Most people use sites like Flightradar24, FlightAware, or ADS-B exchange when they want to track flights. And I don’t blame them, they’re all clean and built for people who just want to know where a flight is and track it.

I wanted something a bit closer to the source, which meant using online radio receivers to decode the communications between aircraft and ground stations. Before online SDR receivers became popular, that meant using my own gear, antenna, and HFDL decoders, and plotting manually based on the message content.

Now, KiwiSDR makes this possible without the expensive gear at home. All you need is a browser, find an SDR that supports HFDL decoding, and see messaging and aircraft position in real time.

I was not trying to replace Flightradar24

I wanted to see the radio side of flight tracking and make the results feel more earned

Like anyone, I still use normal flight-tracking sites for normal purposes. If I want to know where on Earth my flight is when it’s listed as delayed on vacation, I’m not going to break out the SDR and tune into the HFDL messages and track it that way. I’ll use Flightradar24 and other useful vacation apps like everyone else.

The point of this experiment was to try a more hobbyist approach to flight tracking. I opened a remote receiver, tuned into a signal, decoded the messages, and watched the planes appear on the map from that decoded data. HFDL stands for High Frequency Data Link, and it’s used by aircraft to send data over HF radio to ground stations. Some of those messages include the plane’s coordinates, and that’s where it starts to get more interesting.

My own SDR receiver doesn’t pick up much HFDL traffic, so free online KiwiSDR receivers make this much easier because everything runs in my browser. I could pick a public KiwiSDR receiver that sits under flight routes, open up the HFDL extension, and watch the decoder pulling messages straight out of the signal.

Don’t get me wrong, the point isn’t that it looks better, because it absolutely doesn’t. The KiwiSDR HFDL interface looks a bit more like old versions of Microsoft Windows when compared to online flight trackers, but that’s all part of the appeal. Normal online flight trackers give you a nice finished picture, but decoding the raw HFDL messages and watching the map fill up by decoding the raw data.

Watching my first decoded aircraft was really satisfying

It’s one thing to see a plane icon, and another to receive the message that created it

My first attempts at tracking aircraft via radio yielded no good results, and that’s because I was being impatient. Not all HFDL transmissions contain aircraft coordinates, and depending on receiver noise, the signal might not even be clean enough to decode it.

My first proper attempt was after opening a KiwiSDR receiver in Ireland, and getting a decode on AC0923 on 8939.00 kHz. The frequency is actually listed for Seoul Muan’s HFDL ground station, but the message showed a DLS login request involving Reykjavík, Iceland, and showed the aircraft over the North Atlantic:

• GS: Reykjavík, Iceland
• Type: Log on request (DLS)
• Lat: 53.304392
• Lon: -27.108164

This all sounds a bit contradictory, but I remembered this is HF, and I was not tracking Korean airspace, but rather listening to a long-range aeronautical data network. It’s just that the receiver had caught the transmission on the Seoul frequency and handled it.

The decoder that runs as a KiwiSDR extension is called dumphfdl, created by szpajder. It takes the raw HFDL signals and then turns them into human-readable messages and plots aircraft coordinates on a world map. And that’s what happened with AC0923. I saw the message appear in the HFDL output, then watched the aircraft show up on the map.

​​​​​​​Moving between receivers changed what I could see

The aircraft plotted depended on where the radio was listening from

After the first few decoded HFDL messages, I realized that to get more planes plotted on the map; I needed to start moving around between both frequencies and receivers. Each receiver has its own location, antenna, noise floor, and capability. Even when two receivers could tune to the same HFDL frequency, there was no guarantee they could hear the same aircraft or ground stations.

I also needed to consider the time of day. Lower HFDL frequencies around 5MHz, 6MHz, and 8MHz tend to propagate better after dark, which made receiver-hopping not only a matter of proximity to known HFDL stations, but also meant following the sun.

So, rather than zooming around a completed map like an online aircraft tracking site might show, I was changing listening posts to get the signals. One KiwiSDR in New Zealand in the daytime could show me the aircraft around the Oceanic region, while another in Alaska at night could receive signals from Latin America all the way to Japan, and back.

That uncertainty is what makes aircraft tracking by radio so much more rewarding. It made flight tracking feel more like exploring coverage rather than just checking a website.

At first, I used Airframes.io to double-check the aircraft coordinates from wider data-link networks

Then I realized just how accurate the standalone raw HFDL signal decoding was

The aircraft I could track using the HFDL signals received and decoded by the KiwiSDR were from a single source, so I wasn’t sure how accurate these were. To check if what I was seeing was correct, I cross-checked a few flights against the data on the airframes.io platform.

Airframe.io receives data from a wide variety of community sources rather than commercial APIs like AviationStack. Airframes makes use of data including ACARS, VDL, HFDL, and SATCOM, which makes it useful for checking the validity of aircraft tracking produced solely by HFDL decoding over radio.

After confirming the aircraft plotting on the SDR receivers was indeed accurate, it gave me more confidence that using online SDRs this way was both fun and accurate enough to bring more challenge to the hobby than what comes from a site like Flightradar24.

​​​​​​​HFDL tracking is closer to real time, but not “better” overall

KiwiSDR gives me the hunt, but Flightradar24 gives you more data

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The KiwiSDR HFDL view feels immediate because it is. I could see messages appear as the receiver’s plugin decoded them, making it feel much closer to the source of the signal than a normal flight tracker.

Does this make it more accurate overall? In one way, yes, I could see the flight position faster than what was available on the online flight trackers, but that comes with one big caveat. Not every message contains a useful position. Some receivers are full, noisy, offline, or missing the HFDL plugin entirely.

That’s why Airframe.io was useful as a second window. The HFDL decodes showed me what the receiver had directly caught, but Airframes gave me a wider look at all the aviation data-links that come together to provide a more accurate picture overall.

The difference comes down to how the signal gets discovered. With listening for HFDL transmissions only, it’s more like a hunt rather than being spoon-fed the positions via a tracking website. When I combined finding an aircraft’s position via radio with listening in to live air traffic control (ATC) transmissions, also via online SDRs, then I got a far more authentic and real experience.

Basically, Flightradar24 gets me the answer straight away, but KiwiSDR gives me the slower but more rewarding version of aircraft tracking.

​​​​​​​I would still use flight trackers, but KiwiSDR is more fun

It is not the easiest way to track planes, but where’s the fun in that?

Using an online KiwiSDR doesn’t replace Flightradar24, Flightaware, or ADS-B Exchange for everyday flight tracking. If my friend’s plane is delayed, and I need to know when to leave to come pick them up from the airport, then I’m going to use an online tool built for precisely that job.

But that wasn’t why I loved tracking aircraft via HFDL transmission and decoding.

The online SDR receivers let me see the radio and data-link underneath aviation tracking without having to set up my own expensive gear and just hope my location was good enough to receive the messages. But when combined with Airframes.io, it gives me a way more hands-on method of exploring the infrastructure that keeps planes flying safely.

Add in live browser listening for ATC traffic, and the hobby goes from clicking around on a map to actually listening to what’s in the sky. I’ll still use flight trackers when I need a basic, clean answer, but when I want the radio side of the story, the KiwiSDR and HFDL decoder are just far more interesting.