Aircraft Equipment
The heart of the datalink system on board the aircraft is the ACARS Management Unit (MU). The older version of MU is defined in ARINC Characteristic 724B. Newer versions are referred to as the Communications Management Unit (CMU) and are defined in ARINC Characteristic 758.
Aircraft equipment consists of airborne end systems and a router. End systems are the source of ACARS downlinks and the destination for uplinks. The MU/CMU is the router. Its function is to route a downlink by means of the most efficient air-ground subnetwork. In many cases, the MU/CMU also acts as an end system for AOC messages.
Typical airborne end systems are the Flight Management System (FMS), datalink printer, maintenance computer, and cabin terminal. Typical datalink functions are:
- FMS - sends flight plan change requests, position reports, etc. Receives clearances and controller instructions.
- Printer - as an end system, can be addressed from the ground to automatically print an uplink message.
- Maintenance Computer - downlinks diagnostic messages. In advanced systems, in-flight troubleshooting can be conducted by technicians on the ground by using datalink messages to command diagnostic routines in the maintenance computer and analyzing downlinked results.
- Cabin Terminal - Often used by flight attendants to communicate special needs of passengers, gate changes due to delays, catering, etc.
ACARS messages are transmitted over one of three air-ground subnetworks.
- VHF is the most commonly used and least expensive. Transmission is line-of-sight so VHF is not available over the oceans or other vast expansions of uninhabited surface, such as the Amazon.
- SATCOM is a fairly expensive service that provides (near) worldwide coverage. The Inmarsat satellite network does not cover the polar regions. The Iridium satellite network became usable for ACARS transport in 2007 and provides excellent coverage in the polar regions.
- HF is a more recently established subnetwork. Its purpose is to provide coverage in the polar regions where Inmarsat coverage is unreliable.
The router function built into the MU/CMU determines which subnetwork to use when routing a message from the aircraft to the ground. The airline operator provides a routing table that the CMU uses to select the best subnetwork.
Read more about this topic: Aircraft Communications Addressing And Reporting System
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