Development of Policy on Mode S

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Development of Policy on Mode S

33RD ANNUAL CONFERENCE, Ottawa, Canada, 18-22 April 1994

WP No. 92

Development of Policy on Mode S


Secondary Surveillance Radar (SSR) is currently the principle radar surveillance tool for Air Traffic control. Plans for the development and upgrading of SSR services are based on the use of Mode S (Selective SSR). This paper describes the background to SSR systems; the operational need for an upgrade; implementation development stages; and operational benefits of Mode S (with the emphasis on surveillance). Finally the paper presents conclusions and proposes statements of policy.


Current SSR ground stations interrogate airborne transponders by broadcasting alternate coded requests for aircraft identity and flight level. All airborne transponders within the radar beam respond to these interrogations with their altimeter flight level reading and pre-set aircraft identity codes. Processing this information provides controllers with aircraft identity (code and/or callsign), range , bearing and altitude; all of which are automatically updated at the SSR antenna scan rate. the aircraft identity and position permit lateral and vertical separation and provide data for the update of flight plans and the operation of Short Term Conflict Alert (STCA).

International Standards for SSR were originally agreed in the 1960’s. The continued increase in air traffic movements and the increased reliance on SSR has led to a situation where the equipment performance limitations are becoming unacceptable. The introduction of monopulse SSR in the early 1980’s did significantly improve system positional accuracy, but a number of other performance and functionality limitations remain (Section 4 of this paper). These limitations will become operationally significant if the growth of traffic is in line with predictions.

In recognition of these limitations, ICAO amended the International Standards to include Mode S in 1987. The Mode S sub-network (i.e. surveillance plus full data-link capability) standards are due to be completed in November 1993.

In 1990 Eurocontrol published the EASIE ( Enhanced Air Traffic management and Mode S Implementation in Europe) and EATCHIP ( European ATC Harmonisation & Integration Programme) plans for adoption of Mode S. Mode S is thus the standard for SSR surveillance and also establishes the potential of SSR as an air-ground datalink. Whilst the provision of Mode S datalink is an important long term capability; the emphasis of this paper is on the surveillance aspect of Mode S.

The key characteristic of Mode S as a surveillance tool is that each aircraft is uniquely addressable, i.e. each aircraft has the equivalent of its “own telephone number”. This feature leads to operational benefits over “ordinary” SSR (Section 6 of this paper).

International aspects of Mode S

ECAC Strategy

The ECAC En-Route Strategy has an implementation objective that monopulse radar’s are to be upgraded to Mode S capability in the timescale 1995-2000. Another objective is that Mode S datalink is to be operational in central European area from 1998, however most of the ECAC States have no commitment as yet to implement Mode S . the development of the technology and the subsequent system is being pursued by a core group of States within the EASIE Programme, under the management of Eurocontrol.

The EASIE Programme

The EASIE Programme is in two phases: Design and Realisation. The Design phase commenced in 1991 and is scheduled to be completed in 1996. The Realisation Phase follows the Design phase for a five year period.

In October 1992, the EASIE Management Board decided upon the reorientation of the EASIE Programme to focus on the engineering design aspects of Mode S and the Aeronautical Telecommunications Network (ATN). a necessary step is to assess the implications of the introduction of Mode S so that surveillance and initial datalink applications may be accomplished at the earliest opportunity. The EASIE Board endorsed the need for co-ordination between the technical and operational elements of the ATM system and agreed that priority be given to the early implementation of Mode S, taking account of the civil/military SSR environment.

The EASIE Programme is sponsoring a computer model of the Mode S system wherein parameters such as antenna rotation rate, beamwidth, target number, message type, etc. can be varied. Use of the model will demonstrate the Modes S system capacity and which elements can create bottlenecks.

Regulations for mandating the carriage and operation of Mode S transponders in the EUR Region by 1 January 1999 are currently being processed within ICAO. Thus, ground stations should be capable of conducting initial Mode S services by that date.

The main emphasis in development during the EASIE Design phase has been in the Mode S datalink capability. The capacity of an enhanced Mode S system, with the facility to provide additional services and benefits, should become a major contributor to the future management of air traffic. Nevertheless, the surveillance function is a necessary component in the implementation of Mode S. Surveillance characteristics of the system as a whole; multiple interrogators will be required in European airspace. Some of the Mode S Specification applications currently being advocated are apparently focused on encouraging GAT to accept Mode S.


In 1988, the FAA committed themselves to Mode S by ordering 137 new Mode S ground surveillance stations. Furthermore, they mandated that all commercial aircraft of over 30 seats flying in US airspace should be fully equipped to operate with TCAS II by the end of 1993. In effect, this mandates the carriage of Mode S transponders in USA airspace because they are an essential component part of the complete TCAS equipment fit.

There are implications from this for European airspace. For example, the proportion of UK aircraft equipped for Mode S operation operating in controlled airspace has reached 40% and is increasing. However, this figure could be misleading in terms of operational benefit in respect of enhanced Mode S surveillance. Some types of installation require modification to transmit flight identification (callsign) which correlates to its flight plan. Policy on TCAS within Europe is less well defined and is the subject of international discussion.

USA Mode S work has concentrated more on the features of an individual station as opposed to the surveillance characteristics of the system as a whole; multiple interrogators will be required in European airspace. Some of the Mode S Specification applications currently being advocated are apparently focused on encouraging GAT to accept Mode S.

The Operational Need For an Upgrade of The Current SSR System

The design of the current generation of UK monopulse SSR systems is twelve years old. Compared with earlier SSR systems, the monopulse technique has greatly improved track accuracy and has proved very successful. However, the predicted increase in traffic density will increase the effects of a number of limitations in the current SSR system which will in turn limit ATC capacity. These limitations are:

  • Mode A Code shortage and duplication;
  • Over -Interrogation;
  • Erroneous Codes;
  • SSR Reflections;
  • Inadequate altitude Data.

These factors must be relate to the increased reliance by ATC on availability of high quality surveillance data.

Code Shortage

Current SSR systems are limited to the use of 4096 identity codes world -wide, assigned according to the appropriate ICAO Regional Air Navigation Plan. the European plan was developed by the Eurocontrol Radar Application Specialist Panel (RASP) on behalf of the European Air Navigation Planning Group (EANPG), in 1981. The allocation scheme described in the European Plan divides the European Region into five areas, each comprising a number of States, with the aim of ensuring that individual SSR codes are only allocated to one aircraft within an area at any one time.

World-wide traffic growth since 1981 has led to a situation where the demand for discrete codes exceeds the available supply in some areas. Continuous review and adjustment to the code allocation has so far avoided any serious impact on traffic handling capacity; however there is evidence of SSR codes being re-used more frequently than the ICAO rules for re-issue permit. This has led to incidents in Western Europe in which two flights have been using the same SSR code, causing misidentification of aircraft with adverse safety effects.

Recent agreements within the Eurocontrol States for the sharing of designated European Region Allocated SSR codes in Eastern & Southern Europe will further reduce the availability of individual codes in all region areas.


The increased demand for SSR coverage, notably at airports, results in high transponder interrogation rates. This can result in transponder occupancy(i.e. Where SSR cannot interrogate because another is already doing so); desensitisation ( to prevent a transponder overheating, an electronic limiter is placed on the interrogations and replies it can handle, permitting only the stronger signals to be picked up).

Erroneous Codes

SSR code data can be subject to corruption in a number of ways. Firstly, the routine processes of allocation, selection and change of codes are all subject to potential error. Secondly, when two aircraft are close, the replies from two transponders can overlap and cause mutual interference. this results in incorrect decoding and in lost target reports (synchronous garble), which can be potentially hazardous in operations with aircraft separated only by height.

SSR Reflections

Although monopulse SSR has been largely successful in removing reflections, with increasing traffic the inability to discriminate reliable between real and reflected SSR returns is again becoming a problem.

Inadequate Altitude Data

Mode C altitude reports are defined to a minimum increment of 100ft. these relative coarse increments present significant problems in the context of a nominal 1,000ft vertical separation, particularly for the predictive processes associated with STCA.

Implementation of Mode S

Development Stages

Four distinct stages have been identified in the evolution of Mode S to its full potential.

Enhanced Surveillance with Mode S

The term elementary surveillance has been adopted for those surveillance services able to operate with the level of airborne equipment specified in the ICAO transponder carriage requirements i.e. without the installation of an ADLP (Airborne Data Link Processor) on an aircraft.

This level of service comprises:

  • Direct acquisition of the Aircraft Identification i.e. the callsign of the aircraft;
  • Selective interrogation leading to improved efficiency in the 1030/1090 MHz. frequency band;
  • A reduction in Mode 3/A code usage, leading to the possibility of elimination in a total Mode S environment;
  • Aircraft Pressure Altitude reports at 25ft. increments (subject to aircraft capability and the ability of the ground systems to handle it );
  • Airborne status indication;
  • Recognition and potential acquisition of ACAS Resolution Advisory Information i.e. display of RA information to the controller;
  • A means of observing aircraft on the ground.

Operational benefits are discussed in greater detail in section 6 of this paper.

Higher Level Enhanced Surveillance

Within the EASIE participating States of Eurocontrol, the focus of development is on the implementation of Mode S Enhanced Surveillance for the initial stage. the term ” Enhanced Surveillance” is used within the EASIE Programme to indicate the use of certain datalink protocols (e.g. Ground initiated Comm – B, (GICB)) to extract aircraft state vector (true airspeed, heading etc. ) or other information (e.g. future intentions) for application in surveillance related processing. Such data could be included in the broadcast surveillance reports to ATCC’s. This early use of datalink would employ a small subset of the functionality described in the Mode S subnetwork SARPS and would thus require some extra complementary facilities both on the ground and in the aircraft.

Where the ground installation is concerned, it is possible to implement Enhanced Surveillance by the addition of a small mount of processing within the interrogator itself, without necessarily incorporating a full ground Datalink Processor (GDLP). A contract has been let to two companies, within the Design Phase of EASIE, to produce GDLP’s for verification trials in the UK. The GDLP will be the interface between the Mode S sensor and the ground network through which the users communicate with air traffic.

The Enhanced Airborne function can be provided through the early installation of an ADLP or alternatively by the incorporation of extra facilities into an upgraded transponder. The decisions on which path to follow must be guided by industry/operational consultation.

Stand Alone Mode S Subnetwork

This stage would require the operation of the GDLP and support equipment, together with the ADLP in the aircraft, and is a possible configuration for the possible progressive introduction of Mode S.

Mode S Subnetwork Incorporated Into The Aeronautical Telecommunications Network

The full development of the Mode S system envisaged within the EASIE programme which includes the ATN router and ATN applications in addition to the criteria in 5.3.

Operational Benefits of Mode S

Improved Code Allocation

Mode S will provide more than 16 million aircraft addresses thus eliminating the need for ATC to allocate reusable aircraft Mode A identity codes. From a system integrity point of view, the system’s unique aircraft identity avoids the chance of error associated with multiple allocation of codes. the RT and code selection workload would be reduced for both pilots and controllers and safety would be improved by avoiding erroneous selections.

Selective Interrogation

Mode S selectively addresses individual aircraft thereby significantly reducing the overall interrogation rate to each aircraft.

Reduction of SSR Reflections

Mode S will eliminate any code duplication and will thus significantly improve the ability to discriminate against reflections which usually show as code duplications.

Increased Altitude Data

Mode S Surveillance data will provide altitude encoding to 25ft. increments, where the aircraft has the capability. Refinement of the STCA algorithms will enable a reduction in the nuisance alert rates, already a cause of considerable R&D effort to improve the value of STCA to controllers and better prediction of genuine potential conflicts should be achieved.

Data Link Capabilities

This paper emphasises the surveillance aspects of Mode S . However, the ability to provide datalink facilities is inherent in the Mode S system. These datalink facilities are defined at two levels; Mode S; Mode S specific services and full datalink.

Prior to the introduction of full datalink, Mode S specific services will be available to provide data exchange between the aircraft and ground to convey digital information. This has the potential to reduce RT communications significantly and to provide aircraft intention data from Flight Management Computer Systems, which would improve conflict alert systems, which in turn has the potential to give an overall improvement in ground based conflict alert systems. The Defence Research Agency in the UK has used an experimental Mode S datalink to provide aircraft data to ground based STCA algorithms. work to date has been confined to the vertical where benefits have been demonstrated towards both reducing the frequency of ‘nuisance alerts’ and increasing the warning time of real conflicts. (DRA CAD 4 WP 92/12/03 Data Link Enhanced STCA).

The services initially proposed for standardisation and implementation be ICAO are:

  • Aircraft Intention, to contain selected altitude , selected altitude rate and auto pilot status;
  • Ground Reference State Vectors , to contain ground speed, track angle etc.;
  • Aircraft Referenced State Vector, to contain true airspeed , heading etc.;
  • Waypoint Identification , to contain information on active waypoints and desired track etc.;
  • Waypoint Information, to contain bearing to waypoint, time to go and distance to go;
  • Position Report to contain latitude, longitude etc., this information can be used to eliminate reflections due to enhanced processing techniques;
  • Meteorological Report, to contain wind velocity and static air temperature etc.

Development of full datalink will enable more extensive digital communication between the air and ground i.e. air ground integration. Mode S will be an enabling technology for the fully integrated system because of its ability to provide an independent reference for precise aircraft location and identification.

The operational implementation of Mode S must ensure that its primary role of ATC surveillance is safeguarded. ATC surveillance must have priority over other interrogation signals and the saturation of transponders must not result in the loss of data on the ground.

Data link applications are a subject of research in Europe , within the PHARE ( Programme for Harmonised ATM Research in Eurocontrol ) and EASIE Programmes; USA; CANADA; JAPAN.

Implementation Planning

Area Of Operation

The area of Mode S operation, described in the ECAC En-Route strategy as the ‘Core Area’. needs to be precisely delineated in order to determine the required volume of coverage. The dimensions of the airspace are critically important. It is essential that the area has solid and continuous Mode S coverage. Ultimately, only within such an area can Mode S procedures be utilised without the need to continually alternate between these and existing Mode 3/A/C procedures. The establishment of a nucleus area would reduce the need for the assignment of individual four-digit Mode 3/A codes for flights operating totally within it, thus permitting the permanent re-allotment of Mode 3/A codes to non Mode S areas. A mixture of Mode 3/A/C/S procedures would limit the foreseen benefits.

The EASIE engineering plan is for early Mode S surveillance services to be available in Europe over an area roughly encompassing London, Amsterdam, Paris and Northern Germany. This area is considered to be the minimum core size to provide operational benefits from the initial implementation of Mode S.

It will be necessary to improve and maintain code management in the transition period while mixed code SSR operations continue until an overall Mode S environment is established. However, for Mode S to provide operational benefit it must operate over a large enough area for the elimination of Mode 3/A codes to be effective. It is essential that the area of intended Mode S operation is agreed and published at the earliest opportunity with definition of the boundaries especially important. Within Europe this will be co-ordinated by Eurocontrol through EASIE.

Transponder Occupancy

In the longer term , the advent of Mode S installations should permit overall transponder occupancy to decrease even when data link applications are in use. Any interrogation of a transponder incurs a finite occupancy time both for the interrogation and for the reply. That time will be influenced not only by Mode S but also by the Mode 3/A/C interrogations from the ground (during the transition to Mode S) and the operation of ACAS/TCAS equipment. There is concern that use of Mode S for enhanced surveillance and datalink operations will increase transponder occupancy times. Studies (by Eurocontrol and Lincoln Laborities, USA) in a solely Mode S environment have not realised these concerns. Initial results , in a simulated mixed mode environment at Eurocontrol have also not realised such concerns but this work continues.

Distribution of II Codes

In the Mode S system, each interrogator is allocated an Interrogator Identifier (II) code. The number of available II codes has been fixed at 15. Even if it was considered feasible to increase this number it could not be by a large number and, consequently, it severely limits the number of separate interrogators with overlapping cover that can be permitted. The requirements of military interrogators may further decrease the number of II codes available for ATC use. The employment of clustering technique (7.4) will go some way to easing the problem but careful management of the II codes will be necessary to ensure an optimised and equitable distribution.


With only 15 II codes available for the identification of ground interrogator systems, it is necessary to provide some mechanism for all interrogators while remaining within the limitations imposed. The engineering solution to this problem is to ” Cluster” the ground interrogators together where they have overlapping coverage. the cluster concept allows the use of one II code to be shared among several interrogators. By this means, the cluster is treated as an homogenous entity, within which the address of an aircraft, acquired by one of the linked interrogators, is passed to all the other interrogators in the cluster. At the same time the aircraft transponder is’ locked out’ from replying to further ‘all call’ interrogations from the cluster. As it passes into the coverage of another interrogator in the cluster the address and position of the aircraft are already known and it does not need to respond to a further ‘all call’ reply interrogation thus reducing the ‘fruit caused by excessive ‘all call’ reply transmissions.

With lock out there is the facility for the system to deny the display of certain aircraft. however, this problem is recognised and is being addressed.

Aircraft Equipment

Three steps in the development of the aircraft equipment are envisaged, which may be linked to the development stages in section 5.

The first step is seen as the minimum for the operation of Elementary Surveillance and consists of the installation of Level 2 transponders with the ability to transmit the aircraft identity. Some aircraft are already equipped to this standard; others may require modification of the installation.

The second step extends the capability to incorporate support for Enhanced Surveillance which utilises the GICB protocols given in the Mode S Subnetwork SARPS.

The third step would be the installation of a transponder with full support for the Mode S subnetwork. It is to this standard of operation that the engineering development is geared and is seen as the preferred next step for aircraft. equipment even though all the facilities may not be used at the outset. Experimental ADLP’s have already been tested with good results and at lower costs than anticipated.

A fourth step being contemplated comprises aircraft fitted with airborne ATN router to compliment the full functionality of Mode S capability.

Ground Capability

Work on the technical aspects of the interrogator and communications infrastructure is proceeding within the EASIE Programme. At the same time further work is necessary to ensure that data available for Mode S transponders can be exploited for enhanced surveillance through the current surveillance data processing and display systems.

E- Scan Antenna Technology

E (Electronic – Scan antenna technology is already being studied under the EASIE programme. The use of E-Scan will provide the following advantages:

  • Improved surveillance performance;
  • Improved reliability;
  • Reduced datalink delays;
  • Greater flexibility in control of the radar beam pointing direction.

The flexibility offered with this type of system will significantly improve data link capacity of the Mode S data link by controlled, variable dwell time on the aircraft.

To Conclude

Mode S has been established by ICAO 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 is on surveillance, Mode S also establishes the potential of SSR as an air/ground datalink.

Within Europe, the EATCHIP and EASIE Programmes plan for the adoption of Mode S, monopulse SSR is to be upgraded to Mode S capability 1995-2000 and Mode S datalink is to be operational in the core area from 1998.

Requirements for the mandatory carriage and operation of Mode S transponders in the EUR region by 1st January 1999 are being processed by ICAO. Therefore , there will need to be the capability in the ground stations to conduct initial Mode S services by that date.

The FAA have mandated that all commercial aircraft of over 30 seats flying in US airspace be fully equipped to operate with ACAS by the end of 1993. This effectively mandates the carriage of Mode S transponders in USA airspace. this has implications for European operations.

The implementation development stages of Mode S are seen as being:

  • Elementary Surveillance;
  • Enhanced Surveillance – use of some data link protocols to extract aircraft state vector and other information;
  • Mode S Subnetwork;
  • Mode S Subnetwork Incorporated into the ATN.

Initial benefits post 1999 and pre-data link applications availability will be:

  • Improved code allocation – reduction in 4096 code usage;
  • Selective Interrogation – allowing for immediate identification of the address of the aircraft;
  • Reduction of SSR reflections;
  • Increased resolution of altitude data – aiding STCA;
  • Knowledge of airborne status.

In addition to that already described, the implementation of Mode S will require the following future work concerned with operational aspects.

To consider transponder occupancy to avoid saturation of transponders. To define the initial area for implementation and progressive expansion; an area of solid and continuous Mode S coverage, obviating the need for alternating between Mode S and Modes 3/A/C procedures. Code management in the transition period with mixed mode operations To review appropriate ICAO procedures and identification of any necessary changes to cater for Mode S surveillance operations To define a transition plan detailing arrangements and operational procedures required following the initial implementation of Mode S. Such a plan will be essential to achieve operational benefits from Mode S.

Application of Mode S datalink should be a subject for continued R&D work on the benefits to ATC operations, to be carried out in Europe by PHARE and other Programmes.

IFATCA must ensure close co-operation with Programmes such as EASIE, particularly in the areas of defining an Operational Requirement for Mode S coverage and in transition planning.

It is recommended that:

The operational implementation of Mode S must ensure that its primary role of ATC surveillance is safeguarded.

ATC surveillance must have priority over other interrogation signals and the saturation of transponders must not result in the loss of data on the ground.

The Mode S air to ground datalink should be used exclusively for ATM purposes and principally for routine tasks in an automated mode without direct controller intervention.

Mode S operations, for High Level Enhanced Surveillance and the Mode S subnetwork and any applications should be evaluated in a realistic operational environment with the use of operational controllers.

Last Update: September 20, 2020  

December 23, 2019   182   Jean-Francois Lepage    1994    

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