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5.1 ACARS Networks
A high level illustration of FANS ACARS datalink service provider networks is shown below in Figure 1. The networks
each have a datalink processor with connectivity to the following: their own VHF ground stations, the Ground Earth
Station (GES) they use, connectivity to their respective ATSP and Airline customer end systems, and internetworking
function.
Figure 1. FANS-1/A Service
An ATSP is connected to the network of the CSP to whom they have contracted with for FANS-1/A service. A CSP
FANS-1/A service enables a FANS-1/A ATSP access to the datalink networks supporting FANS-1/A through providing
the required connectivity to a FANS-1/A ATSP and providing an internetworking function with other CSP’s.
8
FANS-1/A Datalink Communications Environment
The internetworking function enables an ATSP to address all of their uplinks to a single CSP address such that the
ATSP does not need to keep track of what specific CSP network an aircraft is currently using. The connected CSP
then determines whether or not to attempt delivery over its own datalink network or to send via the internetworking link
to another CSP network for delivery. ATS Internetworking allows airlines to have their choice of CSP and participate
in ATS datalink services regardless of what CSP a given ATSP is connected. Airlines have different preferences with
what CSP they use and sometimes they may also have different CSP preferences based on datalink media and
region. All properly formatted FANS 1/A messages from ATS Facilities Notification (AFN), Controller-Pilot Datalink
Communications (CPDLC), and Automatic Dependent Surveillance (ADS) are supported by CSP’s supporting FANS-
1/A. Downlinks received by another CSP are sent via the internetworking link to the CSP network to the ATS Provider
FANS end system. The ACARS air-ground networks include VHF ACARS, VDL Mode 2/ACARS over Aviation VHF
Link Control (AVLC) (VDL Mode 2/AOA), SATCOM, and, High Frequency Datalink (HFDL).
The following paragraphs further describe the air-ground networks, message routing, and internetworking.
5.2 Datalink Media
5.2.1 Very High Frequency (VHF) ACARS
The initial VHF ACARS media is also referred to as Plain Old ACARS (POA) and operates at a data rate
of 2400 bits per sec (bps). The next generation VHF media is VDL Mode 2/ACARS over Aviation VHF
Link Control (VDL Mode 2/AOA), which operates at a data rate of 31.5 kbps, and is available at various
locations around the world.
The VHF ACARS network service providers include SITA, AVICOM Japan, DECEA Brazil, ARINC, ADCC
China, and AeroThai.
Figures 2 and 3 show the SITA AIRCOM VHF ACARS and VDL Mode 2 coverage.
Figures 4, 5, and 6 show the ARINC GLOBALink® VHF ACARS and VDL Mode 2 coverage.
5.2.2 High Frequency Datalink
High Frequency Data Link, or HFDL, is part of ARINC’s GLOBALink end-to-end communication system
The HFDL system is a segment of the Aircraft Communications Addressing and Reporting System
(ACARS) used to exchange Airline Operational Control (AOC) and Air Traffic Control (ATS) messages
between aircraft end systems and corresponding ground-based stations. GLOBALink/HF provides HFbased,
air-to-ground digital communications with aircraft using ARINC 635 protocol.
The HFDL system uses the principles of geographic diversity (the use of multiple ground stations in
different geographic regions) and frequency diversity (the ability to use different frequencies based on
propagation conditions) to optimize HF propagation. HF propagation is dependent on a region of the
earth’s atmosphere called the ionosphere, which extends approximately 30-375 miles above the earth’s
surface. HFDL is currently the only truly global aeronautical data link capability, and the only data link for
flight routes over the North Pole. Inmarsat satellite SATCOM coverage becomes marginal around 80
degrees north and south latitude.
Figure 7 shows geographical locations of the HFDL ground stations (HGS) around the globe.
Each HGS operates on 2-3 channels and is equipped with a family of HF frequencies ranging from 2-30
MHz. Adaptive Frequency Management techniques are employed to obtain weekly Active Frequency
Tables (AFT). These tables are derived from a combination of real-time ionospheric monitoring, solar and
geomagnetic observational data and HF propagation models. Each week, an updated AFT is uploaded to
each HGS from the ARINC Operations Center (AOC) in Annapolis, Maryland, USA. The AFT instructs the
ground station on which frequencies to operate over the course of each 24-hour period.
9
FANS-1/A Datalink Communications Environment
Each HGS provides primary data link coverage for a radius of 3,000 nautical miles. Overlapping primary
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