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How will you land tomorrow? Continuous technology progress for conventional and satellite-based navigational aids improves safety both in the air and on landing.

The first navigation systems were developed more than one hundred years ago and patents as early as 1907 laid the foundation for the generation of navigational aids as we know them today. Now and certainly in the future three possible precision landing technologies will co-exist.

Firstly the instrument Landing System (ILS), which has been at the forefront of navigational aids and is the basis for today's conventional-based navigational systems. The ILS provides pilots with deviation information from a defined approach path down to and along the runway surface under low visibility conditions. With its continuous improvements, the ILS represents the most common landing aid for Cat I, II and III landings. Thales Air Traffic Management's (ATM) ILS/-420 is the latest generation technology based on 35 years of development and deployment in 160 countries. It easily surpasses the requirements of aviation authorities worldwide to ensure excellent Cat. III signals and safe precision landings every day.

Secondly in 1978, with the increase in airport congestion, a new technology was developed, the Microwave Landing System (MLS), which at that time was seen to be the successor to ILS. The MLS is designed for the highest level of signal accuracy and stability in situations that require additional features to an ILS. The MLS offers wider coverage, missed approach, curved approach guidance and, an increased number of frequency channels that reduce interference probability with other airports. This functionality permits an increased frequency of landings under Cat III conditions.

Finally the optimism of the air transport market for the potential offered by satellite-based navigation systems was seen as the future follow-on from all conventional ground-based navigational aids such as the ILS or MLS. It was assumed that the civil aviation market would make a direct transition from ILS to satellite-guided landing using signals from the US Global Positioning System (GPS) or the currently defined European GALILEO System. This step, nonetheless, has turned out to have taken much longer than anticipated. The issue is that the technology for satellite navigation is still maturing.

In the meantime, the ILS/MLS similarity was exploited for the aircraft approach phase giving a fresh impetus to MLS technology. Until then, the cost for the introduction of the airborne part of the technology had been the major obstacle for the airlines to implement the MLS. The newly-developed 'ILS-Look-Alike' precision approach concept enabled the certification of the airborne system to become economically interesting, as it permitted ILS, MLS or GPS to be used in an identical approach.

Thales succeeded in making the MLS an operational reality in the 1990s. A new type of aircraft receiver, the Multi Mode Receiver (MMR) offered by Thales Avionics provides landing guidance for ILS, MLS and GPS systems, allowing pilots to select the landing guidance system appropriate to the runway in use. Cat III tests with the MMR from Thales are underway. Certification is expected no later than autumn 2004. Co-location and integration of MLS into an existing ILS without degrading the operational performance is a further major benefit of the system. In airports with a high landing density, that can on[y be improved under Cat III conditions, the MLS is selected because of its reduced sensitivity to multipath, interference and environmental conditions. Tests have shown that hourly landing rates can be increased by up to 20%, resulting in significant savings in both cost and time for airlines and airports.

In 1997, Amsterdam Schiphol Airport was equipped with two MLSs supplied by Thales ATM. In 2002, the Italian air force began in parallel its MLS implementation programmes with Thales ATM for NATO air bases. The French civil aviation authority, Service Technique de la Navigation Aerienne (STNA) awarded Thales ATM a contract for a MLS with an option of up to eight further systems. This first MLS is in the Cat III certification process at Toulouse Blagnac International Airport and is used primarily by EADS for Airbus airborne equipment certification. A recent contract award to Thales ATM by the UK's National Air Traffic Services (NATS) comprises four MLSs complemented with an option for up to 40, the first unit will be shortly operational at London Heathrow Airport. This programme is matched with British Airways' and Airbus' MLS airborne certification programme. British Airways will be the first carrier equipped to use MLS and expects immediate benefits when low visibility procedures are in force.

The future alternative satellite precision approach system is the Ground Based Augmentation System (GBAS). The GBAS Ground Station (GS) provides differential correction data and integrity information as well as approach path data via a digital VHF Data Broadcast. The system enables multiple runway coverage and has the capability for surface area navigation and terminal area operations. Hence, in the future, one GBAS GS may substitute several ILS. The flight path is defined by virtual points in space and the GBAS can provide a flexibility in respect to the approach path. Besides 'ILS-Look-Like' precision approaches, functionality such as extended range, curved or tailored approaches in order to avoid obstacles or noise sensitive areas can be added easily. GBAS allows Cat I precision approach at airports that cannot install ILS due to geographic or building constraints.

In 1999, ICAO approved the GBAS including the Global Navigation Satellite System (GNSS) technology under the Standards And Recommended Practices (SARPs) of Annex 10 of the Chicago Convention. GBAS programmes are ongoing with European civil aviation safety agencies, for example ENAV in Italy and the DFS in Germany. Thales ATM's GBAS test-station is installed at ToulouseBlagnac Airport and is used by French DGAC/STNA for the validation of SARPs and the GLS MMR certification.

The GBAS introduction will depend on the airlines' needs as a driving force for the ground equipment, and also the availability of the GPS landing system (=GLS) functions. The Multi Mode Receiver (MMR) allowing ILS, MLS and GLS approaches is certain to foster GBAS installations. Boeing and Airbus are planning to offer the GLS and MLS receivers as options for the MMR in their new aircraft. Airbus has launched a GLS Cat I certification programme and intends to offer GLS capabilities after 2006. Boeing currently is in the process of GLS Cat I certification. The MMR allows GLS approaches with ILS-alike procedures. The ongoing air and ground certifications confirm accurate and safe service in all the envisaged non-ILS operational conditions and will further strengthen the position of GLS.

The International Civil Aviation Organization's (ICAO) strategic plan up to 2020 states operational suitability and cost-effectiveness as the decisive factors for the choice of a specific landing system. ILS will thus remain the most common landing aid for Cat I, II and III. MLS is recommended where operationally required and economically beneficial. The implementation of satellite-based navigation opens the possibility of a low-cost alternative to an I LS precision approach. Satellite-based positioning signals with further augmentation for integrity and accuracy will allow a type of precision approach termed as Approach Operations with Vertical guidance (APV) which will benefit many general aviation airports.

Thales has a leading role in the introduction of Europe's EGNOS (European Geostationary Navigation Overlay System), the first step to the currently defined European Satellite Navigation Programme GALILEO. Thales is positioned as an industrial partner and service developer for these strategic programmes.

Your Choice

The choice of a landing aid for an airport depends on the given environmental and operational specifics and, of course, airline priorities. Experience has shown that the introduction of a new technology generation for safety-critical applications involves long transition periods. Therefore ILS, MLS, GLS, now called xLS, will continue to co-exist.

Landing Precision instrument landings
Categories: are divided into three categories
 according to visibility
 conditions. Performing a CAT
 I or II landing means that the
 pilot must be able to see the runway
 when about 850 or 275m away from
 the runway threshold or abort the
 landing. CAT III has three sub-
 levels (A, 8, C) with IIIC being
 the most demanding as there is
 zero visibility.

GNSS: The ICAO term for navigation
 signals transmitted from a
 variety of satellite-based

GPS: General Positioning System;
 the US satellite navigation

GLS: Landing system using GPS

GALILEO: Europe's currently defined
 satellite navigation programme;
 Galileo represents the future
 European Global Navigation
 Satellite System (GNSS)

EGNOS: European Geostationary Navigation
 Overlay Service; EGNOS
 augments GNSS signals providing
 the availability of navigation
 signals for trans-European
 network applications.

DGAC/STNA: Direction Generale de
 l'Aviation Civile/Service
 Technique de la Navigation

ENAV: Ente Nazionale di Assistenza
 al Volo

DFS: Deutsche Flugsicherung
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Title Annotation:ATM; Instrument Landing System; Microwave Landing System; Ground Based Augmentation System
Author:Schellinger, Ellen
Publication:Airports International
Geographic Code:4EUFR
Date:May 1, 2004
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