(CNN) — On an average day, more than 200,000 flights take off and land across the world. That includes commercial, cargo and charter planes -- which account for about half of the total -- as well as business jets, private aircraft, helicopters, air ambulances, government and military aircraft, drones, hot air balloons and gliders.
Most of them are equipped with a transponder, a device that communicates the aircraft's position and other flight data to air traffic control, and that signal can be captured with inexpensive receivers based on a technology called ADS-B, for Automatic Dependent Surveillance-Broadcast. That's what flight-tracking websites do in a nutshell, providing users with a real-time snapshot of everything that's in the sky (minus a few exceptions).
An ADS-B receiver manufactured by Flightradar24.
That's now reaching far beyond aviation enthusiasts. When a US Air Force plane carrying House Speaker Nancy Pelosi landed in Taiwan in early August, over 700,000 people witnessed the event as it happened, via flight-tracking service Flightradar24.
The plane, a military version of the Boeing 737 called C-40, departed from Kuala Lumpur in Malaysia before embarking on a circuitous path to Taiwan, in order to avoid encounters with the Chinese military, adding hours of flight time. That didn't make it immediately obvious what the final destination would be, sparking online conversations as the plane slowly veered north towards the island. As a result, it was the most tracked flight of all time on Flightradar24, with 2.92 million people following at least a portion of the seven-hour journey.
The website, part of a group of popular flight-tracking services along with FlightAware and Plane Finder, was founded in Sweden in 2006 "completely by accident," says FlightRadar24's director of communications, Ian Petchenik, as a way to drive traffic to a flight price comparison service.
It first gathered global recognition in 2010, when the eruption of an Icelandic volcano grounded thousands of flights and attracted four million visitors: "That was certainly our first foray into international events, and how displaying air traffic to the public in real time could influence how people were thinking about world news," says Petchenik. "The number of visitors we received would have crashed the website, so our saving grace was that there was nothing to show but a hole."
Interest on the rise
Before Pelosi's flight, the record for the most tracked flight on Flightradar24 belonged to Russian opposition leader Alexei Navalny's return trip to Russia, where he was due to be imprisoned. The January 2021 flight was tracked by 550,000 people, beating an earlier record set in April 2020, when almost 200,000 users watched a Boeing 777 draw the crescent and star symbols of the Turkish national flag in the skies above Ankara, to celebrate Turkey's 100th anniversary of sovereignty.
Before then, in September 2017, thousands had watched a brave Delta Boeing 737 fly right into hurricane Irma to land in Puerto Rico, and take off 40 minutes later for JFK by carefully positioning itself in the gaps between the hurricane's arms.
Outside of major events, however, the number of people tracking flights is constantly on the rise: "We see a lot of people using the site to track a loved one, track their own flight, or find the incoming flight that they're going to be on later that day, to make sure the plane's coming," says Petchenik.
"Another use case are folks who are very interested in aviation, or really like to follow certain types of aircraft. They can also go to the airport, pull up the app and see what's coming. Then you have people who are professionally invested in the aviation industry, because they own an aircraft and they rented it out, or because they have a fleet of aircraft and they want to keep tabs on them. Finally, there are people who are professionally invested in having a lot of flight data. This is airlines, airports, aircraft manufacturers that are using large data sets to gain industry insights."
How data is harvested
To gather the data, Flightradar has built its own network of ADS-B receivers, which they now say is the largest in the world at about 34,000 units, covering even remote areas like Antarctica.
Flightradar24 has receivers all over the globe, including remote locations like Antarctica.
About a quarter of the receivers were built by Flightradar24 itself, but the majority are assembled by enthusiasts who provide the data on a voluntary basis. Because building a receiver is relatively cheap -- components cost about $100 altogether -- many have signed up since Flightradar24 started opening up its network to the public in 2009.
A dense array of receivers is essential to track flights globally, but there's an obvious issue with oceans, where the network becomes sparse. So how do you get coverage over open water?
"By finding islands wherever we can and making sure that we have receivers there," says Petchenik. "But more recently we've turned to satellite based ADS-B receivers, to be able to better track aircraft over the ocean. However, the most predominant source of data is still our own terrestrial network."
Having such a granular and localized amount of data can be useful to get an early insight into emergencies and accidents: "We store everything that comes into our servers and if necessary we can go back into a specific receiver and extract the raw data. That's usually done only if there's been an accident or if we have a request from an air navigation service provider or an accident investigation branch," says Petchenik.
Occasionally, the data can reveal the cause of a crash before an official investigation does. In the case of Germanwings Flight 9525, which was deliberately flown into a mountain by the co-pilot on March 24, 2015, the data suggested a very clear picture: "One of the parameters that comes across in the fullest set of data, which we received in the case of the Germanwings flight, is something called MCP ALT -- that's the knob that is turned to tell the aircraft's autopilot what altitude to fly at. Looking at the data on that aircraft, that altitude value was set to zero."
Not all data is available for every aircraft, however, as that depends on the type of transponders and receivers involved.
Aircraft owners or operators can also decide to prevent their data from being publicly displayed, most commonly for military, government or private planes. For example, they can sign up to a program such as LADD, for "Limiting Aircraft Data Displayed," which is maintained by the Federal Aviation Administration: "We abide by that list," says Petchenik.
"It allows operators to have their data displayed differently, anonymously or, in some instances, not displayed at all. Out of the total number of aircraft that we track on a daily basis, about 3% have some type of data display regulation."