How ADS-B Flight Tracking Works: A Complete Guide

ADS-B flight tracking works because aircraft announce their own position. Each equipped aircraft reads its location from satellite navigation and broadcasts it, with altitude, speed and identity, about once per second on a public radio frequency. Anyone with a receiver in range can hear those messages, and flight tracking sites combine thousands of receivers into one live map.

That one idea explains most of what you see on a tracker: why coverage is dense near cities and thin over oceans, why some aircraft appear as a hex code with no flight number, why a few jump around, and why some are missing entirely. This guide walks through the radio link, the message format, the receiver networks, multilateration, satellite ADS-B and the limits of the data.

By the end you should be able to look at any flight on the live flight tracker and know where its position came from, how fresh it is and how much to trust it.

Open the live flight map

What is ADS-B?

ADS-B stands for Automatic Dependent Surveillance-Broadcast. "Automatic" because it needs no pilot input once set up, "dependent" because the position comes from the aircraft's own navigation system rather than a radar on the ground, and "broadcast" because it is sent to everyone in range, not to one station.

The FAA splits it into two services. ADS-B Out is the transmitting side: according to the FAA's ADS-B page, it broadcasts GPS position, altitude, ground speed and other data to ground stations and other aircraft once per second. ADS-B In is the receiving side, which shows traffic and weather on cockpit displays. Flight tracking websites are, in effect, very large ADS-B In systems on the ground.

In the United States, ADS-B Out has been required since January 1, 2020 in Class A, B and C airspace and in much of Class E above 10,000 feet, under 14 CFR 91.225. Other countries set their own equipage rules, so coverage of light aircraft varies by region, while airliners and business jets flying in controlled airspace are the aircraft you can most reliably expect to see.

How ADS-B works, step by step

1. The aircraft's GPS or other satellite navigation receiver computes latitude and longitude.
2. The transponder or a dedicated ADS-B unit packs that position, plus pressure altitude, velocity and identity, into short digital messages.
3. Those messages are broadcast on 1090 MHz (or 978 MHz for UAT, covered below) with no request from the ground.
4. Air traffic control ground stations, other aircraft and hobbyist receivers within line of sight decode them.
5. Tracking networks forward the decoded messages to servers that merge duplicates, add aircraft and route data, and draw the result on a map.

The key difference from classic radar is step 1. Secondary surveillance radar measures an aircraft's range and bearing by interrogating it; ADS-B simply trusts the aircraft's own position report. That makes it precise and cheap to receive, and it also means a degraded GPS signal produces a degraded position, a point we return to in the limits section.

1090ES vs UAT: the two ADS-B links

There are two ADS-B radio links in use, and which one an aircraft uses affects who can see it.

The split is written into the regulation: above 18,000 feet the FAA rule accepts only 1090ES equipment, while below it either link qualifies (14 CFR 91.225). The FAA ADS-B FAQ also notes that FIS-B weather is broadcast only on the 978 MHz UAT link because of bandwidth.

For flight tracking, this means a small US piston aircraft using UAT will only appear on networks that run 978 MHz receivers. Flightradar24 describes UAT as a growing data source for smaller aircraft flying below 18,000 feet in the United States.

What is inside an ADS-B message?

A 1090ES message is 112 bits long. According to Junzi Sun's open textbook The 1090 Megahertz Riddle, it is made of a 5-bit downlink format, a 3-bit capability field, the 24-bit ICAO aircraft address, a 56-bit data payload and 24 bits of parity for error checking. Civil ADS-B uses downlink format 17; format 18 is used by non-transponder devices.

A type code at the start of the payload says what the message carries. The main families are:

• Identification (type codes 1–4): the callsign, such as BAW117, and a category describing the kind of aircraft.
• Surface position (type codes 5–8): for aircraft moving on the airport surface.
• Airborne position (type codes 9–18 with barometric altitude, 20–22 with GNSS height): compressed latitude and longitude.
• Airborne velocity (type code 19): ground speed, track and vertical rate.
• Status (type codes 28, 29 and 31): aircraft status, target state and operational status, including the accuracy and integrity values that tell a receiver how far to trust the position.

Position and velocity messages are each sent about twice per second while airborne, according to the same textbook, which is why a well-covered aircraft updates smoothly. The FAA rounds this to "once per second" in its description.

The ICAO hex code

The ICAO 24-bit address is the single most useful identifier on a flight map. It is written as six hexadecimal characters, such as 4CA7B5, and the 24-bit space allows about 16.7 million unique values (SKYbrary). ICAO gives each state a block of addresses; the state assigns one to each registered aircraft. When an aircraft moves to another country's register, it gets a new address from that country's block.

Because the hex code belongs to the airframe, not the flight, it is the most reliable way to follow one aircraft across many flights. Our guide on how to track a flight explains when to search by hex, callsign, registration or flight number.

Who receives the signals? Volunteer receiver networks

Air navigation service providers run their own ground stations, but public flight maps rely mostly on volunteers. A typical home receiver is small: adsb.fi lists a Raspberry Pi, a USB software-defined radio, an antenna and its feeder software as the parts you need.

The networks are large. Flightradar24 says it combines more than 50,000 receivers on seven continents and in space. ADS-B Exchange describes a network of over 25,000 active receivers. The OpenSky Network, a non-profit association based in Switzerland that started as a research project in 2012, collects data from volunteer, industrial and academic sensors and archives it for research. Community aggregators such as adsb.lol and adsb.fi publish their data openly.

Each receiver only hears aircraft above its horizon. A Euronews report on Flightradar24 put typical ground receiver range at roughly 300 kilometers; in practice, an airliner at cruise altitude is heard from much farther away than a helicopter at 1,000 feet behind a hill.

What is MLAT and why does it matter?

Not every aircraft broadcasts its position. Older transponders and many military aircraft reply with Mode S messages that include identity and altitude but no latitude or longitude. Multilateration (MLAT) recovers a position anyway.

MLAT works on timing. When four or more receivers hear the same message, the tiny differences in arrival time can be solved for the point where the signal came from. Flightradar24's support documentation says four or more receivers must hear the same aircraft, and that MLAT coverage generally begins somewhere between about 3,000 and 10,000 feet, because low aircraft are heard by too few stations.

MLAT positions are usually slightly less smooth than ADS-B positions, and trackers often label them separately. They matter most for older business jets, some general aviation and many military aircraft, which is why our explainer on military aircraft tracking spends time on them.

How flights are tracked over oceans

A ground receiver cannot hear an airliner 1,500 kilometers out over the Atlantic. Two things fill that gap.

Space-based ADS-B puts receivers on satellites. Aireon operates a system hosted on Iridium's constellation of 66 satellites and sells the data to air navigation service providers and other customers; it has been in service since 2019. Flightradar24 says it also ingests satellite ADS-B to extend coverage over oceans.

Estimated positions fill the rest. When no receiver hears an aircraft, some trackers extrapolate its path from its last known speed and heading or from its filed route, and mark it as estimated. An estimated position is a forecast, not an observation.

How often do positions update?

There are three separate clocks to keep in mind:

On GodsView AI, the aircraft layer pulls from the OpenSky Network first and falls back to adsb.lol and adsb.fi, refreshing about every 10 seconds. That is fast enough to watch arrivals and holding patterns, while a professional data feed will be faster. The methodology page lists every source and refresh rate we use.

See where the flight data comes from

Where ADS-B tracking has gaps and blind spots

ADS-B is very good, and it is still a partial picture. Expect these gaps:

• Low altitude and terrain. Helicopters, crop sprayers and light aircraft in valleys often vanish because no receiver has line of sight.
• Thin receiver regions. Large parts of Africa, Central Asia, the Amazon and the polar regions have few volunteer receivers.
• Aircraft that do not transmit. Not every aircraft is required to carry ADS-B Out everywhere, and the FAA rule allows sensitive government missions to switch it off (14 CFR 91.225(f)).
• Filtered aircraft. The FAA's LADD program lets owners keep their aircraft off displays that use FAA data feeds, and the Privacy ICAO Address program lets eligible US aircraft use a temporary address that is not linked to them in the public registry.
• Bad positions. Because ADS-B trusts the aircraft's GPS, jamming and spoofing can make tracks jump, freeze or loop. Our guide to GPS jamming and spoofing explains how this shows up, and the GPS jamming map shows known interference zones.
• Data entry errors. Callsigns are typed by crews and registration databases lag behind sales, so labels can be wrong even when positions are right.

What a flight map cannot tell you

A dot tells you where an aircraft said it was, a moment ago. It does not tell you why it is there, who is on board, or whether a strange pattern is a problem. Circling is usually a holding pattern, a training flight or a survey job. A squawk of 7700 means the crew declared an emergency (FAA AIM 6-2-2), which can range from a sick passenger to an engine issue; it is not a prediction of what happens next.

The same caution applies to missing aircraft. A track that stops may mean the aircraft landed, dropped below coverage or flew into a jamming zone. Treat any single observation as a lead to verify, and use official sources for anything that affects safety. If you are curious about the rules around watching and sharing this data, see is flight and ship tracking legal?.

How to see ADS-B data live

You can watch all of this in a browser. Open the live flight tracker, turn on the aircraft layer, and click any aircraft to see its hex code, callsign, altitude and speed. On the 3D globe, the same data sits alongside ships, satellites and weather. You can replay an aircraft's track from adsb.lol traces at 1× to 40× speed, and Pro adds following an aircraft in 3D and the military flight tracker layer, which uses the adsb.lol military feed.

If you are choosing between the big consumer trackers, our ADS-B Exchange vs Flightradar24 comparison covers how their data and filtering differ.

The short version

ADS-B is aircraft broadcasting their own GPS position, about once or twice per second, on 1090 MHz or (in the US, at low altitude) 978 MHz. Volunteer receivers pick those messages up, MLAT fills in aircraft that only send Mode S replies, and satellites cover the oceans. The six-character ICAO hex code ties it all together.

Read any flight map with three questions in mind: how was this position measured, how old is it, and what might be missing? If you keep those in view, ADS-B is one of the most open and useful public datasets you can watch live.

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