The Development of the Microwave Landing System ( MLS)

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The Development of the Microwave Landing System ( MLS)

34TH ANNUAL CONFERENCE, Jerusalem, Israel, 27-31 March 1995

WP No. 85

The Development of the Microwave Landing System (MLS)


The development of the Microwave Landing System ( MLS) and the Global Navigational Satellite Systems (GNSS) is continuing with these systems intended to replace ILS as the primary approach and landing aids. It was originally intended to replace ILS with MLS, and ICAO has prepared a Transition Plan to enable this to take place. Since the Transition Plan was agreed, recent developments in GNSS have indicated that it may also be appropriate to consider GNSS as a replacement for ILS.

The discussion of these systems must take place in the context of the ICAO COM/OPS divisional meeting in MARCH 1995. This meeting will discuss the ILS/MLS Transition Plan and consider if it is still appropriate. It will also consider how GNSS and other issues such as the use of Required Navigation Performance (RNP) for approach and landing should be included in the development of a future strategy for new approach and landing aids. It is hoped that this meeting will adopt a strategy for the replacement of ILS with other aids.

The European ILS/MLS Transition Group (IMTEG) has proposed strategy for the replacement of ILS in Europe. It recommends that GNSS should be developed to replace CAT I ILS and that, if GNSS cannot provide CAT II/III facilities where this standard of ILS is required, the Europe will consider adopting MLS for CAT II/III runways.

In the USA, the FAA has been at the forefront of GNSS development, with trials of the Global Positioning System (GPS), demonstrating its use for non-precision and CAT I approaches. It is expected that CAT I certification will be achieved soon.

The FAA also has a program aimed at demonstrating that it will be possible for GPS, with suitable augmentation systems, to achieve CAT II/III standards. It is anticipated that the results of this study will be available in 1995.

Although the study has not yet been completed, the FAA has announced that it is terminating development of MLS. Should GPS fail to achieve CAT II/III standards in the required timeframe. then MLS may be purchased to make up any shortfall in capability.

Other states have been reluctant to adopt GPS as a world-wide GNSS, due to the lack of any standards to guarantee the availability and reliability of the system to international users. ICAO has now set up the Global Navigation Satellite Systems Panel (GNSSP) to prepare standards for GNSS. When these standards are adopted then it will hasten the use of GNSS on a world- wide basis.

The AWOP proposed Strategy

AWOP has developed a proposed strategy which will be presented to the COM/OPS divisional meeting to replace the existing plan. This proposes that there should be a transition from ILS to the Required Navigational Performance (RNP) standard. RNP doe not specify which approach aids should be used, but rather defines the navigational standards that should be achieved.

The strategy proposes that, when a complete transition to RNP has taken place, then ILS will be withdrawn. The strategy does not set any firm timescales, but suggests that CAT III certification of GNSS should be achieved by 2015. The proposal also suggests that the strategy should be implemented on a regional basis, with each region developing specific plans within the overall global strategy. The strategy encourages the use of a Multi Mode Receiver (MMR) to maintain aircraft interoperability.

Automation of ATC

In addition to the proposed strategy, AWOP has considered the implications of introducing RNP. In many areas this has been good, noting the additional equipment that may be required by ATC, the changes to separation criteria, and the need for controller training. Of concern is that it has been assumed that the introduction of RNP will result in automation of the ATC system with the controller being reduced to a monitoring role. This is clearly not a factor in RNP, which is strictly related to the means of navigating the aircraft. There will have to be changes to the ATC system as a result of the introduction of RNP, but these will depend on factors such as the type of operation and the traffic density. Developments to the ATC system should not be dictated by any specific navigational requirements. The design of the ATC system must be developed to provide a safe and efficient service, independent of the navigational systems used. It is also noted that automated systems have been included in the ADS standards being developed by ICAO. In each case the type of automation varies, but there must be concern that the introduction of automation may occur in a random manner in association with new procedures, rather than as an integrated plan.

To Conclude

It appears from the current situation that both MLS and GNSS will be implemented, with regional variations. ILS will be retained in many areas where there are no economic benefits of changing to the new aids. This will significantly increase ATC workload at busy airfields where more than one landing aid is in use, and could affect the capacity of the ATC system. This situation will continue until the full implementation of RNP. After this, the withdrawal of ILS and the availability of advanced procedures should provide benefits in terms of runway and airspace capacity. In busy airspace, this will require the introduction of automated systems that may significantly change the role of ATC. The introduction of these systems will require careful planning to ensure that the role of the controller is clearly defined and that safety is maintained.

The introduction of automated ATC systems is being considered by many as a prerequisite of new concepts such as RNP and ADS. IFATCA must be vigilant to ensure that ATC is not “railroaded” into adopting automated systems as these concepts are introduced.

Last Update: September 28, 2020  

February 12, 2020   784   Jean-Francois Lepage    1995    

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