


If the GDC has received the oceanic clearance from Gander OACC, the clearance is sent to the aircraft as a datalink message. If the GDC has not received the oceanic clearance from Gander OACC, a datalink message is sent to the aircraft indicating that the oceanic clearance has not been received from Gander OACC and that the oceanic clearance may be requested again in 10 minutes. Multiple oceanic clearance requests may be sent until 25 minutes prior to entry into oceanic airspace. Oceanic clearances are valid for 30 minutes beyond the issue time and voice readback of oceanic clearances is required.


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Oceanic Route Clearance Authorisation (ORCA)

Delivery of oceanic clearances via datalink for westbound transatlantic flights for the Shanwick Oceanic Control Area (OCA) is available from the Prestwick Oceanic Area Control Centre (OACC) to Epic CMF-equipped aircraft. This service is known as Oceanic Route Clearance Authorisation (ORCA) and requires that the aircraft be registered with the GDC for the service.


The flight crew should request the clearance via datalink between 30 and 90 minutes prior to entry into the Shanwick OCA. Shanwick normally responds to the clearance request with a message indicating that the clearance should be received within the next 15 minutes. Shanwick then sends the clearance to the aircraft, which contains the aircraft registration or callsign, entry point, ETA at the entry point, Mach number, flight level, route, and destination. The flight crew must promptly acknowledge the clearance via datalink by line selecting ACKNOWLEDGE on the message page containing the clearance. Failure to promptly acknowledge the clearance results in cancellation of the clearance transaction and requires that Shanwick be contacted by voice. Upon receipt of the clearance acknowledgement, Shanwick sends a message to the aircraft confirming the clearance. If this message is not received, Shanwick must be contacted by voice.


If the flight crew requests a new clearance or if Shanwick requires a change to an existing clearance, one or more reclearances may be received by the flight crew. These reclearances will be annotated "RECLEARANCE 1", "RECLEARANCE 2", etc., although may not necessarily be numbered consecutively.


Please note that aircraft must not enter the Shanwick OCA without a clearance. If at any time the flight crew is in doubt regarding the oceanic clearance transaction, Shanwick must be contacted by voice using the phrase "(AIRCRAFT REGISTRATION) ORCA CONTACT". If any clearance or reclearance is not terminated by the phrase "END OF MESSAGE", Shanwick must also be contacted by voice using the phrase "(AIRCRAFT REGISTRATION) ORCA CONTACT". If no clearance has been received by 15 minutes prior to entry into the Shanwick OCA, Shanwick and Air Traffic Control (ATC) for the airspace in which the aircraft is operating must be contacted by voice. All clearances and reclearances must be acknowledged.

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Automatic Position Reports

Enabling automatic position reports allows the Epic CMF to automatically send position reports to the GDC at 15, 30, or 60 minute intervals. These position reports serve two functions. First, in order for the GDC to send an uplink to an aircraft, the position of the aircraft must have been updated within the preceding 15 minutes. Any manual downlink from the CMF, such as a flight plan request or a free-text message, includes the aircraft position, which allows the GDC to respond immediately with the corresponding uplink. If the position of the aircraft is updated only from irregular manual downlinks, however, periods may exist when the GDC cannot send an unsolicited uplink message, such as an eastbound transatlantic oceanic clearance, because the last known position of the aircraft is no longer current.


The GDC therefore recommends enabling automatic position reports with a 15 minute interval in order to regularly provide the GDC with the current position of the aircraft. Automatic position reports may also be enabled with a 30 or 60 minute interval or be disabled completely in order to reduce datalink transmission costs, although the GDC would not be able to send an unsolicited uplink message to the aircraft during any period 15 minutes after the last downlink is received. If the GDC cannot send an unsolicited uplink message to an aircraft, the message is stored for up to seven days or until a downlink is received from the aircraft providing its current position, which then allows the stored message to be sent.


Second, automatic position reports, as well as reports for all other downlinks, are accessible through AFISCOM Express software in both text and graphic form. These flight following reports allow users to track aircraft progress and review previous flights from the ground. Please refer to the AFISCOM Express User’s Guide for instructions to obtain and display flight following reports.

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Aircraft equipped with a printer, such as the Miltope TP-4840, interfaced to the Epic CMF may print information from many displays. Displays which contain information that may be printed include a PRINT prompt at the lower right. Depending on printer status, the prompt also shows READY, PRINTING, PAPER OUT, TEST, QUEUE FULL, or FAIL indicators. As an example, the CURRENT FLIGHT page of the TIMES/FUELS display would be printed as follows:

1551Z    01 FEB 05
filler filler filler filler filler filler
Registered Users

Related Services and Applications
GDC Datalink Services

Epic CMF

Epic MAU
Epic APM
GDC Datalink Coverage Map
GDC Air Traffic Services Airports
PDC and OCD Procedures & Registration
ORCA Procedures & Registration