Call Sign Similarity (CSS) Service

The use of similar call signs by aircraft operating in the same area, on the same radio frequency gives rise to potential and actual flight safety incidents.

The Call Sign Similarity (CSS) project, has been initiated by EUROCONTROL with the aim of establishing pan-European CSS solutions centred on a coordinated service operated by EUROCONTROL’s Network Manager Operations Centre (NMOC). The aim is to reduce the level of operational call sign confusion events and therefore improve levels of safety.

The issue - Call Sign Confusion

The use of similar call signs by aircraft is referred to as “call sign similarity”. The danger of an aircraft taking and acting on a clearance intended for another due to call sign confusion is a common occurrence. This was identified in the European action plan for air ground communication safety as a significant contributor to air-ground communication issues.

Further studies have indicated that the best defence against call sign confusion consists of eliminating, or reducing the chance of having two (or more) aircraft with similar call signs on the same radio frequency at the same time.


How can the Network Manager help aircraft operators?

A stepped-approach is being taken:

The Network Manager has put in place a Service Level 0 that includes a Call Sign Similarity Management Cell (CSMC). The CSMC provides the aircraft operators with the similarity rules to be applied in the process of the detection and resolution of conflicts.

March 2012 saw the formal launch of Call Sign Similarity Service (CSS) Service Level 1 operations, i.e. the detection and deconfliction of call sign similarities with a single aircraft operator’s schedule using the EUROCONTROL Call Sign Similarity Tool (CSS Tool).

If Service Level 1 is successful, Service Level 2 will provide de-confliction between multiple aircraft operator schedules.


The main benefits associated with the implementation of the CSS Project are: 

  • Elimination over 80% of the CSS incidences
  • Improvement of safety
  • Economies of scale

Monitoring Call Sign Similarity Service and Tool Performance

The CSMC has established procedures with participating aircraft operators to monitor the operational effectiveness of the CSS Service/Tool. The anticipated improvements in safety performance will be monitored by the EUROCONTROL Voluntary ATM Incident Report (EVAIR) scheme.

Call Sign Similarity User Group

To drive the development of the CSS Service and its support tool (CSS Tool), a users group has been established. The target membership of this users group is:

  • ANSPs – Operational Flight Planning staff and Safety Managers
  • Airspace Users – Flight Schedulers, Operations staff and Safety Managers
  • Airports – Operations staff
  • Military Stakeholders as appropriate
  • IATA and ICAO Paris office are also observers to the CSS users Group.

Register to become a member of the Call Sign Similarity Users Group.


Service Levels

Service Level 0 - the Call Sign Management Cell

The Call Sign Management Cell (CSMC) is based at EUROCONTROL, Brussels. Its role is primarily for assisting the airlines in the de-confliction of Similarities within the Aircraft Operator’s own schedule.

The main objectives include active participation in raising awareness about call sign similarity reduction processes. In particular this involves providing the following:

  • Publication of the CSS Rules to be applied in the process of detection and resolution of conflicts.
  • Support to aircraft operators in implementing call sign similarity reduction processes.
  • Acting as a facilitator between aircraft operators.
  • Feedback and monitoring of the results of the implementation and of the use of the de-confliction strategies during the season.

Call sign Similarity (CSS) Service - Service Level 1

Service Level 1 corresponds to the de-confliction or prevention of similar call signs within a single aircraft operator’s schedule with the support of a dedicated software called CSS Tool. This has to be performed by the aircraft operator prior to the start of the summer and winter season.

The role of the CSMC is to facilitate and monitor the whole process.

The CSS Tool is available to aircraft operators via a special NOP Portal url and simplifies the various detection and de-confliction operations needed. Specific procedures to use this application have been developed.

CSS Service Level 1 began with a pilot phase during winter 2011/12 and was formally launched in March 2012. It is now available to all aircraft operators in the ECAC zone.

Call Sign Similarity (CSS) Service - Service Level 2

Service Level 2 corresponds to the detection and de-confliction of similar call signs between different aircraft operators’ schedules performed by the CSMC to which the aircraft operators are willing to send their schedule data. This will be performed in advance of the summer and winter season and could only be performed by a centralised unit such as the CSMC.

Service Level 2 would need the support of an improved version of the CSS Tool.

Service Level 2 is dependent of the success of Service Level 1 operations.


Call Sign Similarity Rules

The ultimate aim of the call sign similarity service (CSS) is to reduce the number of similarity events (similar call signs on the same frequency at the same time) thereby reducing the likelihood of call sign confusion occurring and therefore improve levels of safety.

One key element in reducing the number of call sign similarities (CSS) is the publication of an agreed set of Call Sign Similarity Rules i.e. agreed definitions of what constitutes a ‘similarity’. These Rules have been identified by analysis of safety reports concerning call sign confusion and by the Call Sign Similarity User Group (CSS UG), which includes representatives from aircraft operators (AOs), air navigation service providers (ANSPs) and other aviation organisations’ (e.g. ICAO and IATA).

In addition to the rules for detection of similarities, the CSS UG, in considering how to modify a call sign to create a solution, have determined some basic principles that describe the acceptable formats for the creation or correction of call signs.

Aircraft operators actively engaged or wishing to engage in prevention of call sign similarity without making use of the on-line tool provided by EUROCONTROL are strongly encouraged to apply, in so far as possible, the rules described here.

Single AO Similarity Rules

The single AO Similarity Rules describe a set of rules for detecting similarities between two (or more) call signs belonging to the same AO i.e. having the same AO designator and usually present within the same schedule file. These Rules are usually applicable to all flights in all airspaces.

Local Similarity Rules

Specific constraints on the composition of call signs can come from ANSPs. For example, an ANSP may not be able to accept mixed alphanumeric call signs or may wish to avoid confusion with other data included within a radar label. By definition such a rule or constraint is applicable to a specific airspace and is therefore referred to as a ‘local’ rule which applies only to flights passing through that airspace. These Rules may, depending upon the specific circumstances, differ in terms of their importance.

Single Call Sign Similarity Rules

A similarity event does not have to involve two call signs. A call sign can be similar to different entities commonly broadcast within an ATC clearance e.g. runway designator, a flight level or VHF frequency.

Depending upon the detail these type of similarity rules may be general i.e. applicable to all flights, or some may be applied locally. While these type of similarities can certainly generate confusion they are generally considered less severe as they will normally not cause one aircraft to take the clearance intended for another.

They are also more predictable for pilot and/or controller as the entities concerned are always present. In addition, as there is usually only one aircraft involved, recovery is easier.

The Call Sign Management Cell (CSMC) will provide support and assistance where needed.

Reules diagram

Rules diagram