Understanding ADS-B: A Complete Guide for Aviation Enthusiasts

If you have ever watched a live map fill up with thousands of aircraft moving in real time, you have seen ADS-B at work. Automatic Dependent Surveillance-Broadcast is the backbone of modern flight tracking, and understanding it explains almost everything about how services like PlaneTrack.ai know where every plane is. This guide walks through the technology from the ground up: what it broadcasts, how that signal reaches a map, why aviation authorities mandated it, and how enthusiasts run their own receivers to expand global coverage.

What Is ADS-B?

ADS-B stands for Automatic Dependent Surveillance-Broadcast, and each word describes exactly what it does:

• Automatic: the aircraft transmits continuously with no interrogation or pilot action required.
• Dependent: the position data depends on an onboard navigation source, almost always a GNSS receiver such as GPS.
• Surveillance: it provides surveillance information such as position, altitude, and velocity.
• Broadcast: the data is broadcast openly, so any suitably equipped receiver can decode it.

Unlike traditional radar, where a ground antenna sends out a pulse and waits for a reflection, ADS-B has the aircraft do the work. The plane figures out its own position from satellites and then shouts that position to anyone listening, several times per second.

1090 MHz ES and 978 MHz UAT

ADS-B is carried on two different radio links depending on the region and aircraft. The dominant one worldwide is 1090 MHz Extended Squitter (1090ES), an extension of the existing Mode S transponder that most commercial aircraft already carry. A second link, 978 MHz Universal Access Transceiver (978 UAT), is used mainly by general aviation aircraft in the United States that fly below 18,000 feet. UAT can also carry free weather and traffic information services to the cockpit, which is one reason it exists alongside 1090ES.

For flight tracking purposes, 1090ES is by far the most important, since it is used globally and covers essentially all airline traffic. Most hobby receivers focus on 1090 MHz for that reason.

How the Broadcast Works

The chain starts with the aircraft's navigation system. A GNSS receiver on board computes a highly accurate latitude and longitude, typically within a few meters. The ADS-B transmitter combines that position with data from the aircraft's air data and inertial systems, then encodes it into short digital messages.

On 1090ES, each message is 112 bits long and is transmitted roughly twice per second. Position, velocity, and identity are spread across different message types, so a receiver assembles a complete picture from several messages over a couple of seconds. Because the signal is a simple line-of-sight broadcast, it can be picked up by any receiver within range, whether that is an air traffic control ground station, another aircraft, a satellite, or a hobbyist with a small antenna.

Ground Stations and Satellite Reception

On land, a network of ground stations receives these broadcasts and forwards them to air traffic control and to tracking services. Ground reception is limited by the horizon, so coverage is excellent over populated land and much weaker over oceans, deserts, and polar regions.

To fill those gaps, space-based ADS-B uses satellites in low Earth orbit that carry 1090 MHz receivers. Because a satellite can see a huge area at once, it can pick up aircraft over the middle of the ocean where no ground station could ever reach. This is how tracking of transoceanic flights became possible without relying on slower position reports relayed through other systems.

ADS-B Out vs ADS-B In

There are two sides to ADS-B, and they are easy to confuse:

• ADS-B Out is the transmitting side. The aircraft broadcasts its identity, position, altitude, and velocity. This is what makes an aircraft visible to controllers and to trackers, and it is the part that regulators have mandated.
• ADS-B In is the receiving side. The aircraft receives broadcasts from nearby traffic and from ground services, enabling cockpit displays of surrounding aircraft and, on UAT, free weather and traffic information.

For flight tracking, ADS-B Out is what matters. Every aircraft that shows up on a live map is transmitting ADS-B Out, and receivers around the world are listening.

Why ADS-B Replaced Radar

Radar served aviation for decades, but it has real limitations that ADS-B addresses directly.

• Accuracy: radar estimates position from the angle and timing of a returned pulse, which becomes less precise with distance. ADS-B reports a GNSS-derived position that stays accurate to within meters regardless of range.
• Update rate: a rotating radar only sees a target each time the antenna sweeps past it, often every several seconds. ADS-B updates roughly twice per second.
• Cost: primary and secondary radar installations are expensive to build and maintain. An ADS-B ground station is far cheaper, which makes it practical to add coverage in remote areas.
• Coverage over oceans: radar simply cannot reach across an ocean. Combined with satellite reception, ADS-B provides continuous coverage over water where radar never could.

The result is more precise, more frequent, and more widely available surveillance at a lower cost, which is exactly why aviation authorities moved to adopt it.

The Mandate

Regulators made ADS-B Out equipage mandatory to modernize airspace. In the United States, the FAA required ADS-B Out for aircraft operating in most controlled airspace starting January 1, 2020. In Europe, EASA introduced a comparable requirement for many aircraft on the 1090ES link. Similar mandates have rolled out in other regions around the world.

Because of these mandates, the overwhelming majority of commercial and a large share of general aviation aircraft now transmit ADS-B Out, which is what makes comprehensive real-time tracking feasible today.

What Is Inside an ADS-B Message

Decoded ADS-B data contains the fields that power every flight tracker. The most important ones are:

• ICAO 24-bit address: a unique hardware identifier assigned to each aircraft, shown as a 6-character hexadecimal code (for example, A12345). This is the key that ties every message to a specific airframe.
• Callsign: the flight identification, often the flight number or a registration.
• Latitude and longitude: the GNSS-derived position.
• Barometric altitude: altitude derived from air pressure, the standard used for separation.
• Geometric altitude: altitude derived from GNSS, which can differ from the barometric value.
• Ground speed: horizontal speed over the ground.
• Track: the direction the aircraft is actually moving, in degrees.
• Vertical rate: the rate of climb or descent in feet per minute.
• Squawk code: the 4-digit transponder code assigned by air traffic control, including emergency codes such as 7700, 7600, and 7500.

Limitations of ADS-B

ADS-B is powerful but not perfect, and it helps to know where it falls short:

• Line of sight: signals travel in straight lines, so a ground receiver cannot see aircraft below the horizon. Low-flying aircraft far away simply are not received until they climb or come closer.
• Not everything transmits: some military aircraft and certain state flights disable or filter their broadcasts for security reasons, and a portion of older general aviation aircraft may not be equipped at all.
• Dependent on onboard navigation: because the position is self-reported, a faulty navigation source or a jammed GNSS signal degrades the reported position.
• Congestion: in very busy airspace, the shared 1090 MHz frequency can become crowded, causing some messages to be lost.

Good tracking services work around these gaps by combining many receivers, blending in other data sources, and smoothing tracks between updates.

Running Your Own ADS-B Receiver

One of the best parts of ADS-B is that anyone can receive it. A basic setup is inexpensive and genuinely useful, both for you and for the wider tracking community. You need three things:

• An RTL-SDR USB dongle, a low-cost software-defined radio that tunes to 1090 MHz. A model with a 1090 MHz filter and preamp works best.
• An ADS-B antenna tuned for 1090 MHz, mounted as high and as clear of obstructions as possible. Line of sight is everything, so height matters more than almost anything else.
• Decoding software such as dump1090 or the more modern readsb, typically running on a Raspberry Pi or a small Linux computer.

The software listens on 1090 MHz, decodes the raw messages into positions and identities, and serves the results on a local web interface. From there you can forward your feed to tracking networks, which is where the community aspect comes in. Every additional receiver extends coverage, especially in areas that ground stations reach poorly.

How PlaneTrack.ai Uses ADS-B

Everything on the PlaneTrack.ai live map begins as an ADS-B broadcast. We operate our own receivers and combine them with feeds contributed by the community, then process that firehose of messages into clean, deduplicated aircraft positions in real time. There are no artificial delays on any tier: what you see is as current as the data arriving from the receivers.

Developers can access the same underlying data through our API. A simple request to the live endpoint returns current aircraft with the exact fields ADS-B provides:

curl -X GET "https://planetrack.ai/v1/live?limit=100" \
  -H "X-API-Key: your_api_key_here"

The response wraps the aircraft in a data array alongside a meta object:

{
  "data": [
    {
      "icao24": "A12345",
      "callsign": "UAL123",
      "lat": 37.7749,
      "lon": -122.4194,
      "altitude": 35000,
      "speed": 450,
      "heading": 180
    }
  ],
  "meta": {
    "count": 100
  }
}

The API Free tier includes 2,000 credits per month, with Starter at $29 per month and Pro at $199 per month for higher volumes. See the API documentation and pricing for the full details.

Conclusion

ADS-B turned flight tracking from a closed radar system into an open, precise, and global network. By having each aircraft compute its own position from satellites and broadcast it several times per second, it delivers accuracy and coverage that radar never could, at a fraction of the cost. Whether you simply want to understand the technology behind the map or you want to build a receiver and contribute to global coverage, ADS-B is the foundation to start from.

Explore live aircraft on the PlaneTrack.ai map, or create a free account to start building with the data yourself.