Radar Monitoring in the En-route Phase

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Radar Monitoring in the En-route Phase

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

WP No. 102

Radar Monitoring in the En-route Phase


At the 32nd. IFATCA Conference the subject of Radar Monitoring was put on the SC 1 Work Programme. the subject was divided into two aspects – the En Route phase and the Approach phase. This paper addresses the En Route aspects.

The principle of radar monitoring has long been established and is defined by ICAO as ‘ the use of radar for the purpose of providing aircraft with information and advice relative to significant deviations from nominal flight path’. (ICAO Doc 4444 Part 1). It has not been possible to establish what is classed as a significant deviation, or what parameters the term is based on.

ICAO provides for radar monitoring as one of the functions in the provision of an air traffic control service , (ICAO Doc 4444 part X 2.1.1.b).

A radar controller with only one aircraft under his (or her ) control has no real need of radar solely to perform the ATC task in controlled airspace. The aircraft can be cleared from point A to point C via point B, given climb or descent clearance as necessary and handed over to the next frequency without the need to ‘see’ the aircraft. However, even in this situation, it can be argued that the radar controller has a responsibility to use the equipment provided to its best advantage, by watching the progress of this single aircraft in case it deviates from its cleared route, or an unknown aircraft is detected which could conflict with the aircraft being controlled.

When dealing with more than one aircraft – and the possibility of conflicts – radar is generally considered as the means of reducing the separation required in order to expedite the traffic flow, and a primary means of resolving conflicts. The controller will, primary task permitting , monitor the aircraft under his control even when conflicts have been resolved, and other means of separation are being employed. However, it is not unusual for a controller to have so many aircraft under his control that maintaining separation between them becomes the sole concern. Since some sectors cover hundreds, if not thousands of square kilometres, and potential conflicts are occurring in all part of the sector , the controllers full attention is focused on keeping all the aircraft separated by the required margin. Even under these circumstances, a controller should consider radar monitoring as a secondary function, devoting attention to that aspect if time allows.

Modern avionics have extremely accurate navigational capabilities. As routes are planned closer together to take advantage of this (and put more aircraft into the same volume of airspace) the ability to detect deviations on current En Route radar displays , which often cover a large geographical area, will decrease. This becomes important when considering closely spaced routes, since ‘ time to conflict point’ becomes shorter.

In the En Route phase of flight aircraft are generally travelling at or near their cruising speeds ( compared with the approach phase where they are more usually at slower speeds). This speed factor adds to the difficulty of detecting deviations in time to prevent losses of separation.

As SSR becomes more widespread around the world, with its Mode C altitude reporting capability, controllers will be expected to monitor the height keeping aspects as well as the lateral flight path of aircraft under their control, since level deviations can be at least as serious as lateral ones.

Radar equipment is available for Approach operations which allows precise monitoring of an aircraft’s track, and presents the controller with the information on deviations more quickly. In the first instance, any warning should be generated on the flight deck to indicate to the crew that the aircraft is deviating from the required flight profile. This would leave the navigation on the flight deck where it properly belongs. However, the radar controller should still be able to detect such deviations, as one aircraft straying from its cleared flightpath could have a serious effect on other aircraft in the vicinity, of which the ‘deviating’ crew would possibly be unaware. This raises a question of philosophy concerning radar monitoring and vectoring – whither or not aircraft which have a capability to fly a very accurate flight profile should need to be vectored by controllers at all. This question will need to be answered as it is fundamental to the development of procedures based on the use of Precision RNAV. If Precision RNAV routes are to be introduced in the near future – whilst ATC En Route Units still have their current generation of Area Radar’s – it would be less than sensible to align them such that the distance between the routes was less than current ‘standard separation’ minima.

Another development which is certain to lead to an increase in capacity is data-link. As more aircraft operate in the same volume of airspace routes will necessarily have to be placed closer together to accommodate the volume of traffic – or aircraft will wish to use routes which have been placed closer together to take advantage of weather systems, such as those using the North Atlantic tracks. If controllers are going to be required to monitor these flights for deviations, equipment must be provided which will be adequate for the task.

To Conclude

As the number of aircraft using modern avionics increases, and the volume of air traffic continues to grow, there will be a requirement to use the capability of such avionics to reduce the spacing between routes, in order to make the most effective use of the finite volume of airspace and keep delays to a minimum.

Radar controllers will be faced with more aircraft in airspace under their control and will need better means of detecting deviations from cleared flight paths than current area radar displays can provide.

There will undoubtedly be commercial pressure to introduce routes more closely spaced to accommodate the volume of traffic anticipated in the future. It is considered that such routes would be potentially unsafe if controllers will be expected to ‘radar monitor’ flights on them, unless the controllers are provided with equipment which will detect any deviation from a cleared flight path with sufficient accuracy, and in sufficient time, to allow appropriate remedial action to be taken which will prevent a loss of separation.

The introduction of Precision RNAV routes in areas where current generation ATC Area Radar’s are being used could cause difficulty to ATC if controllers will be required to radar monitor the progress of flights on the Precision routes, in addition to dealing with other aircraft on airways or random RNAV routes. If two Precision RNAV routes – or even worse , more than two – were to be positioned close together, the time available to a controller to recognise that a deviation was occurring and then take action to remedy this situation, would almost certainly be insufficient to prevent a loss of separation. Even with some form of deviation alert , it is likely that the general control task could be degraded whilst immediate attention was given to resolving this priority problem.

It is recommended that:

Separation standards should not be reduced below those that would otherwise be required purely because of the use of radar monitoring.

Any introduction of Precision RNAV routes should ensure that these routes are spaced at such a distance from each other that the required separation minima are not likely to be infringed if an aircraft on one of the routes deviates from its cleared path.

Last Update: September 20, 2020  

December 23, 2019   254   Jean-Francois Lepage    1994    

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