ADS-B stands for Automatic Dependent Surveillance – Broadcast. It is worth unpacking that name piece by piece, because each word describes something real about how it works:
- Automatic — it transmits continuously on its own. No controller has to interrogate the aircraft, and the crew does not press anything to send a report.
- Dependent — it depends on the aircraft's own navigation equipment. The position you see is the position the aircraft believes it is at, usually from GPS. The ground has not independently measured it.
- Surveillance — its purpose is to tell air traffic control, and other aircraft, where it is.
- Broadcast — it is sent out to nobody in particular. There is no handshake and no recipient list. Anyone within range with the right receiver can hear it.
That last word is the one that matters for us. Because ADS-B is an unencrypted broadcast, a hobbyist with a £25 USB dongle can receive exactly the same transmissions that air traffic control does. That is the entire basis of every public flight tracking site, including this one.
The frequency: 1090 MHz
Almost all ADS-B traffic you will see over the UK and Europe is carried on 1090 MHz, in a format called 1090 Extended Squitter — usually shortened to 1090ES. It rides on the same transponder hardware that has answered secondary radar interrogations for decades, which is why adoption was practical: manufacturers extended an existing box rather than inventing a new one.
There is a second ADS-B link called UAT on 978 MHz, but it is a United States arrangement for light aircraft operating below 18,000 ft. You will not encounter it in British airspace, and a UK receiver tuned to 1090 MHz will never see it.
A "squitter" is simply a transmission sent without being asked. Aircraft squitter several times per second, which is why a fast-moving jet tracks smoothly across the map rather than jumping.
What an aircraft actually broadcasts
ADS-B messages are short — 112 bits — so no single message carries everything. Instead, different message types cycle round, each carrying a slice of the picture:
| Message type | What it carries |
|---|---|
| Aircraft identification | The callsign or flight number entered by the crew, plus a broad category (light, heavy, rotorcraft, and so on) |
| Airborne position | Latitude, longitude and barometric altitude |
| Airborne velocity | Ground speed, track, and vertical rate (climb or descent) |
| Surface position | Position and ground speed while taxiing |
| Operational status | Equipment capability and the quality of its own position data |
Underneath all of them sits the ICAO 24-bit address — a unique hexadecimal identifier
burned into the aircraft's transponder, such as 4CA1FA. It belongs to the airframe, not the
flight. The callsign changes every day; the hex address does not. That is why trackers use it as the key
for stitching messages together into a single aircraft.
Notice what is not in the list: destination, origin, aircraft type and airline name are never broadcast. Trackers add those by looking the hex address or callsign up in a separate database. When a flight tracker shows the wrong aircraft type or a blank route, that is a database gap — not a fault in the radio signal.
Why position needs two messages
This is the part that surprises people. An ADS-B position message does not contain a plain latitude and longitude. There is not enough room in 112 bits to send full-precision coordinates often enough to be useful, so ADS-B uses a scheme called Compact Position Reporting (CPR).
CPR encodes position relative to a grid, and it alternates between two slightly different grids, known as even and odd frames. To resolve a position from cold, a receiver needs one of each, received close together in time. Once it has locked on, it can keep updating from single frames.
The practical consequence: when an aircraft first drifts into range at the edge of coverage, it may be heard for several seconds before it can be plotted. It is transmitting fine — the receiver simply has not yet caught the matching frame.
How far can a receiver hear?
1090 MHz is line-of-sight. The limit is not really transmitter power, it is the horizon: the Earth curves away and the signal does not follow it. Higher aircraft are visible from further away, and a higher antenna sees further in every direction.
As a rough guide, a well-sited antenna with a clear view will typically reach 150–250 nautical miles for aircraft at cruising altitude. The same receiver might manage only 20 or 30 miles for something low and slow. This is why coverage maps are shaped like flowers rather than circles — hills, buildings and trees each carve out their own notch.
Why some aircraft are missing
A blank patch of sky on a tracker rarely means empty sky. The common explanations:
No ADS-B Out fitted
Plenty of older light aircraft, gliders and microlights have no ADS-B transmitter at all. They may carry a basic transponder that replies to radar interrogation, but they broadcast nothing on their own, so a passive receiver has nothing to hear.
Military and state aircraft
Military traffic frequently operates with ADS-B transmission disabled, or transmits an address that is not tied to any public registry. Air ambulance, police and some government flights are handled similarly or are filtered out by trackers.
Terrain and range
The aircraft is simply below the receiver's radio horizon. Low-level traffic in a valley twenty miles away can be completely invisible while an airliner two hundred miles away comes through cleanly.
Coverage gaps over water and wilderness
ADS-B needs someone on the ground listening. Mid-ocean, there is nobody, which is why transatlantic flights appear to freeze halfway across on many trackers and then reappear near Ireland. Satellite-based ADS-B reception now fills much of this in for commercial services, but ground-fed community networks still show the gap.
Because ADS-B is dependent, it inherits any error in the aircraft's own navigation solution. Messages include a quality indicator so controllers know how much to trust the figure. In normal operation the position is accurate to a few metres — but a tracker is repeating a claim, not making a measurement.
ADS-B is not the same as radar
It is worth being precise about this, because "radar" is used loosely on tracking sites — including in the name of ours. Conventional radar transmits a pulse and measures the reflection or the transponder reply, so it works whether or not the aircraft cooperates. ADS-B only works because the aircraft chooses to tell you. Air traffic control uses both, precisely so one covers the other's weaknesses.
For an enthusiast on the ground, though, ADS-B is the one that is realistically receivable — and it carries far richer information than a radar return ever did.