ADS-B
ADS-B is a radio technology that allows aircraft to continuously broadcast their own position, altitude, and speed. Because these signals are unencrypted, anyone with a cheap receiver can pick them up – which is what flight-tracking sites like Flightradar24 rely on.
Today, an aircraft knows very precisely where it is. It determines its position via satellites, much like a phone with a navigation app. ADS-B is the technology by which the aircraft then broadcasts this position into its surroundings by radio. About twice a second, it sends out a short data packet: identification, position, altitude, speed, climb or descent rate. This message goes out to anyone listening – to air traffic control on the ground, to other aircraft nearby, and to any private individual with a suitable receiver. The abbreviation stands for “Automatic Dependent Surveillance – Broadcast”: automatic because no pilot has to trigger anything, and broadcast because the signal goes out undirected to everyone.
From the radar screen to the open data stream
Classic radar works the other way around. A ground station sends out radio waves and waits until they bounce back off the aircraft. This is elaborate, requires large antennas, and works poorly over oceans, deserts, and mountain ranges. Simply put, there is no one there to operate a radar station.
ADS-B reverses the responsibility: the aircraft reports itself. As a result, a receiver is much cheaper than a radar installation. That is precisely why the US and the EU have, over the years, made ADS-B mandatory. In Europe, larger commercial aircraft have been required to be able to transmit since the late 2010s. This also allows air traffic control to see areas that used to be gaps.
For the tech world, something else is decisive. The data is neither encrypted nor signed. This means the movement of almost every commercial aircraft worldwide is a publicly accessible data source. This has given rise to an entire industry of flight-tracking services – and regular debates about whose private jet one is actually watching.
What’s inside an ADS-B packet
The device on board is called a transponder and transmits on 1090 megahertz. It sends very short messages of 112 bits in length, so roughly 14 characters of actual information. Because there is so little space, different message types alternate: first the identification, then the position, then the speed. A receiver assembles these fragments over a few seconds into a complete picture.
A distinction is made between two directions. ADS-B Out is transmitting, ADS-B In is receiving. Only Out is mandatory. Aircraft equipped with In can display their neighbors in the vicinity directly in the cockpit, without the detour via the ground.
The letter D in the abbreviation stands for “dependent” – and it is the actual weak point. The system depends on the aircraft providing correct data. Anyone who wants to can send fabricated packets, so-called spoofing, or simply switch off their own signal. Radar, by contrast, detects an aircraft even when it doesn’t want to say anything. That’s why ADS-B doesn’t fully replace radar, but complements it.
Flightradar24, antennas in the backyard, and jet tracking
Anyone who has ever checked where a relative’s flight currently is has used ADS-B. Services like Flightradar24, FlightAware, or the open project ADS-B Exchange build their maps almost entirely from these signals. The receivers involved are often not owned by the companies themselves. Thousands of volunteers operate a small antenna by the window and forward the data.
The hardware for this costs little. A USB stick originally intended for digital television, plus a small computer like a Raspberry Pi, is enough. It is precisely this combination that has made ADS-B a popular entry-level project for hobbyists. Over open ocean, however, a gap remains because there are no antennas there. Some providers therefore additionally purchase reception data from satellites.
In the news, ADS-B mostly comes up in connection with private jets. Journalists and activists use this data to calculate the emissions of individual flights or to track the travels of billionaires and politicians. A well-known case was Elon Musk’s dispute with a student who automatically published Musk’s jet movements. Observers of military conflicts also use ADS-B to detect transport and reconnaissance flights. The reason is always the same: the system was designed for safety, not for secrecy.