Open Flightradar24 and thousands of little planes crawl across the map in near real time. It feels like magic, or like there's one giant radar in the sky watching everything. There isn't. What you're seeing is a fusion of several different data sources — mostly a global network of small radio receivers listening for signals the aircraft themselves broadcast. Here's how the whole thing actually works, source by source.
ADS-B: the primary source
The backbone is ADS-B (Automatic Dependent Surveillance–Broadcast). Modern aircraft continuously transmit their own GPS position, altitude, speed, and identity over a 1090 MHz radio signal — no interrogation needed, they just broadcast. Flightradar24 (and every tracker like it) runs a network of thousands of small ground receivers, many of them hosted by volunteers, that pick up those broadcasts and forward them to a central server. Wherever a receiver can "hear" an aircraft, the plane appears on the map almost instantly.
This is why coverage maps look the way they do: dense over Europe, North America, and populated coastlines where receivers are thick on the ground, and thin over oceans and remote regions where there's no one to listen. ADS-B is line-of-sight — a receiver has to be within radio range (roughly a few hundred miles at altitude) to catch the signal.
MLAT: filling in the older aircraft
Not every aircraft broadcasts a GPS position. Older ones may have a Mode S transponder that replies with identity and altitude but no position. For those, trackers use MLAT — multilateration. When four or more ground receivers hear the same transponder reply, the tiny differences in when each receiver hears it (time difference of arrival) can be solved geometrically to compute the aircraft's location. It's the same principle GPS uses in reverse.
MLAT only works where receiver density is high enough that four stations hear the same aircraft at once, and it needs the plane to be high enough for that many receivers to have line of sight — so it works over well-covered land and fades out at low altitude or in sparse areas. It's the quiet workhorse that keeps non-ADS-B aircraft on the map.
The other feeds
ADS-B and MLAT do most of the work, but a few more sources fill the gaps:
- Satellite-based ADS-B. Receivers on satellites pick up the same 1090 MHz broadcasts over oceans and remote regions where ground stations can't reach — this is how transatlantic and transpacific flights stay visible mid-ocean.
- Radar / air-traffic data feeds. In some regions, trackers ingest official surveillance or flight-data feeds (for example FAA data in the US), usually with a short delay, to cover aircraft the receiver network misses.
- Schedule and status data. Airline and airport feeds supply flight numbers, gates, and scheduled times — the labels wrapped around each position track.
The tracker's job is to fuse all of this: dedupe the same aircraft seen by multiple sources, stitch it into one continuous track, and decorate it with schedule data.
Why some planes are missing
Once you know the mechanism, the gaps make sense. An aircraft is invisible if no receiver can hear it (mid-ocean with no satellite coverage, or below the horizon of nearby stations), if it isn't ADS-B equipped and too few receivers hear it for MLAT, or if it's deliberately blocked or has its transponder off (some military and privacy-sensitive flights). "Live flight tracking" is really "live flight tracking where someone is listening."
Getting the data itself, not the app
Flightradar24 is a consumer app built on top of that receiver network. If you're a developer who wants the underlying data — positions, altitudes, callsigns — to build your own product, you don't scrape the app; you use an ADS-B API. SkyLink API exposes the same class of data (live aircraft positions, filtered by area, enriched with registration and type) through the ADS-B endpoint, so you can build a tracker, a map, or an analytics tool without running your own global receiver network.
If you want the deeper background, What Is ADS-B? explains the broadcast itself, Mode S hex codes covers the identifier every one of these signals carries, and our Flightradar24 API alternatives guide compares the ways to get the feed.
SkyLink API gives you a free tier of 1,000 requests/month to build a tracker against, with paid plans starting at $19/mo for production traffic. It's available through the free trial — sign up, grab a key, and pull live aircraft positions straight from the source.