Tutorial HFDL

Tutorial: HFDL (aircraft data link on HF)

What it is

HFDL is the high-frequency data link that gives aircraft on oceanic and polar routes a data connection to ground stations when VHF is out of range. Aircraft log on to a ground station, send position reports, ACARS text and ADS-C, and ground stations broadcast short "squitter" frames that list the frequencies in use. OnAir decodes every HFDL channel inside your capture at once and shows messages, a map, aircraft, ground stations and channel activity.

What you need

• A radio that can receive HF. A plain RTL-SDR can do it only in direct-sampling mode, which OnAir offers in the radio settings, and the result depends on the antenna. SDRplay and Airspy HF+ radios are better suited. See Supported radios and the pages for your radio.
• An HF antenna: a long wire, a loop or an active whip placed away from computer and mains noise. HFDL is received as upper sideband, so antenna quality and local noise decide how much you hear.
• Frequencies from the project's database (assigned ground-station carriers, 3 to 22 MHz). Examples: 5.451 and 5.502 MHz, 6.529 and 6.535 MHz, 8.825, 8.912 and 8.948 MHz, 10.027 and 10.063 MHz, 11.306 and 11.318 MHz. The mode starts at 8.9 MHz with 2 Msps (the mode table allows 2 to 30 MHz; the lowest working rate is 24 ksps).

Before you start

• Which band is open depends on the time of day and the solar cycle. Higher bands (11 to 22 MHz) are usually better in daytime, lower bands (3 to 8 MHz) at night.
• The decoder was corrected so that real off-air bursts decode (release 0.2.4). Use a current version.
• Keep gain low enough to avoid overload by shortwave broadcasters, which are far stronger than HFDL.

Step by step

1. Choose the group Aviation, entry Data link, 8.9 MHz (mode HFDL).
2. In the tuner open Band, choose presets, and pick a band: "5.5 MHz (5.45 - 5.72)", "6.6 MHz (6.53 - 6.71)", "8.9 MHz (8.82 - 8.98)", "10.06 MHz (10.03 - 10.09)", "11.3 MHz (11.18 - 11.39)", "13.3 MHz (13.26 - 13.36)", "17.94 MHz (17.90 - 17.99)" or "21.96 MHz (21.93 - 22.00)". Every ground-station frequency of the built-in table that lies inside the sample band is then decoded at once.
3. Or type dial frequencies in kHz in Channels kHz (upper sideband), separated by spaces. Empty means "the station table".
4. Press Start. The status line says "Listening", then "Decoding" when a burst is read.

What you should see

When a band is open, HFDL bursts come every few seconds. If you see "nothing decoded yet: HFDL bursts come every few seconds when the band is open", the band may be closed or the antenna too noisy. Try another preset.

Reading the screen

• Messages: time (UTC), the aircraft or station, frame kind (logon or link, squitter, ACARS, position), SNR, rate in bit/s and a filter (ICAO, registration, flight or ground station). Click a message to see every layer of it, and use the ACARS text view for the free text.
• Map: aircraft positions, plus ground stations. The position tooltip shows prediction age and uncertainty.
• Aircraft: ICAO address, flight, position, last heard, message count, channel.
• Ground stations: each station, whether it was heard directly or named by another station (in squitters), its frequencies in use and last heard.
• Channels: per channel signal, bursts, PDUs good and bad, preambles, squitters and rate.

Options and settings

HF simulated signals offer "clean channel", "HF moderate (1 ms, 0.5 Hz)" and "HF poor (2 ms, 1 Hz)" fading presets for trying the screen without a radio. Predict positions on the map dead-reckons an aircraft between its infrequent reports.

Troubleshooting

• Nothing decoded: wrong band for the time of day, or no antenna. Try 8.9 MHz and 11.3 MHz first, and give it several minutes.
• Channels shown as "outside the sample band": the radio's sample rate does not cover the channel you listed. Choose a preset or raise the rate.
• Strong noise floor: move the antenna away from the computer; use a ferrite on the cable.
• RTL-SDR: nothing at all: direct sampling has to be on; read your radio page.
• Decodes with errors: the signal is faded; a better antenna helps more than gain.

Tips

Run ACARS or VDL2 on the same aircraft ID to see how the same flight moves between links. Satellite ADS-C and HFDL positions together give a view of the oceans.

Legal and privacy notes

HFDL messages are unencrypted but may contain personal and operational details. Receiving is allowed in many countries; check your rules. See Legal and Privacy.

Under the hood

HF upper sideband, channels about 3 kHz wide, PSK modulation at 300, 600, 1200 and 1800 bit/s with interleaving and convolutional coding, bursts located by a known preamble; ARINC 635 defines HFDL and ICAO Annex 10 vol. III covers the link.

Related

HFDL mode page, Tutorial: ACARS, Tutorial: VDL2, Live map, Troubleshooting.