Receiving HFDL: aircraft positions over shortwave, even in the middle of the Atlantic

Your ADS-B receiver on the roof stops hearing somewhere over the North Sea or the English Channel. A few hundred kilometers beyond the coast at the latest, every aircraft disappears from your own map. With HFDL over shortwave, you keep listening where 1090 MHz gives up: position reports from long-haul jets in the middle of the Atlantic, received with an SDR and a piece of wire in your yard. This guide explains what HFDL is, what you can realistically see with it and how to build your first station.

HFDL in one paragraph

HFDL (High Frequency Data Link) is a digital data radio on shortwave that airliners use to exchange messages with a worldwide network of ground stations. It carries airline ACARS messages, text-based communication with air traffic control via CPDLC and automatic ADS-C position reports. Because shortwave is reflected back to Earth by the ionosphere, an HFDL signal covers thousands of kilometers. That is exactly what makes it exciting for spotters: you receive aircraft flying far beyond any line of sight.

An important note on expectations: HFDL is not a replacement for ADS-B. An ADS-B transponder broadcasts its position several times per second, while HFDL messages containing a position arrive only at longer intervals. On your map, you will therefore see not smooth tracks but individual points – but from regions you could never otherwise reach.

Where HFDL fits in the data link family

Aircraft have several ways to send data to the ground. Four of them matter for spotters with an SDR, and it helps to keep them apart:

Method	Frequency	Range from the ground	What you get	Position density
ADS-B	1090 MHz	Line of sight, usually up to about 300–400 km	Position, altitude, identification, speed	several times per second
ACARS (VHF)	around 131 MHz	Line of sight	Airline messages in plain text	occasional
VDL Mode 2	around 136 MHz	Line of sight	ACARS messages, CPDLC, sometimes positions	occasional
HFDL	2.9–22 MHz	thousands of kilometers via the ionosphere	ACARS, CPDLC, ADS-C, position reports	sporadic

You may already know how decoding VDL Mode 2 works. HFDL is, so to speak, its big brother for the oceans: it carries similar content, but on a frequency band that reaches far beyond the horizon. The second technology for ocean positions is space-based ADS-B – but you cannot receive that yourself, only use it through commercial providers. HFDL, on the other hand, lands directly in your receiver.

How the HFDL network is built

The network consists of around 16 ground stations spread across the globe, with overlapping coverage. Each station operates on several frequencies and, depending on the time of day and propagation conditions, activates some of them. Aircraft automatically pick the best station and frequency; the crew does not have to set anything.

The station list that comes with the dumphfdl decoder includes these locations, among others:

• Europe and the Atlantic: Shannon (Ireland), Reykjavik (Iceland), Canarias (Spain)
• North America: San Francisco (California), Riverhead (New York), Barrow (Alaska), Molokai (Hawaii)
• Central and South America: Albrook (Panama), Santa Cruz (Bolivia)
• Africa and the Middle East: Johannesburg (South Africa), Al Muharraq (Bahrain)
• Asia and the Pacific: Krasnoyarsk (Russia), Muan (South Korea), Hat Yai (Thailand), Agana (Guam), Auckland (New Zealand)

For you in central Europe, Shannon, Reykjavik and the Canary Islands are the most important stations. They serve the North Atlantic, where long-haul flights between Europe and North America run every day. The name Shannon is a household name in transatlantic aviation anyway, as our profile of Shannon Airport shows.

The squitter: your control signal

Every ground station sends a short status message every 32 seconds, the squitter. It tells aircraft which station is currently active on which frequencies. For you, the squitter is worth its weight in gold: it arrives regularly, it is comparatively strong, and as soon as your decoder shows squitters from a station, you know that antenna, SDR and frequency selection work in principle. The aircraft themselves transmit much more weakly and irregularly.

What you can realistically receive

HFDL traffic is more varied than the name suggests. A typical log contains:

• Ground station squitters with the station ID and active frequencies.
• Performance and logon messages from aircraft logging on to or off from a station.
• ACARS messages with airline content: weather, fuel levels, maintenance reports, arrival times.
• Position reports that dumphfdl extracts from suitable messages and outputs as a Basestation data stream – for example for Virtual Radar Server.

The aircraft you hear this way are mostly long-haul types: Airbus A350-900, Boeing 777-300ER and the like on their way across the Atlantic. This is especially exciting early in the morning, when the overnight flights from North America are still over the ocean. When these waves arrive in Europe is described in the article Long-haul arrival times in Europe.

The hardware: SDR and antenna

Which SDR is good enough for HFDL?

This is the first hurdle for ADS-B tinkerers: your trusted 1090 MHz stick is often the wrong choice for HFDL. Classic RTL-SDR sticks only start at around 24 MHz – HFDL lies entirely below that. The options at a glance:

Receiver	HF capability	Suitability for HFDL	Note
Classic RTL-SDR (R820T)	no	unsuitable	does not go below 24 MHz
RTL-SDR Blog V3	direct sampling only	limited	signals often too weak, an upconverter helps
RTL-SDR Blog V4	yes, built in	usable for getting started	inexpensive, but not optimized for HF
Airspy HF+ / HF+ Discovery	yes, HF-optimized	very good	tested by the dumphfdl author
SDRplay RSP1A / RSPdx	yes	very good	requires the SDRplay driver on the host

Which sticks work well for ADS-B is covered in the SDR stick comparison, and how to listen to voice radio with the same stick in the article Receiving aviation radio with an SDR stick. For HFDL, anyone who wants to stick with it seriously is better off with an HF-optimized receiver. The V4 stick is enough, though, to find out whether the topic grabs you.

The antenna: a piece of wire, placed right

Shortwave antennas are bigger, but not more complicated. These have proven themselves:

• Dipole: two wire legs of equal length, fed in the middle. Easy to build and easy to predict.
• End-fed wire: one long wire that connects to the coax cable at one end through a transformer. Handy when you only have one mounting point.
• Active magnetic loop or mini-whip: compact and interesting for a balcony or attic when there is no room for wire.

Far more decisive than the design is the noise level. On shortwave, switch-mode power supplies, LED lamps, chargers and routers badly pollute reception. An antenna outdoors, as far from the house as possible, almost always beats an elaborate antenna in the living room. The basic mounting rules from the article Finding the right antenna location apply by analogy – except that on shortwave, sources of interference inside the house matter even more than a clear view.

The software: setting up dumphfdl

The standard decoder is dumphfdl, an open-source program under the GPL-3.0 for Linux and macOS. It talks to your SDR directly through SoapySDR, so it needs no virtual audio cables, and it decodes many HFDL channels at once. CPU load depends mainly on the sample rate, not on the number of channels.

A typical command with an SDRplay receiver at 250,000 samples per second and ten channels in the 8.9 MHz band looks like this according to the project documentation: dumphfdl --soapysdr driver=sdrplay --sample-rate 250000 8834 8885 8894 8912 8927 8939 8942 8948 8957 8977. The numbers are channel frequencies in kilohertz.

Two pitfalls with the sample rate

• Only use supported rates: If you request a sample rate your SDR does not support, the system may silently accept it – and decoding fails without a clear error message.
• Keep the edges free: Only about 80 percent of the bandwidth is truly usable, because the receiver’s filters attenuate the edges. So do not place your channels at the outermost edge of the window.

Finding the right frequencies automatically

Because the active channels change with the time of day, a fixed band is not always optimal. The docker-dumphfdl container therefore includes a scanner: it tries different frequency groups in turn, stays with the most productive one and switches again when the number of messages falls below a threshold. If you already feed several networks with Ultrafeeder, this container fits nicely into your existing Docker setup.

Day, night and sun: understanding propagation

Shortwave behaves differently from anything you know from ADS-B. Whether a signal arrives does not depend on line of sight but on the state of the ionosphere, and that changes constantly.

This breakdown is a rule of thumb. Geomagnetic storms can knock out shortwave almost completely for a while; your log then simply stays empty, and it is not your station’s fault. Interestingly, because HFDL needs only a fraction of the bandwidth of voice radio, data messages often still get through when shortwave voice radio is barely intelligible under difficult conditions.

What to do with the data

Putting positions on the map

dumphfdl outputs positions in Basestation format, which many mapping programs understand. That is how the Atlantic flyers end up in Virtual Radar Server or similar applications. How to set up flight tracking on a PC is shown in the article Flight tracking on the desktop with tar1090 and Virtual Radar. Expect positions to jump around and to be several minutes old at times – that is inherent to the method.

Analyzing ACARS content

If you already receive ACARS messages with an SDR and use acarshub for that, you can feed HFDL into it. VHF ACARS, VDL2 and HFDL then end up in one common interface, and you can see how one and the same aircraft communicates first over shortwave and later, shortly before the coast, over VHF.

Community networks

Similar to ADS-B Exchange for ADS-B, there is Airframes.io, a community project that collects ACARS, VDL2 and HFDL data from many receivers. The Docker container supports forwarding via your own feed ID. Before you share data, though, it is worth looking at the legal side.

HFDL in everyday spotting

For photography at the fence, HFDL is not a tool for the last ten minutes before landing – that is what ADS-B and flight tracking for spotters are for. Its strength lies in the lead time: early in the morning, you see which aircraft are currently moving across the ocean toward Europe. If an unusual registration shows up, you still have hours to check the runway direction in use and the light and drive to the right spot.

Then there is the appeal of the hobby itself. A squitter from Reykjavik or a position report from a jet between Newfoundland and Ireland, received with a wire in your own yard, is a very different experience from a position you simply read off a website. If you already feel at home with MLAT and feeder statistics, HFDL offers a new playground with a radio flavor.

Conclusion

HFDL opens a window onto air traffic where ADS-B is blind from the ground: over oceans, deserts and polar routes. The network of around 16 ground stations, the squitters every 32 seconds and the robust shortwave transmission make it possible to hear long-haul jets in the middle of the Atlantic with an HF-capable SDR, a wire antenna and dumphfdl. Do not expect smooth tracks, but individual messages – and take your time to get to know the quirks of shortwave.