Surveillance Applications Policy – Conformance Monitoring

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Surveillance Applications Policy – Conformance Monitoring

46TH ANNUAL CONFERENCE, Istanbul, Turkey, 16-20 April 2007

WP No. 90

Surveillance Applications Policy – Conformance Monitoring

Presented by TOC


Conformance monitoring is undertaken by controllers as an integral part of their routine responsibilities. This manual conformance monitoring is however becoming relied upon more and more as mitigation to enable, for example, routes to be spaced closer together. In order to reduce the amount of manual monitoring needed, some advanced ATC platforms are introduced automated conformance monitoring. These functions will enable both actual conformance and intended conformance to be monitored for several tasks. For example, conformance monitoring can be applied to routes, where the lateral, longitudinal and vertical profiles can be automatically monitored with any unexpected deviations triggering an alert to the controller.


1.1  The Technical and Operations Committee (TOC) has been tasked to investigate conformance monitoring. The term conformance monitoring can cover a wide range of functions and the purpose of this paper is to outline some of the aspects of conformance monitoring.

1.2  In the next few years, some next generation Air Traffic Management (ATM) systems are due to be introduced into operational service. Examples of such systems are iFACTS in the UK and Vaforit in Germany. These systems include automated conformance monitoring. Hence the need for TOC to investigate conformance monitoring.


2.1  A useful start to the discussion is an examination of what is meant by ‘conformance’. A look in the dictionary reveals that the meaning of the word ‘conformance’ is “the act of conforming”. The verb ‘conform’ has the following interpretations, “to act in accordance or harmony; to comply with rules” and “to act in accord with the prevailing standards, attitudes, practices”. A straight-forward translation of these explanations into the ATC environment is possible.

2.2  Conformance monitoring is not a new concept. It is a core task in ATC operations to determine whether aircraft are adhering to trajectories assigned by ATC instructions. The ability to determine if an aircraft is adhering to an assigned trajectory is important for many reasons. These include:

  • to ensure that tactical instructions;
  • to ensure separation are properly executed; and
  • to ensure strategic conflict detection and resolution schemes are valid.

2.3 Monitoring categories

2.3.1  Conformance monitoring can be divided into two broad categories. These are monitoring of actual performance and intent monitoring. The monitoring of actual performance covers events that have actually happened, e.g. an aircraft cleared to climb FL210 is observed at FL217 having had a level bust. Intent monitoring on the other hand covers events that could happen, e.g. the Selected Flight Level (SFL) for an aircraft is observed via Mode-S downlink to be FL220 when the Cleared Flight Level (CFL) is FL210. In this case the intent of the aircraft is being monitored.

2.3.2  As a result non-conformance criteria are generally based upon positional deviations from assigned trajectories. Another example by which non-conformance can be detected is the penetration of restricted airspace, e.g. the Non Transgression Zone (NTZ) on a Precision Radar Monitored (PRM) approach.

2.4 Manual conformance monitoring

2.4.1 In the majority of the ATC environments that exist today, conformance monitoring is a manual process. In a non-radar environment, manual conformance monitoring may, for example, be achieved by ensuring that aircraft cross fixes within a specified parameter of the estimated time. In a radar environment, controllers use the positional information provided on the radar systems to check for conformance against ATC instructions, such as an assigned heading.

2.5 Automated conformance monitoring

2.5.1  Some modern ATC systems, either in service or under development, have the capability of automated conformance monitoring. The automation of this task alleviates controllers from having to manually undertake the conformance monitoring function.

2.5.2  Automated conformance monitoring can be applied to both actual conformance and intent monitoring.

2.6  There are many uses of conformance monitoring. Some of the applications of conformance monitoring are detailed in the following paragraphs.

2.7  Lateral conformance monitoring

2.7.1  Conformance monitoring can be applied to the lateral profile of an aircraft. This can be applicable for both when an aircraft is on an assigned heading and also when it is on its own navigation along an ATS route or procedure.

2.7.2  In the case of an aircraft on an assigned heading, the radar data processing system is used to monitor conformance of the aircraft’s heading within a defined tolerance from the assigned heading. In an automated system, an alert will be presented to the controller, normally via the Track Data Block (TDB), in the event a deviation exceeding the defined tolerance from the assigned heading is detected. Such functionality is dependent upon the assigned heading being known to the system and this is normally achieved by the controller entering the assigned heading via the system Human Machine Interface (HMI).

2.7.3 Conformance monitoring can also be applied to an aircraft on its own navigation on an ATS route or procedure. The route or procedure, such as a Standard Instrument Departure (SID) or Standard Arrival Route (STAR), is known by the flight data processing system. The trajectory predictor will establish a projected profile for the flight. If this predicted trajectory exceeds a defined tolerance from the intended lateral profile, an alert will be presented to the controller, normally via the TDB. This form of conformance monitoring can be intent monitoring as the trajectory predictor is being used to estimate the aircraft’s position at a future point and the alert can be presented to notify the controller of a potential deviation, i.e. one that is about to occur, as well as an actual monitoring, i.e. presenting an alert once a deviation has occurred. Similar functionality can also be applied to an aircraft on a direct routing between any two points as the trajectory predictor can still determine if the aircraft will deviate by more than a defined parameter from a track between the two points.

2.8 Longitudinal conformance monitoring

2.8.1  There are some current systems that already feature longitudinal conformance monitoring. This system is known as Estimated Time Over Deviation Alert (ETODA). For example, in Australia where there are large areas of procedural airspace, the ATM system has a function that generates an alert if an aircraft deviates outside of specified tolerances.

2.8.2  Variations in aircraft speed, that have an impact on the aircraft’s longitudinal position on a given route or procedure, can also be the subject of automated conformance monitoring.

2.8.3  In the current day environment, controllers can verify aircraft indicated air speed by asking the pilot. However, the availability of downlink parameters through Enhanced Mode S enables the indicated air speed to be fed to the ATM system. The ground system can then undertake a comparison between the assigned speed and the actual speed and ensure conformance on the part of the aircraft. Any deviations could be presented to the controller for action.

2.9 Vertical conformance monitoring

2.9.1  Conformance monitoring can also be used for vertical profiles. Conformance to cleared altitude / flight level can be monitored for both actual deviations and potential deviations through the use of intent data. For instance, SFL is currently downlinked from aircraft and displayed to controllers via the TDB at the London Terminal Control Centre (LTCC). The controllers at LTCC currently undertake a manual comparison of the SFL, as provided from the aircraft using Enhanced Mode S, against the CFL that they have issued. In the future, it is intended that the system will undertake automatic conformance monitoring by checking the SFL against the CFL. Differences between the two would be advised to the controller to alert them about a potential level bust.

2.9.2  Actual deviations of altitude / flight level against cleared altitude / flight level can also be tracked by conformance monitoring. For instance, if the actual Mode C exceeds the altitude / Flight Level by more than the amount permitted by the level occupancy rules, some systems can provide an alert to the controller advising that a level bust situation has occurred. For such functionality to work, it is necessary for controllers to enter the cleared altitude / flight level into the system, which is normally achieved through interaction with the HMI.

2.9.3 Aircraft adherence to assigned vertical speed can also be the subject of conformance monitoring. Most radar data processing systems have a vertical tracker incorporated into them. This tracker can determine an aircraft’s vertical speed. The aircraft’s actual vertical speed can also be obtained through downlinked information via Mode S. If the assigned vertical speed is entered into the system by the controller then the aircraft’s adherence to that restriction can be monitored. A deviation from the assigned vertical speed that exceeds a defined parameter can be alerted to the controller.

2.10 Trajectory Prediction

2.10.1 Ground based trajectory predictors are often used to anticipate aircraft behaviour. Such trajectory prediction is core to many aspects of intent monitoring, especially in the lateral profile. These trajectory predictors are based upon aircraft performance criteria. In the event that aircraft performance is observed by the system to be outside of predetermined tolerances, a deviation from the expected performance can be flagged to the controller through the HMI, such as on the TDB.

2.11 IFATCA Policy

2.11.1  IFATCA already has Policy (WP92, Melbourne 2005) with regard to Route Conformance Monitoring System. The current Policy states:

”A ROUTE CONFORMANCE MONITORING SYSTEM (RCMS) is a function of an Automated ATS System that monitors the position of an aircraft to detect when it deviates from its route. An RCMS is considered to be a Controller Tool.

A ROUTE DEVIATION ALERT (RDA) is an alert provided to a controller to notify that an aircraft’s position is displaced outside the tolerances defined within RCMS. Note: Certain processing may be suspended.

An ESTIMATED TIME OVER DEVIATION ALERT (ETODA) is an alert provided to a controller to notify a controller that a new estimate is outside specified parameters when compared to a previous estimate.”


2.11.2  The above Policy was accepted, based on the following statement in WP92, Melbourne 2005:

“For the purpose of this paper, a route is considered to be a two dimensional path, i.e. lateral and longitudinal.”

Whilst this statement was agreed upon by TOC at the time of the discussions, there had been debate as to whether a route, which includes procedures, should be defined as three dimensional. For instance, SIDs and STARs, although procedures are also considered as routes. These procedures can incorporate a vertical path defined through altitude / level restrictions published at certain waypoints.

2.11.3  It is now evident that the new generation of ATM systems will include the capability to monitor in the vertical plane, as well as the horizontal ones. The statement in the IFATCA Manual therefore needs to be amended to reflect this.

2.11.4  It is anticipated that there will be many new alert types generated by the automated conformance monitoring functions. At present, it is not clear what form these alert types will take. However, it can be anticipated that IFATCA Policy will need to be updated in the next few years as these specific functions start to enter operational service.

2.11.5  IFATCA Policy on Conflict Detection Tools (CDTs) already makes reference to conformance monitoring. The Policy statement reads:

“CDTs can provide conformance monitoring to ensure that aircraft comply with instructions issued to resolve a detected conflict.” (WP 90, Hong Kong, 2004).


This statement is still valid and there is no need to amend it based upon the research undertaken for this paper.

2.11.6  Automated conformance monitoring is a form of an automated system. IFATCA already has Policy in place to protect controllers using automated systems. This Policy reads:

“The legal aspects of an air traffic controller’s responsibilities must be clearly identified when working with automated systems.” (WP74, Port of Spain 1991).


This statement is equally valid for automated conformance monitoring.

2.11.7  In addition, a further Policy statement reads:

“A controller shall not be held liable for any incident or accident resulting from the total or partial failure of any Air Traffic Control System. Guidelines and procedures shall be established in order to prevent incidents occurring from the use of false or misleading information provided to the controller.” (WP 155, Marrakech, 2000).


2.11.8  It would appear therefore that Policy statements already exist to safeguard controllers for when they use automated conformance monitoring.


3.1  Conformance monitoring can be either a manual or an automated process.

3.2  Automated conformance monitoring is an integral part of some next generation ATM systems. The automation of conformance monitoring will be beneficial to controllers as it will assist with workload by removing some of the current tasks involving manual conformance monitoring from the human to the system.

3.3  Conformance monitoring can be applied in many areas, including the lateral, longitudinal and vertical profiles. Routes should be considered as being either a two dimensional path or a three dimensional profile. There is a need to revisit the use of route in Policy statements.

3.4  There are two categories of conformance monitoring. These are the monitoring of actual performance, e.g. Mode C flight level, and intent monitoring based on predictions, e.g. SFL (as provided by Enhanced Mode S).


It is recommended that;

4.1. This paper is accepted as Information Material.

Last Update: September 29, 2020  

April 9, 2020   204   Jean-Francois Lepage    2007    

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