CNS/ATM resource kit – Chapter 1: Overview of CNS/ATM

CNS/ATM describes a new approach to integrated air traffic management (ATM) in the satellite age.

It brings together voice, satellite and digital Communications, performance-based Navigation (PBN) and automatic dependent Surveillance broadcast (ADS-B), as well as ground-based systems such as radar and fixed navaids.

This chapter provides an introduction to CNS/ATM, explores its benefits and components and outlines the relevant rules and regulations.

CNS: Don’t be nervous about the acronyms

In the medical world, CNS is the acronym for central nervous system, vital to sustaining life. And so, in a sense, it is in aviation.

Clear Communication, accurate Navigation and Surveillance-based control and guidance have long been important for safe and efficient flying, but as skies become more crowded, any one of them can become a limiting factor in airspace capacity.

As the International Civil Aviation Organization (ICAO) puts it, CNS/ATM is:

Communications, navigation and surveillance systems, employing digital technologies, including satellite navigation systems together with various levels of automation, applied in support of a seamless global air traffic management system.

Many of these digital technologies have been around for some time. For example, global navigation satellite systems (GNSS) receivers have been in Australian civil aviation use since 1995.

What is new, however, is thinking of these technologies as an integrated system of air traffic management.

In Australia, regulations mandate the fitting of CNS equipment such as Mode S transponders and automatic dependent surveillance-broadcast (ADS-B) to instrument flight rules (IFR)-operated aircraft. These are used in combination to implement performance-based navigation (PBN).

Benefits of CNS/ATM

For pilots and operators, CNS/ATM offers significant efficiencies and improvement in safety, particularly (but not only) for those operating under IFR.

For those involved in airspace management, such as air traffic controllers, it means being able to process more aircraft more efficiently and safely.

The need for change: limitations of legacy technologies

Previous technologies have a number of limitations and effects.

This table contains previous technologies have a number of limitations and effects
Limitation Effect
Radar and VHF have limited range Over oceans, aircraft are out of radar range and use unreliable and inefficient HF radio for voice communication.
Transmit limited amount of information Legacy technologies can be insufficient for automated systems to operate effectively.
Expensive infrastructure Legacy technologies require large and costly structures on the ground, each one of which serves only a limited area.

Advantages of modern CNS/ATM technologies

The advantages and benefits of this new system are shown in the following table:

This table contains advantages of modern CNS/ATM technologies
Benefit Effect
Global Many operators and pilots are now using a worldwide satellite network.
High bandwidth The ability to transmit and receive large volumes of digital data allows position reports and weather forecasts to be transmitted automatically and frequently, rather than relying on the limited information of a voice transmission.
Accurate Global navigation satellite systems, such as GPS, the Chinese BeiDou, the Russian GLONASS, and the European Union’s Galileo have accuracies of single metres or less when augmentation systems are used.

Components of CNS/ATM

The relationship between the various components of communication, navigation, surveillance and air traffic management systems can be seen in the following diagram.

Components of CNS/ATM
Components of CNS/ATM



Voice communication using very high frequency (VHF) radio remains an essential part of routine and emergency air to ground (and air to air) communication. In emergency operations, voice tone and nuance provide valuable information. Many pilots have spoken of the reassuring effect of hearing a calm controller’s voice. However, VHF can transmit information only as fast as a person can speak coherently, and it cannot handle multiple transmissions on the same frequency. Technology such as controller-pilot data link communications (CPDLC) can significantly reduce the demand for bandwidth and time. Increasingly, routine air traffic management (ATM) air–ground communication services will use data communications, with voice for real-time, critical communication.

Aircraft can reply to ATC with a standard format message or in free text. Messages from a controller normally follow a standard format, with response required to most messages. CPDLC’s advantages include:

  • reduced congestion of voice channels
  • fewer communication errors
  • lower workload for pilots and controllers.

Communication is discussed in more detail in Chapter 2 of this guide.


Performance-based navigation (PBN) is the ‘N’ in CNS. PBN standards require a particular level of navigation accuracy. Required navigation performance (RNP) equipment must have on-board performance monitoring and alerting systems to provide assurance the system is working properly. As with all CNS standards, there is no specified technology. However in Australia, GNSS such as GPS is the only practical means of performing en route, terminal and approach operations.

PBN’s accuracy and cost advantages make it hard to justify maintaining an extensive network of old-technology navaids, such as VHF omnidirectional range (VORs) and non-directional beacons (NDBs). About half of these were decommissioned in 2016, but Airservices Australia is maintaining some as a backup navigation network (BNN). Read more about PBN and its practical applications in Chapter 6.


The S in CNS stands for surveillance, such as automatic dependent surveillance-broadcast (ADS-B). This technology uses GNSS equipment and a transponder-like broadcaster to determine the aircraft’s height, position and speed, and broadcasts this, along with its identity, twice per second.

Australia has had an operational ADS-B network since 2009. It uses a network of ground stations to ‘listen’ to these aircraft broadcasts and transmit information to ATC and to aircraft with ADS-B IN equipment. Aircraft fitted with ADS-B IN can also receive aircraft transmissions and display them to the pilot for situational awareness. ADS-B is seamless between countries.

In some parts of Australia, a system known as multilateration (MLAT) uses existing aircraft transponders and a network of ground receivers as an alternative to radar and/or ADS-B.

Air traffic management

The main air traffic management benefit of CNS is reduced aircraft separation in controlled airspace. Aircraft can now be operated closer together with no compromise to safety. Separation standards in oceanic airspace have reduced from 180 nm to 50 nm and then to 30 nm, with the prospect of satellite-based ADS-B systems allowing minimums of less than 15 nm.

Transforming airspace management: the Cold War to hot technology

This table contains transforming airspace management: the cold war to hot technology
1960s Global navigation satellite systems (GNSS) had their roots in the Cold War when the United States and Soviet Union launched the first systems. Designed for military applications, including missile guidance, the first-generation technology was crude.
1970 The US launched its global positioning system (GPS), and the Soviets, GLONASS. Before long, the technology was being used in just about every aspect of civilian life, including aviation.
1994 CASA’s predecessor, the Civil Aviation Authority, approved the use of GPS as a supplemental IFR en route navigation aid, putting Australia at the forefront of regulation of GNSS technology.
1998 The development of GPS non-precision approaches (NPAs) began when a non-precision approach (NPA) for Goulburn Airport, near Canberra, was given the green light. Fuelled by the low cost of GNSS and safety enhancement of straight-in approaches, NPAs proliferated and many Australian airports now have them. The first approvals for GPS approaches were based on technical standard order (TSO) C129 equipment, but that technology had reached its limits by the turn of the century.
2000s Superseded by units delivering gains in accuracy, integrity and continuity of service, the TSO C145, C146 and C196 receivers enabled general aviation pilots to spend more time utilising data from satellites 20,000 km above the Earth’s surface. GNSS equipment with augmentation was designed to allow precision approaches and automatic landing.
2002 The US introduced the first satellite-based augmentation system (SBAS) and authorised localiser performance with vertical guidance (LPV) approaches with decision altitudes similar to ILS Cat 1.
2004 ICAO set the global direction for PBN, establishing area navigation (RNAV) and required navigation performance (RNP) specifications.
2007 ADS-B avionics were first used for in flight operations in Australia, with non-complying ADS-B units disabled.
2009 Australia submitted a PBN implementation plan to ICAO and tabled a 2010–2016 timetable for implementation of approaches with vertical guidance.
2012 Australian rules for ICAO-standard PBN specifications published. Australia-specific navigation specifications withdrawn from use.
2016 Mandate requiring all Australian-registered aircraft operating under IFR to be fitted with GNSS avionics came into effect. Airservices Australia turned off around 180 ground-based navigation aids as part of the transition to GNSS-based navigation.


The main benefits of CNS/ATM are:

  • increased aircraft capacity, especially in congested airspace
  • increased schedule flexibility
  • better flight path efficiency
  • less disruption due to delays and diversions
  • increased efficiency from reduced separation minimum.

Managing airspace safely and effectively

Improvements in air traffic management can help reduce aviation fuel burn and as a result, reduce the levels of aircraft engine emissions. This can be achieved by:

  • establishing more direct routes
  • reducing use of holding patterns
  • improving access to optimal cruising levels and constant descent approaches where engines can be throttled back for a steady near-glide to the threshold.

The seamless integration to which ICAO refers is ‘rapid and reliable transmission between ground and airborne system elements’. The promise is that more accurate and reliable navigation systems will also allow aircraft to navigate in all types of airspace and operate closer together with the same or better safety than under current separation standards.

The figure opposite shows the reduction in distance and elevation between aircraft over a 25-year period.

Reduced separation minimums over time

Separation standards refer to the minimum vertical and horizontal distance that aircraft operating in controlled airspace and at airports with an operational control tower must observe. Different separation standards apply to aircraft operating under instrument flight rules (IFR) and visual flight rules (VFR).

Satellite navigation allows an aircraft to navigate to any location using optimum flight paths. It opens up short cuts in the sky and sets aircraft free of the fixed routes required with ground-based navigation aids.

How close can they go-IFR aircraft

In Australia, aircraft flying under IFR in controlled airspace up to flight level (FL)290 must be separated by 1000 ft vertically, unless they are separated horizontally. Above FL290, the vertical separation increases to 2000 ft, except in airspace where reduced vertical separation minimums (RVSM) is applied. When aircraft are separated vertically, horizontal separation can be reduced without compromising safety.

In controlled en route airspace, the horizontal separation standard between aircraft flying at the same altitude is 5 nm. In terminal area airspace, the minimum separation is 3 nm.

Reduction in distance and elevation

Reduction in distance and elevation
Reduction in distance and elevation

Within the confines of an airport control zone, the separation can be as close as practicable as long as the aircraft remain separated. In airspace not monitored by radar or other satellite-based navigation services, aircraft separation is achieved by the use of procedural rules including time and estimated position.

How close can they go-VFR aircraft

Visual separation depends on where aircraft are flying. For example, over Sydney Harbour, sightseeing helicopters use ‘see and avoid’ principles, where pilots maintain their own separation. For general aviation aircraft outside controlled airspace, separation can be as close as 500 ft vertically and horizontally.

Loss of separation occurrences

A loss of separation assurance (LOSA) occurs when there has not been a clear application of a separation standard. This can happen for a number of reasons and does not necessarily mean there has been an infringement of separation.

CNS and human factors

Human factors research seeks the best possible fit between people and the systems in which they operate by applying knowledge of human capabilities and limitations. In aviation, human factors are the social and personal skills, such as communication and decision making, which complement technical skills, and which are important for safe and efficient operations.

The ICAO addressed the importance of human factors considerations in the design of CNS/ATM systems as long ago as 1994 when it looked at the impact of automation and advanced technology on the human operator. ICAO stated that automation must meet the needs and constraints of designers, purchasers and users of the system. Chapter 8 examines GNSS-related human factors issues.

The rules

CASA is moving towards a complete set of operating rules for private operations, and will supplement the rules applicable to corporate/business, air experience, aerial work and air transport operations.

For an updated list of applicable rules see the CASA website at: rules and regulations

Key points

  • CNS brings together satellite and digital communications, performance-based navigation (PBN) and automatic dependent surveillance broadcast (ADS-B). The advantages of modern CNS technologies are that they are global, high bandwidth and accurate.
  • The use of satellite navigation systems where the user performs on-board position determination from satellite information has been adopted as global navigation satellite systems (GNSS).
  • The chief air traffic management benefit of CNS is reduced aircraft separation in IFR. Aircraft can now be operated closer together, with no compromise to safety.
  • Controller-pilot data link communications (CPDLC), used in Australia since 1998, is a means of communication between air traffic control (ATC) and pilot, using a data link instead of voice. Its three main advantages include reduced congestion of voice channels, fewer communication errors and reduced workload for pilots and controllers.
  • Required navigation performance (RNP) equipment must have on-board performance monitoring and alerting systems to provide assurance the system is working properly.
  • ADS-B uses a network of ground stations to ‘listen’ to these aircraft broadcasts and transmit information to ATC twice per second and, if they have ADS-B IN equipment, to the aircraft.


Further reading

  • CASA (2014). SMS for Aviation-A Practical Guide (2nd edition). Canberra.
  • CASA (2016). Communications, navigation, surveillance/air traffic management. Retrieved April 2017.
  • ICAO (2012). Global Navigation Satellite System (GNSS) Manual. Doc 9849 AN/457. Retrieved April 2017.


  • Airservices Australia (2015). Separation standards Retrieved April 2017.
  • CASA (2016). Bringing it all together: CNS/ATM. Flight Safety Australia, January−February. Retrieved April 2017.
  • CASA (2016). CASR Part 91 General operating and flight rules. Retrieved April 2017.
  • CASA (2006). Civil Aviation Advisory Publication 179A-1(1). Navigation using Global Navigation Satellite Systems (GNSS) Canberra.
  • CASA (2015). Meet the experts: CNS/ATM. Flight Safety Australia, January−February. Retrieved April 2017.
  • CASA (2006). Overview. Global Navigation Satellite Systems Canberra.
  • CASA (2017). Rules and regulations. Retrieved January 2017.
  • CASA (2014). Stress and the human factor. Flight Safety Australia, July−August. Retrieved April 2017.
  • ICAO (International Civil Aviation Organization) (1994). Human factors digest no 11. Human Factors in CNS/ATM systems. Circular 249-AN/149. Montreal, Canada.
Published date: 15 June 2021
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