HFDL (aircraft data over shortwave)
A shortwave data link that lets airliners exchange messages when far from land, for example over oceans.
What does it sound like?
Short bursts of dense digital noise, plus regular 'squitter' announcements from ground stations.
Who uses it?
Airlines, aircraft and a worldwide network of ground stations.
Where and when can I hear it?
Many frequencies between about 2 and 22 MHz, for example 8.927 and 10.081 MHz.
The science
HFDL is sent on upper sideband with a 1440 Hz audio subcarrier that is phase-shift keyed at 1800 symbols per second, using BPSK, QPSK or 8-PSK to give user data rates of 300, 600, 1200 or 1800 bits per second; the rate is chosen to suit propagation at the moment. Time on each channel is cut into 32-second frames of 13 slots. The first slot is the squitter, sent by the ground station to announce system status, provide a timing reference and handle protocol control. Stations offset their squitters in time so aircraft can compare several stations before logging on, and traffic then moves in assigned time slots so two aircraft do not transmit at once.
Why use it?
Shortwave signals are carried by the ionosphere far beyond VHF range, so HF ground stations can provide data link to properly equipped aircraft over oceans and remote regions where line-of-sight radio cannot reach. Choosing among several data rates lets the link slow down and keep working when conditions are poor.
Lore
HFDL is also called HF ACARS because it grew out of the ACARS system. Its protocols are set out in ARINC Specification 635, and ARINC runs it as part of its GLOBALink service; in 2019 the network had 16 ground stations, from Reykjavik to Auckland.
What do the decoded parts mean?
- Aircraft and ground station: Which plane and which ground station are talking.
- Position reports: Where aircraft are.
Easily confused with
- ACARS (aircraft messages), from the same family
- VDL Mode 2 (aircraft data link), from the same family