What Is ADS-B? The Technology Behind Flight Tracking Explained
Every time you open a flight tracking app and see a live aircraft moving across a map, you're looking at ADS-B data. But what exactly is ADS-B, how does it work, and why is it so much better than what came before? This guide breaks it all down.
The Short Answer
ADS-B stands for Automatic Dependent Surveillance–Broadcast. It's a surveillance technology where aircraft automatically transmit their GPS position, speed, altitude, heading, and identity to anyone who's listening — no radar required.
Every modern commercial aircraft is equipped with an ADS-B transponder. It broadcasts a signal roughly every half-second, and that signal can be received by ground stations, other aircraft, and even hobbyists with a cheap USB radio receiver connected to a laptop.
Why Was ADS-B Developed?
Traditional radar has been the backbone of air traffic control since the 1950s. It works — but it has real limitations:
- It's infrastructure-heavy. You need radar stations every 200 miles or so to maintain coverage, and they require enormous amounts of power and maintenance
- It's imprecise. Primary radar bounces radio waves off an aircraft and measures the return — this tells you roughly where something is, but not what it is or how high
- It has dead zones. Over oceans, mountains, and remote areas, radar coverage is patchy or nonexistent. Historically, aircraft disappeared from screens over the Atlantic and Pacific
- It doesn't scale well. As air traffic grew, traditional radar struggled to keep up with density
ADS-B solves all of these problems. It was mandated for all aircraft flying in controlled US airspace from January 2020, and similar mandates exist across Europe and other regions.
How ADS-B Works: Step by Step
1. The Aircraft Gets Its Position
The aircraft's GPS receiver continuously calculates its exact position to within a few metres. This is many times more accurate than what primary radar can determine.
2. The Transponder Broadcasts
Every 0.5 seconds or so, the ADS-B Out transponder broadcasts a signal on 1090 MHz. This signal contains:
- Aircraft identifier (ICAO 24-bit address — a unique hex code assigned to each aircraft)
- Flight number (where the aircraft is squawking a callsign)
- GPS latitude and longitude
- Altitude (both GPS altitude and barometric altitude)
- Ground speed
- True track/heading
- Vertical rate (climb or descent rate)
- Aircraft category (light, medium, heavy, etc.)
3. Ground Stations Receive the Signal
A network of ground receivers picks up the signal. These range from official ATC receivers to the thousands of hobbyist receiver stations that feed data to networks like ADS-B Exchange, FlightAware, FlightRadar24, and ADSB.lol.
4. Data is Aggregated
All those individual signals are aggregated and matched up. Multiple receivers may hear the same aircraft, and the system uses timestamps to cross-check positions and fill gaps in coverage.
5. You See It on Your App
The aggregated, cleaned data is served via APIs to flight tracking applications — including What Plane? — which display it as a live map of aircraft positions.
ADS-B Out vs ADS-B In
There are two directions to ADS-B:
ADS-B Out is what we've been describing — the aircraft transmitting its position. This is mandatory on modern aircraft.
ADS-B In is the aircraft's ability to receive ADS-B signals from other aircraft. This enables cockpit displays showing nearby traffic, improving situational awareness. ADS-B In is not yet universally mandated.
MLAT: Filling in the Gaps
Not every aircraft has ADS-B. Older or smaller aircraft may only have a Mode-S transponder, which sends a less detailed signal. For these aircraft, tracking networks use MLAT — Multilateration.
MLAT works by having multiple receivers time-stamp when they receive a signal from the aircraft. Since the speed of radio waves is known and constant, the slight difference in arrival times at each receiver can be used to calculate the aircraft's position geometrically — a bit like GPS in reverse.
MLAT requires at least three receivers within range. It's less precise than ADS-B but fills important coverage gaps.
Who Collects ADS-B Data?
Several organisations collect and aggregate ADS-B data:
ADSB.lol — a community-run, open network that provides free access to real-time ADS-B data. This is the data source used by What Plane?.
FlightAware — one of the original flight tracking platforms, with a large receiver network.
FlightRadar24 — perhaps the most widely known flight tracking platform.
ADS-B Exchange — a community network with a commitment to unfiltered data (they don't remove military or sensitive flights from the feed).
OpenSky Network — a research-focused network associated with various European universities.
Can You Set Up Your Own ADS-B Receiver?
Yes — and it's surprisingly easy. The basic setup is:
- A Raspberry Pi (or similar single-board computer)
- A DVB-T USB dongle (a cheap SDR — software-defined radio — often sold as a TV tuner). A popular option is the RTL-SDR
- A 1090 MHz antenna (either bought or homemade)
- Software like dump1090 to decode the signals
- Optional: feeding data to a network like FlightAware, ADSB.lol, or FlightRadar24
With this setup, you can receive ADS-B signals from aircraft within roughly 200–300 km (depending on your antenna height and location) and see them on your own local display in real time.
ADS-B vs Radar: Why Both Still Exist
Despite ADS-B being superior in many ways, radar hasn't disappeared — and for good reason:
- ADS-B is dependent on the aircraft's GPS. If GPS is jammed or spoofed (a growing concern near conflict zones), ADS-B position data becomes unreliable
- ADS-B only works if the transponder is on. Some incidents involve transponders being deliberately switched off
- Primary radar requires nothing from the aircraft — it simply bounces a signal off whatever is there
For these reasons, ATC systems use both in combination. ADS-B is the primary source; radar is the cross-check and backup.
Privacy and ADS-B: Can Aircraft Hide?
Most can't, but some do. Some aircraft owners — particularly private jets — apply to have their aircraft's ICAO code blocked from public tracking databases. This isn't perfect: ADSB Exchange, for instance, has a policy of not honouring such blocking requests, maintaining that public airspace produces public data.
Military aircraft almost never broadcast ADS-B signals. Some do, particularly transport aircraft on non-sensitive routes, but fighters, surveillance aircraft, and most military helicopters operate without ADS-B or with it switched off.
How What Plane? Uses ADS-B
What Plane? for iPhone uses live data from ADSB.lol to show you the nearest aircraft to your location in real time. The app:
- Automatically finds the closest aircraft to you using your GPS position
- Shows the exact aircraft type, airline, flight number, altitude, and speed
- Displays a compass bearing (the Compass Ring) so you know exactly which direction to look
- Updates regularly so you're always seeing current data
The home screen widget means you don't even need to open the app — your nearest aircraft is always a glance away.
Download What Plane? free on the App Store.
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