The typical image people have of air traffic control (ATC) is that of a group of people in an airport tower who coordinate aircraft activity by staring at radar screens that use points of light to represent aircraft. While not fundamentally incorrect, this isn't a fair representation of the extent of ATC operations. This article will flesh out that simplistic image and introduce you to the equipment, technologies, and procedures that go into keeping aircraft and air travelers safe in the air and on the ground. We'll look at the way air traffic control is organized, and explore the communication technologies that air traffic controllers use to keep in touch with air crew and ground personnel. We'll also look at the radar technologies used to keep track of aircraft, and we'll end with a brief look at some next-generation technologies.
Safety and wake turbulence
The primary mandate of air traffic control is to ensure the safe transport of people and cargo by keeping aircraft at a safe distance from each other and expediting the flow of traffic. Air traffic controllers have access to sophisticated radar systems that provide an overview of the airspace they control, and they have communication tools to coordinate flight paths with the air crew. Pilots lack the tools necessary to get an overview of the airspace, so they have to rely on air traffic controllers to guide the aircraft through congested airspace.
Since aircraft travel at significantly higher speeds compared to other common modes of transport, the time available for pilots to react to a dangerous situation can be quite short. Thus it's essential that flight paths are carefully planned and managed to minimize the risk of a collision. This is especially true around major airports where the density of aircraft in a given volume of airspace is higher than average.
Turbulence created by wingtip vortices and exhaust gases from jet engines can be significant when aircraft are in close proximity. This phenomenon, called "wake turbulence," can adversely affect trailing aircraft if the distance between them falls below a certain limit. This limit depends on the mass of the two aircraft. For instance, a light aircraft following a heavy aircraft is more susceptible to wake turbulence than a heavy aircraft in the same situation. Therefore, aircraft approaching an airfield have to be carefully sequenced in a manner that takes such factors into consideration. Additionally, weather conditions such as low cloud, heavy rain, or snow blizzards can mean that pilots can't see other aircraft in the vicinity and have to use their instruments and instructions from air traffic control to navigate.
Types of ATC
Air traffic controllers are organized into various groups, each of which is in charge of handling a distinct portion of the aircraft's flight. Each group has a designated airspace that it controls, and aircraft are handed off to the next group of controllers as it approaches the limits of the prior group's airspace. The airspace controlled by each group is further divided into sectors that are themselves handled by individual controllers. The way these groups are organized varies from country to country and depends on the extent of controlled airspace and number of aircraft handled.
The Potomac TRACON facility controls approaches and departures in the airspace surrounding Baltimore Washington International, Washington Dulles, and Washington National airports.
The tower controllers are the most visible group. From their vantage point on the airport tower, they have a visual overview of all the important parts of the airport tarmac, such as runways and taxiways. Tower controllers monitor the airspace surrounding the airports and keep track of approaching and departing aircraft. At well-equipped airports, they may even have access to surface movement radar systems to monitor aircraft and support vehicles as they move on the ground.
Once the aircraft is in the air and clear of the airfield, tower control hands the aircraft off to a departure controller. These controllers are typically based at facilities a good distance from the airport. With the use of surveillance radars, they are able to monitor air traffic around the airport. These facilities are called Terminal Radar Approach Control (TRACON) facilities in the US. In an area like New York, where airports are close to one another, a TRACON facility can service multiple airports. The controllers here ensure that the planes approaching and departing the airspace they control are following designated flight paths and speeds. TRACON facilities also sequence the aircraft that are coming in to land, in order to ensure that they are adequately separated to minimize any wake turbulence effects. Departure controllers also need to take into account aircraft that may be flying through their airspace, and keep them separated from aircraft landing or taking-off.
As the aircraft exits the TRACON airspace, a facility known as an Area Control Center (ACC) takes over. These facilities monitor the aircraft's flight while in controlled airspace through remote radar stations. Each ACC will have a designated airspace that it supervises. An aircraft may fly through multiple ACC sectors as it flies to its destination, with each ACC handing off control of the aircraft to the next ACC as it exits the former's airspace. Once an aircraft gets closer to its destination airport, the ACC controllers hand off responsibility to the approach controllers at the local TRACON, who guide and sequence the aircraft to the active runway, and finally to the tower controllers.
ATC Radar Systems
Air traffic controllers use radar systems positioned at or near the ATC facility to get a real-time overview of the aircraft flying in the airspace they control. Radar technology for detecting aircraft first became popular during the wars in the first half of the last century and played a vital role in their outcome. First-generation radar systems served as early warning systems; these systems had relatively poor resolution, and their only purpose was to alert their operators to the presence of flying objects in the radar's field of view. These early radars operated by emitting a continuous radio signal and listening for any echos, but they weren't able to use these echos to gauge the size of the aircraft, calculate its ground speed or altitude, or determine if the aircraft belonged to an ally or the opposition.
After the war, radar technology was advanced with improved electronics and materials for antenna construction. This allowed for systems that were much more efficient and had higher resolution. The Air Traffic Controllers today are served by many types of radar equipment such as Primary Surveillance Radars (PSR), Secondary Surveillance Radars (SSR), and Mode S for monitoring traffic in the air, and Surface Movement Radars (SMR) for traffic on the ground.
Primary Surveillance Radar (PSR)
The Primary Surveillance Radar is the traditional form of radar that most people are familiar with. The radar sends a directed pulse into the atmosphere, and when that pulse encounters an object it gets reflected back to the radar station. By having precise knowledge of the orientation of the radar and the time between the sending and receiving of the radio pulse, the bearing of the object with respect to the radar station and its approximate distance can be calculated. The radar is typically enclosed in a dome to protect it from adverse weather. The PSR cannot determine the altitude or elevation of the aircraft relative to that of the radar station. For this to be possible, a second radar that sweeps the sky vertically would be required. However, such technology is typically only found in Precision Approach Radar installations at military facilities to assist pilots with landing aircraft.
The potential for an aircraft to be detectable by the PSR depends on its Radar Cross Section (RCS). The RCS depends on a number of factors, including the distance between the aircraft and the radar, and the size of the aircraft. A larger aircraft, for instance, would be visible at a greater distance than a smaller aircraft due to the larger surface area available for reflecting the radar waves. Other factors, such as reflectivity properties of the aircraft skin, the wavelength of the radar signal, and the angle at which the radar signal is incident on the aircraft (and thereby reflected), also play a part in how visible an aircraft is to the PSR.
Secondary Surveillance Radar (SSR)
The Secondary Surveillance Radar system—or Air Traffic Control Radar Beacon System (ATCRBS) as it is sometimes referred to—is comprised of the ATC radar installation and a transponder that rides onboard the aircraft being monitored. The origins of SSR lie in the "identify friend or foe" system used by the military to distinguish allied and enemy aircraft. While a Primary Surveillance Radar listens for reflected radio signals, the Secondary Surveillance Radar listens for messages from the aircraft's transponder. The radar rotates about the vertical axis similar to a PSR, but transmits a specific signal on 1030 MHz. This signal is subsequently received by the aircraft's onboard transponder, which responds with a reply on 1090 MHz. Much like the PSR, the bearing and distance of the aircraft with respect to the radar installation can be calculated with precise knowledge of the orientation of the radar when the signal was transmitted.
The SSR system has many advantages over PSR. Firstly, since it doesn't rely on radio waves being reflected back by the aircraft, the radar cross section of the aircraft does not form a part of the equation. All aircraft in range of the radar, regardless of size, composition, or distance from the radar, can be "heard" equally well. Secondly, since the signal received by the radar originates on the aircraft, the signal is subject to less attenuation compared to a PSR signal. This is because the reflected PSR signal has to travel twice as much as the SSR signal. This also implies that the signal transmission power for an SSR can be much lower than that of a PSR.
While SSR has many advantages, the prime disadvantage is that the system needs a properly functioning transponder on the aircraft for it to work. An aircraft without a transponder would effectively be invisible to the SSR. For this reason, PSRs are still used as a backup mechanism in most parts of the world. The Federal Aviation Regulations (FAR) in the US (and similar regulations in most countries) require aircraft operating in air traffic controlled airspace to have an operational transponder.
SSR Modes
The signal transmitted by the SSR is termed the "interrogation signal." There are different modes of interrogation that compliant transponders respond to. Civil aircraft typically respond to interrogation modes A and C.
In Mode A, the transponder responds with its squawk code, a unique identifier for the aircraft assigned by ATC comprising of four octal numbers. This code enables ATC to differentiate between the various aircraft being monitored, though it can also be used to discretely communicate the existence of an emergency situation onboard the aircraft. For instance, a squawk code of 7500 indicates that the aircraft has been hijacked, and a squawk code of 7600 indicates that the pilots are unable to communicate with ATC.
In Mode C, the transponder responds with the aircraft's pressure altitude, which is the altitude above sea level as calculated using the barometric reading at the aircraft's altitude. The altitudes reported are in 100-feet increments. This information is vital for ATC to ensure that aircraft flying in close proximity have adequate vertical separation.
Mode S is an improved secondary surveillance mode, and it operates over the same radio frequencies as SSR, making it backwards compatible. While Modes A and C in the legacy SSR system "broadcast" an interrogation message to all aircraft in the path of the radar beam, Mode S selectively interrogates individual aircraft using a 24 bit identifier that is assigned to the aircraft upon registration.
Mode S has a number of advantages over Modes A and C, the first of which is that its selective interrogation approach reduces the workload on the system, as it might be inundated with SSR replies in heavily congested airspace. Secondly, the legacy SSR system allowed for a transponder identification code of only four octal digits (12 bits) that yielded just 4096 codes (not including reserved codes) which were assigned by ATC. To complicate matters, this could be different for the same aircraft in different ATC sectors. As the number of aircraft increased over the past few decades, there was a scarcity in the number of available transponder codes, but move to a 24 bit addressing system alleviated the problem. Finally, the altitude reporting in Mode C was in increments of 100 feet, but this was improved to increments of 25 feet, allowing controllers to have a more accurate picture of aircraft positions.
Surface Movement Radar (SMR)
The airfield can be a busy place, with pushback tugs, tractors with baggage containers in tow, refuelling trucks, catering trucks, airport security vehicles, and (of course) aircraft. While the PSR and SSR provide controllers with an overview of aircraft in the air, the surface movement radar provides a real-time view of aircraft and support vehicles on the ground at airports. Most modern airports have Ground Control in charge of ensuring that critical patches of the airport tarmac such as active runways and taxiways are safe for moving aircraft.
Ground Control can observe all moving vehicles and aircraft on a radar screen overlaid on a map of the airport. As the objects being tracked by an SMR are relatively smaller than those tracked by a PSR or SSR, the radar uses a much shorter wavelength (and correspondingly higher frequency) with a narrow beam for a higher resolution result. Depending on the size of the airport, multiple installations of this sort may be required to cover all the critical parts of the airport. Enhancements to this basic system include having airport support vehicles installed with transponders that can be queried to ascertain location. Similarly, aircraft transponders can be queried to augment the radar display with call signs. Information from the tower radar can also be incorporated to display approaching aircraft. Newer systems can even aurally warn controllers of potential runway incursions and conflicts, so that action can be taken in time to avoid disaster. Such a system is generally called an Airport Movement Area Safety System.
Unfavorable weather conditions such as heavy rain and fog can lead to a reduction in visibility, making it difficult to monitor the tarmac. Runway incursions are a constant danger in such challenging conditions. A runway incursion is described by ICAO (International Civil Aviation Authority) as the incorrect presence of an aircraft, vehicle, or person on the protected area of a surface designated for the landing and take-off of aircraft. The Tenerife Airport Disaster, the deadliest accident in aviation history, was as a result of two aircraft colliding with each other on the runway. The incident occurred in heavy fog and at a time when the air traffic controller on duty could not see the two aircraft, nor could the two pilots see each other. A more recent incident that highlights the importance of the surface movement radar in emergency situations is the crash of british airways flight 038 on the threshold of runway 27L at London Heathrow Airport.
Air-Ground Voice Communication
In the early years of aviation, when there were fewer planes in the sky than we have today and there was not much need for pilots to communicate with ground personnel, signalling was often done using lights and flags. But with an increase in aircraft, a more efficient and unambiguous two-way communication system became necessary. At the same time, radio technology was progressing, and it became feasible for aircraft to have radio transceivers on board.
Aircraft communication in the early years was over the HF (High Frequency) range of the radio spectrum. In the US, each airline company had its own dedicated radio frequency over which company pilots communicated with their operators on the ground. But over time, with an increase in airliner companies and air traffic, this system soon led to a depletion in the available frequencies on the spectrum. The problem was resolved by setting up a common entity that provided air traffic coordination services. This allowed for a better use of the available radio spectrum, as pilots communicated with air traffic personnel over common frequencies. Over time, this system evolved to the air traffic control system we have today.
Modern civil aviation uses the HF (High Frequency) and VHF (Very High Frequency) parts of the spectrum for communication between aircraft and ATC. Military aviation in various countries are also known to operate in UHF (Ultra High Frequency). Early air to ground voice communication was over HF, but VHF started to get adopted in the 1930s and 1940s.
VHF communication
VHF is the predominant frequency range used by civil aviation in most parts of the world, and communication usually happens over the 118 MHz to 138 MHz frequency range via an amplitude modulated (AM) signal. (FM radio, in comparison, is over frequencies from 87 MHz to 108 MHz in most countries, and is of course, frequency modulated.) The frequency 121.5 MHz is reserved for emergencies in the VHF frequency range.
VHF transmissions rely heavily on a line-of-sight between the transmitter and receiver. This doesn't necessarily mean a visual line-of-sight, however, but a "radio line-of-sight" in the VHF part of the spectrum. Solid structures, such as buildings or the earth's surface, tend to attenuate the signal (or reduce its strength). Additionally, atmospheric layers do not refract or reflect VHF waves well. Due to these characteristics of VHF propagation, and due to the curvature of the earth, an aircraft below a VHF transmitter's radio horizon would typically fall outside its range. Thus the nature of the terrain and the height of the aircraft above the ground plays a part in determining whether it is in range of a given ground station operating over VHF frequencies.
For instance, the range would be severely curtailed for low-flying aircraft in hilly areas or urban areas with tall skyscrapers. In such cases, signal repeaters can be used to increase coverage. Theoretically, an aircraft at cruise altitude, flying in ideal weather conditions with a good quality transceiver can expect to communicate with a station unobstructed by hilly terrain about 200 nautical miles away (370 kilometers or 230 miles). In less than ideal real-world conditions however, this range can be significantly lower.
VHF operating frequencies are allocated to ATC stations in such a way that they do not interfere with each other, and since VHF relies on line-of-sight, frequencies can be reused by other distant stations. A typical activity for controllers is to pass on the frequency used by the next ATC sector to the pilot as the aircraft exits the ATC's airspace.
HF communication
When flying over large expanses of water, VHF communication, due to its line-of-sight nature becomes unusable. Additionally it would be impractical to construct, operate and maintain VHF relay stations in the middle of the ocean. So in these situations, HF can be used to communicate with an aircraft below the station's horizon and often many thousands of nautical miles away. In some cases, HF transmissions are known to have been successfully received on the other side of the planet. Thus pilots flying certain oceanic routes that are outside radar coverage use HF to periodically report their position to the oceanic control station for the sector they are flying in.
HF has some major drawbacks that make it impractical for more widespread applications. First, HF signal propagation is highly dependent on atmospheric conditions and solar activity, making HF communication comparatively noisy and unreliable. Due to this variability, ATC stations operating over HF have a number of alternative frequencies that they can operate over, and they switch to the frequency with the best signal propagation characteristics at the time.
It's also the case that HF signals have a longer wavelength than VHF signals, so the antennas used for transmitting and receiving are much larger, as well. And HF transmitters also operate at a higher power, since the receivers may potentially be hundreds of nautical miles away.
Next-generation Technologies
The aviation industry has been relatively conservative in its approach to adopting new technologies, so a number of the systems described so far are based on technology that has been around for at least a few decades. This conservatism is primarily attributable to the safety and reliability considerations that must be taken into account when making changes to existing equipment and procedures.
While tried-and-true systems do provide the required safety, this safety may at times come at the cost of efficiency. Critics of the current air traffic control systems claim that efficiency gains of many percentage points remain to be realized by more intelligent routing in controlled airspace that allows for lower separation distances between aircraft.
Currently, the Federal Aviation Administration (FAA) in the US is studying the implementation of various next-generation technologies to improve the efficiency of the ATC system while retaining or improving the level of safety. Next generation will incorporate global positioning satellites, digital communication networks, data networking, and improved weather forecasting to improve efficiency. Of all the technologies being considered, Automatic Dependant Surveillance (ADS-B) is being billed as the the future of air traffic control and as the backbone of the NextGen system.
Automatic Dependant Surveillance—Broadcast (ADS-B)
ADS-B is a relatively new technique (compared to use of primary and secondary surveillance radar) to monitor aircraft. The technique uses the global positioning system (GPS) to provide an accurate report of an aircraft's position. As the name suggests, this is a broadcast technique where an aircraft equipped with an ADS-B transponder routinely broadcasts data. Using similar information from all aircraft, the air traffic controllers can build an accurate picture of aircraft positions. ADS-B is able to provide information not unlike an SSR, but without the requirement for a radar installation or transmissions from a ground station.
The aircraft typically transmits its identity, current position, speed, and direction of travel (among other parameters) over a digital link, twice every second. Due to the broadcast nature of the data, other aircraft in the region can also receive this information and provide their pilots with an overview of the traffic in the neighborhood as well. One of the advantages of the ADS-B system is that the receiver can be relatively simple and inexpensive. Another advantage is that ground vehicles at the airport can use the same system to report their location on the airport tarmac which can be incorporated into the Airport Movement Area Safety System described earlier.
One of the disadvantages of ADS-B is that the system relies on the GPS system for accurate reporting of position information. Loss or degradation of the GPS signal could potentially put lives in danger. This can be mitigated to an extent by alternative sources of positioning information such as the European Galileo project, the Russian GLONASS project or the Chinese Beidou project, when they become fully operational. The other disadvantage is that a malfunctioning or inoperative transponder could render the aircraft invisible, or worse, broadcast false information. This is one reason for the continued use of surveillance radars as backup. Given the relative simplicity and cost-effectiveness of building an ADS-B transponder (compared to surveillance radars) and the open nature of the system, critics fear it is also a security hazard as it would in theory be possible to spoof data to represent aircraft that don't actually exist.
Listening to ATC and Tracking Aircraft
ATC and pilots communicate over open, well-advertised frequencies. Since VHF communication takes place using frequencies between 118 MHz to 138 MHz, a frequency range not commonly available on general purpose radio receivers, a scanner that can tune into these frequencies is required. It isn't unusual for serious aviation enthusiasts to invest in a good quality scanner to listen to their local air traffic controllers and pilots. Sites such as live ATC stream ATC broadcasts from various ATC facilities in the world. One must, however, exercise caution and check local laws first, as listening to ATC is illegal in certain jurisdictions. HF communications also occur over open, well-advertised frequencies and, unlike VHF, general purpose shortwave radios often have the frequency range to tune into oceanic ATC. Due to the nature of HF propagation, it may be possible to listen to controllers thousands of miles away, but the reception quality will vary.
The ADS-B system, as it operates currently, is an open system as well. It is possible to purchase an ADS-B receiver for as little as $600. A number of enthusiast sites such as flightradar24 (Scandinavian region), Zurich University of Applied Sciences' School of Engineering radar site (Switzerland) and Casper(Netherlands) provide an overview of aircraft in their region by listening to ADS-B transmissions.
Safety and wake turbulence
The primary mandate of air traffic control is to ensure the safe transport of people and cargo by keeping aircraft at a safe distance from each other and expediting the flow of traffic. Air traffic controllers have access to sophisticated radar systems that provide an overview of the airspace they control, and they have communication tools to coordinate flight paths with the air crew. Pilots lack the tools necessary to get an overview of the airspace, so they have to rely on air traffic controllers to guide the aircraft through congested airspace.
Since aircraft travel at significantly higher speeds compared to other common modes of transport, the time available for pilots to react to a dangerous situation can be quite short. Thus it's essential that flight paths are carefully planned and managed to minimize the risk of a collision. This is especially true around major airports where the density of aircraft in a given volume of airspace is higher than average.
Turbulence created by wingtip vortices and exhaust gases from jet engines can be significant when aircraft are in close proximity. This phenomenon, called "wake turbulence," can adversely affect trailing aircraft if the distance between them falls below a certain limit. This limit depends on the mass of the two aircraft. For instance, a light aircraft following a heavy aircraft is more susceptible to wake turbulence than a heavy aircraft in the same situation. Therefore, aircraft approaching an airfield have to be carefully sequenced in a manner that takes such factors into consideration. Additionally, weather conditions such as low cloud, heavy rain, or snow blizzards can mean that pilots can't see other aircraft in the vicinity and have to use their instruments and instructions from air traffic control to navigate.
Types of ATC
Air traffic controllers are organized into various groups, each of which is in charge of handling a distinct portion of the aircraft's flight. Each group has a designated airspace that it controls, and aircraft are handed off to the next group of controllers as it approaches the limits of the prior group's airspace. The airspace controlled by each group is further divided into sectors that are themselves handled by individual controllers. The way these groups are organized varies from country to country and depends on the extent of controlled airspace and number of aircraft handled.
The Potomac TRACON facility controls approaches and departures in the airspace surrounding Baltimore Washington International, Washington Dulles, and Washington National airports.
The tower controllers are the most visible group. From their vantage point on the airport tower, they have a visual overview of all the important parts of the airport tarmac, such as runways and taxiways. Tower controllers monitor the airspace surrounding the airports and keep track of approaching and departing aircraft. At well-equipped airports, they may even have access to surface movement radar systems to monitor aircraft and support vehicles as they move on the ground.
Once the aircraft is in the air and clear of the airfield, tower control hands the aircraft off to a departure controller. These controllers are typically based at facilities a good distance from the airport. With the use of surveillance radars, they are able to monitor air traffic around the airport. These facilities are called Terminal Radar Approach Control (TRACON) facilities in the US. In an area like New York, where airports are close to one another, a TRACON facility can service multiple airports. The controllers here ensure that the planes approaching and departing the airspace they control are following designated flight paths and speeds. TRACON facilities also sequence the aircraft that are coming in to land, in order to ensure that they are adequately separated to minimize any wake turbulence effects. Departure controllers also need to take into account aircraft that may be flying through their airspace, and keep them separated from aircraft landing or taking-off.
As the aircraft exits the TRACON airspace, a facility known as an Area Control Center (ACC) takes over. These facilities monitor the aircraft's flight while in controlled airspace through remote radar stations. Each ACC will have a designated airspace that it supervises. An aircraft may fly through multiple ACC sectors as it flies to its destination, with each ACC handing off control of the aircraft to the next ACC as it exits the former's airspace. Once an aircraft gets closer to its destination airport, the ACC controllers hand off responsibility to the approach controllers at the local TRACON, who guide and sequence the aircraft to the active runway, and finally to the tower controllers.
ATC Radar Systems
Air traffic controllers use radar systems positioned at or near the ATC facility to get a real-time overview of the aircraft flying in the airspace they control. Radar technology for detecting aircraft first became popular during the wars in the first half of the last century and played a vital role in their outcome. First-generation radar systems served as early warning systems; these systems had relatively poor resolution, and their only purpose was to alert their operators to the presence of flying objects in the radar's field of view. These early radars operated by emitting a continuous radio signal and listening for any echos, but they weren't able to use these echos to gauge the size of the aircraft, calculate its ground speed or altitude, or determine if the aircraft belonged to an ally or the opposition.
After the war, radar technology was advanced with improved electronics and materials for antenna construction. This allowed for systems that were much more efficient and had higher resolution. The Air Traffic Controllers today are served by many types of radar equipment such as Primary Surveillance Radars (PSR), Secondary Surveillance Radars (SSR), and Mode S for monitoring traffic in the air, and Surface Movement Radars (SMR) for traffic on the ground.
Primary Surveillance Radar (PSR)
The Primary Surveillance Radar is the traditional form of radar that most people are familiar with. The radar sends a directed pulse into the atmosphere, and when that pulse encounters an object it gets reflected back to the radar station. By having precise knowledge of the orientation of the radar and the time between the sending and receiving of the radio pulse, the bearing of the object with respect to the radar station and its approximate distance can be calculated. The radar is typically enclosed in a dome to protect it from adverse weather. The PSR cannot determine the altitude or elevation of the aircraft relative to that of the radar station. For this to be possible, a second radar that sweeps the sky vertically would be required. However, such technology is typically only found in Precision Approach Radar installations at military facilities to assist pilots with landing aircraft.
The potential for an aircraft to be detectable by the PSR depends on its Radar Cross Section (RCS). The RCS depends on a number of factors, including the distance between the aircraft and the radar, and the size of the aircraft. A larger aircraft, for instance, would be visible at a greater distance than a smaller aircraft due to the larger surface area available for reflecting the radar waves. Other factors, such as reflectivity properties of the aircraft skin, the wavelength of the radar signal, and the angle at which the radar signal is incident on the aircraft (and thereby reflected), also play a part in how visible an aircraft is to the PSR.
Secondary Surveillance Radar (SSR)
The Secondary Surveillance Radar system—or Air Traffic Control Radar Beacon System (ATCRBS) as it is sometimes referred to—is comprised of the ATC radar installation and a transponder that rides onboard the aircraft being monitored. The origins of SSR lie in the "identify friend or foe" system used by the military to distinguish allied and enemy aircraft. While a Primary Surveillance Radar listens for reflected radio signals, the Secondary Surveillance Radar listens for messages from the aircraft's transponder. The radar rotates about the vertical axis similar to a PSR, but transmits a specific signal on 1030 MHz. This signal is subsequently received by the aircraft's onboard transponder, which responds with a reply on 1090 MHz. Much like the PSR, the bearing and distance of the aircraft with respect to the radar installation can be calculated with precise knowledge of the orientation of the radar when the signal was transmitted.
The SSR system has many advantages over PSR. Firstly, since it doesn't rely on radio waves being reflected back by the aircraft, the radar cross section of the aircraft does not form a part of the equation. All aircraft in range of the radar, regardless of size, composition, or distance from the radar, can be "heard" equally well. Secondly, since the signal received by the radar originates on the aircraft, the signal is subject to less attenuation compared to a PSR signal. This is because the reflected PSR signal has to travel twice as much as the SSR signal. This also implies that the signal transmission power for an SSR can be much lower than that of a PSR.
While SSR has many advantages, the prime disadvantage is that the system needs a properly functioning transponder on the aircraft for it to work. An aircraft without a transponder would effectively be invisible to the SSR. For this reason, PSRs are still used as a backup mechanism in most parts of the world. The Federal Aviation Regulations (FAR) in the US (and similar regulations in most countries) require aircraft operating in air traffic controlled airspace to have an operational transponder.
SSR Modes
The signal transmitted by the SSR is termed the "interrogation signal." There are different modes of interrogation that compliant transponders respond to. Civil aircraft typically respond to interrogation modes A and C.
In Mode A, the transponder responds with its squawk code, a unique identifier for the aircraft assigned by ATC comprising of four octal numbers. This code enables ATC to differentiate between the various aircraft being monitored, though it can also be used to discretely communicate the existence of an emergency situation onboard the aircraft. For instance, a squawk code of 7500 indicates that the aircraft has been hijacked, and a squawk code of 7600 indicates that the pilots are unable to communicate with ATC.
In Mode C, the transponder responds with the aircraft's pressure altitude, which is the altitude above sea level as calculated using the barometric reading at the aircraft's altitude. The altitudes reported are in 100-feet increments. This information is vital for ATC to ensure that aircraft flying in close proximity have adequate vertical separation.
Mode S is an improved secondary surveillance mode, and it operates over the same radio frequencies as SSR, making it backwards compatible. While Modes A and C in the legacy SSR system "broadcast" an interrogation message to all aircraft in the path of the radar beam, Mode S selectively interrogates individual aircraft using a 24 bit identifier that is assigned to the aircraft upon registration.
Mode S has a number of advantages over Modes A and C, the first of which is that its selective interrogation approach reduces the workload on the system, as it might be inundated with SSR replies in heavily congested airspace. Secondly, the legacy SSR system allowed for a transponder identification code of only four octal digits (12 bits) that yielded just 4096 codes (not including reserved codes) which were assigned by ATC. To complicate matters, this could be different for the same aircraft in different ATC sectors. As the number of aircraft increased over the past few decades, there was a scarcity in the number of available transponder codes, but move to a 24 bit addressing system alleviated the problem. Finally, the altitude reporting in Mode C was in increments of 100 feet, but this was improved to increments of 25 feet, allowing controllers to have a more accurate picture of aircraft positions.
Surface Movement Radar (SMR)
The airfield can be a busy place, with pushback tugs, tractors with baggage containers in tow, refuelling trucks, catering trucks, airport security vehicles, and (of course) aircraft. While the PSR and SSR provide controllers with an overview of aircraft in the air, the surface movement radar provides a real-time view of aircraft and support vehicles on the ground at airports. Most modern airports have Ground Control in charge of ensuring that critical patches of the airport tarmac such as active runways and taxiways are safe for moving aircraft.
Ground Control can observe all moving vehicles and aircraft on a radar screen overlaid on a map of the airport. As the objects being tracked by an SMR are relatively smaller than those tracked by a PSR or SSR, the radar uses a much shorter wavelength (and correspondingly higher frequency) with a narrow beam for a higher resolution result. Depending on the size of the airport, multiple installations of this sort may be required to cover all the critical parts of the airport. Enhancements to this basic system include having airport support vehicles installed with transponders that can be queried to ascertain location. Similarly, aircraft transponders can be queried to augment the radar display with call signs. Information from the tower radar can also be incorporated to display approaching aircraft. Newer systems can even aurally warn controllers of potential runway incursions and conflicts, so that action can be taken in time to avoid disaster. Such a system is generally called an Airport Movement Area Safety System.
Unfavorable weather conditions such as heavy rain and fog can lead to a reduction in visibility, making it difficult to monitor the tarmac. Runway incursions are a constant danger in such challenging conditions. A runway incursion is described by ICAO (International Civil Aviation Authority) as the incorrect presence of an aircraft, vehicle, or person on the protected area of a surface designated for the landing and take-off of aircraft. The Tenerife Airport Disaster, the deadliest accident in aviation history, was as a result of two aircraft colliding with each other on the runway. The incident occurred in heavy fog and at a time when the air traffic controller on duty could not see the two aircraft, nor could the two pilots see each other. A more recent incident that highlights the importance of the surface movement radar in emergency situations is the crash of british airways flight 038 on the threshold of runway 27L at London Heathrow Airport.
Air-Ground Voice Communication
In the early years of aviation, when there were fewer planes in the sky than we have today and there was not much need for pilots to communicate with ground personnel, signalling was often done using lights and flags. But with an increase in aircraft, a more efficient and unambiguous two-way communication system became necessary. At the same time, radio technology was progressing, and it became feasible for aircraft to have radio transceivers on board.
Aircraft communication in the early years was over the HF (High Frequency) range of the radio spectrum. In the US, each airline company had its own dedicated radio frequency over which company pilots communicated with their operators on the ground. But over time, with an increase in airliner companies and air traffic, this system soon led to a depletion in the available frequencies on the spectrum. The problem was resolved by setting up a common entity that provided air traffic coordination services. This allowed for a better use of the available radio spectrum, as pilots communicated with air traffic personnel over common frequencies. Over time, this system evolved to the air traffic control system we have today.
Modern civil aviation uses the HF (High Frequency) and VHF (Very High Frequency) parts of the spectrum for communication between aircraft and ATC. Military aviation in various countries are also known to operate in UHF (Ultra High Frequency). Early air to ground voice communication was over HF, but VHF started to get adopted in the 1930s and 1940s.
VHF communication
VHF is the predominant frequency range used by civil aviation in most parts of the world, and communication usually happens over the 118 MHz to 138 MHz frequency range via an amplitude modulated (AM) signal. (FM radio, in comparison, is over frequencies from 87 MHz to 108 MHz in most countries, and is of course, frequency modulated.) The frequency 121.5 MHz is reserved for emergencies in the VHF frequency range.
VHF transmissions rely heavily on a line-of-sight between the transmitter and receiver. This doesn't necessarily mean a visual line-of-sight, however, but a "radio line-of-sight" in the VHF part of the spectrum. Solid structures, such as buildings or the earth's surface, tend to attenuate the signal (or reduce its strength). Additionally, atmospheric layers do not refract or reflect VHF waves well. Due to these characteristics of VHF propagation, and due to the curvature of the earth, an aircraft below a VHF transmitter's radio horizon would typically fall outside its range. Thus the nature of the terrain and the height of the aircraft above the ground plays a part in determining whether it is in range of a given ground station operating over VHF frequencies.
For instance, the range would be severely curtailed for low-flying aircraft in hilly areas or urban areas with tall skyscrapers. In such cases, signal repeaters can be used to increase coverage. Theoretically, an aircraft at cruise altitude, flying in ideal weather conditions with a good quality transceiver can expect to communicate with a station unobstructed by hilly terrain about 200 nautical miles away (370 kilometers or 230 miles). In less than ideal real-world conditions however, this range can be significantly lower.
VHF operating frequencies are allocated to ATC stations in such a way that they do not interfere with each other, and since VHF relies on line-of-sight, frequencies can be reused by other distant stations. A typical activity for controllers is to pass on the frequency used by the next ATC sector to the pilot as the aircraft exits the ATC's airspace.
HF communication
When flying over large expanses of water, VHF communication, due to its line-of-sight nature becomes unusable. Additionally it would be impractical to construct, operate and maintain VHF relay stations in the middle of the ocean. So in these situations, HF can be used to communicate with an aircraft below the station's horizon and often many thousands of nautical miles away. In some cases, HF transmissions are known to have been successfully received on the other side of the planet. Thus pilots flying certain oceanic routes that are outside radar coverage use HF to periodically report their position to the oceanic control station for the sector they are flying in.
HF has some major drawbacks that make it impractical for more widespread applications. First, HF signal propagation is highly dependent on atmospheric conditions and solar activity, making HF communication comparatively noisy and unreliable. Due to this variability, ATC stations operating over HF have a number of alternative frequencies that they can operate over, and they switch to the frequency with the best signal propagation characteristics at the time.
It's also the case that HF signals have a longer wavelength than VHF signals, so the antennas used for transmitting and receiving are much larger, as well. And HF transmitters also operate at a higher power, since the receivers may potentially be hundreds of nautical miles away.
Next-generation Technologies
The aviation industry has been relatively conservative in its approach to adopting new technologies, so a number of the systems described so far are based on technology that has been around for at least a few decades. This conservatism is primarily attributable to the safety and reliability considerations that must be taken into account when making changes to existing equipment and procedures.
While tried-and-true systems do provide the required safety, this safety may at times come at the cost of efficiency. Critics of the current air traffic control systems claim that efficiency gains of many percentage points remain to be realized by more intelligent routing in controlled airspace that allows for lower separation distances between aircraft.
Currently, the Federal Aviation Administration (FAA) in the US is studying the implementation of various next-generation technologies to improve the efficiency of the ATC system while retaining or improving the level of safety. Next generation will incorporate global positioning satellites, digital communication networks, data networking, and improved weather forecasting to improve efficiency. Of all the technologies being considered, Automatic Dependant Surveillance (ADS-B) is being billed as the the future of air traffic control and as the backbone of the NextGen system.
Automatic Dependant Surveillance—Broadcast (ADS-B)
ADS-B is a relatively new technique (compared to use of primary and secondary surveillance radar) to monitor aircraft. The technique uses the global positioning system (GPS) to provide an accurate report of an aircraft's position. As the name suggests, this is a broadcast technique where an aircraft equipped with an ADS-B transponder routinely broadcasts data. Using similar information from all aircraft, the air traffic controllers can build an accurate picture of aircraft positions. ADS-B is able to provide information not unlike an SSR, but without the requirement for a radar installation or transmissions from a ground station.
The aircraft typically transmits its identity, current position, speed, and direction of travel (among other parameters) over a digital link, twice every second. Due to the broadcast nature of the data, other aircraft in the region can also receive this information and provide their pilots with an overview of the traffic in the neighborhood as well. One of the advantages of the ADS-B system is that the receiver can be relatively simple and inexpensive. Another advantage is that ground vehicles at the airport can use the same system to report their location on the airport tarmac which can be incorporated into the Airport Movement Area Safety System described earlier.
One of the disadvantages of ADS-B is that the system relies on the GPS system for accurate reporting of position information. Loss or degradation of the GPS signal could potentially put lives in danger. This can be mitigated to an extent by alternative sources of positioning information such as the European Galileo project, the Russian GLONASS project or the Chinese Beidou project, when they become fully operational. The other disadvantage is that a malfunctioning or inoperative transponder could render the aircraft invisible, or worse, broadcast false information. This is one reason for the continued use of surveillance radars as backup. Given the relative simplicity and cost-effectiveness of building an ADS-B transponder (compared to surveillance radars) and the open nature of the system, critics fear it is also a security hazard as it would in theory be possible to spoof data to represent aircraft that don't actually exist.
Listening to ATC and Tracking Aircraft
ATC and pilots communicate over open, well-advertised frequencies. Since VHF communication takes place using frequencies between 118 MHz to 138 MHz, a frequency range not commonly available on general purpose radio receivers, a scanner that can tune into these frequencies is required. It isn't unusual for serious aviation enthusiasts to invest in a good quality scanner to listen to their local air traffic controllers and pilots. Sites such as live ATC stream ATC broadcasts from various ATC facilities in the world. One must, however, exercise caution and check local laws first, as listening to ATC is illegal in certain jurisdictions. HF communications also occur over open, well-advertised frequencies and, unlike VHF, general purpose shortwave radios often have the frequency range to tune into oceanic ATC. Due to the nature of HF propagation, it may be possible to listen to controllers thousands of miles away, but the reception quality will vary.
The ADS-B system, as it operates currently, is an open system as well. It is possible to purchase an ADS-B receiver for as little as $600. A number of enthusiast sites such as flightradar24 (Scandinavian region), Zurich University of Applied Sciences' School of Engineering radar site (Switzerland) and Casper(Netherlands) provide an overview of aircraft in their region by listening to ADS-B transmissions.
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