Flight Tracking, or How to Find Planes Over the Ocean

The tragedy of Malaysia Airlines Flight MH370 remains a mystery.

The plane departed from Kuala Lumpur heading to Beijing, but 40 min after takeoff, it changed its route towards the west. Communication was lost with the pilots, and air traffic control could only track their position through scattered military radars that followed it until it entered the Indian Ocean. Six hours later, a satellite received a log-in request from the plane, meaning that it was still operational. Unfortunately, such messages do not contain any location details. That was the last interaction with the plane.

Rescue missions were quickly sent, but they faced an impossible task: to sweep the ocean over a radius of about 5500 km (the distance the plane could cover in 6 hours). The decision was to focus on the section of the Indian Ocean west of Australia. Nothing was found. One year later, a 2 m flap was found in the Island of Réunion near Madagascar, which was later confirmed to belong to the MH370 flight.

The MH370 sent an important message to the aviation industry: to improve safety and make rescue missions useful, better flight tracking was required. Such plans were already on the table, but the MH370 flight accelerate them.

So, how are planes currently tracked?

The primary radar

Radar was invented in 1935 by a group of British scientists to detect possible air raids over the United Kingdom. They gave it the name RADAR as an acronym for RAdio Detection And Ranging, but due to its widespread use, radar is now a noun on its own.

Aviation radars are constantly spinning and emitting electromagnetic waves. If any of these hit a plane, the time difference between the sent and reflected beams can be used to determine the plane's location. Radar can also estimate the size of the plane since larger planes reflect more energy than smaller ones. Such types of radar are known as “primary radar”.

But there are several limitations with the primary radar. First, smaller planes might be missed since they reflect too little. Second, the inaccuracy of their measurements can be high, specially for the plane altitude. This is because if the plane is far from the radar, a change in altitude of a few hundred meters means a very small change in the angle at which the radar “sees” the plane. Such small changes can be of a similar magnitude as the atmospheric effects that distort the radar signal (storms, temperature, wind, etc.), making it almost impossible to distinguish between the two.

Luckily, secondary radars were invented to resolve this issue.

The secondary radar

All commercial aircraft are equipped with transponders. These are electronic devices that, when receiving a radar beam at a certain frequency, they reply with another that specifies their position and flight information. The plane computes it's own coordinates using GPS in a similar way as we do with our smartphones (trilateration with satellites). Radars capable of processing this data beams are known as “secondary radars”.

Secondary radar can be categorized as Mode A, C or S. In the older Modes A and C, when a radar sends an interrogation beam all planes in range reply with their altitude. This is a problem since high number of replies can cause frequency congestion, reducing the efficiency of the system. On the other hand, the newer Mode S can perform selective interrogation by sending the unique 24 digit code of the plane that wants information from, and only the plane that matches it will reply. Moreover, the response is no longer the the altitude alone but the complete coordinates and flight information.

Primary and secondary radar are still used together by air traffic control, and are often found one on top of each other.

ADS-B

The main drawback of radar systems is that they cannot track planes far from the land. Moreover, radars are expensive to operate and maintain, making them unattractive for sparsely populated islands or harsh terrain.

The Automatic Dependent Surveillance–Broadcast (ADS-B) works in a completely different way than radar. Instead of requesting the flight information by sending a beam, planes equipped with ADS-B are constantly broadcasting their coordinates and flight information. To track over the sea, the ADS-B data can be picked up by satellites, which will in turn send it to any air traffic control station. This closes the loop for a complete world coverage of planes

The ADS-B data is usually not encrypted, so anyone within range can pick up the information with a small antenna designed to capture the frequency at which the plane emits (usually 978 or 1090 MHz). Even private individuals can read such data. For example, flight tracking aggregators such as Flightradar24 rely on a giant network of individuals that volunteer to install antennas at their homes in exchange for free Flightradar24 accounts.

Due to its precision and flexibility, ADS-B is expected to replace radar soon. The European EASA requires all aircraft to be equipped with ADS-B since 2017, and the United States FAA enforced the same rule since 2020.

How often are planes tracked?

When using radar, air traffic control monitors the plane position every 5-12 seconds, whereas ADS-B enables almost every second

The precise tracking of ADS-B over the oceans also means that the busy corridors such as the North Atlantic can be planned more efficiently. That is, instead of requesting the planes to follow tracks very far apart from each other (needed to compensate for the uncertainty of their actual plane locations), more tracks can be added in between.

The usual lateral separation minima for North Atlantic track is about 60 nautical miles (110 km). With the adoption of ADS-B, this distance can be reduced to 25 nautical miles (45 km) without compromising safety. This means that more planes can fit inside a easterly jet stream when going towards Europe, saving a lot in fuel and emissions.

Traffic collision avoidance system

Air traffic control is in charge of planning the routes and suggesting adjustments over their course. However, pilots have the ultimate decision on the plane movements. This is why planes are equipped with the Traffic Collision Avoidance System (TCAS), a system that is completely independent from any ground stations.

Similar to a secondary radar, the TCAS is constantly interrogating neighboring planes for their coordinates and processing any incoming ADS-B data. These two data sources are combined in the pilot's screen, and, in case the pilot misses it, it creates an automatic alerts for collision.

The TCAS is mandatory for every plane weighting more than 5,700 kg or authorized to take more than 19 passengers. Therefore, mid air collisions are most common between small private planes.

What is air traffic control?

Air traffic controllers are split into three main categories:

Whereas the tower and approach controller are located at the control tower of an airport, area controllers can be placed in any office-type location. Another important difference is that, while airport controllers usually focus on one flight at the time, area controllers need to monitor all flights within their sector and make sure they remain well separated. They also monitor the weather and provide alerts, but flight separation is the main role.

Air traffic control can only make suggestions to pilots. These are usually accepted, but when pilots disagree they reply stating their reason, and a polite exchange of short messages continues until an agreement is found. Quick actions are sometimes needed, so being calm and clear is a key requirement for both pilots and controllers.