How are planes tracked when they fly around the world?
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This week marked the 9th anniversary of the tragic and mysterious disappearance of MH370, a Boeing 777 flying between Kuala Lumpur and Beijing on March 9, 2014. To this day, there is no solid explanation of what happened to the aircraft.
What happened to the aircraft is shrouded in confusion and controversy; how such a piece of multi-million dollar machinery and all of its passengers and crew can disappear into thin air is one of the most perplexing elements of the story.
It’s a common belief that every aircraft always shows up as a little blip on the Air traffic Controllers (ATC) screen. We’ve all seen enough movies where that blip dramatically disappears, spelling disaster for the aircraft involved.
However, it may come as a surprise that aircraft aren’t always visible to ATC as that little blip. Indeed, in some parts of the world, ATC doesn’t have the equipment to detect the aircraft and, thus no screen on which to display them.
That said, no matter what facilities are available to ATC, there are always several layers of safety procedures to ensure that aircraft are kept safely separated from each other. In fact, the aftermath of the disappearance of MH370 resulted in the global body for civil aviation making changes to how aircraft are tracked.
In the early hours of March 8, 2014, MH370, a Boeing 777-200, took off from Kuala Lumpur International Airport for a scheduled flight to Beijing. After climbing to 35,000ft and approaching the edge of Malaysian airspace, the crew were instructed to contact Vietnamese ATC at 0119 Malaysian time.
This consists of ATC passing the pilots a new radio frequency which they repeat back to ATC to confirm that they have understood it correctly, which they did. It would then be normal procedure for the pilots to change the radio from the Malaysian frequency to the new Vietnamese frequency and then contact Vietnamese ATC. However, that radio call was never made on the new frequency.
Three minutes later, at 0122, the flight disappeared from ATC’s screens and was never heard from again.
The most common way ATC keeps track of aircraft is by radar. The origin of this system dates back to the Second World War, when British scientists developed technology to detect German aircraft inbound to the UK mainland.
Primary Surveillance Radar
This simple form of radar is known as Primary Surveillance Radar (PSR). It uses a beam of electromagnetic wave energy that is sent out into the sky by an emitting station. As this wave travels out toward space, anything that it might bump into, like an aircraft, acts as a deflector. This causes some of the waves to be bounced back to the ground, where a receiving station picks up the signal.
This radar return is then displayed to the radar operator on a screen, who can determine how far away the aircraft is and what relative bearing.
Through the use of multiple stations, over time, it became possible to overlay the signals and also work out the approximate altitude of the aircraft.
However, this system has its limitations in both range and accuracy. The radar signal will create a return if it hits anything; this includes thunderstorms, hills and even small objects such as birds. As a result, it could often be difficult for the radar operator to be sure of exactly what the return was indicating.
This was fine in the early days of commercial aviation, but as the skies became busier and airspace more congested, ATC needed a better, more accurate way of tracking aircraft that would ensure flight safety.
Secondary Surveillance Radar
Instead of sending out a signal from the ground and relying on it hitting aircraft to find them, the Secondary Surveillance Radar (SSR) uses an interrogator station and a piece of equipment on the aircraft called a transponder.
Before departure, pilots are given a specific 4-digit number to enter into their transponder, identifying that aircraft to ATC. This is known as the squawk code. The interrogator station can then send signals to the specific aircraft, requesting certain pieces of information.
When the transponder receives these requests, it sends back a coded signal containing the demanded information.
The transponder can be operated in two modes. Mode C sends back the aircraft’s position, squawk code and altitude to 100ft. Mode S, in addition to the position and squawk code, sends the altitude accurately to 25ft and also the aircraft’s callsign (e.g. “United 26LP”), the magnetic heading, indicated airspeed, groundspeed and rate of turn. It also shows the controller the altitude to which the pilots have instructed the autopilot to climb or descend.
All this extra information lets ATC know exactly what the aircraft is doing without wasting time asking the pilots. It’s particularly useful to controllers sequencing aircraft on the final approach to a runway as it allows them to ensure that one aircraft isn’t catching up with the one ahead.
In addition, the fact that the controller can see what altitude the pilots have demanded from the autopilot allows ATC to pick up o any errors before they become a major threat.
There is still one major weak point of Secondary Surveillance radar and that is that it still requires a ground-based radar system to detect the aircraft’s position. This is fine over land where radar stations are plentiful, but as soon as the aircraft flies over large areas of rainforest, desert or oceans, the aircraft can no longer be tracked.
A good example of this is over the Atlantic Ocean.
On either side of the pond, as part of Canadian and Irish/Scottish ATC, good radar coverage allows for accurate position determination by radar. However, as aircraft fly further away from land, they can no longer be detected by radar.
Instead, a very simple procedure known as Procedural Control is used.
Before the invention of a more sophisticated system like SSR, a very simple way of determining aircraft positions was by the pilot passing a position report. This would comprise of a message with their position, altitude and speed. ATC would then keep a note of this and would be able to build a picture of where aircraft were and where they would be over the next minutes and hours.
When flying over the Atlantic, a similar system is used.
The busiest part of the North Atlantic is divided into a series of motorways called tracks; each track has entry and exit points.
Before starting the crossing, pilots tell ATC their requested altitude and speed at which to cross and when they will reach the entry point. ATC collates this information from all the flights wishing to make the crossing and assigns each aircraft an altitude and speed depending on when they reach the entry point.
Pilots must then fly exactly as instructed by ATC.
The result is that aircraft will either be separated by at least 1000ft or, if at the same altitude, they will fly at the same speed, ensuring that one aircraft doesn’t catch up with another. So, once the aircraft leaves the radar coverage zone, ATC knows that they will remain safely separated until they are picked up by radar on the other side.
The procedural system works well, but large safety buffers must be built into the system due to the inability to track aircraft in real-time. This results in fewer aircraft being able to cross the Atlantic in a set time, sometimes resulting in delays.
What was needed was a system that could enable ATC to know the exact position of an aircraft, even if it is outside of radar coverage. The answer to this was ADS-B.
Automatic Dependent Surveillance-Broadcast (ADS-B) works the other way around from Secondary Surveillance Radar. Instead of a ground station interrogating the transponder, the transponder itself sends out a signal which can be picked up by stations on the ground or, indeed other aircraft.
The name may sound somewhat complicated, but it describes exactly how the system works. ADS-B is Automatic because it does not require interrogation by another system; it automatically sends out its own data. It is dependent because this information is dependent on data provided by the aircraft systems itself, such as the aircraft position, to send out surveillance information to other stations. Finally, this data is broadcast, and the sender has no idea who is picking the signal up.
The major benefit of ADS-B information is that instead of requiring the reach of a ground-based radar station, the signal can be transmitted up into space, where satellites then redirect it back to ATC.
This means that ATC can now see, almost in real time, where exactly an aircraft is and what it is doing. A great example of this is flight tracking programs such as Flight Radar 24.
Flight Radar 24
Sites such as Flight Radar 24 use a variety of data sources to allow the user to see not only the position of an aircraft almost anywhere in the world but also data such as its altitude, speed and heading. This is because they have access to ADS-B receivers enabling them to get such accurate data.
Most of this data comes from ground-based ADS-B receivers, but when aircraft are out of their range, for example, when over the Atlantic, they are also able to access data transmitted by satellites. This means that the information is so accurate that you can watch an aircraft pass over you on your phone at the same moment it physically passes over your head.
Modern-day technology on commercial airliners is so good that ATC is able to track the aircraft’s position even when over the middle of an ocean. The improvements in ADS-B accuracy have meant that ATC has been able to reduce the minimum lateral separation of aircraft over the Atlantic from 60 miles down to 30 miles.
As a result, there can now be almost double the number of North Atlantic Tracks at anyone one time, allowing more aircraft to fly their most optimum and fuel-efficient flight profile.
In addition to this, most aircraft will have some form of satellite communication. Most commonly for pilots to make phone calls to any station worldwide or for passengers to stay connected. As a result of the disappearance of MH370, these independent systems now send the aircraft position to a ground station every 15 minutes.
Featured Image – Flight Radar 24
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