Review Policy on Mode S – Operational Requirements

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Review Policy on Mode S – Operational Requirements

41TH ANNUAL CONFERENCE, Cancun, Mexico, 15-19 April 2002

WP No. 85

Review Policy on Mode S – Operational Requirements

Presented by SC1


1.1  The present IFATCA Policy on Mode S was developed mainly when ICAO started to discuss amending the International Standards to include Mode S in 1987 and 1993. Several WPs have been developed by IFATCA through the years on the subject contributing to the definition of present IFATCA Policy (i.e.: WP 57 – Amsterdam 1982, WP 92 – Ottawa 1994, WP 81 – Jerusalem 1995, WP 93 – Tunis 1996 and WP 91 – Geneva 2001).

1.2  There has been little progress since 1987, however in Europe the shortage of SSR codes has reached critical status and Eurocontrol is focusing on Mode S as the enabler to solve the problem.

1.3  The EATMP Mode S programme is a two-step programme:

  • Elementary Mode S implementation, which purpose is to overcome the Mode A code shortage by removing the necessity to assign a discrete code to every flight and instead use the Mode S aircraft ID to identify aircraft.
  • Enhanced Mode S, which include the possibility of Downlinked Aircraft Parameters (DAP), and Controller Access Parameters (CAP). This should enable a decrease in controller workload by reducing the number of transmissions per flight and thereby increase airspace capacity. Parameters such as heading, speed and selected altitude could be provided to the controller via Mode S datalink and ATC-systems could benefit from parameters such as vertical rate or ground speed.

1.4  The initial implementation of Mode S elementary radar surveillance in Europe is foreseen for March 31, 2003. European States which will undertake the initial co-ordinated implementation of Mode S are:

  • Germany
  • The Netherlands
  • Belgium
  • Switzerland
  • France (eastern portions)

1.5  In Eurocontrol, the development of the operational concept for Mode S surveillance is being undertaken by the Operational Requirements and Data Processing Team (ODT) within both its Mode S FDP Requirements Task Force (MOFR-TF) and the Surveillance Concept and Operational Requirements Task Force (SCORS-TF).

1.6  The purpose of this paper is to review, with particular emphasis to the European area, the issues concerning the introduction of Mode S and the implications for IFATCA policy.


2.1  The provision of ATS has placed increasing reliance on SSR based radar services and applications. Almost all European states comply with the requirement for the carriage and operation of transponders using civil Mode A, military Mode 3 (Mode 3/A) together with Mode C altitude reporting for aircraft flying in controlled airspace. SSR provides data on the position, identification and altitude of the aircraft, permitting individual flights to be tracked in three dimensions.

2.2  Deficiencies associated with SSR Mode 3/A and C include:

  • Interference due to overlapping replies from two or more transponders (garbling).
  • Interference at one interrogator caused by replies from a transponder in response to interrogations from another interrogator (fruiting).
  • Interference at one transponder caused by over-interrogation resulting in transponder dead time and consequent unavailability.
  • Reflections from obstacles or high terrain causing ghost targets to be displayed.
  • Availability of only 4096 codes in Mode 3/A for identification purposes.

2.3  To ensure the efficient handling of increasing traffic volumes in a safe, orderly and expeditious manner, progressive implementation of SSR Mode S, in conjunction with other system improvements, was defined as necessary to satisfy the medium to long term surveillance requirements in Europe.

ICAO has established Mode S as the standard for SSR surveillance due to those performance and functional limitations of the present system, which are becoming increasingly significant. Whilst the initial implementation emphasis was on surveillance, Mode S also established some potential of air/ground datalink.

2.4 Eurocontrol is developing Mode S concept of operations to meet the operational requirements of the users of IFF (Identification of Friend or Foe) / SSR systems. The overall user requirement is to overcome the above specified limitations of existing SSR systems and to:

  • Facilitate the efficient handling of increased traffic volumes in a safe, orderly and expeditious manner.
  • Permit the alleviation of SSR (4096)-code shortage.
  • Allow for a progressive reduction in R/T workload per aircraft.
  • Provide continued access to SSR Mode 3/A and C through sustained backward compatibility.
  • Ensure that displayed radar target accuracy remains at least equal to or superior to that provided by existing SSR monopulse systems.

In order for these requirements to be realised, according to Eurocontrol Strategy, a sufficiently uninterrupted and contiguous area of airspace should be made available in which Mode S can be effectively employed.

2.5  The introduction of Mode S as a means of surveillance will have impact on ATC when it comes to identifying aircraft and correlating tracks. ICAO accepts reception of aircraft ID from a Mode S transponder as a means of identification, but the question how to correlate the tracks within the ATC-system arises. In a Mode A environment, the ATC-system assigns a unique Mode A code to each FPL. When a track with the corresponding Mode A code is recognised correlation takes place. The procedure is different with Mode S. The Mode S 24-bit address may only be used for technical correlation and identification and not operationally. Correlation will happen by comparing the received aircraft ID with FPL ́s known in the ATC-system. It remains unclear how controllers distinct the correlated aircraft from others, when every Mode S aircraft is transmitting the aircraft ID.

2.6  Another issue is mixed mode operations. In order to save Mode A codes it is planned to let Mode S aircraft squawk a conspicuity code (a non discrete SSR-code, such as 7000). That makes no difference to the controller in the Mode S unit, since identification and correlation is based on the aircraft ID. But if there are other non-Mode S units monitoring the same airspace, i.e. military units, all they will see is a bunch of aircraft with the same code. This has led to specific military requirements such as:

  • Permanent, uninterrupted and reliable access to real-time SSR data including Mode S information both for flight safety reasons and in support of the compilation of the Recognised Air Picture (RAP).
  • Maintenance of operational flexibility.
  • Provision for the continued use of military IFF.
  • The ability to employ ground selectable Mode S aircraft addresses.
  • The ability to disable selected Mode S functions on military platforms when demanded by the operational situation.

All these requirements do not contain the practical solutions needed. Do these requirements for example imply that all units monitoring airspace with Mode S traffic should be Mode S equipped or will it be possible to transfer the data?

2.7 The transition to Mode S needs also to be addressed. In Eurocontrol there is ongoing work within the SCORS Task Force to develop a Transition Strategy. A document entitled ‘Transition to Mode S Surveillance’ (SUR.ET2.ST02.3000-GUI-01-00, Edition 0.97, 23/02/99) contains the most recent SCORS view and recommendations on a number of transitional issues. Work to refine this document is ongoing.

The Transition document defines itself as guidance material and states that:

“The guidance material and proposals for the development of operational procedures are applicable to all current and future users of SSR, including military Identification Friend or Foe (IFF) systems operating in 1030/1090 MHz. In particular, they apply to the organisations, people, automated systems, infrastructure and existing rules concerned in the operation of IFF/SSR and the data derived from the ground and airborne elements involved.”

Examining the transitional environment, the following issues are unsolved:

  • Co-existence of Mode 3/A+C and Mode S SSR facilities ATS operations in an environment containing a mix of Mode S and non-Mode S equipped aircraft.
  • Effects concerning the operational implementation of Mode S Enhanced Surveillance in a mixed civil/military Mode S/Mode A/C environment.

2.8  Covering handover procedures in a mixed Mode S / Non-Mode S environment might also be difficult. Especially interesting are the handover procedures (which will need to be developed and verified) for aircraft being transferred from a Mode S ATS Unit to a Mode A/C ATS Unit. A unique Mode A code will be required to support this, and the procedures on assigning this code should be included in the development of handover procedures. This is also applicable to issues of full and partial transponder failure.. Mode A codes will continue to be required to support ‘special’ purposes such as emergency, communications failure, hijack and a range of other reserved codes (some of these are State specific / regional). Therefore, in the foreseeable future, Mode S aircraft will require to continue transmitting Mode A codes, whether under control of a Mode S ATS unit or not.

2.9  When it comes to Enhanced Mode S, some other items need to be addressed especially the impact on ATC procedures and on controller workload. The reception of parameters such as speed, magnetic heading and selected altitude via Mode S datalink might decrease R/T workload but this doesn’t automatically imply that overall workload decreases. And the impact on workload sharing between planner and executive is also unknown.


3.1  IFATCA must continue to follow closely the development of MODE S procedures, implementation and development.

3.2  Due to the fact that the ICAO 24-bit address may only be used for technical correlation and identification and not operationally, the purpose of maintaining a database and receiving the 24bit address does not exist any longer. Correlation in ATC-systems should be done by comparing any transmitted aircraft ID with an existing FPL known to the system.

3.3  The fact that all aircraft in a MODE S environment are transmitting an aircraft ID can cause confusion to the controller. Although ICAO accepts reception of a MODE S aircraft ID as a means of identification, the controller will still need to know whether the aircraft is known by the ATC- system or not. The appearance of aircraft on controller displays must reflect this clearly.

3.4  The use of ”Conspicuity Codes” of a Mode S identified aircraft within the Mode S area must be studied in depth as other implications such as level 3 co-ordination with military units, possibly having no Mode S equipment, must be studied with care.

3.5  The impact on controller workload, and sharing of workload between executive and planner when introducing Enhanced MODE S must be addressed.


It is recommended that IFATCA policy on MODE S be amended as follows:

4.1 That the following sentences are deleted:

Prior to the operational use of Mode S that ATC have in place the necessary database correlating 24-bit address with airframes.

That ATC receives the 24-bit address.

4.2 That the following sentence is added:

The controller HMI shall clearly distinguish between correlated aircraft and aircraft only transmitting aircraft ID.

Last Update: September 29, 2020  

March 14, 2020   836   Jean-Francois Lepage    2002    

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