Also known as: HFDL, HF Data Link
HFDL (High Frequency Data Link) is a long-range aviation datalink that carries ACARS traffic over HF shortwave using phase-shift keying, reaching aircraft thousands of kilometres from any ground station by sky-wave ionospheric propagation.1 Where VHF datalink is line-of-sight and satcom needs a satellite in view, HFDL exploits the ionosphere’s ability to bend HF signals around the curvature of the Earth, making it the economical datalink for oceanic and polar routes.
Overview
HFDL is run as a single global network: a handful of HF ground stations, each on several frequencies across the HF spectrum, together provide near-worldwide coverage. Aircraft log onto whichever station and frequency propagate best at the current time of day, and the network hands messages to and from the same ACARS back-end that VHF and satcom use. Because HF conditions swing with the ionosphere, ground stations broadcast a squitter advertising which frequencies are usable.
Technical characteristics
| Property | Value |
|---|---|
| Band | HF, roughly 2.6–22 MHz (upper sideband) |
| Modulation | 2/4/8-PSK, single tone |
| Data rates | 300, 600, 1200, and 1800 bps (condition-adaptive) |
| Channel bandwidth | ~3 kHz (voice-channel compatible) |
| Access | TDMA within a 32-second frame (slot map) |
| Ground stations | Global network on shared frequencies |
| Payload | ACARS (incl. ADS-C, position, ops) |
| Standard | ARINC 635 |
The modulation is single-tone PSK whose order and rate adapt to propagation — from robust BPSK at 300 bps in poor conditions up to 8-PSK at 1800 bps when the channel is strong. A 32-second TDMA frame divided into slots lets many aircraft share one ground station’s frequency, coordinated by the station’s slot assignments.
History
HFDL was standardised in ARINC 635 and entered service in the late 1990s as a low-cost alternative to HF voice position reporting and to satcom, particularly for operators crossing oceanic and polar airspace where VHF cannot reach. Its ground network grew to give effectively global coverage.
Deployment
HFDL is widely fitted to long-haul aircraft and is popular with shortwave listeners because, like VHF ACARS, it is unencrypted and decodable with a capable HF SDR and software. It commonly carries ADS-C position contracts over regions with no other datalink coverage, complementing satcom.
Decoding it with GopherTrunk
Not decoded. HFDL is an HF datalink, outside GopherTrunk’s land-mobile VHF/UHF and 1090 MHz ADS-B scope, and it requires an HF-capable receiver GT does not target. Enthusiasts decode HFDL with dedicated HF tooling; this page is included for honest context on the aviation datalink family alongside ACARS and VDL Mode 2, and to point at the sky-wave propagation that makes it work.
Sources
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High Frequency Data Link — Wikipedia, for HFDL’s PSK HF physical layer, adaptive 300–1800 bps rates, 32-second TDMA slot map, global ground-station network, and its role carrying ACARS over oceanic and polar routes. ↩