# What is ACARS?

**Aircraft Communications Addressing and Reporting System** (**ACARS**) is a datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite. The protocol was designed by ARINC and deployed in 1978, using the Telex format.

ACARS is available over four types of data links:

* Analog ACARS
* VDL
* HFDL
* SATCOM

### Types of ACARS Datalink Systems

**Analog ACARS** is the original protocol and is heavily used by aircraft and ground stations, and it is commonly the form that people think of when referring to ACARS. But ACARS is actually a technology that is used within other transmissions system formats as well. These signals are easy to receive with standardized easy-to-acquire antennas, and will even permeat indoor spaces.

**VHF Digital Link** (**VDL**) is essentially the digital successor to ACARS and is also widely used for transmission of short messages between aircraft and ground stations. It has several enhancements over Analog ACARS. These signals are also easy to receive with standardized easy-to-acquire antennas, and will even permeat indoor spaces.

**High Frequency Data Link** (**HFDL**) is an ACARS communications medium used to exchange data such as **Aeronautical Operational Control** (**AOC**) messages, **Controller Pilot Data Link Communications** (**CPDLC**) messages and **Automatic Dependent Surveillance** (**ADS**) messages between aircraft end-systems and corresponding ground-based HFDL ground stations. Using the unique propagation characteristics of high-frequency radio waves, the ground stations provide data link communications to properly equipped aircraft operating anywhere in the world. As a result, pilots can always communicate with someone on the ground. These signals reach further than the VHF variants, but require more specialized antennas to receive them and will generally not work indoors. HFDL is especially useful in the polar regions where SATCOM and ground stations are not available.

**Satellite Communications** (**SATCOM**) is an ACARS datalink system that uses a constellation of satellites to exchange data similar to analog, VDL, and HFDL ACARS, but also often adds additional voice and data capabilities. SATCOM is primarily used for oceanic regions of the world where there are no ground stations for aircraft to communicate with. It does not tend to have any polar reach. It is also commonly referred to as **AERO**.


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