How ADS-B Works, Explained Simply

Short answer: ADS-B (Automatic Dependent Surveillance-Broadcast) is a system in which aircraft use satellite navigation to work out their own position and then broadcast it, along with altitude, speed and identity, several times a second on 1090 MHz or 978 MHz. Anyone with a suitable receiver can pick up those unencrypted broadcasts, which is how live flight maps work.

7 min read · Updated October 11, 2026

What ADS-B stands for

ADS-B is short for Automatic Dependent Surveillance-Broadcast. Each word means something:

Automatic: no pilot action is needed once it is switched on.
Dependent: the position depends on the aircraft's own navigation system, usually GPS, rather than on a radar measuring it from the ground.
Surveillance: the purpose is to let air traffic control and other aircraft see where it is.
Broadcast: the message is sent to anyone listening, not addressed to a single station.

That last word is why a site like ours can exist. The signals are public, unencrypted radio transmissions, and volunteers around the world receive and share them.

What an aircraft actually broadcasts

An aircraft equipped with ADS-B Out transmits a stream of short digital messages. The important ones are:

Airborne position: latitude, longitude and altitude, sent about twice per second.
Airborne velocity: ground speed, track and vertical rate, also about twice per second.
Identification: the callsign and the emitter category (light aircraft, large jet, rotorcraft and so on), sent about every five seconds.
Status: the current squawk code, emergency status and information about how accurate the position is.

Every message carries the aircraft's ICAO 24-bit address, a unique code assigned to the airframe, usually written as six hexadecimal characters such as A1B2C3. US-registered aircraft have addresses beginning with A. That address ties all the messages from one aircraft together, and it can be looked up against registration databases to find the aircraft type and registration.

A typical feed therefore updates an aircraft's position roughly once per second once the receiver has heard enough messages, which is far faster than the 4.5 to 12 second sweep of traditional radar.

Two frequencies: 1090 MHz and 978 MHz

There are two ADS-B "links" in use:

1090ES (1090 MHz Extended Squitter) is the worldwide standard. It reuses the frequency and message format of Mode S transponders, so ADS-B is really an extension of the transponder. Airliners, business jets and most aircraft that fly high use it.
UAT (Universal Access Transceiver) on 978 MHz is used only in the United States, and only by aircraft flying below 18,000 ft (FL180). It is popular with small private aircraft because UAT equipment can also receive free weather and traffic information.

A receiver needs to listen on the right frequency to hear each kind, which is why some feeders run two receivers.

Where the altitude comes from

ADS-B messages usually carry two altitudes. The primary one is barometric (pressure) altitude, measured by the aircraft's air data system against the standard pressure of 29.92 inHg (1013.25 hPa). This is the same number air traffic control sees, and it is what we show by default. Messages can also include geometric altitude, the GPS height. The two can differ by hundreds of feet depending on the weather. Our guide to altitude numbers explains why.

Who has to carry ADS-B?

In the United States, the FAA has required ADS-B Out since January 1, 2020 in "rule airspace". In broad terms that covers:

Class A airspace (18,000 ft MSL and above);
Class B and Class C airspace around busy airports;
Within 30 nautical miles of the major Class B airports (the "Mode C veil"), from the surface up;
Above 10,000 ft MSL across the country, except within 2,500 ft of the ground;
Some airspace over the Gulf of Mexico.

Aircraft that stay out of those areas, such as many gliders, balloons and antique aircraft without electrical systems, are not required to equip. Europe, Canada, Australia and many other regions have their own mandates, mostly focused on larger aircraft. This is why nearly every airliner shows up on ADS-B maps while some small aircraft do not. More in why some planes don't show up.

From radio wave to dot on the map

Here is the path a position takes before it reaches the live sky tool:

The aircraft's GPS computes its position.
The transponder or UAT unit broadcasts it.
A ground receiver within radio range decodes the message. Receivers range from professional installations to a volunteer's small antenna on a roof connected to a cheap software-defined radio and a single-board computer.
The receiver, or feeder, forwards decoded messages over the internet to an aggregator. Our data comes from airplanes.live, a community network of these feeders.
The aggregator merges reports from many receivers, removes duplicates and publishes the latest state of each aircraft.
Our site requests the aircraft near your location and calculates where to look for each one.

The whole chain usually takes a few seconds. The tool refreshes about every 10 seconds.

Range: why line of sight matters

1090 MHz and 978 MHz signals travel in straight lines and do not bend around hills or buildings much. The distance to the radio horizon in nautical miles is roughly 1.23 times the square root of the height in feet. Add the horizon distance of the receiver antenna and of the aircraft to get the maximum range:

Aircraft altitude	Radio horizon (approx.)
1,000 ft	39 nm
10,000 ft	123 nm
35,000 ft	230 nm

So a well-placed receiver can hear airliners at cruise from 200 nm or more, but a helicopter at 500 ft behind a ridge may be invisible to every receiver nearby. Low aircraft are where coverage gaps show up first.

What about planes without ADS-B? MLAT

Aircraft that have a Mode S transponder but no ADS-B position still reply to radar interrogations. If four or more receivers with precise timing hear the same reply, the network can calculate the aircraft's position from the tiny differences in arrival time. This is called multilateration (MLAT). MLAT positions are less precise and update less often, and they only work where receiver coverage is dense.

ADS-B In

ADS-B In is the receiving side. Aircraft equipped with it can display nearby traffic in the cockpit. In the US, ground stations also rebroadcast traffic (TIS-B) and weather (FIS-B, on UAT only) to equipped aircraft. A volunteer feeder is, technically, an ADS-B In receiver on the ground.

Limits and privacy

ADS-B is not perfect. Positions depend on GPS, so GPS interference can produce bad positions. Some aircraft broadcast a wrong callsign or none. And in the US, owners can ask the FAA to limit how their aircraft is displayed in FAA-provided data through LADD or to use a temporary, anonymous address through PIA. Community networks receive the raw broadcasts directly, so policies differ from commercial trackers that rely on FAA data feeds.

Frequently asked questions

Is ADS-B encrypted?

No. ADS-B broadcasts are unencrypted and intended to be received by anyone, including air traffic control, other aircraft and ground receivers.

How often does an aircraft send its position over ADS-B?

Airborne position messages are sent about twice per second. After a receiver decodes them, a typical feed updates each aircraft about once per second.

What is the difference between 1090 MHz and 978 MHz ADS-B?

1090 MHz (1090ES) is the worldwide standard used by airliners and most high-flying aircraft. 978 MHz (UAT) is used only in the US, and only below 18,000 ft, mostly by smaller aircraft.

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