ADS-B, explained

The name, decoded

ADS-B is four ideas, one per letter. Together they say something simple: the aircraft announces where it is, all the time, to whoever cares to listen.

Automatic

The aircraft transmits on its own, unprompted, with no interrogation from the ground.

Dependent

The position does not come from a ground radar: it depends on the aircraft's own instruments, first of all its GPS.

Surveillance

It is a way to track aircraft, like radar, but built the other way round.

Broadcast

The message goes out to everyone at once, not to a single recipient. Nothing stops you from listening.

Radar, turned inside out

Classic radar works like a lighthouse: the ground sends out a wave, it bounces off the aircraft, and the echo reveals its range and bearing. The aircraft is passive, it is measured.

ADS-B flips this around. The aircraft computes its own position, using GPS, then announces it over the radio. The ground only has to listen. No spinning antenna, no powerful transmitter: a simple receiver is enough. That reversal is what put flight tracking within reach of an amateur, from a rooftop.

The radio: 1090 megahertz

In Europe and most of the world, ADS-B travels on 1090 MHz, the same frequency as the Mode S transponders already on board. It is called 1090ES, for Extended Squitter: the aircraft sends short bursts on its own, without being interrogated.

A burst is brief: one message lasts about 120 microseconds, less than two ten-thousandths of a second. The information is coded by the position of pulses within one-microsecond slots. In the United States a second channel exists for light aviation, on 978 MHz (UAT), but the principle is the same.

A single aircraft can send several messages per second. A busy sky means tens of thousands of bursts crossing on 1090 MHz every second.

What a message carries

The most common ADS-B message is 112 bits, that is fourteen bytes, preceded by a short 8-microsecond preamble that lets the receiver lock on. The message itself fits into five fields, in a fixed order:

1. Format (5 bits)the link type. The value 17 flags an ADS-B Extended Squitter.
2. Capability (3 bits)a few service bits about the transponder's capabilities.
3. ICAO address (24 bits)the aircraft's unique identifier, in hexadecimal. It is the "hex" this site shows on every record.
4. Data (56 bits)the useful payload. A type code says whether it carries the callsign, the position, or the velocity.
5. Checksum (24 bits)a fingerprint that lets a damaged message be thrown away.

Position deserves a trick. To fit into so few bits, latitude and longitude are sent in two complementary versions, called even and odd. You need both, received close together, to recover the exact spot on the globe. This is CPR (Compact Position Reporting), and it is why an aircraft needs both an even and an odd message before it appears on the map.

From antenna to log

This site listens to ADS-B with an official Flightradar24 box; a homemade program turns it into the log you browse. Every contact, every track, every first appearance comes from that antenna, nothing is bought or invented.

The full path, from antenna to screen→

What about silent aircraft? Multilateration

Not every aircraft transmits its position. Older ones only send a Mode S signal with no coordinates. They can still be located, by multilateration (MLAT).

When several ground stations hear the same aircraft, the signal does not reach them at exactly the same instant: it takes a little longer to reach the farther one. By comparing these tiny differences, with synchronised clocks and at least four stations, the position can be recomputed. It is geometry on the travel time, the same idea as GPS but inverted: here the ground listens, not the sky.

Range is all about the horizon

1090 MHz travels in a straight line, like light: it goes around neither hills nor the curve of the Earth. Range is therefore the radio horizon, and that horizon recedes with altitude.

An aircraft cruising at 35,000 feet can be heard more than 400 km away; the same aircraft low and behind a ridge stays inaudible. That is exactly what the station's coverage chart shows: range grows band by band with altitude, sector by sector, because the physics of the horizon demands it.

What ADS-B is not

ADS-B is open by design, which is both its strength and its limit. It is neither encrypted nor authenticated: anyone can listen, and nothing proves, in theory, that a message truly comes from the aircraft it names (spoofing is a real security concern). The position is only as good as the aircraft's GPS. Some sensitive flights do not transmit, or are filtered out.

On privacy, the registration is public, but tying a tail number to a person is another matter. That is why this site lets an owner request that their aircraft be removed: the data is open, showing it stays a choice.

Why it is fascinating

From an antenna, a receiver and a little software, a living, worldwide picture of aviation is rebuilt, in real time, shared among enthusiasts. Nothing is hidden, everything decodes. This site, its log and its map are only what the sky, patiently, was willing to say.