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5.3.3 Current data link technology
5.3.3.1 VHF data link
VHF datalink is the traditional basis for AOC data services provided as part of the ACARS
service. Aeronautical Mobile Satellite Service (AMSS) also supports ACARS and is also
used as the bearer for Future Air Navigation System 1/A (FANS 1/A) 20 in the Pacific and
North Atlantic 21 . High Frequency Data Link (HFDL) also supports ACARS.
5.3.3.2 High Frequency Data Link
High Frequency Data Link (HFDL) is a global end-to-end packet data communication
system designed to function as a sub-network of the Aeronautical Telecommunications
Network (ATN), and to provide a world-wide coverage based on operations at around 10
HFDL ground stations.
The use of HF and the wide coverage of each ground station (5000 km radius) means
that HFDL is suited for the transmission of information data (neither tactical, nor strategic)
in oceanic and remote areas.
The characteristics of HFDL are summarised in Table 5-1 below.
Parameter Value Notes
Service topology Point-to-point
ATN compliance Yes
Frequency band HF spectrum: 2 to 32
MHz
RF Channels Carrier centred at 1440
Hz and modulated at an
1800 Hz rate (symbol
speed is always 1800
baud).
Modulation scheme M-PSK modulation (2-PSK, 4-PSK or 8-PSK)
Bit rate 300, 600, 1200, or 1800
bps.
Channel access
method
System with 4 to 6 families of 5 to 6 HF frequencies
each, based on operations at 8 to 10 HF Datalink
ground stations for world wide coverage
Four data rates are usable, based on signal quality,
depending on the atmospheric conditions (impacting
the SNR):300, 600, 1200, and 1800 bps.
Reserved or random Each slot can be designated either for uplink, for
downlink for a particular aircraft (reserved slot) or for
contention downlink (random access).
Uses a protocol consisting of a combination of
Frequency Division Multiple Access (FDMA) and slotted
Time Division Multiple Access (TDMA)
20
FANS 1/A is the system, including avionics, to support digital controller-pilot dialogue
primarily in the oceanic region
21
The ACARS VHF Datalink is also used to support the FANS1/A applications where
there is coverage.
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