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What is HFDL?

HFDL is a HF data link protocol, defined in ARINC spec 635-3. The HFDL service is operated by Rockwell Collins (since 2014) through a worldwide network of HF stations. See this PDF file from the UDXF for more.

  • Transmissions on HF are in USB on a sub carrier of 1440 Hz with a symbol speed of 1800 baud.
  • Modulation is 2-PSK, 4-PSK or 8-PSK with effective bit rates of 300, 600, 1200 or 1800 bits/sec.

This topic can be discussed in our SW Data Decoding forum.

Current Usage

This mode is utilized exclusively by fixed aeronautical stations in communications with planes, often at great distances - too far for VHF radio to accomplish. See the Related Links section for a link to a list of frequencies and sites.


  • HF Datalink
  • ARINC 635



PC-HFDL is a Windows software to decode HFDL data traffic and has been written by Charles Brain. Just feed the HF receiver speaker's output into the soundcard of a Windows PC. It can be downloaded from the HFDL Yahoo group.

PC-HFDL screen

The best monitoring results will be achieved with the following PC-HFDL settings:

  • PREAM ON - to control the audio offset of the receiver
  • SPDU OFF - only needed temporarily ON to see the squitters of a new unknown ground station
  • MPDU ON - see the MPDU packets
  • LPDU ON - see the LPDUs within the MPDUs
  • BDU ON - see the BDUs
  • VERB ON - see more details
  • HFNPDU ON - see the routine position reports
  • HEX OFF - only needed for debugging purposes

So in short, with all options ON except SPDU and HEX, the complete session can be seen: LOG-ON of the aircraft to the ground station giving its 24bit ICAO-ID, the assignment of the 1byte sequence ID number to the given ICAO-ID by the ground station referenced in all subsequent traffic, clear distinction of MPDU packets containing more than just one LPDU, and full traffic details.

The HFDL Yahoo group can be found here. Version 2.042 can be downloaded from this stub page

Other software that can decode HFDL

Hobby Level Software

Professional Equipment/Software

Audio Example and Waveform

Related Links

  • The current version (Sept. 5 2014) of the HFDL database table is 49. It's available from the UDXF Modes page here in PDF.
  • The .dat file that can be used with the registered version of PC-HFDL can be had by joining the PC-HFDL Yahoo group and requesting one via private mail. It will NOT work with the free version.
  • go2Monitor HFDL Description

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