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Multilateration (MLAT): The second major source is multilateration using Flightradar24 receivers.As of 2021, Flightradar24 has the largest ADS-B network in the world with over 20,000 connected receivers. ADS-B signals can also be received and uploaded by a low-cost Software-defined radio. Typical ADS-B receivers include Kinetic Avionic's SBS-1 and AirNav-systems's AirNav and these receivers are run by volunteers, mostly aviation enthusiasts. 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. As of 2019, about 80% of aircraft in Europe are equipped with ADS-B and 60% in the US.
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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. 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.
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The Guardian considers the site to be "authoritative". The service is available via a web page or mobile device apps. It aggregates data from multiple sources but, outside of the United States, mostly from crowdsourced information gathering by volunteers with ADS-B receivers, and from satellite-based ADS-B receivers.
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It can also show time-lapse replays of previous tracks and historical flight data by airline, aircraft, aircraft type, area or airport. It includes flight tracking information, origins and destinations, flight numbers, aircraft types, positions, altitudes, headings and speeds. Flightradar24 is a Swedish internet-based service that shows real-time commercial aircraft flight tracking information on a map.
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