Use of Datalink in A Microwave Landing System

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Use of Datalink in A Microwave Landing System

41TH ANNUAL CONFERENCE, Cancun, Mexico, 15-19 April 2002

WP No. L021

Use of Datalink in A Microwave Landing System


There are plans by several European airports to introduce MLS in the near future. During the Committee B debate on MLS procedures at conference in Geneva, the subject of datalink was raised. The MLS equipment has a datalink capability and this is described in this paper.

In addition, where ILS and MLS are both in use at an airport, ATC may need to ascertain which approach aid the pilot will use. At busy airports this would create considerable extra RTF with the associated workload for pilots and controllers. This paper will also consider the use of Controller Pilot Datalink Communication (CPDLC) for this application.


MLS datalink

Data Communications (or Data link) refers to any exchange of digitized information between end-users (including broadcast from one to all). MLS equipment has a data link facility which can send (uplink) data to the aircraft. However, this data link facility cannot support direct controller-pilot communications.

The MLS datalink was principally designed to support curved approaches and other advanced procedures, such as computed centreline approaches. The waypoints defining these approaches are stored in a database on the ground and transmitted to each aircraft when it arrives within range of the MLS ground station. The aerodromes in Europe that are currently installing MLS plan to conduct straight in ILS look-alike operations. These approaches can be performed by the aircraft receiver without the information from the datalink. As a result, the aircraft will not equipped with receivers for the MLS datalink.

In addition, it was originally intended that the MLS datalink could be used to transmit other information to the aircraft, such as ATIS and RVR. There are no plans to implement this.

Use of CPDLC in the Terminal Area

In most cases where MLS is installed in Europe, ILS will also be in use on the same runway. Some airports have decided to ask all aircraft if they will be making an ILS or MLS approach. At other airports, controllers will only require this information in certain specific circumstances e.g. where ILS and MLS aircraft are to use different runways, or in LVP where closer spacing is applied to MLS aircraft.

An MLS equipped aircraft will indicate this equipment on the flight plan and the information will be displayed to the controller on the appropriate flight strip or data display to assist with advanced planning.

Unfortunately, this information cannot always be relied upon as a late aircraft change or equipment failure en-route may render it invalid. Even where the information is correct, it only indicates that MLS is fitted to the aircraft, not that the aircraft will carry out an MLS approach. This means that controllers need to ask the pilot which approach aid he will be using. This is essential where safety is involved e.g. closely spaced MLS operations in LVP.

At present, the only way to achieve this confirmation is for the controller to ask each aircraft individually by RTF, with the additional workload and frequency congestion that would result. In order to address some of these issues, the use of datalink may be appropriate.

A possible solution to the problem could be provided by the Controller-pilot data link communications (CPDLC) application – a means of communication between controller and pilot, using data link for ATC. This would allow ATC to request a report of the actual approach aid to be used. A reply would be provided without any RTF communication and updated into the ATC data display system.

It must be noted that pilots have often stated that they do not want to use CPDLC during approach (high workload) using current HMIs due to the additional head-down time. In the future, a better solution would probably be to introduce the MLS requirement into the PPD (Pilot Preferences Downlink) service.

Note: The PPD Service allows aircrew in all phases of a flight to provide the controller with information not available in the filed flight plan. It automates the provision to controllers of selected pilot preferences even before the aircraft reaches their sector.

Downlinked Aircraft Parameters

Some equipment, such as Mode S, provides the facility for aircraft to automatically downlink information from onboard the aircraft. This could be used to transmit the equipage state of the aircraft to ATC, but this facility is not currently specified for Mode S. Such a system would not cater for the case of an equipment failure en-route. It would still require the pilot to report which approach aid was actually being used during safety critical operations.

Other future developments

The introduction of MLS has proceeded very rapidly in recent years, resulting in an urgent need to address this problem, but it is not the only development that will require increased information to be passed from the pilot to the controller. There are other future developments in the Terminal Area such as GNSS (APV, Cat I Cat II and Cat III), RNAV SID’s and STAR’s, Mode S, where the actions of ATC will depend on information from the aircraft. These are all cases where ATC requires a knowledge of the aircraft equipage and the pilots intentions to know how best to handle the aircraft.


The datalink facility within the MLS equipment will not be used in the current implementation of MLS in Europe.

Many developments in the TMA over the next few years will require additional information to be passed between the pilot and controller in order for ATC to handle the aircraft in the most appropriate manner. While MLS is currently the most urgent of these, consideration should also be given to these other requirements in order to develop a balanced approach to the introduction of datalink in the Terminal Area.

It would appear that the most appropriate action would for all those involved in Terminal Area developments to be aware of these requirements when planning the introduction of new technology and procedures. The development of datalink should be planned alongside these other items to ensure that all the relevant requirements are taken into account (e.g. the provision of appropriate messages and protocols). This will enable datalink to be introduced into Terminal Area in a planned in a timely and efficient manner.

IFATCA is co-ordinating with the ODIAC group of Eurocontrol about initial datalink applications within Europe.


That this paper is accepted as information.

Last Update: September 29, 2020  

March 13, 2020   666   Jean-Francois Lepage    2002    

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