Recent Developments in Mode S

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Recent Developments in Mode S

35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996

WP No. 93

Recent Developments in Mode S


This paper brings conference up to date with the recent developments in Mode S, with particular emphasis to the core area of Europe.

The European Strategy is the development and installation of three pre-operational ground systems in the UK, France and Germany, followed by an evaluation and verification of the interpretability of these systems.

Documentation has been developed by members of the Strategy Development Task Force representing Belgium, Eurocontrol, France, Germany, Italy, The Netherlands and the UK, including representatives of the Civil/Military Task Force.

Mode S Overview

Mode S relies on the concept of selective addressing in which each aircraft is identified by a unique 24 bit address, hard coded into, and reported by the transponder. Mode S enables ground-air digital communications through two types of communication services, collectively referred to as the Mode S Sub-Network:

a) Switched Virtual Circuit (SVC): interoperable with other A/G data links and enables the Mode S sub-network to be integrated into the Aeronautical Telecommunications Network (ATN);

b) Specific Services: capable of data transfer with lower overheads than SVC. One specific service is the Ground Initiated Comm. B (GICB) protocol. This allows the extraction of real time air derived data by a Mode S interrogator. Other specific services include broadcasts and MSP’s (a form of ‘Datagram’). Unlike SVC , Specific Services are not interoperable with other data links.

Elementary Surveillance – This system only requires Mode S ground stations and Mode S transponders.

NOTE :- The full Mode S sub-network requires Ground and Airborne Data Link Processors.

Enhanced Surveillance – Employs the use of the GICB protocol in order to add airborne information (e.g. to ‘State Radar Vector’ or ‘Aircraft Ident.’) It lies between elementary surveillance and the full Mode S sub-network.

On the Ground – It is proposed to implement the necessary functionality, known as Specific Service Entity (SSE) as part of the preoperational ground station. The remainder of the GDLP may be added subsequently to upgrade to the full Mode S Subnetwork.

On the Aircraft – This is only a sub-set of the ADLP and is strictly necessary to support Enhanced Surveillance. The minimum requirement is a function to extract the required aircraft data from the avionic busses and to load this data into the transponder registers which are referenced by the BDS number. This function would have to be entirely replaced by the full ADLP for upgrading to the full Mode S sub-network. The solution may be to fit the full ADLP at the outset.

Operational Implications – Mode S will facilitate the crossing of international boundaries, introducing a seamless surveillance services throughout the ‘Critical Area’ of high density air traffic areas, permitting improvements in the overall separation between aircraft enroute and coping with the forecast in traffic growth. Mode S will also ease the increasing difficulties concerning SSR Mode 3/A Code shortages and VHF communications overloading.

Legal Requirements – The Council of ICAO approved the inclusion of Mode S operating practices in Amendment No.5 to PANS-RAC (Doc 4444 – RAC / 12). Requirements for the mandatory carriage and operation of Mode S transponders for international flights in the EUR region by 1st January 1999 have been incorporated in ICAO Regional Supplementary Procedures (Doc 7030).

Operational Benefits

Apply to the ATC system, its controlling staff and its users:

1. Improved and unambiguous correlation between radar and flight plan data through the use of aircraft identity;

2. Elimination of synchronous garbling and a reduction in fruit leading to improved discrimination of aircraft in close proximity;

3. Availability of certain aircraft ‘State Vector’ information leading to an improved air situation picture;

4. Reduction in the use of Mode 3A codes;

5. ACAS resolution advisory information;

6. Reduction in RTF occupancy, when utilising datalink;

7. Reduction in controller workload;

8. Refined data for STCA and MSAW algorithms.

Direct benefits to the Airspace User:

1. Improved flight management;

2. Reduction in crew workload;

3. Support for the EATCHIP flexible use of airspace concept, assuming minimum constraints on airspace sharing.


The highest priority task is the development of the Pre-Operational ground stations. Preparation has already begun, in the form of the establishment of the Institutional Arrangements to refer this task to National administrations. The development of the functional specifications for these systems is being progressed by a Task Force (FSTF).

The development of the operational concept from which requirements will be derived for the enhanced surveillance applications and the necessary airborne equipment.

Attention has to be given to the technical concept. This activity includes interrogator coordination, II code allocation etc. The standards and tools to support the implementation programme will be developed within this area.

A validation programme which includes all functions shall precede the operational introduction in the core area. The adaptation and co-ordination of the related (end) systems like ARTAS and the existing ATC systems are a prerequisite to a successful validation.

Air Traffic Management Considerations

The ability of ATM and the system to accommodate mixed Mode S and classical SSR transponder equipped aircraft within the initial area of Mode S operations and adjacent airspace is essential.

In planning for the operational use of the initial implementation of Mode S, consideration needs to be given to the extent of surveillance services possible in the area of Mode S coverage which will be available. The initial implementation will be based on a phased approach commencing with the provision of surveillance in the en-route structure. However, the aim is to provide a Mode S “ ramp to ramp” environment which ensures the benefits are realised throughout the entire flight envelope. Furthermore, it must be assumed that ultimately Mode S will supersede the use of classical SSR in the environment as a whole.

It will therefore, be necessary to determine which airfields are absorbed into the Mode S structure and, for those that are not, operational procedures will have to be developed to ensure full interoperability exists. It will be necessary to establish common procedures for the transfer of control of aircraft crossing into and out of the area of Mode S operations.

Military Operational Considerations

Military participation in Mode S implementation is being achieved through active involvement in EATCHIP, internal discussion between state civil and military authorities and through the involvement of the appropriate groups within NATO and CEAC. Although Mode S was not originally designed to accommodate specific military requirements the need for civil/military interoperability haws been endorsed at ECAC Ministerial level.

The following military issues should be noted for consideration in transition and implementation planning:

  • The military require access to Mode S interrogation data primarily to satisfy flight safety requirements;
  • A variety of options are potentially available to fulfil military Mode S interrogator requirements and the following factors will affect the choice of solution for each given application;
  • Type of platform (ship, airborne, static or mobile land based);
  • Extent of civil Mode S coverage;
  • Area of operation, geographical location, (e.g. “blue water” remote AEW), and size;
  • Acceptability of solution to national and international civil aviation authorities;
  • SSR environment considerations;
  • Ability/desirability for upgrade to full datalink;
  • Practicality of implementation;
  • The capability of military Mode S transponders will be such that the interest on national security will be protected.

Transition Principles

The phased implementation of Mode S will be achieved over a period of time which will necessitate the development of a comprehensive transition plan which is effectively managed throughout the period to maintain operational effectiveness. As Mode S is progressively introduced within the EUR Region the degree of benefit will be substantially increased with a reduction in the need to accommodate non-Mode S equipped elements. However, throughout the entire transition period, planning and execution must be consistent.

The ability of ATM and the system to accommodate mixed mode S and classical SSR transponder equipped aircraft within the initial area of Mode S operations and adjacent airspace is essential. This must be supported by harmonised arrangements for the provision of surveillance services to categories of aircraft which are exempt or partially exempt from Mode S transponder carriage requirements. However, the level of service provided to exempt or partially exempt aircraft may differ from that provided to aircraft which comply fully with Mode S carriage requirements.

Procedures are to be developed to maintain the interoperability between Mode S equipped ground stations and those which are not. This will be particularly important in respect of accommodating airfields within the developing Mode S structure. Furthermore, the procedures must ensure that safe and effective co-ordination can take place between control agencies.

During the transition period a systematic re-allocation and notification of Mode 3/A codes should occur in accordance with a pre-planned Mode S/ORCAM rationalisation scheme.

Transition planning must have the objective of realising the benefits of Mode S for Mode S equipped users whilst taking care to prevent unnecessary increases in workload due to interim arrangements, including Mode 3A code changes.

Operational Concepts and Planning

The operational concepts for the employment of Mode S in Europe and operational planning considerations will be developed by the appropriate groups within the EATCHIP structure and published in a Concept of Operations document.

System Design Issues

The current strategy puts priority on the development of the pre-operational Mode S interrogator, since this work is perceived as lying on the critical path to achieving the operational introduction of Mode S in the required time scale. However, the Mode S system involves a number of elements in addition to the interrogators ( e.g. surveillance co-ordination, datalink functions, datalink applications) and there is a need to carry out an overall system design to integrate these components in a cost effective way to meet user requirements, while at the same time providing as much flexibility as possible to support future expansion of system capability.

In order to prevent mutual interference between Mode S interrogators having overlapping coverage, it is necessary to co-ordinate their operation. In the European region, the most effective way to achieve this is believed to be grouping of interrogators into clusters (typically between three and six interrogators) , in which the necessary co-ordination takes place by means of a surveillance co-ordination protocol over aground network linking the interrogators of a cluster. The interrogators clustered in this way would operate normally with a single II code, whereas overlapping clusters would operate with different II codes. Clustering of interrogators in this way both reduces the need for II codes, and also produces a superior RF environment through reductions in FRUIT.

The functionality and protocols for co-ordinating the operation of clustered Mode S interrogators by means of ground networks are not specified by ICAO, and hence are considered as a local issue. A consolidated protocol will be incorporated into the Functional Specification of the preoperational Interrogator which supports three modes of operation:

(a) Central Mode , based upon a central cluster controller. This is based upon an experimental implementation by UK CAA.

(b) Distributed mode, in which ground stations co-ordinate between themselves to ensure correct cluster operation. This is based upon studies carried out for the French STNA, but has not yet been implemented in experimental ground stations.

(c) Autonomous mode, where each station operates independently from a cluster.

Comparative studies of these various modes will be undertaken, with the aim of establishing the comparative strengths and weaknesses. Their behaviour under conditions of real network operation ( i.e. including delays, message loss, failure etc.) will be determined, and the protocols validated as far as possible using modelling and analytical methods. Guidelines will be formulated on the use of different modes of any constraints on the architecture or performance of the ground surveillance co-ordination needed to support them.

RF Loading Analysis

Previous studies have shown that in a pure civil Mode S environment, FRUIT levels remain very low, and the transponder occupancy high, even under heavy data link use. Consequently, configuration of interrogators into clusters in such an environment would be expected to be driven by II-code allocation and network implementation considerations.

It is likely that military users may also have requirements for use of Mode S surveillance and data link functions, including the possible use of deployable and mobile interrogators. Particular difficulties arise in co-ordinating these interrogators, and various solutions have been considered including the use of a process known as ‘stochastic acquisition’. This technique would allow mobile interrogators to acquire targets without co-ordination with other overlapping interrogators, but at the expense of increased loading on the RF channel.

A number of enhancements are being made to the current modelling of the RF loading and FRUIT parameters of the Mode S environment. This aims to take into account known requirements and intention of military users.

II code allocation

Previous studies have shown that the most efficient allocation of the 15 II codes available is achieved by performing the allocation over a larger region, rather than individual states. An allocation technique known as the Nearest Neighbour Method has been developed which allocates codes based on an interactive method together with highlighting the need for any reductions in protected range ( i.e. lockout range). This method has been demonstrated with the Initial Mode S Siting Plan.

Current work is aimed at further development of the II code allocation method by appealing to graph theory to provide a deterministic approach capable of optimising both the allocation of II codes, as well as protected range reductions. The method also provides for allocation of any necessary spare codes to individual interrogators clusters. The method is being incorporated into a software tool which runs under a PC/Windows environment.

Following completion of the development and testing of the II code allocation tool, the method will be comprehensively validated by applying it to a number of different scenarios for Mode S implementation, including foreseen military intentions.

Data Link Design Issues

The objectives of initial Mode S implementation include the provision of a limited data link capability to support Enhanced Surveillance applications. This would exploit the so called “ Mode S Specific Services” which are a subset of the full Mode S Subnetwork.

It is perceived that the marginal costs associated with including the initial Enhanced Surveillance data link capability in the ground system are low in comparison with those surrounding Mode S implementation as a whole. Furthermore, Enhanced Surveillance applications are seen as having potential benefits to the ATM system, and thus may contribute to the overall cost/benefit case for Mode S implementation.

However, it is foreseen that within the framework of the future European Air Traffic Management System (EATMS), the full Mode S Subnetwork, which provides a reliable point – to – point connection orientated communication service will play a leading role. The initial phases of EATMS are foreseen for implementation in the timescale of 2005. Consequently the initial implementation of Mode S should be planned so as to lay the foundation for transition to the full Mode S Subnetwork supporting EATMS. Ideally, this transition should not involve substantial changes to equipment already implemented ( i.e. add to the system, but avoid changing what is already there).

The Functional Specification for the Pre-Operational interrogator will incorporate certain elements of data link functionality, most notably the Specific Services Entity (SSE) and Frame Processing, with the aim of offering a high degree of flexibility in the implementation of data link both initially and in subsequent upgrades. Nevertheless, there remain a number of design issues at the system level, which while not directly affecting the functionality of the interrogator itself, should be resolved in order to achieve a cost effective implementation of enhanced surveillance and at the same time promote subsequent migration to the full Mode S Subnetwork.

Application Hosting

A range of options exist for the hosting of enhanced surveillance applications:

  • as part radar display processing located in ATC centres;
  • as part of a specific server ( e.g. ARTAS or FDPS);
  • located at the interrogator.

The most appropriate solution for a particular application will depend on the characteristics of the application, the potential users, and the impact on network configuration.

Handling of multiple applications

The initial applications foreseen for enhanced surveillance involve the extraction of data from transponders registers by means of the GICB protocol, and the transfer of the resulting data to the application by means of the surveillance data line. Where multiple applications require data from a given interrogator, the principle requirements are to merge the requests for GICB data from individual applications, and to eliminate duplicated requests to avoid unnecessary overhead on the Mode S link.

In the event that multiple applications were to make use of other Mode S Specific services, in particular MSP.s an additional requirement would arise to provide a means to route downlink data to the appropriate application. Furthermore, co-ordination might also be required between applications using the same MSP channel, to eliminate the possibility that mutual interference between applications addressing the same aircraft could occur.
Alternative means to achieve the necessary handling of multiple applications can be foreseen, which include the possible implementation of a separate co-ordination processor.

Data Link Network Configuration

In the case of initial enhanced surveillance applications, the transfer of data extracted by GICB’s will take place by means of the surveillance data lines themselves. However, applications must be able to specify to the interrogator the GICB data they wish to receive for which an information flow to the interrogator is required.

For more sophisticated applications making use of other Specific services, information flows carrying uplink and downlink data to/from the interrogator would also be required. Alternative ways of supporting these information flows are available, which include point-to-point links and/or wide area networks.


The development and introduction of Mode S continues. The required ICAO documentation has been produced and the regional approach to development will see initial benefits in due course.

Last Update: September 28, 2020  

December 25, 2019   631   Jean-Francois Lepage    1996    

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