The ADS-B technology itself is best explained by the image to the right.
Aircraft gets its location from a GPS navigation source (satellite)
The ADS-B transponder on aircraft transmits signal containing the location (and much more)
ADS-B signal is picked up by a receiver connected to Flightradar24
Receiver feeds data to Flightradar24
Data is shown on
www.flightradar24.com and in Flightradar24 apps
ADS-B is a relatively new technology under development which means that today it's rarely used by Air Traffic Control (ATC). Our estimations show that roughly 65% of all commercial passenger aircraft (75% in Europe, 35% in the US) are equipped with an ADS-B transponder. For general aviation this number is probably below 20%. But this percentage is steadily increasing as ADS-B will become mandatory for most aircraft in most airspaces around the world, by year 2020. When mandatory, ADS-B will replace primary radar as the primary surveillance method used by ATC.