Watching the skies

How do we know where planes are in the sky, and why do we occasionally lose them?
13 August 2015


The sheer scale of global aviation is incredible. Every day, around 100,000 flights take place, transporting nearly nine million passengers. Yet despite the skies being so busy, air travel is also the safest form of transport per kilometre travelled.

But just how do you keep track of so many flights and make sure that aircraft are a safe distance apart? This is no mean feat considering busy airports, such as London Heathrow, where there is a plane landing and taking off every 45 seconds.

Heathrow AirportAeroplane

Introducing radar

Aircraft are tracked by a system known as radar. This was developed as a way of detecting enemy aircraft during World War II and originally stood for RAdio Detection And Ranging. The idea is very simple: you send out a short pulse of radio waves from a transmitter and then you listen out for any signals that are reflected back. Large metal objects like aeroplanes reflect radio waves very effectively, making them easy to pick out from the surrounding noise, known as 'clutter'.

Radars are very good at detecting how far away something is, because the further away the object is, the bigger the gap between the transmitted pulse and the received one. This time gap can be measured very accurately, making it easy to precisely calculate the distance to the object.

AIr traffic control radar

It is also very easy to use radar to work out how quickly an aircraft is moving toward or away from it. Any moving object causes a change in the frequency of the reflected signal known as a 'Doppler shift'. This happens for the same reasons that an ambulance siren has a higher pitch as it races toward you, and a lower pitch as it speeds away. Simply measuring how much the frequency of the reflected signals has changed as they return to the radar is enough to work out how quickly the aeroplane is moving toward it.

For all their sophistication, even modern aircraft radars have some major limitations: They are not very good at working out the height of an aircraft or its direction from the transmitter. For this reason, air traffic control radars use a tightly focussed beam, which sweeps around continuously. Each time a pulse is transmitted, the direction the radar is pointing in is known, and any reflections received by it are assumed to have come from that direction.

In the time that it takes the radar to revolve once, a complete snapshot of the airspace surrounding the radar can be built up. Unfortunately, this means that the radar is only looking in a particular direction once every 10 seconds or so. That's a long time when you're flying at 500 mph - enough to travel well over a mile!

There is also a limit to how far a radar can 'see', because the received signals become harder and harder to detect. As the radio waves travel away from the transmitter, their power decreases by a physical rule known as the 'inverse square law'. This means that the power of these transmitted radio waves drops by three quarters, every time the distance from the transmitter doubles. When one of these pulses of radio waves eventually hits a plane, only a small amount of its remaining power is reflected back toward the transmitter and even then, its power drops off according to that 'inverse square law' on the return trip.

This means that the power of the signal received by the radar is a tiny fraction of the power of the signal that was transmitted. As the distance between a radar and aeroplane increases, this signal evenutally becomes too small to detect. To get around this problem, a network of radar stations is needed. When a plane gets too far from one radar to be detected, it is 'handed over' to the next radar station able to detect it, in a process co-ordinated by air traffic controllers.

Continually 'handing' over from one air traffic controller to the next is labour intensive. To make the process manageable, the airspace is divided up into a series of 'airways'. These are defined routes that planes are required to follow - effectively motorways in the sky. This means that, rather than being able to take the most direct route from its origin to its destination, an aircraft must often follow a series of airways before turning off into international airspace, sometimes adding hundreds of extra miles to an aircraft's journey.

Radar systems also have the problem that, although they are very good at detecting metal objects, they can't tell you what they are. Even worse, if there are two objects, one behind the other, the first causes a radar 'shadow' that makes the second object completely invisible to the radar. In the last few years, this has caused big problems when wind farms have been planned near airports. A wind turbine is simply large, moving, metal object as far as a radar is concerned and so many radars simply can't tell the difference between a wind farm and an aircraft, whilst any aircraft flying behind the wind farm are undetectable.


When radar is not enough...

Fortunately, a lot of these problems with simple radars can be simply overcome by using a device called a transponder. This is a radio beacon that sits in the nose of an aircraft and listens out for radar pulses. Each time it hears a radar transmission, it responds with a short package of information, including an identification code for the aircraft and its altitude. Modern transponders are even able to transmit the aircraft's precise GPS co-ordinates, making possible to identify its position to within a few metres.

These extra pieces of information are invaluable for air traffic controllers. The identification code makes it easy to identify individual planes from complicated radar data, which is extremely helpful in busy areas, such as near airports. Using the accurate altitude information from the transponder, aircraft can safely be made to fly above and below each other, making it possible for many aircraft to be 'stacked' in a small area, ready to come into land.

Even better, using a transponder increases the range of air traffic control radars. Instead of just 'listening' for reflections from and aircraft, the radar station can also listen out for its transponder responses. These responses are much louder than any radar reflections off the 'plane, so the radar station can detect them from much further away.

As well as ensuring our safety when we fly, the information from aircraft transponders can be useful to all of us. If you've ever tracked the flight of a friend or relative online, the exact location of the plane is picked up by 'listening in' to these aircraft transponder broadcasts.

Its not just aircraft that can be seen with radar, it is also possible to detect the weather. The movement of water droplets within clouds cause very distinctive radar reflections, which make it possible to work out whether rain or snow is falling, and how heavily. Many larger aircraft carry weather radars, capable of identifying weather conditions over 350 miles ahead. This information can then be used to by the pilot to avoid severe weather conditions, whilst we as passengers benefit from a smoother flight with less turbulence.

But what about when it all goes wrong? The disappearance of MH370 in March 2014 showed that our current tracking systems are far from perfect: How is it possible for an entire plane to go missing and why was it not possible to track the flight once it left its course?

It's all a question of range. Air traffic control radar is only capable of surveying the airspace within a few hundred miles of its base. Once an aircraft is outside this range, it is invisible to a radar. This means that aircraft in international airspace over the oceans are rarely tracked. In the case of MH370, its transponder was also disabled, which made it much harder to spot.


The sky's the limit

So, where might this all go in the future? On the ground, radars will become more sophisticated and make use of ever-increasing computing power to track planes better than ever before. One technology, known as 'phased array radar' may even be able to keep a constant watch on the skies, by scanning the whole surrounding area simultaneously. This would make it possible for air traffic controllers to watch aircraft movements on a second-by-second basis and see beyond obstructions such as wind farms.

In the sky, real-time reporting of aircraft GPS position is likely to become standard, as improving satellite data links make it possible to continually feed back data to air traffic control. This would mean that the precise location of each aircraft could be known at every point in its flight and should hopefully make disappearing planes a thing of the past.

So, next time you take off from a busy airport, take a glance at that strange revolving antenna near the control tower and settle back safe in the knowledge that radar is keeping an watchful eye on you as you fly.


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