Study Mode S Transponders

Study Mode S Transponders

52ND ANNUAL CONFERENCE, Bali, Indonesia, 24-28 April 2013

WP No. 91

Study Mode S Transponders

Presented by TOC

Summary

Due to limitations with legacy Secondary Surveillance Radar Systems (Mode A/C), SSR Mode Select (Mode S) was developed and introduced. The purpose of this paper is to identify possible ways forward of getting the most out this new system, and also to enhance and improve the operational use of Mode S (including more safety).

In this WP it is also attempted to pin-point the operational and technical difficulties that the new system is posing, as well as to make a review of the current IFATCA policies in regard to Mode-S.

Introduction

1.1  Conventional SSR systems employ technical principles that were developed and implemented fifty years ago. Since that time the ATC operational environment and the traffic density have changed quite significantly.

1.2  Due to limitations of the surveillance technique used, such as for instance the maximum number of targets that can be detected and handled simultaneously, RF pollution, garbling, interference, lack of SSR codes, frequent change of SSR-codes, including the inability to cope with future traffic loads – this all is putting more and more demands on the ATM-System. These changes and system improvements are becoming inevitable – in fact they are very much welcome.

1.3  In the mid 1990s Eurocontrol, but also ICAO have started to roll-out the Mode S technology. In Europe it was Eurocontrol, as a coordination body and the initial work consisted of developing the specifications and the standards for the new system, including the necessary technical and safety validations. Then, the operational roll- out of Mode S was started with the aim to replace slowly but surely the old SSR Mode A/C technology.

1.4  Nowadays – at least in Europe – controllers are working in a so-called mixed mode, where both technologies do still co-exist.

Discussion

2.1 Basic principles

2.1.1  A Mode-S Transponder is not only a system transmitting surveillance data to the outside world (to airborne flights suitably equipped, as well as to ground ATC), but this transponder is also a fully-developed communication data- link system and so able to send-out and receive short data-link messages. It possesses the standard functions of a normal SSR-transponder (Mode A and Mode-C), but is “augmented” with additional data sets and communications functions (“registers”) that are specific to Mode S.

2.1.2  Many Mode-S transponders nowadays do possess some additional features and “registers”, such as ADS-B (to squit-out). They are capable of broadcasting automatically data based on GPS/GNSS).

2.1.3  Mode S radar sensors (radar heads) have two different methods of transponder interrogation, they are:

  • the “All-Call”
  • the “Selective Call”

2.1.3.1  The all-call interrogations of a ground radar-head are transmitted regularly and periodically by a Mode-S radar antenna at a steady rate in a similar way to the conventional SSR radar. This is to make sure that any Mode S transponder in range (which is not “locked out’ – as already known and under a surveillance contract) will reply to an all-call interrogation of a Mode-S interrogator, making its presence known to the radar head.

2.1.3.2  The aim of this process is to acquire targets, which were previously not detected. Once a transponder is known to the interrogator and its track has been established, it can be ‘locked out’ (meaning it will not anymore reacting to “All Call” inquiries of that same radar head). The target is considered to be under a surveillance contract and will be passing the required surveillance data in a selective and distinct way to the radar site (on a “contract”).

2.1.3.3  While traditional SSR-stations will interrogate all stations (Transponders) in range, Mode S radar employ selective and directly-addressed interrogations with aircraft with which they have concluded so-called surveillance contracts. This is done with help of the unique 24-bit aircraft address that is permitting the selective handling, interrogating and addressing of the surveillance messages – including the interrogations.

2.1.4  Aircraft Identification feature (ARCID) permitting flight crews to set the Aircraft Identification ID, according either to flight plan and/or according to the ship/tail number of the airframe. This feature permits the correlation of a given Flight Plan to a distinct flight – or Mode S radar blip. The Aircraft Identification transmission must correspond with the aircraft identification specified in item 7 of the ICAO flight plan, or, when no flight plan has been filed, with the aircraft registration.

2.1.5  The frequency band used is exactly the same as the one for the normal legacy SSR Mode A/C transmissions (1030/1090 kHz) – for the interrogations and replies. Selective interrogation not only improves the quality and the integrity (robustness) of the detection, but as well the altitude reporting of the flights (25 ft increments, instead of 100 feet for Mode A/C).

2.1.6  ICAO classifies the different Mode-S transponders into different Levels (1-5), with different capabilities. It identifies the communication protocol capabilities of each transponder.

2.1.7  The functional components of Mode-S are the up-link format (UF) and the down-link format (DF). UF is a specific interrogation format originating from SSR (from the ground). It can also come from another airframes asking specific, addressable information about a given aircraft. DF is the reply from the airframe.

2.1.8  UF 0, 4, 5, 11 and 16 are part of basic surveillance, and basic surveillance messages are comprised of the airframe address, the parity bits and a 56-bit data word, also known as the “short” interrogations and replies. UF/DF 0 is mainly used by the ACAS-System (TCAS).

2.1.9  The Transponder maintains avionics data in 256 different 56 bit-wide Binary Data Store (BDS) Registers that can be loaded with information and read-out by the ground system. Sometimes BDS Registers are also referred to as GICB, or “Ground Initiated Comm B transactions”. They are specified in the ICAO document “Manual on Mode S Specific Services” (Doc 9688).

2.1.10  Each register contains the data payload of a particular Mode S reply or extended squitter. If registers have not been updated within a fixed period; they are cleared by the transponder.

2.1.11  Squitter is an unsolicited reply-format – an automatic (and periodical) transmission that is totally independent from interrogation. Transmission is done at least once – or more – per second. It has its origins in the Distance-Measuring Equipment (DME). These “unsolicited replies”, or squitters, are used to permit that TCAS II- equipped airframes are able to acquire the discrete address of the squittering airframe (being in this case a possible intruder). This enables the TCAS II system to acquire, and then track the airframe – using all the various mode S formats UF/DF0 and UF/DF16.

2.1.12  Mode-S Extended Squitter, or ADS-B is also called DF 17, as this is the BDS- register used for the transmission of the ADS-B data (GNSS-based data) by the Mode-S transponder.

2.1.13  This ADS-B transmission is normally done by a modified Mode-S Transponder that is enabled to handle as well ADS-B registers and data. As DF 17 is a squitter (or broadcast), it does not need an interrogation to function and will report its information regardless of any ground station, or airframe asking for it – triggering it.

2.1.14  In Europe Mode S is implemented in two stages:

  • Mode S Elementary Surveillance (ELS);
  • Mode S Enhanced Surveillance (EHS).

2.1.15  The purpose of Elementary Mode S implementation 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. Aircraft compliant with Mode S ELS provide the following operational benefits:

  • Unambiguous aircraft identification.
  • Improved integrity of surveillance data. Selective interrogation and the superior resolution ability of Mode S over existing SSR and MSSR installations will:
    • Eliminate synchronous garble.
    • Resolve the effects of over interrogation.
    • Simplify aircraft identification in the case of radar reflections.
    • Improve air situation picture and tracking.
    • Alleviate Mode 3/A code shortage.
    • Improve Safety Nets (e.g. STCA).
    • Increase target capacity.

2.1.16  Enhanced Mode S, includes the benefits of ELS together with the possibility of Downlinked Aircraft Parameters (DAPs). 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 data-link and ATC-systems could benefit from parameters such as vertical rate or ground speed. In addition to the benefits of Mode S ELS, which are identified above, aircraft compliant with Mode S EHS also provide the following operational benefits:

  • Improved situation awareness. (A clearer air situation picture, enhanced tracking and access to pertinent information direct from the aircraft. It enables the controller to benefit from quicker and more accurate recognition of airborne events).
  • Progressive reduction of R/T workload per flight. (There is scope for R/T usage between controller and individual flight under service to be reduced following the progressive introduction of Mode S Enhanced Surveillance).
  • Safety enhancement. (Access by controllers to a/c intent DAPs, such as selected altitude enable cross-checking of climb/descent instructions and helps the early identification of potential level bust incidents).

2.2 Operational use of Mode-S down-linked aircraft parameters

2.2.1 Mode S has the capability of sending out and so to down-link many airborne (cockpit) aircraft parameters. This is often referred to as Mode-S DAPS (Down-Link Aircraft Parameters). Ground ATC-Systems – those being fed with Mode-S radars – are able to pick up these automatic data sets and make them available at the relevant Controllers Working Positions (CWPs).

2.2.2  More and more ATCOs are working with modern ATM-System where DAPs-data is shown to them via the radar – or surveillance screens. There is an urgent need that clear and unambiguous rules and procedures are to be laid down on how to use these data in safe and efficient manner in every days work. ICAO and all the main Safety Regulators in general have remained very silent about the operational use of these down-linked airborne data and DAPs by the ATM-system.

2.2.3  The Enhanced Surveillance functionality will need to ensure, through Ground Initiated Comm-B (GICB) protocols defined by ICAO in Annex 10 Volume III, Part 1 (by Amendment 77), that the extraction and the transmission of information contained in the following standardized transponder registers is always assured. The data transmitted for Mode S Enhanced Surveillance will need to have the following minimum data:

  • BDS 6,0 (Heading and Speed Report), Magnetic heading, Indicated airspeed, Mach Number, Vertical rate (Barometric rate of climb/descend or baro-inertial)
  • BDS 5,0 (Track and Turn Report) Roll angle, Track angle rate (or True Airspeed, True track angle, Ground speed
  • BDS 4,0 (Selected Vertical Intention) “Selected Altitude”

2.2.4 The use of BDS 4,0, the “Selected Altitude”, is possibly the easiest issue that has to be resolved in the whole equation regarding the safe operational use of Mode-S DAPs.

2.2.4.1  BDS 4,0 “Selected Altitude” is in several documents and working papers regarding Mode-S referred to as “Intention data”. This is fully in-line with the list above, where Register 4,0 is described as Selected vertical intention. But this is only part of the automatically transmitted aircraft-DAPs to the outside world.

2.2.4.2  The ATM Procedures Development Sub-Group (APDSG) of Eurocontrol makes in its documents very firm statements by saying that the data (or information) of BDS 4,0 (the Selected Altitude of a controlled flight) cannot and shall not be used by ATC to provide “separation”. They also state that this indication is in no-way a substitute for a voice read-back of a Cleared Flight Level (or CFL) of a controlled flight.

2.2.4.3  “Selected Altitude” is considered a “Tool” – able to work as additional safety feature (or as a Safety Net). For instance by alerting in time when the CFL and the SEL ALT on the flight deck don’t match. APDSG and ICAO insist in saying that it is necessary that the limitations of selected altitude/level must be properly understood by the air traffic controllers, as:

a) data presented indicates intention, only. Hence, it cannot be used for separation;

b) there may be situations where, despite the flight crew complying with ATC clearances and instructions, the displayed selected level varies from the cleared level:

i) along SID/STARs with vertical restrictions pilots may select the final cleared level, and utilize the aircraft flight management system to achieve the vertical constraints.

ii)  on final approach, where at given moments pilots may pre-select the missed approach point (MAP) altitude. The ATC system should normally inhibit the display of the selected altitude in the track label before this occurs to avoid any confusion. If the system detects that the aircraft has broken off the approach and is climbing then the selected altitude will automatically be displayed again providing the controller with the aircraft’s revised intention.

iii)  when the aircraft is being flown manually, in particular in response to vertical avoiding action maneuvers.

iv)  Where there is an incorrect barometric pressure setting in the FCU.

2.2.4.4  The position adopted by APDSG and the ICAO Paris-office is for sure “safe” and so very “reasonable”. We should insist that also other data-sets, beside SEL ALT, send-out by Mode-S transponders are considered for operational use.

2.2.4.5  One of the questions is whether the use of a “Tool” – e.g. to alarm automatically the ATCOs of a control sector in case the SEL ALT of a controlled flight does not match the CFL of an IFR-under control – if this in itself is not already an “operational use”?

2.2.5  ICAO and the other major aviation safety regulators should initiate a discussion and adopt fully applicable and safe procedures/rules about the operational use of all the other down-linked airborne DAPs becoming available by Mode-S and/or ADS-B systems as soon as possible.

2.2.6  The European office of ICAO in Paris writes the following:

“Modern surveillance technologies provide additional information to ATS systems for improving the predictability, and so to enhance the tracking of flights within the area of coverage. Some of these parameters provide indication on the intentions of the flight crew (the downlink of the selections made by the pilot in the MCP/FMC)”. Modern ATM- Systems are currently upgraded and so enabled to receive these airborne data-sets from the flights being under control.”

2.2.7  Many other DAPs are send-out and received by ATC-unit today. Many of them include as well actual or actualized airborne data, so fully current and actualized operational airborne data. This is in particular true for BDS Register 6,0 of Mode S, that contains many current and actualized ATC-relevant aircraft and flight-data, they are:

  • Magnetic Heading
  • Indicated Airspeed (IAS)
  • Mach Number
  • Vertical Rate (Rate of Climb or Rate of Descent)

2.2.8  For this reason it is very important and urgent that operational ATCOs, get the procedures saying what they are allowed to do with these data-sets – and what they are not allowed to do.

2.2.9  It cannot be denied that some of the data sets sent-out (or squitted by ADS-B) have some safety issues/safety concerns attached to them. So it’s of utmost urgency that procedures and rules are determined and published by relevant Safety Regulators and by ICAO.

2.2.10  In a recently published NPA (Notice of Proposed Amendment) in Europe – a text widely distributed to all stakeholders which are discussing new and future Standards or Rules applicable for the European airspace, the following is written:

The transmission of incorrect information or invalid data to be processed by ANSP operational ground surveillance systems can lead to incorrect aircraft identification resulting in an increased workload for ATC personnel with a potential safety risk to the aircraft and other nearby aircraft. To mitigate these risks, the transmission of all parameters should be adequately validated during the certification process. It is possible for some ELS installations to transmit EHS parameters, therefore, the certification requirements for ELS installations have been extended to ensure that those EHS parameters transmitted are also validated.

2.2.11  It is very encouraging that the European Safety Regulators do recognize the safety- issues that the transmission of erroneous airborne data can pose – not only for ADS-B – but in particular also for Mode-S DAPs. So, there is a clear requirement for stringent certification standards, including processes to monitor and identify as quickly as possible airframes transmitting erroneous or wrong data. Only well- established and stringent safety processes in this matter will permit a widespread, extensive and most of all safe use of these DAPs-data sets by the ground ATM- Systems and the ATCOs.

2.2.12  One of the currently received data sets by ground-systems concern is the current (actual) Magnetic Heading of a controlled IFR-flight. A heading can be quite relevant for separation purposes (for a “radar-controller” – and for a controller using other surveillance, such as ADS-B).

2.2.12.1 So far the approved ICAO-procedure is to inquire the crew about the current Heading – for instance by Voice (or CPDLC) – and then to instruct the crew either to continue on the present heading (until further notice) and/or to turn the flight on new and different heading in order to provide a horizontal (radar) separation.

2.2.12.2Using modern ATM-Systems and DAPs, current heading-information can be retrieved quite easily using Mode S – (or ADS-B data) which is automatically down- linked to the ATM-system.

2.2.12.3 From operational point of view the question is whether the reading of a heading via Mode-S data on a surveillance display is as “good” – and so fully acceptable and “safe” – as the one obtained verbally from the crew?

2.2.13  These questions need to obtain as soon as possible clear and unambiguous answers from ICAO. So far, all the European ANSPs, which have made these airborne DAPS available to their controllers, have taken a very restrictive and safe approach to this question by saying in the procedures that this DAPs-data shall not be used “operationally”.

2.2.14  This is an unsatisfactory solution, as many ANSPs and Safety Regulators have by now understood that these DAPs data-sets can be used operationally, and that it can help ATCOs to handle traffic not just safely (through Safety Nets), but also efficiently (e.g. by reducing the R/T load).

2.2.15  A new issue, that is hindering the correct identification of Mode-S aircraft via the ARCID-feature, was detected. It concerns a wrong implementation of the Mode S Comm B broadcast in one type (brand) of transponder.

2.2.15.1 There is a need to clarify and strengthen the Mode S Comm – B broadcast protocol – this is expected to correct the air-ground interoperability (as some flights are still not correctly reporting their Aircraft Identification).

2.2.15.2 When a wrong Aircraft Identification is detected by ATC, controllers often ask pilots to correct the Aircraft Identification (to have an error-free correlation with the ground flight plan and/or the flight Plan Data Processing System (FDPS)).

2.2.15.3 Eurocontrol and some air Navigation Service Providers have now detected that there can be two different reasons for having a wrong Aircraft identification shown on radar- or other surveillance displays:

  • No or wrong input of the Aircraft Identification in the aircraft avionics (normally a human-factor issue)
  • Technical avionics failure

2.2.15.4 ICAO decided to work on the Comm-B broadcast protocol in order to review the second point of failure-which is exclusively of technical nature.


2.3 ICAO

In ICAO DOC 9924 AN/474 Aeronautical Surveillance Manual we can find SSR Mode S overview:

SSR Mode S overview

5.3.3.1 Mode S allows selective addressing of aircraft through the use of a 24-bit aircraft address that uniquely identifies each aircraft and has a two-way data link between the ground station and aircraft for the exchange of information. It was designed to be backward compatible with and supports all functions of Mode A/C. Appendix F shows details of Mode S and Mode A/C compatibility.

5.3.3.2 The Mode S data link allows additional information such as airspeed, heading, ground speed, track angle, track angle rate vertical rate and roll angle to be obtained from the aircraft. Such aircraft derived data may be used to improve the tracking of the aircraft and to alleviate the need for radio calls for obtaining the information. Other information that may be obtained via the Mode S data link includes the aircraft ID, the altitude selected by the flight crew on the aircraft’s mode control panel and an ACAS RA report. Appendix I shows details of Mode S specific services.

5.3.3.3 In parts of Europe, there have been mandates issued requiring all aircraft that fly into designated airspace to be equipped with a Mode S transponder capable of supplying aircraft derived data. These requirements are known as ELS and EHS; mandates have been introduced in Chapter 2.

5.3.3.4 The Mode S data link also allows information to be uplinked or sent from the radar to aircraft. This uplink is used in a function known as TIS where an aircraft is provided, upon request, information on the aircraft detected by the radar to be in its vicinity. Appendices G, H and J contain detailed descriptions of Mode S protocols and implementation considerations.


2.4 IFATCA policy

2.4.1 The following IFATCA policy is related to this subject:

AAS 1.3 MODE S DEVELOPMENT

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 data-link

IFATCA Policy is:

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

The Mode S air to ground data-link 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 sub- network and any applications, should be evaluated in a realistic operational environment with the use of operational controllers.

The use of ”Conspicuity Codes” of a Mode S identified aircraft within the Mode S area must ensure that the safe operations of all other non-mode S units in that airspace, e.g. military operations, are not compromised.

The correlation of a departing flight using Mode S must be as safe and easy as the procedures used nowadays with SSR.

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

*ID is the callsign of the flight as filed in the ICAO flight plan e.g. AZA611.

ATC systems must validate the Flight ID transmitted by an aircraft’s Mode S transponder and indicate to the controller any discrepancy with the ICAO aircraft identification in the flight plan.

Any broadcast of incorrect ATM data should be corrected or if unable then:

  • switched off, or
  • marked as invalid.

ATC surveillance systems must be able to process all data, regardless of the volume or type, necessary to provide ATC Services safely.

 


2.5 Review of the current IFACTA policies in regards to Mode-S

2.5.1 As some time has passed since the IFATCA policy has been adopted by Conference – also as a lot of progress was make, including new developments – it’s high time that the policies of the Federation are reviewed and checked for relevance.

2.5.2

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 data- link.

This policy statement is still fully correct and of actuality, so no changes are proposed.

2.5.3

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

Operational experience has shown that Mode-S is much more than just an ATC- surveillance system. The short data-link message capabilities, including the making available of DAPS should not be underestimated and a lot of experience was gained in the past years in regards to the operational use of Mode-S.

It became apparent that the crucial point for the safe use of Mode-S is not the priority (and relevance) for the data-transmission, but more the protection of the Mode-S spectrum (1030 and 1090 MHz) from over-loads, as well as from excessive data interrogations.

Currently, extensive international efforts are underway to tackle all these burning issues, including the burning spectrum protection issues – and it looks like that indicates that solutions and interesting technical improvements are being found.

This IFATCA-policy is therefore superfluous and should be deleted.

2.5.4

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

From an ATCO perspective the statement is certainly correct and fully understandable, but it should not be forgotten that Mode S data-link is not only used by ground ATM-systems, but also by other “parties”, i.e. other airborne parties (other aircraft in the vicinity).

Bearing this in mind, this policy statement seems to be way too restrictive – even close to unnecessary. This, as furthermore it must acknowledged that these data- broadcasts are all occurring in an automatic manner – as squitters – and done fully independently from any interrogations.

The only interrogation load that is put on the Mode-S system (and the Transponders that are essential to it) are either coming from ground ATC-radars, or from other airborne systems (such as TCAS, which is resorting to active interrogation).

Data-Link is also used by these systems, and many of them are very essential (such as ACAS/TCAS Coordinations), and so they have a valid requirement to continue to communicate and interrogate the Mode-S System, including the use of short data-link messages.

So, this policy statement lost all its relevance and therefore should be proposed for deletion.

2.5.5

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

The main thrust of Mode S implementation and fine-tuning is more situated in the safe and efficient use of the data, which is coming out of the Mode-S system.

Mode S system, in particular the use of this system is now fully understood and in “operations”. It can’t be denied that some fine-tuning of the Mode-S system is still required and must continue to occur.

This particular policy statement, which is formulated in a very restrictive manner is not needed anymore, and so is proposed for deletion.

2.5.6

The use of ”Conspicuity Codes” of a Mode S identified aircraft within the Mode S area must ensure that the safe operations of all other non-mode S units in those airspace e.g. military operations are not compromised.

The use of a ”Conspicuity Code” for a Mode-S equipped aircraft is the trigger-event for the ATM-units (being civil or military) that the identification of this particular aircraft – or radar blip – is achieved via the Aircraft Identification (Flight ID) feature (or ADS-B).

One of the most important pre-requisite for a safe and efficient coordination between civil – and military ATM-units working together in the same area or region, is that the aircraft flying in this area/region can be identified unambiguously and also distinctively.

The use of ”Conspicuity Codes” (e.g. A1000 in Europe), assorted with a radar identification via the Aircraft ID feature of Mode-S (or ADS-B) shall only be used if all the stakeholders have means to see and identify all targets and flights evolving in a that given airspace.

This policy is still fully valid and of actuality. No changes are proposed.

2.5.7

The controller HMI shall clearly distinguish correlated aircraft and aircraft only transmitting aircraft ID*, *ID is the callsign of the flight as filed in the ICAO flight plan e.g. AZA611.

The intent of this statement is clearly to make a very clear distinction between flights and radar blips which are identified (and correlated) via the ARCID feature (via Mode-S or ADS-B), or using other methods (such as Mode A, where a given distinct number is allocated, and the identification/correlation is normally done by the FDPS (Flight Plan Data Processing System).

Furthermore, this policy statement needs to be seen in conjunction with the following ICAO statements, discussed under 2.5.8 – where other issues are mentioned:

Whenever it is observed on the situation display that the aircraft identification transmitted by a Mode S-equipped aircraft is different from that expected from the aircraft, the pilot shall be requested to confirm and, if necessary, re-enter the correct aircraft identification.

If, following confirmation by the pilot that the correct aircraft identification has been set on the Mode S identification feature, the discrepancy continues to exist, the following actions shall be taken by the controller:

a) Inform the pilot of the persistent discrepancy;

b) Where possible, correct the label showing the aircraft identification on the situation display; and

c) Notify the erroneous aircraft identification transmitted by the aircraft to the next control position and any other interested unit using Mode S for identification purposes.

2.5.8  Whereas the above policy is forming part of ICAO Document 4444 (PANS-ATM), it must be questioned if this procedure is really safe?

This, as with the development of ADS-B OUT and the installing of CDTI-displays in aircraft cockpits more and more aircraft are giving to their crews the possibility to see and identify surrounding traffic (on a cockpit traffic display).

ATSAW or Airborne Traffic Situational Awareness is part of ASAS (Airborne Separation Assurance System (ASAS), and it makes use of a Cockpit Display of Traffic Information (CDTI) that is fed by data coming either from ADS-B, or traffic information broadcast from the ground (TIS-B), and it is allowing that the crew is able to perform some tasks related to separation or spacing, and so thereby reducing the workload of the controller. It also provides pilots with a real-time picture of the surrounding traffic during all phases of flight.

The perceived risk is when pilots see an ARCID on their CDTI-screen which is different from the label shown to ATC (as ATC-label is either based on Mode A – correlated via an FDPS-system, or other using means of making a correlation (for instance in Australia the 24-bit address is used for this) and the well-identified risk/hazard is that this discrepancy can go unnoticed from the controllers. Worse even that it can lead to confusion, and in the end going as far as facilitating miss- identifications of targets and traffic seen visually (by the air crew).

For this reasons the two IFATCA policy statements of 2.5.6 and 2.5.7 do remain very valid.

It must even be questioned whether the label-renaming procedures, described by ICAO in the PANS-ATM is really a safe and a desirable safe procedure?

In particular bearing in mind that future ASAS- and ATSAW – applications are due to be introduced and some are even nowadays already in use (e.g. ITP, or visual separation on approach).

In the same context – attempt to mitigate the risk of confusion and miss- identifications, the IFATCA policy statement requiring that the labeling of the blips on the radar-display / the HMI must be distinct – making it very clear whether the aircraft ID is used as source of the “correlation” is a very valid and important requirement.

These two IFATCA policies must be not only be retained – but remain unchanged.

2.5.9

The correlation of a departing flight using Mode S must be as safe and easy as the procedures used nowadays with SSR.

This policy statement is not needed anymore. Mode S is currently a proven technology implemented in many control centres, being correlation achieved as with SSR.

2.5.10

ATC systems must validate the Flight ID transmitted by an aircraft’s Mode S transponder and indicate to the controller any discrepancy with the ICAO aircraft identification in the flight plan.

Again, a very valid and important statement. The radar screen HMI and the ATM- System must be tuned in such a way that this is done automatically and in the end be “seen” by the controller team working the CWP (Controller Working Position).

This policy statement must be retained.

2.5.11

Any broadcast of incorrect ATM data should be corrected or if unable then:

  • switched off, or
  • marked as invalid.

This is also a very important policy statement, which must be retained.

2.5.12

ATC surveillance systems must be able to process all data, regardless of the volume or type, necessary to provide ATC Services safely.

This is also a valid policy statement that must remain untouched.

Conclusions

3.1  Depending on system abilities and aircraft equipage we may get following information and display it on a controller working position:

  • Selected altitude
  • Ground speed
  • IAS/Mach no.
  • Roll, track angle
  • Magnetic heading
  • Vertical rate (barometrical rate of climb/descent)

3.2  Down-linked airborne data (DAPs) sets can be shown at CWPs (Controller Working Positions) of advanced ATM-Systems. Clear and stringent procedures should be published by ICAO to regulate how these down-linked data-sets shall be used operationally for the provision of safe and efficient ATS.

3.3  “Local procedures” and ANSP-actions (via procedures) are not good and sufficient to solve this problem. The need to find harmonized and fully global solutions are of urgency. This common approach has also the advantage to avoid Mixed-operations and/or disperate and different implementation – and would also permit an international monitoring process (chasing the airframes and operators which are not complying with the standards and/or sending out systematically wrong or erroneous data-sets).

3.4  Due to many improvements in recent years and to the fact that more and more ATCOs are using mode S derived data on a daily basis operationally, ICAO and the other major aviation safety regulators should initiate discussion and adopt fully applicable and safe procedures about the operational use of all the other down- linked airborne DAPs becoming available by Mode-S and/or ADS-B systems as soon as possible.

3.5  For operational ATCOs, is of utmost importance and urgent to get the procedures saying what they are allowed to do and what not.

3.6  It is convenient for IFATCA to review in depth its policies in Mode S.

Recommendations

It is recommended that;

4.1 IFATCA Policy:

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

is deleted.

4.2 IFATCA policy:

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

is deleted.

4.3 IFATCA policy:

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

is deleted.

4.4 IFATCA policy:

The correlation of a departing flight using Mode S must be as safe and easy as the procedures used nowadays with SSR.

is deleted.

4.5 It is recommended that the IFATCA Executive Board brings the issue of lack of clear procedures to regulate the operational use of down-linked airborne data from Mode-S to the attention of ICAO.

References

Doc 9924, Aeronautical Surveillance Manual.

Doc 4444, Air Traffic Management.

Annex 10, Volumes 3 & 4.

Report of the Aeronautical Surveillance Panel (ASP 12), Montreal, April 23 – 27, 2012, Christoph Gilgen.

Mode S Elementary Surveillance (ELS) Operations Manual, Eurocontrol.

Concept of operations Mode S in Europe, Eurocontrol.

Eurocontrol Mode S programme http://rfdesign.com/military_defense_electronics/radio_understanding_mode_technology/

WP from previous IFATCA conferences.

Last Update: September 30, 2020  

April 22, 2020   269   Jean-Francois Lepage    2013    

Comments are closed.


  • Search Knowledgebase