Page 26 - ITU-T Focus Group on Aviation Applications of Cloud Computing for Flight Data Monitoring - Avionics and Aviation Communications Systems

ITU-T Focus Group on Aviation Applications of Cloud Computing for Flight Data Monitoring
Avionics and Aviation Communications Systems

11.1.2.1 ACARS – Aircraft communications addressing and reporting system (ACARS)

ACARS character-oriented protocol has been in use since the late 1970s, having been designed for transmission
over narrow bandwidth pipes such as VHF radios. Linked to this are ground networks hosted by Rockwell
Collins Information Systems (ARINC) and SITA, allowing aircraft to send reports of up to 220 characters in
length either automatically or upon request. This allows aircraft and airline operation centers to exchange
information such as equipment health and maintenance data, flight relevant events such as out, off, on, in
(OOOI) status, or other en-route flight data such as engine performance, speed, altitude, flight plans, and
numbers and city pair destinations.

The ACARS unit or function is not a data link system in itself that processes the character-oriented messages
on board the aircraft, but rather a short text message router that uses available data link systems that may
be installed and connected. These links include:

i) VHF data link or VHF digital link (VDL Mode 2);

ii) HF data link;
iii) Inmarsat Classic Aero SatCom systems;
iv) Iridium SatCom.
These links all are narrowband. HF provides 600 bps, while VDL Mode 2 provides 31.5 kbps and Analog VHF
Data and SatCom links are limited to only 2.4 kbps when used for ACARS. The actual throughput data rate for
VDL Mode 2 is less than 20 kbps. This means these ACARS data links are suited to sending short character
oriented messages as they were designed for, but they are not suited for, streaming full black box data from
modern aircraft generating over 5 MB per flight hour. It is feasible and it has been demonstrated that flight
data parameters can be streamed over VDL Mode 2 and Iridium at a lesser rate that matches older black box
data standard recording rates.

VHF or VDL Mode 2 is the most widely used overland, while Classic Aero SatCom is the most widely used on
oceanic routes. HF data link is used to a much lesser extent and Iridium is increasingly being used too. Typically,
airline's will configure their ACARS systems to utilize the lowest cost link when available which is usually VDL
Mode 2, then SatCom, then HF data link but the airline preferences may vary based on their negotiated data
services costs.

The diagram below also illustrates that many avionics systems are connected to the ACARS router as clients
or "end-system" peripherals on board the aircraft. Systems such as the flight management system (FMS),
aircraft condition monitoring system (ACMS) and maintenance and fault monitoring (CMC) as well as many
other avionics are connected. The ACARS unit itself also includes an airline operational communication
(AOC) application and the ACARS system is the core messaging protocol for FANS, controller-pilot data link
communication (CPDLC) and ADS-C air traffic applications.

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