Flightradar24 is a Swedish internet-based service that shows real-time aircraft flight information on a map. It includes
flight tracks, origins and destinations, flight numbers, aircraft types, positions, altitudes, headings and speeds. It can also show time-lapse replays of previous tracks and historical flight data by airline, aircraft, aircraft type, area or airport. It aggregates data from multiple sources but, outside of the United States, mostly from crowdsourced information gathering by volunteers with ADS-B receivers. The service was founded as a hobby in 2006 by two Swedish aviation enthusiasts. It is available via a web page or mobile device apps. The Guardian considers the site to be "authoritative".

Flightradar24

Type of site: Aircraft tracking
Available in: English
Headquarters: Sweden Stockholm, Sweden
Country of origin: Sweden
Owner: Flightradar24 AB
Revenue: 59m SEK; EBIT 10m SEK
Website: Flightradar24.com
Alexa rank: Decrease 2,247 (November 2017)
Commercial: Partially
Views: 3D & 2D
Registration: Freemium
Launched: 2006
Current status:Active

Flightradar24 aggregates data from three sources:
Automatic dependent surveillance-broadcast (ADS-B). The principal source is a large number of ground-based ADS-B receivers, which collect data from any aircraft in their local area that are equipped with an ADS-B transponder and feed this data to the internet in real time. The aircraft-based transponders use the GPS and other flight data input to transmit signals containing aircraft registration, position, altitude, velocity and other flight data. Currently, about 65% of aircraft in Europe are equipped with ADS-B but only 35% in the US. For example: all Airbus aircraft are ADS-B equipped but Boeing 707, 717, 727, 737-200, 747-100, 747-200, 747SP do not come equipped and are not generally visible unless retrofitted by their operators. Typical ADS-B receivers include Kinetic Avonics′s SBS-1 and AirNav-systems's AirNav and these receivers are run by volunteers, mostly aviation enthusiasts. ADS-B signals can also be received and uploaded by a low-cost Software-defined radio, such as those based on the $15 R820T tuner.
Multilateration (MLAT). The second major source is multilateration using Flightradar24 (FR24) receivers. All aircraft types will be visible in areas covered by MLAT, even without ADS-B, but while 99% of Europe is covered, only parts of the US are. At least four receivers are needed to calculate the position of an aircraft.
Federal Aviation Administration. The shortfall in the US is mostly made up from 5 minute delayed data from the Federal Aviation Administration (FAA) but this may not include aircraft registration and other information.