If you have ever watched a live flight map and wondered how a website knows exactly where a jet is over the middle of the ocean, the answer is almost always ADS-B. So what is ADS-B, exactly? It is the surveillance technology that replaced decades-old radar as the primary way aircraft positions get tracked, shared, and displayed in real time — and it is the reason flight-tracking APIs exist at all.

This post covers what ADS-B stands for, how it works, ADS-B Out vs. In, why it beats radar, where it's mandatory, and why it powers SkyLink API and similar products.

What Is ADS-B? The Definition

ADS-B stands for Automatic Dependent Surveillance-Broadcast. Each word describes a piece of how it works:

  • Automatic — it transmits on its own, with no pilot or controller input required.
  • Dependent — the position it reports depends on the aircraft's own onboard navigation system (GNSS/GPS), not an external sensor.
  • Surveillance — it exists to let others (air traffic control, other aircraft, tracking services) know where the aircraft is.
  • Broadcast — it is sent out openly, to anyone with a receiver in range, not to a single requesting party.

In plain terms: ADS-B is a system where an aircraft determines its own precise position via satellite navigation, then continuously radios that position out into the open, unprompted, for anyone equipped to listen.

How ADS-B Actually Works

This is the part that makes ADS-B fundamentally different from traditional radar.

  1. The aircraft determines its own position. Using GNSS (GPS and similar satellite constellations), the aircraft's avionics calculate latitude, longitude, altitude, and velocity with high precision.
  2. It broadcasts that data automatically, about once per second. No one has to ask. The aircraft transmits its position, identity, altitude, speed, and heading continuously while in flight.
  3. Ground stations and satellites listen in. Any properly equipped receiver within range — a ground antenna, another aircraft, or a satellite — picks up the broadcast and decodes it.

Large jetliner passing through clouds, the kind of aircraft that broadcasts its position via ADS-B

Two link types carry these broadcasts. 1090ES (1090 MHz Extended Squitter) is the global, all-altitude standard used by most commercial and international traffic. 978 MHz UAT (Universal Access Transceiver) is a US-only option typically used by general aviation below 18,000 feet. Both update roughly once per second.

This is the opposite of how radar works. A ground station actively sends out a signal and either reads the reflection (primary radar) or interrogates an onboard transponder for a reply (secondary radar). With ADS-B, the aircraft already knows where it is and simply tells everyone — nobody has to ask.

ADS-B Out vs. ADS-B In

These two terms get mixed up constantly, but the distinction is simple:

  • ADS-B Out is the equipment that transmits the aircraft's position, identity, and flight data to the outside world. This is the half that air traffic control and tracking networks rely on, and what mandates require.
  • ADS-B In is the equipment that lets an aircraft receive broadcasts from other aircraft and from ground services, displaying nearby traffic to the pilot in the cockpit.

A given aircraft can have just ADS-B Out, or both. Flight-tracking products — including APIs like SkyLink — are essentially large-scale ADS-B In systems, built for data consumers instead of cockpits.

Why ADS-B Is More Accurate Than Radar

Two practical advantages explain why ADS-B has overtaken radar as the primary surveillance method:

  • Update frequency. Ground radar typically sweeps once every 4–12 seconds, and that interval grows with distance from the antenna. ADS-B broadcasts roughly once per second regardless of range, giving a far smoother, more current picture.
  • Position source. Radar estimates position from a reflected or interrogated signal, which introduces imprecision at range. ADS-B position comes straight from the aircraft's own GNSS receiver, which is generally more accurate and doesn't degrade with distance the way radar does.

The net effect: less lag, tighter precision, and coverage that doesn't disappear just because an aircraft is far from the nearest antenna.

Where ADS-B Is Mandatory

ADS-B Out is no longer optional in most controlled airspace. In the US, the FAA's mandate took effect January 1, 2020, wherever a transponder was already required. The EU followed under EASA rules, with the mandate (after a COVID-related extension) taking full effect by December 2020. Most other major aviation authorities have adopted comparable requirements, which is why ADS-B data now covers the vast majority of commercial and business traffic worldwide.

Jet aircraft flying through blue sky leaving a contrail, equipped with ADS-B Out avionics

Why ADS-B Became the Backbone of Flight-Tracking APIs

Before ADS-B, building a flight-tracking product meant licensing radar feeds from air navigation service providers or running your own receiver network — expensive and out of reach for smaller companies. ADS-B changed the economics. Because it is broadcast openly on a known frequency, anyone can build, or rent access to, a network of low-cost ground receivers picking up the same signals controllers use.

Coverage improved further with satellite-based ADS-B receivers — most notably the Aireon network on Iridium's satellite constellation — which pick up broadcasts over oceans, polar routes, and other areas with no ground infrastructure at all. Combined, terrestrial and satellite networks now deliver close to global coverage, which is what modern flight-tracking APIs are built on top of. ADS-B is the raw signal; an API turns thousands of those signals into a clean, queryable feed of live aircraft positions.

This is also why SkyLink's live ADS-B tracking feature exists — it sits on top of this kind of receiver network, so you don't have to build or rent one yourself. See our tutorial on building a real-time flight tracker with Python and ADS-B data, and our 2026 flight-tracking statistics post for where the numbers stand.

Get Live ADS-B Data

SkyLink API exposes live ADS-B aircraft positions — callsign, altitude, speed, heading, registration — refreshed roughly every 5–10 seconds. The free tier includes 1,000 requests/month, with paid plans starting at $18.59/mo, available through the free trial. If you're building anything that needs to know where an aircraft is right now, that's the data layer to start from.