In the context of aviation and aerospace, the "Aircraft Communications Addressing and Reporting System" (ACARS) is a digital communication system used to transmit and receive data between aircraft and ground stations. It was developed in the 1970s and is still widely used today.

ACARS allows aircraft to send and receive short messages, typically limited to a few hundred characters, using a combination of radio frequencies and satellite communication systems. The system uses a unique address for each aircraft, which is used to identify the aircraft and route messages to and from it.

ACARS is used for a variety of purposes, including:

1. Flight planning and management: ACARS is used to send and receive flight plans, weather updates, and other operational information between aircraft and air traffic control (ATC) or flight dispatch centers.
2. Position reporting: Aircraft use ACARS to report their position, altitude, and other flight parameters to ATC, which helps to track the aircraft's location and ensure safe separation from other aircraft.
3. Weather reporting: ACARS is used to send and receive weather updates, including wind, temperature, and precipitation data, which helps pilots to make informed decisions about flight routes and altitudes.
4. Maintenance and troubleshooting: ACARS can be used to send and receive maintenance-related messages, such as fault reports and diagnostic data, which helps maintenance personnel to troubleshoot and repair issues with the aircraft.
5. Crew communication: ACARS can be used for crew communication, such as sending and receiving messages between pilots and dispatchers or other crew members.

ACARS messages typically consist of a header, which includes the aircraft's address and other identifying information, followed by a data field, which contains the actual message. The system uses a combination of fixed and variable-length messages, depending on the type of information being transmitted.

While ACARS is still widely used, it is being gradually replaced by more modern communication systems, such as the "Controller-Pilot Data Link Communications" (CPDLC) system, which uses digital communication protocols to transmit and receive data between aircraft and ground stations. However, ACARS remains an important part of the aviation communication infrastructure, particularly for older aircraft that do not have the capability to use newer communication systems.