How are planes tracked in flight?

Air traffic control has various means of tracking aircraft in flight, so as to be able to deal with any eventuality, even if it can still happen that an aircraft accidentally disappears from radar.

Long gone are the days when radio was the only means of communication between aircraft and air traffic control, and when radar allowed us to locate them only on rare occasions. Today, there are a multitude of technologies available on both the control and aircraft sides to ensure that, whatever the circumstances, an aircraft is never “out of sight”.

And yet the disappearance of flight MH370 has raised a host of questions: how can one of the world’s most modern aircraft literally disappear in one of the world’s most frequented and surveilled areas?

If we don’t have the answer to this question, and won’t engage in dubious suppositions like Netflix did it in its series on flight MH370, we can, however, explain how things work under normal circumstances, so you’re free to make up your own mind.

This is, of course, a popularization article, and professionals will forgive us in advance.

Visual observation

We won’t dwell on this aspect, but in the early days of aviation, to find out where the planes were, you used your eyes and a pair of binoculars.

Primary radar

Primary Surveillance Radar (PSR) is the most basic and primary tool used to track an aircraft.

The radar emits an electromagnetic wave which reflects off the aircraft and returns to the radar.

This signal allows the operator to see that there is an object (not necessarily an aircraft…), how far away it is and what its direction is.

It doesn’t tell which aircraft it is nor what its altitude is. When the radar mesh is large enough, altitude can be deduced by taking into account the signals from several radars simultaneously.

Very useful in the early days of aviation, primary radar, even if still in use today, quickly became inadequate in the face of growing traffic, especially as it suffered from a number of limitations.

It is indeed better at detecting devices of a certain size, ideally metallic (it has trouble with composite materials), its operation can be altered by weather conditions (rain, snow…), its range can be affected by relief (mountains…) and it can pick up false positives (mountains, trees, birds, cars).

Secondary radar

A secondary radar or SSR (Secondary Surveillance Radar) works in conjunction with the aircraft to inform air traffic controllers.

Its introduction into air traffic control dates back to the 1950s, when aircraft were equipped with a device known as a transponder (a contraction of transmitter and responder).

Before the flight, air traffic control assigns a code (squawk) to each flight. The pilot enters it into the transponder, and it then serves to identify which flight one is dealing with.

The secondary radar transmits a signal asking the aircraft to identify itself, and in return the transponder sends back certain information depending on the mode in which it is operating.

– Mode C: aircraft code, position and altitude to within 100 feet.

– Mode S: aircraft code, position, altitude to within 25 feet, callsign, magnetic heading, indicated airspeed, ground speed and rate of turn. It also indicates the instructions the pilots have given to the autopilot.

The transponder can also be used by the crew to transmit emergency messages without having to communicate verbally with controllers (e.g. 7500 for a hijacking, 7600 for a radio failure, 7700 for a general emergency or distress situation).

It’s important to understand that primary and secondary radars don’t replace each other, but complement each other – at least for the time being! The secondary radar is in fact dependent on the transponder, and even the flight instruments, working properly: if for any reason the speed or altitude indications are wrong, then the transponder will transmit erroneous information.

And if the transponder is switched off or doesn’t work, only primary radar will provide a view of the aircraft, provided it’s within range.

But be aware that the range of these radars is limited, and today only 10% of the earth’s surface is covered! But rest assured, even in such an area, the plane is not invisible.

Radio connection

Before the advent of secondary radars and more advanced devices, the only way for air traffic control to know where a plane was was to regularly ask pilots, over the radio, where they were, and even, for that matter, which flight it was and where it was going.

Not perfect, but there were no other options, and even today, pilots are required to make regular contact with air traffic control when they arrive at certain points on their itinerary (reporting points). When an aircraft leaves a control zone, it is invited to establish radio contact and make itself known to the controllers in charge of the neighboring zone.

ADS (Automatic dependent surveillance)

ADS is the most modern aircraft tracking system, and is also based on the use of transponders.

But instead of responding to a secondary radar, the transponder calculates its position using GPS, and automatically sends information to ground stations and other aircraft in the vicinity.

The ADS can be used in two modes: B and C. B stands for Broadcast and C for Contract.

– ADS-C: a connection is manually property between the aircraft and the station tracking it, and the aircraft regularly sends its information.

– ADS-B: the aircraft sends its signals publicly, and anyone interested can receive them (ground stations, other aircraft, etc.) within a range of around 450km (280 miles). To go beyond this limit, satellites are used to relay signals to ground stations.

Although there are still white ADS areas, or connections are lost for a few seconds when switching from one satellite/radar zone to another, or the transponder stops transmitting for a few seconds because it is busy with an incoming call or because it has momentarily lost the GPS signal, it is rare for an aircraft to be completely out of sight for more than a few dozen seconds.

If the ADS connection is lost, air traffic control relies on the flight plan to calculate an estimated position based on the last known position. It therefore has a simulated, ” fictive ” position, until the connection is restored.

Since it uses radio waves, ADS makes radar unnecessary, enabling coverage of almost 100% of the globe and drastically reducing the number of “white areas”. It is considered to be the future of guidance, since it can guide an aircraft in all visibility conditions, both on the ground and in flight, with an accuracy of one meter, allowing the aircraft to situate itself in the surrounding traffic. It has been mandatory in Europe and the USA since 2020.

Of course, it suffers from certain limitations similar to those of secondary radar, since it relies on data transmitted by the aircraft. If for any reason they are wrong, then air traffic control and other aircraft will receive incorrect data.

Since ADS-B is not encrypted, anyone can access the data, including sites like Flightradar24, which we’ll talk about later.

ACARS messages

ACARS means Aircraft Communication Addressing and Reporting System. It’s not a localization system, but it’s still worth mentioning here.

This is a communication system between the aircraft and ground stations, which constantly sends automated messages on the status of the aircraft and its equipment, to reduce the pilots’ workload. In the other direction, it can receive information on the weather, traffic and points of vigilance in a given area. It also enables to send manual messages and receive replies on a printer.

If the ACARS is silent for too long, it receives an interrogation “ping” from ground stations, to which it responds automatically.


Flightradar24 is a very popular site that allows you to follow the flight of an aircraft in real time, with a wealth of data (speed, altitude, squawk, etc.). Of course, it’s not a tool for air traffic control, but it is for all aviation fans.

How does Flightradar24 get all this information?

Unsurprisingly, most of the data comes from ADS-B! But not only.

Let’s start with ADS-B. As mentioned above, its data is accessible to anyone with a receiver. Flightradar has created a collaborative network of stations with a community of volunteer users, who have installed over 30,000 receivers in their homes.

For areas not covered by its community, Flighradar24 uses other means.

Firstly, multilateration, which enables it to estimate the position of an aircraft in relation to various known points. It is used for aircraft not equipped with ADS-B.

Then all the radar data for flights in North America.

Finally, they have partnerships with certain satellite ADS-B service providers.

So if Flightradar isn’t 100% reliable, it’s what’s closest to the data available to air traffic control.

Case studies: flights AF447 and MH370

It’s all very technical and theoretical, but let’s take a look at how it works (or doesn’t work) through two concrete cases: flights AF447 and MH370.

Flight AF447

The flight in question was the Air France flight from Paris to Rio, which tragically disappeared on June 1, 2009.

The point is not to discuss the causes of the aircraft’s disappearance, but the moment when it was realized that there was a problem.

After the aircraft has taken off, several ADS-C connection attempts will fail. Twice because the air traffic control system did not have the aircraft’s flight plan, and once because the aircraft’s full registration was not included in the flight plan.

A call from air traffic control goes unanswered with no cause for concern. Ten minutes later, a final radar contact was made off the Brazilian coast, south of French Guiana. From that moment on, in the absence of radio communication, air traffic control will, as we have seen, only have a simulation of its trajectory and position, a fictitious view.

It wasn’t until several hours later, in the absence of any contact with the crew who had failed to show up at a reporting point and remained unreachable, that concerns arose. The crew will never respond to air traffic control or to any other aircraft trying to reach them. ACARS messages will then be sent and rejected. The rest, or rather what happened in the meantime, is well known.

If we know the reasons for the failure of the ADS-C connection, it can be surprising that ADS-B wasn’t used. The reason is simple: it’s 2009, and the aircraft wasn’t equipped with. It has been mandatory in Europe since 2016 for new aircraft, and 2020 for older aircraft.

In this case, all that was left was radio communications, and it’s a wonder that the crew’s lack of response didn’t catch attention sooner. But as the aircraft was spotted by radar a few minutes later…

As for the ACARS data transmitted by the aircraft, it confirmed that the plane had experienced serious problems with its flight controls and pitot probes. They were received in real time by maintenance, but no one paid them any further attention, as there had been precedents in the past for aircraft flying through thunderstorms that were not serious. It wasn’t until the airline’s operations center, after being alerted by air traffic control and having tried in vain to contact the aircraft for over an hour, that they began to gather all the information available (which is a normal procedure, ACARS’ primary vocation being maintenance).

Let’s be clear: none of the devices mentioned here could have saved the flight. On the other hand, precious hours could have been saved if, for example, there had been survivors to rescue. And the wreck would certainly have been discovered more quickly. In fact, for two hours it looked as if all was well, as the aircraft appeared on radar screens even though it had already crashed at sea.

Flight MH370

Nor is there any need to present flight MH370, which disappeared on March 8, 2014. Here, things are both simpler and more complicated. Complicated because the incident remains unexplained, simple because we only have the facts.

The aircraft left the Malaysian air traffic control zone, waving at the controllers but making no contact with Vietnamese air traffic controllers.

The ADS-B connection was then lost, which is nothing to worry about, as it happens in the area. However, it was never restored, and this is what alerted the authorities. At this stage it may be a question of a faulty or deliberately cut connection or transponder. Or crash.

The aircraft also disappeared from radar, possibly because it was too far away, too low or missing (primary radar), and its transponder was not working (secondary radar).

Last but not least, there’s the enigma of ACARS messages. An hour after the flight disappeared, the airline reportedly sent a ping to the aircraft. To no avail. However, according to satellite supplier Inmarsat, the aircraft responded to pings from ground stations for at least 6 hours. By response, it means that the device said it was connected, but didn’t send any location information. However, the response time allowed to try to estimate a trajectory. Data reliability still questionable, complex calculations with high margins of error: nothing conclusive has come of it, and we don’t even know if the data are reliable.

As in the case of the Air France flight, we can see that these devices, while they can’t prevent anything, can help to understand things in real time or a posteriori…provided they work. What we learn from this is that, because they did not work in order to provide answers, they simply ask questions and offer a few clues to help answer them.

Bottom line

There are many ways of tracking and locating aircraft, both external (radar) and internal (transponders, ACARS), whether by radio, electromagnetic waves or satellite.

If there are still white areas, they are increasingly rare, and any loss of contact is short-lived. Unless an external factor interferes…

That said, it’s important to put things into perspective. For every two flights about which there is something to say, how many of flights did have devices working perfectly, or even preventing a critical situation from arising? All the rest.

Image : Airborne06, CC BY-SA 3.0, via Wikimedia Commons

Bertrand Duperrin
Bertrand Duperrin
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.

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