CNS/ATM technical support

Air Traffic Management today is unthinkable without special facilities. The requirements for high reliability, redundancy, accuracy, continuity of operation, safety nets and monitoring, recording and play-back as well as regular maintenance and testing, that ICAO (International Civil Aviation Organization – ) sets up for aviation technical systems, are very stringent. High quality technical support is based on the appropriate maintenance system as well as the qualification and competence of the personnel.

CNS/ATM department ensures the operability of the equipment and its compliance with the requirements above.

According to the operating principles all aviation systems can be divided into the so-called conventional, that are currently in use, and CNS/ATM (Communication, Navigation, Surveillance and Air Traffic Management) technology based (the ICAO term is FANS – Future Air Navigation Services) – their extensive implementation has been recently started. The main difference between the systems is that CNS/ATM Systems are based on data exchange networks and satellites. CNS/ATM systems potential is really huge, whereas the conventional equipment cannot cope with the forecasted traffic growth nor ensure the required capacity and accuracy of the air traffic management system.

Two major programmes  are established for developing and implementing new technologies: SESAR in Europe and NexGen in the USA. The European Commission regulation 552/2004 shall be followed for ensuring the systems interoperability. Implementing Rules prescribe the implementation of several technologies in order to ensure simultaneous and harmonised activities in the European region.



  • Radio communication equipment is used for exchanging instructions, requests, readbacks and other information between pilots and air traffic controllers.

Radio transmitter and receiver are geographically separated from each other – the purpose is to achieve better signal quality. Some radio relay stations installed in different locations enable radiocommunications in Estonian airspace even at lower altitudes; all of them are integrated into a common network. Every ATS unit has a separate, designated to it, radio frequency. The importance of radio communications prompted installation of so-called last resort independent radio stations for each frequency – we are not restricted to only one radio communication network any longer.

The existing voice communication will be replaced by CPDLC – Controller-Pilot Data Link Communication, but still it remains as back-up.

  • Ground communication system includes all telephone lines and other direct channels required for cooperation between different ATS units and Centres.  The information is exchanged through voice communication. EANS has regular connection with ATC Centres in Finland, Latvia, Russia and Sweden. TDM-based analogue equipment has been replaced by digital; presently, there is a trend in the world to replace point-to-point channels with corporate IP-based data exchange networks. Companies invest more and in purchasing VoIP technology based systems for ground communications both between air traffic control units and radio stations.
  • Air-Ground data communication is part of the CNS/ATM technology, where the information exchange applications (including instructions) use either VDL (VHF Data Link) or Mode S extended squittertechnology accordingly. The idea is to use the limited resources (actually limited radio frequencies are) in a more efficient way, to handle the continuously increasing amount of information as well as to update the information more frequently and to reduce misinterpretation of the presented information by users.
  • Ground-ground data exchange is one of the CNS/ATM technology key elements: in the future it will replace voice communications between different ATS units. Even nowadays WANs and LANs are used in different spheres – systems’ remote control, surveillance data exchange, exchange of flight messages – AFTN/AMHS, aeronautical information exchange, although their formats differ and their networks are not integrated into a common system. In Europe there is a EROCONTROL ANSP led initiative – PENS – Pan European Network Service – it is supposed to meet the above mentioned requirements for information exchange.


For navigating in the air, aircraft in flights need to know accurate information regarding their position and the fixed points to head for. The information required for navigating can be conditionally divided into:direction and distance from a known point. This information is transmitted from the ground-based network of navigation aids, which, in case of Estonia, consists of direction beacons: NDBs (Non-directional Radio Beacons), VORs (VHF Omnidirectional Radio Beacons) and ground based distance measuring equipments – DMEs. The airborne equipment receives information from several facilities simultaneously, processes it and determines the aircraft position in the air.

But already now satellite-based navigation GNSS elements are widely used, GPS system being most popular. GNSS procedures are based on such systems like ABAS (Aircraft Based Augmenting System),SBAS (Satellite Based Augmenting System – WAAS, EGNOS), GBAS (Ground Based Augmenting System).

NDBs and VORs are slowly vanishing – they are used at aerodromes only.


Surveillance facilities are used for creating the real airspace picture. The major subgroups are the radars and emerging ADS. During last years MLAT solutions started to spread all over the world and became third subgroup.

  • According to the working principle the radars can be divided into primary and secondary ones. Primary radars (PSR – Primary Surveillance Radar) display echoes reflected by all objects, including aircraft and some meteorological phenomena. In secondary radars a set of radio interrogations is transmitted by a ground interrogator and answers received by the airborne transponder, allowing the controller to see the identification of the aircraft, its altitude and groundspeed. Secondary radars are most popular nowadays in civil aviation. Estonia has two of them: in Tallinn and in the western part of the country.

Mode S (Mode Select) radar is an advanced secondary radar. This radar allows to interrogate transponders selectively (also different requests to different transponders), the data exchange is digital and it is considerably more accurate compared to previous ones; through the transponder it is possible to transmit readings of different airborne systems.

  • With the help of ADS (Automatic Dependent Surveillance) the airborne equipment sends GPS data to the ATC Centre through data-base exchange channels. These data are processed and added to the general picture of the traffic situation. This is the technology of the future, but some solutions are already implemented in several countries.
  • MLAT (Multilateration) is a modern equivalent of old hyperbolic triangulation systems. In MLAT 4 or more receivers, positioned at known sites, receive transponder signals.

The time of signal reception is fixed with high accuracy by a centralprocessor which derives computations on the precise 3D position of aircraft (transponder).

Multilateration system can operate passively (when transponder signals from other radars are used) or actively (MLAT set has its own interrogator, which is less sophisticated than radar). Similarly to radars and ADS system, the data are collected and processed from different aircraft (MLAT messages) composing the real-time picture of the traffic situation.

MLAT systems are divided into aerodrome (ground movements), approach (landing and departing traffic) and wide area (covering entire national airspace) systems.

WAM – Wide Area MLAT – will soon replace the so-called air route surveillance radars, ensuring at the same time extensive surveillance coverage.


The data coming from all technical facilities are collected, verified, afterwards processed and finally presented to air traffic controllers. The most crucial of these systems are:

  • VCS (Voice Communication System) – integrates all incoming and outgoing voice lines, radio and emergency channels and provides user-friendly manipulation for controllers.
  • FDPS (FDPS – Flight Data Processing System), which processes all the data on the traffic situation.
  • SDPS (Surveillance Data Processing System) – it is used for surveillance data processing and creating the real picture of the traffic situation.
  • ODS (ODS – Operational Display System) – together with Safety Nets and Flight Monitoring software helps to reduce potential conflicts.
  • Traffic Flow planning tools (MAESTRO, FMP positions, etc.)
  • Common Clock System
  • Recording and Replay System


The following technical solutions are supporting the provision of air navigation services:

  • Additional information system for ATCOs;
  • Pre flight data preparation,
  • Transmission of information about routes and meteorological conditions;
  • Electronic aeronautical information preparation, exchange and dissemination (eAIP);
  • Post-flight data processing, billing system
  • Testing and training environment