How Flight Tracking Apps Work: Volunteers

Flight tracker apps, initially popular among aviation geeks, rely on some of those same geeks for their data.

How Flight Tracking Apps Work: Volunteers
[Photo: Flickr user Jeff Kubina]

Aviation disasters tend to make headlines, particularly because they are so rare and can involve so many fatalities. Statistically, however, flying is still one of the safest ways to travel, and that doesn’t appear to be changing. In fact, despite the number of recent high-profile fatal airplane disasters, 2014 saw the fewest number of plane crashes in the modern era.


But a curious phenomenon has emerged recently in the way these disasters are reported: Watch a news report about a downed or missing plane, and you’re likely to see graphics that show the locations of thousands of other planes in the sky, sourced from free flight-tracking apps that are already popular among airlines, airports, and aviation geeks.

The first time I noticed the mainstream media using flight-tracking apps was after Malaysia Airlines flight MH17 was shot down over the Ukraine in July 2014. The BBC kept showing an image of Europe in which the continent looked like it was being swarmed by yellow insects–in all countries except the Ukraine. Virtually all planes were not entering Ukrainian airspace. In the corner of the image, I noticed a logo for a company called Flightradar24.

When I switched to CNN, I saw a blue map, this time showing the flight path of MH17 from its origin, Amsterdam, to the place where it was shot down over the Ukraine. This blue map was from a company called FlightAware.


Whenever there’s an event of interest, such as a snowstorm that shuts down New York or a tragic accident, “we definitely see a surge in traffic,” says Daniel Baker, a pilot and computer programmer who founded Texas-based FlightAware in 2005. Besides the media and aviation enthusiasts, the 30-person company counts a number of airports, manufacturers, and carriers among its customers, who use it to “predict delays, handle irregular operations, and analyze route efficiency.”

“In light of recent accidents,” says Baker, “there has been a huge interest–and some regulatory requirements–from our aircraft-operator customers to improve flight tracking. Some of the same interest applies to consumers, and the number of commercial airlines and air travelers is increasing every day, as well as the number of people with Internet connectivity.”

Flightradar24, which also offers free and enterprise services, says it saw a 50-fold traffic increase following the downing of MH17. In the days that followed, its free and paid apps topped the App Store charts in the U.K., Germany, and the Netherlands. Since it became a standalone company two and a half years ago–it started life as a popular page on a Swedish price-comparison site in 2006–Flightradar24 has grown to 12 employees and seen “at least a four-time increase on every relevant metric,” says CEO Fredrik Lindahl.

Daniel Baker, CEO of FlightAware

The growing interest in flight tracking relates in part to a lack of information about air tragedies, like last month’s Germanwings flight 4U9525 in the French Alps and Malaysia Airlines MH370, presumed lost somewhere over the Indian Ocean in March 2014.

After the downing of MH17, as we watched the news coverage, a friend posed a simple question: “If a website can show the flight path and all those little yellow planes in real time, how can they not know where the other Malaysia Airlines flight [MH370] went down? I mean, I can lose my iPhone and find it with its GPS.”

Answering that involves understanding a little about how flight-tracking sites work, where they get their data, and the limitations of existing technologies. It also involves appreciating the value of a relatively new approach that both companies are rushing to expand, a global sensor system known as ADS-B, or automatic dependent surveillance broadcast. Broadcasting updates of aircraft GPS data in real time, it’s slowly superseding the ground-based radar systems that have been used for decades, becoming central not only to flight tracking but also to the future of flight safety too.


And it’s hosted, in part, by thousands of dedicated aviation hobbyists around the globe.

How An App Tracks Planes Overhead

FlightAware, says Baker, ingests data from over 100 sources in addition to ADS-B, including “from the air traffic control systems in over 50 countries, from airlines’ operations systems, and directly from airplane cockpits over datalink.”

A screenshot from Flightradar24 showing near-empty airspace in the Ukraine after the downing of MH17

The data that FlightAware first receives for most commercial flights is sometimes delivered a year in advance via schedules published by the airlines. That data then usually remains unchanged until a few hours before the flight. Pilots or air traffic control operators controlling airspace on the route of flight will push out new data with a flight plan that provides specific details about the planned routing, altitude, and speed of the plane. But that’s where the static data end and the real-time data begin.


“When the door is shut and the parking brake is released by the pilots, the aircraft or airline will send us an ‘out’ message that indicates it’s pushed back from the gate, and we know that departure is imminent,” says FlightAware’s Baker. “As soon as the weight is lifted off the landing gear, we often get an ‘off’ message from the airlines or the aircraft itself that indicates the plane is airborne or a departure notice from air traffic control.”

Once en route, FlightAware continues to get position updates from the plane via radar installations at air traffic control centers and via ADS-B, along with continued updates on the plane’s route from air traffic control. All of this information is then fused together by FlightAware’s software, which uses it to determine the plane’s estimated time of arrival, and then displays the aggregate data for the flight on its website and in its apps.

“We detect the landing either by seeing the plane slow down below flying speed, by getting an ‘on’ message from the airplane, or by receiving an arrival notice from air traffic control. From there, we may have surface coverage to track the taxiing on the ground, and eventually receive the ‘in’ message from the airline indicating it’s safely parked at the gate,” says Baker.


Now multiply all the above steps by the thousands, or one for each plane in the sky across the world at any given time, and you’ll begin to appreciate the amount of data that fight tracking websites’ servers and algorithms are crunching each and every second.

How ADS-B Works

On the leading edge of flight tracking tech is ADS-B, which relies partly on sensors operated by commercial and government entities, and partly on rooftop and window receivers run by thousands of flight tracking enthusiasts across the globe. Think of them as the bird watchers of the aviation age.

ADS-B differs from radar in that it doesn’t rely on the bounced radio waves from fixed earthbound antennas to tell air traffic control where an object is in the sky. Instead, it works like the GPS in our phones, allowing the aircraft to determine its own position from a satellite overhead. The aircraft then transmits its location and identification number along with other ADS-B data from its ADS-B transponder, which can be picked up by anyone with an ADS-B receiver–on the ground or in the air–within 200 miles. (Aircraft owners and operators have the option of blocking their tail numbers from release through public databases, and military flights are typically concealed.)

Automatic dependent surveillance–broadcast, or ADS-B, relies on a combination of satellites and receivers to provide information about the position and velocity of airplanes in the area.Boeing

ADS-B offers a precision that radar can’t, says Baker, and is less likely to degrade due to adverse atmospheric conditions or range. “For flights that we’re tracking with government radar, it’s accurate to about 500 meters,” says Baker. “If we’re using ADS-B, it’s less than 10 meters.”

ADS-B isn’t just helping track flights: It’s also part of a system that could help airlines fly more direct routes at more efficient speeds and altitudes, give pilots better awareness of nearby aircraft, and bring safer flows of traffic to the world’s increasingly crowded skies.

Right now, over 70% of planes in Europe and Australia are quipped with ADS-B transponders; By 2020, the FAA will require every plane flying in most U.S. airspace to be equipped with ADS-B transponders as part of a decade-old project called NextGen to upgrade air traffic control.


The technology was initially deployed to cover areas with no radar coverage in the southwestern part of Alaska. After the system was put in place, fatal accidents in the state dropped by 47%. Last year, the FAA completed the construction and deployment of more than 630 new radio stations on the ground—the baseline for a national ADS-B ground infrastructure. In the Gulf of Mexico, the FAA partnered with oil and natural gas companies to install ground stations on floating oil platforms.

Despite the FAA regulations—and the fact that ADS-B is more affordable for carriers to operate than existing satellite communications—the rollout of the system has been hobbled by delays and budget overruns, according to an inspector general report to Congress last year. The FAA spends nearly $1 billion annually on NextGen, and the president’s 2016 budget asks to double that number.

Some aircraft owners have also been slow to adopt to NextGen. “Airplanes have service lives of decades and the upgrades can be extremely expensive,” says Baker. “For a four-seat Cessna 172, an upgrade of $5K to 15K is not uncommon–for airliners, hundreds of thousands or potentially more than a million in some cases.”


Another concern: The system as currently imagined is not equipped to handle the influx of unmanned aerial vehicles forecast for U.S. skies in the coming years. A few companies are making ADS-B transponders for drones in addition to Google, which is hoping to develop its own drone delivery fleet, and is also developing a UAV transponder.

To make improvements to their own services, both FlightAware and Flightradar24 aren’t waiting for governments or commercial entities to develop the ADS-B system. They’re relying instead on the global network of hobbyists who have built up much of the ground-based system so far, using low-power, soda-can-sized receivers that typically cost anywhere from $400 to the high thousands of dollars.

Currently FlightAware draws its own ADS-B data from over 2,600 aviation enthusiasts who feed their ADS-B data into its database, in exchange for its premium services. It also designs and builds its own receivers; each month, it distributes between 75 and 100 of them at no cost to sites that currently have limited or no ADS-B coverage. It also provides instructions on how to build your own receiver using a Raspberry Pi, and keeps a “leaderboard” highlighting the users who have seen the most planes in the past 30 days.


Flightradar24 uses a similar model to gather ADS-B data, which it now relies on almost exclusively for flights outside the U.S. Lindahl says the company sends out 50 to its supporters every week, particularly in places where coverage is currently minimal, and has built the world’s largest network of ADS-B receivers, over 6,000 strong. Together, the two companies’ ADS-B systems span 90 countries across the globe, with over 125,000 square miles of coverage, or about 80% of the world’s landmass.

While ADS-B receiver can transmit data freely to any database, depending on how its operator configures its software, the companies have taken different approaches here: FlightAware has made its ADS-B protocol and software open source and offers an API, but ADS-B enthusiasts have noted that Flightradar24 tends to keep the data collected through its software within its own network. Baker disagrees with that approach. “In general, we feel like it’s good for the industry to make the data readily available and for the industry to be cooperative,” he says. “Everyone wins with our model.”

How A Plane Disappeared From Flight Trackers

Given the precise nature of ADS-B, I ask Baker my friend’s question: If I can track my lost iPhone with GPS, why can’t we quickly find a lost plane—especially one equipped with an ADS-B transponder?


He’s heard this question a lot. “Over populated land, it’s pretty easy with radar, but most of the Earth is water and lots of the Earth isn’t inhabited. Although new planes have GPS and older airplanes have countless navigation systems, just because they know where they are doesn’t mean that people on the ground do.”

The only way airplanes can broadcast their GPS data is via a network of dedicated ADS-B receivers on the ground, the ocean, or on nearby aircraft. “Unfortunately, GPS is just one-way,” says Baker, who notes that a smartphone can derive its location using GPS satellites, but the GPS satellites don’t actually receive any information from the phone. (When a lost phone “beams” its location back to an app, it’s relying on cellular or Wi-Fi networks to send that information.)

MH370’s last ADS-B broadcast according to Flightradar24Image: Flightradar24

The area where MH370 disappeared is covered by ADS-B receivers, and the last ADS-B reading from MH370 as it flew from Kuala Lumpur to Beijing was about 40 minutes after takeoff, cruising at 35,000 feet at 471 knots.


At that point, writes aviation analyst David Cenciotti, the Boeing 777 either disintegrated midair or plunged more than 5,000 feet in 30 seconds, a steep dive of 10,000 feet per minute that could bring it below 30,000 feet, where ADS-B coverage may not work. Or, he says, “the aircraft onboard ADS-B transponder was switched off or failed.” (From then on, the sole evidence of the airplane’s fate is a handful of simple electronic exchanges between the plane and a satellite.)

ADS-B surveillance over the South China Sea is improving, thanks to ADS-B systems that have recently been installed by Singapore, Vietnam, and Indonesia. But most of the world’s oceans remain outside ADS-B coverage. Flightradar24 is experimenting with low-cost receivers attached to buoys and, eventually, perhaps, with high-altitude balloons and small satellites. Baker of FlightAware says his company is also working hard to grow its network of crowdsourced ADS-B receivers to over 5,000 by next year.

Another reason it’s difficult to track flights that have gone missing is that a large majority of planes still don’t have ADS-B transponders. The FAA’s 2020 deadline for carriers to install ADS-B, along with the growing number of volunteers in the ADS-B receivers network, means that flight tracking sites like FlightAware will only get more comprehensive and accurate as time goes on.

In addition to a premium app, FlightAware’s ad-supported website and free app draw 9 million users a month and bring in millions in advertising revenue each year, he says. The company also offers a dozen paid commercial services for airports and carriers. Flightradar24 largely relies on ad revenue from its site and free app and from downloads of its paid app, which has reached the top of Apple’s App Store charts in 118 countries, and become the company’s main revenue driver, says Lindahl.

While the popularity of flight tracking services grows, however, their continued growth may depend on finding new revenue streams. “We have a lot of different services so we reach a broad audience,” says Baker. “Everyone remotely involved in aviation or travel—consumers, pilots, dispatchers, airplane owners, car services, maintenance companies”–is a potential customer.

Despite the mainstream attention to air disasters that flight trackers may bring, Baker reminds me that flying is the safest form of transportation, and notes FlightAware tracks tens of millions of safe arrivals every year.

“Travelers and family members should be far more concerned about the drive to and from the airport than the flight itself. Just last week, I drove myself to the airport in torrential rain and low visibility during rush hour–it was really a harrowing experience. As I walked from my car to the terminal to board an Air France 777 for an overnight flight to Paris, I remember thinking, ‘Okay, I’ve done the hard part, now you guys fly me to France.’”

Technological precision aside, Baker urges users not to be worried if the plane they’re tracking suddenly disappears from their favorite flight tracker. “Just like any software, there can be glitches,” he says, “so I wouldn’t jump to any conclusions if you see a plane disappear or briefly point the wrong direction.”


About the author

Michael Grothaus is a novelist, journalist, and former screenwriter. His debut novel EPIPHANY JONES is out now from Orenda Books. You can read more about him at