HF Data Link (HFDL) protocols are defined in the ARINC (Aeronautical Radio, Inc.) Specification 635-3, published on December 29, 2000. The development of HF Data Link service builds on the experiences of ACARS (Aircraft Communications Addressing and Reporting System), therefore HFDL is also known as HF-ACARS.

The protocols are compatible with the OSI model and enable the ground stations and avionics systems on aircraft to communicate with each other in a bit-oriented message format.

HF Data Link employs M-ary Phase Shift Keying (M-PSK) to modulate a carrier centered at RF+1440 Hz, where RF is the nominal HF carrier frequency. The carrier is modulated with 1800 symbols per second (Baud). However, the rate at which user data is transmitted can be selected to be 300, 600, 1200 or 1800 bps.

HFDL air-ground protocols employ a slotted Time Division Multiple Access (TDMA) protocol.

Every frame has duration of 32 seconds and is divided into 13 slots. The first slot is called a Squitter, which carries various types of information, including slot acknowledgement and assignment codes.

The next 12 slots of a frame are called Medium access Protocol Data Units (MPDU). They are used by aircrafts and ground stations to exchange different kinds of data, e.g., aircraft logon/logoff requests, aircraft position, frequency assignment etc. Each MPDU contains several Link Protocol Data Units (LPDU), which may consist of Basic Data Units (BDU).

Each slot (Squitter or MPDU) has the same structure:

The following diagram shows the TDMA slotted frame structure.

The HF-ACARS decoder detects each slot with the characteristic pre-key (1440 Hz single tone), uses the preamble to synchronize the slot, eliminates various distortions in the HF communication channel, configures several components of the decoder, demodulates the user data section and decodes it according to the HFDL protocol interpreter.

Each message is finally output on the screen in Squitter (SPDU) format, Uplink MPDU format (from ground station to aircraft) or Downlink MPDU format (from aircraft to ground station).

Only when a HFDL slot is successfully detected, the decoder enters the Traffic state, otherwise it remains in the Sync state.

Usually an HFDL station sends the signal with USB setting. However the decoder has the possibility to process signals in both USB and LSB settings. This can be done by setting the polarity in the menu neither to NOR for USB and INV for LSB signals.

The center frequency of the decoder should be set to 1440 Hz when the receiver is set to the nominal frequency of the ground station. A small frequency difference will be automatically compensated by the decoder. However, the center frequency can be tuned by ±400 Hz from its nominal setting.

There are two internal system configuration files for the display of HFDL messages, hfacars.txt and hfacars.dat. These files should not be modified by the user.