ref gali-swed-dd045 galilei ate 06.11 · galilei ref : date : gali-swed-dd045 06.11.2002...

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Complementary Systems: VDL Mode 4 ISSUE : 1.1 PAGE: 1

Sustainable Mobility and IntermodalityPromoting Competitive and Sustainable Growth

GALILEI

Complementary Systems: VDL Mode 4

Written by Responsibility - Company Date Signature

Jens Redeborn, Abdul Tahir Swedavia 2002-11-06

Verified by

Niclas Gustavsson Swedavia 2002-11-06

Certified by

Umberto Guida Alenia Spazio 08-11-02

WBS Code : C.2.B.8Classification : Final Issue

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THE INFORMATION IN THIS DOCUMENT IS PROVIDED AS ISAND NO GUARANTEE OR WARRANTY IS GIVEN THAT THE

INFORMATION IS FIT FOR ANY PURPOSE. THE USER THEREOFUSES THE INFORMATION AT ITS SOLE RISK AND LIABILITY.

FURTHERMORE, DATA, CONCLUSIONS ORRECOMMENDATIONS IN THIS REPORT ARE PROVIDED ON THE

BASIS THAT SUCH INFORMATION IS SUBSEQUENTLY, ANDPRIOR TO USE, VERIFIED BY THE PARTY WISHING TO USE

THAT INFORMATION.

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CHANGE RECORDS

ISSUE DATE § : CHANGE RECORD AUTHOR

0.6

2002-06-07

2002-06-25

First draft issue.

Draft issue (Internal updates)

0.8 2002-07-01 Draft issue (Internal updates) J. Redeborn

1.0 2002-07-02 Final Issue J. Redeborn

1.A 2002-07-15 Update per comments from Industrial Team A. Tahir

1.B 2002-10-14 Update per comments from Industrial Team as well ascomments from Alenia Spazio during telecon

A. Tahir,

J. Redeborn

1.C 2002-10-30 Update based on email from Alenia Spazio A.Tahir,

J.Redeborn

1.1 2002-11-6 Issue 1.1 A.Tahir,

J.Redeborn

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TABLE OF CONTENTS

1 PRESENTATION OF THE STUDY.............................................................................................. 7

1.1 SPECIFIC REFERENCES......................................................................................................... 81.1.1 Acronyms...................................................................................................................................81.1.2 Reference Documents .............................................................................................................10

2 VDL MODE 4 ................................................................................................................................. 11

2.1 SYSTEM DESCRIPTION........................................................................................................ 112.2 LOCAL FUNCTIONS AND SERVICES................................................................................ 172.2.1 Communication Service..........................................................................................................182.2.2 Functions and applications.....................................................................................................182.2.2.1 Surveillance applications .....................................................................................................192.2.2.2 Communication applications...............................................................................................202.2.2.3 Navigation applications........................................................................................................212.2.2.4 Secondary Navigation in VDL Mode 4 based on triangulation........................................212.2.2.5 Operational benefits of secondary navigation ...................................................................242.3 VDL MODE 4 LOCAL PERFORMANCES .......................................................................... 252.4 VDL MODE 4 COVERAGE .................................................................................................... 252.5 VDL MODE 4 TRANSCEIVER ASPECTS ........................................................................... 262.5.1 VDL Mode 4 Transceiver.......................................................................................................262.5.2 Hybrid TRANSCEIVER .......................................................................................................282.5.2.1 Spectrum for Hybrid Transceiver ......................................................................................30

3 COMBINATION IN CRITICAL ENVIRONMENT.................................................................. 31

3.1 CRITICAL ENVIRONMENT ................................................................................................. 313.2 VDL MODE 4 AND GNSS IN CRITICAL ENVIRONMENT ............................................. 313.3 EXTENSION TO GALILEO ................................................................................................... 313.3.1 Brief Description of VDL4- Augmentation with GNSS.......................................................313.3.2 VDL4-Augmentation with Galileo for Aviation...................................................................323.3.3 VDL4-Augmentation with Galileo for Multimodal Applications.......................................363.3.3.1 Universal Automatic Identification System for Maritime ................................................36

4 RECOMMENDATIONS ............................................................................................................... 39

4.1 INTEROPERABILITY ............................................................................................................ 394.2 CERTIFICATION AND STANDARDISATION................................................................... 404.2.1 Certification Objective ...........................................................................................................404.2.2 Certification Process...............................................................................................................40

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4.2.3 Development and Certification Standards ...........................................................................444.3 LEGAL ASPECTS .................................................................................................................... 454.4 SECURITY ISSUES.................................................................................................................. 45

5 CONCLUSION............................................................................................................................... 47

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LIST OF FIGURES

Figure 2-1 Time-slots in VDL Mode 4.................................................................................... 11Figure 2-2 Ground station transmissions ................................................................................. 13Figure 2-3 Layered Structure of VDL Mode 4 ........................................................................ 15Figure 2-4 Services Supported by VDL Mode 4 ..................................................................... 18Figure 2-5 Overview of GNSS Navigation Concept ............................................................... 23Figure 2-6 Secondary Navigation in VDL Mode 4 ................................................................. 24Figure 2-7 VDL Mode 4 Coverage from Flight Test at Stockholm/Arlanda Airport.............. 26Figure 2-8 Illustration of Cell Shrinkage using Robin Hood principle.................................... 26Figure 2-9 Block Diagram of a Typical VDL Mode 4 Transceiver for CNS applications...... 28Figure 2-10 Detailed Hybrid GNSS Transceiver.................................................................... 29Figure 2-11 VDL Mode 4 and Galileo Spectrum Allocation .................................................. 30Figure 3-1 Basic VDL4-Augmentation with Galileo Concept ................................................ 34Figure 3-2 VDL Mode 4 Airborne and Ground Subsystem for CNS/ATM ............................ 35Figure 3-3 Overview of the AIS .............................................................................................. 38Figure 4-1 Typical Development/Certification Process used in Civil Aviation ...................... 41Figure 4-2 Top Level Safety Allocation Process..................................................................... 42Figure 4-3 Typical Safety Assessment Process Model............................................................ 43Figure 4-4 Software Life Cycle Process .................................................................................. 44

LIST OF TABLES

Table 2-1 VDL Mode 4 Target Performance for CNS Services.............................................. 25Table 3-1 VDL4-Augmentation Performance Improvement progression ............................... 34Table 4-1 Certification Authorities for Various Segments ...................................................... 42

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1 PRESENTATION OF THE STUDY

The objective of this study is to provide the basic principle of VDL Mode 4 as part of Galileoand its use in Communication, Navigation and Surveillance (CNS) in Aviation. The use ofVDL Mode 4 is not limited to aviation; it provides similar functionality for multi-modalapplications as it does for aviation. These include maritime uses such as AutomatedIdentification System (AIS). The principle focus of this presentation is VDL Mode 4 andhow it compliments Galileo for achieving enhanced performance for CNS usage. The targetobjective is to achieve these capabilities during all phases of flight i.e. for entire gate-to-gateoperations. VDL Mode 4 is a datalink developed to provide a high throughput and optimaldata broadcast with minimal losses..

VDL Mode 4 is a robust Self-organising TDMA datalink, which can be used for CNSfunctions. Conversely, navigation data derived from Galileo would be used in aviation forsurveillance for ADS-B and other aviation applications. The precise timing available fromGalileo provides precise UTC signal for the synchronisation of .VDL Mode 4 broadcasts. Theprecise timing also forms a basis for a secondary navigation function which can be derivedfrom the VDL Mode 4 as a backup navigation source when a GNSS based primary navigationfunction is not available due to lack of satellite visibility or other causes. Hence VDL Mode 4and Galileo form a proper compliment to Galileo whereby navigation and precise timing areavailable for complimentary use and a convenient way to field Galileo receivers early.

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1.1 SPECIFIC REFERENCES

1.1.1 ACRONYMS

ADS Automatic Dependent SurveillanceADS-B Automatic Dependent Surveillance Broadcast

AIS Automatic Identification SystemAPV APproach with Vertical guidanceASAS Airborne Separation Assurance SystemsA-SMGCS Advanced Surface Movement Guidance and Control SystemATC Air Traffic ControlATM Air Traffic ManagementATS Air Traffic ServicesCAA Civil Aviation AuthorityCDTI Cockpit Display of Traffic InformationCFIT Controlled Flight Into TerrainCNS Communication, Navigation and SurveillanceCPDLC Controller-pilot data link communicationsDAS Delegated Airborne SeparationDCL Departure CLearanceDGNSS Differential GNSSDGPS Differential GPSDLS Data Link ServicesDoS Directory of ServicesEC European CommissionEGNOS European Geostationary Navigation Overlay ServiceESA European Space AgencyFAA Federal Aviation AdministrationFAS Final Approach SegmentFIS-B Flight Information Service BroadcastGALA GALILEO overall Architecture studyGALILEO Global navigation satellite systemGBAS Ground Based Augmentation SystemGEMINUS GALILEO European Multimodal Integrated Navigation User ServiceGEO GEOstationary satellite

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GIS Geographic Information SystemGLONASS GLObal NAvigation Satellite System (Russia)GNSS Global Navigation Satellite SystemGNSSP GNSS PanelGPRS General Packet Radio ServiceGPS Global Positioning System (USA)GR General Request messageGRAS Ground based Regional Augmentation SystemGSC Global Signalling ChannelICAO International Civil Aviation OrganisationISO International Organisation for StandardisationLME Link Management EntityMAC Media Access ControlMEDUP Mediterranean Update ProgramMFF Mediterranean Free FlightMOPS Minimum Operational Performance StandardsNEAN North European ADS NetworkNEAP North European CNS/ATM Application ProjectNLTB Northern Latitude Test BedNPA Non Precision ApproachNPV Non Precision approach with Vertical guidanceNUP NEAN Update ProgramPVT Position Velocity TimeQoS Quality of ServiceRF Radio FrequencyRNAV aRea NAVigationRNP RadioNavigation PlanSAR Search And RescueSARPS Standards And Recommended PracticesSBAS Satellite Based Augmentation SystemSMGCS Surface Movement Guidance and Control ServerSTDMA Self-organising Time Division Multiple AccessTIS-B Traffic Information Service BroadcastTRAN Terrestrial Regional Augmentation NetworkTTA Time To AlarmUMTS Universal Mobile Telecommunication System

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UTC Universal Co-ordinated TimeVDL VHF Data LinkVOR Very high frequency Omni-directional Radio rangeVSS VDL Mode 4 Specific ServicesWAAS Wide Area Augmentation System

1.1.2 REFERENCE DOCUMENTS

RD 1 DD020 : Required Performances for Local ApplicationRD 2 DD021 Complimentary Systems: Functions and PerformanceRD 3 MRD Mission Requirements Document Issue 1RD 4 SRD System Requirements Document Issue 1RD 5 MASPS for A-SMGCS, EUROCAE; EurocaeRD 6 ED 108 VDL Mode 4 MOPS; EurocaeRD 7 DRAFT ICAO SARPS for GNSS Amendment Letter 77; November 2001; ICAORD 8 ED-12B Software Considerations in Airborne Systems and Equipment

Certification, EurocaeRD 9 "Secondary navigation in VDL Mode 4", Larry Johnsson, Aeronautical Mobile

Communications Panel, Madrid, April 1997.RD 10 Study on the options of time synchronisation in the VDL Mode 4 Datalink System;

Issue 1.2; EurocontrolRD 11 DO 242 Minimum Aviation System Performance Standards For Automatic

Dependent Surveillance Broadcast (ADS-B), RTCARD 12 VDL Mode 4 Master Document; Issue 2, Swedish CAA, September 2001RD 13 ED 78A Guidelines for Approval of the Provision and Use of Air Traffic Services

supported by Data, EurocaeRD 14 ED 72A MOPS for Airborne GPS receiving equipment for supplemental means of

navigation, Eurocae

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2 VDL MODE 4

2.1 SYSTEM DESCRIPTION

VDL Mode 4 is an ICAO standardised1 STDMA VHF data link, providing digitalcommunications between mobile stations (aircraft and airport surface vehicles) and betweenmobile stations and fixed ground stations. It was developed for CNS/ATM aviationapplications, supporting various services including broadcast applications (e.g. ADS-B, TIS-B, FIS-B and GNSS) as well as point-to-point communications (e.g. ADS-C). VDL Mode 4protocols support ADS-B and similar applications through the broadcast of short repetitivemessages, with graceful adaptation to increasing traffic loads.

The core of VDL Mode 4 is the ADS-B Service, other services such as TIS-B, FIS-B, GNSSetc., are add on services that are being considered to enhance the use of VDL Mode 4 foraviation. Navigation applications of VDL Mode 4 is currently not being considered forstandardisation by ICAO

VDL Mode 4 transmits digital data in a standard 25 kHz VHF communications channel anddivides the communication channel into a large number of time slots. The start of each slot isan opportunity for a station to transmit as shown in Figure 2-1.

4500 slots per minute

Figure 2-1 Time-slots in VDL Mode 4

VDL Mode 4 is built on the Self-organising Time Division Multiple Access (STDMA)technology, in which the time-slots are synchronised, to UTC. A possible source for preciseUTC is GNSS including Galileo. The stations advertise their intention to transmit in aspecified time-slot by means of a reservation protocol carried in a prior transmission seefigure 2-2. For convenience, a group of contiguous time slots spanning a period of 60 secondsis termed a “superframe”. The superframe contains 4500 slots (equivalent to 75 slots per 1 VDL Mode 4 is currently standardised within ICAO for Surveillance applications only.

1 2

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second). Each time slot may be used by a radio transceiver (mounted on aircraft, groundvehicles or at fixed ground stations) for transmission of data. The exact timing of the slots andplanned use of them for transmissions are known to all users in range of each other, so thatefficient use of the data link can be made and users do not transmit simultaneously. As aresult of this ‘self-organising’ protocol, VDL Mode 4 is capable of operating outside thecoverage of a ground infrastructure and can therefore support air-air as well as ground-airdata communications and applications. In high density airspace, a ground infrastructure maybe used to manage the system and improve overall performance.

VDL Mode 4 operates in the aeronautical VHF spectrum band, i.e. 108-136.975 MHz. Thediscrimination property of VHF, which allows a station to select the stronger of twooverlapping signals, enables efficient re-use of time slots and spectrum. A pair of GlobalSignalling Channels (GSC) will be allocated for worldwide use. These channels will besufficient to support ATM in most areas, but may need to be complemented by LocalSignalling Channels (LSC) needed in busy terminal areas and at high-density airports tosupplement the GSCs for ADS-B, and possible additional VHF-channels required for uplinkand downlink of application data. Principles for assigning VDL Mode 4 channels are yet to bedeveloped. Appropriate frequency management techniques must be used when determiningthe set of physical VHF frequencies in a certain area, considering the co-channel and adjacentchannel interference (CCI/ACI) characteristics of the VDL Mode 4 transceivers.

Ground stations will generally transmit bursts containing several different types of messages.There are several EC sponsored programs one of them called NUP (NEAN Update Program)was set up to broadcast different types of messages for various different services an exampleof that is shown in Figure 2-2. The figure shows uplink transmissions containing:

• ADS-B messages (synchronisation bursts);

• Directory of Services messages (DoS);

• TIS-B messages;

• FIS-B messages;

• GNSS augmentation data

General Request messages (GR).

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FIS-B

Ground stationtransmissions

DoS TIS-B GRSystem control GNSSAugmentation

Blockedarea

Figure 2-2 Ground station transmissions

The figure also shows:

• Directed reporting of some aircraft. These are aircraft that are reporting under directionof the ground station and they have been directed into the slots immediately after theuplink transmission.

• Ground quarantine slots. These are the slots immediately following a ground stationtransmission or a transmission from a mobile in directed reporting. They are not used forADS-B messages by autonomous mobiles.

• Blocked slots. These are typically reserved slots for ground station transmissions only, inorder to protect the transmitted information from unintentional interference from othertransmitters, but mobile stations may also be directed into blocks. In an area with severalactive ground stations their pre-planned transmissions are co-ordinated and the blockedarea is covering all transmissions from these ground stations.

As shown in Figure 2-3 VDL Mode 4 sub-system implements the three lower layers of theOSI model:Layer 1 (Physical layer) provides transceiver frequency control, bit exchanges over the radiomedia, and notification functions. These functions are often known as “radio” and“modulation” functions.The ICAO VDL SARPs define the physical layer for VDL Mode 4: The modulation schemeis Gaussian Filtered Frequency Shift Keying (GFSK), at a nominal bit rate of 19,200 bits/s.Layer 2 (Link Layer): is split into three sublayers and a management entity:• The Media Access Control (MAC) sublayer provides access to the Physical layer by a

simple Time Division Multiple Access (TDMA) algorithm under the control of the nexthigher sublayer. It also provides system time functions to co-ordinate the TDMA channelaccess.

• The VDL Mode 4 specific services (VSS) sublayer provides control of channel accessusing a self-organising mechanism. The VSS also support a number of ground controlled

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access protocols. The basic services are built on reserved, random and fixed access to theTDMA slots and support broadcast and point-to-point communication.

• The Data Link Services (DLS) sublayer is composed of the Aviation VHF Link Control(AVLC) derived from the High Level Data Link Control (HDLC) protocol (ISO 3309)whose main functions are frame exchanges, frame processing and error detection. TheDLS protocols are adapted to make best use of the unique VSS channel access protocols.

The Link Management Entity (LME) is in charge of the links between peer DLS sublayersand also the maintenance of the broadcast link functions.

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Transmission,nominal start oftransmission

Received transmission,Channel busy/idle notification

Bursts and frames,Time to send,Access control(reserved or random)

Sync burst data,VSS userparameters

Received burst and frame data,Unsent random transmissions

Burst and frame data,VSS user parameters

Received burst and frame data,VSS user status information

XID frame data, DLSuser parameters

VDL Mode 4specific

applications

ATN

Frames and DLSuser parameters

Link control dataand parameters

SubnetworkVDL Mode

VHF

PhysicalFrequency control,Data encoding

Transmission timing,Transmitter shutdown

Data decoding,Signal quality notification,Channel sensing,Arrival time measurement

MACSlotted TDMA,Time synchronization

Transmission formatting,Burst error detection

Slot occupied/not occupiedprocessing,Arrival time measurement

Burst formatting,Self organizing TDMA

Slot selection,Reservation table,Burst encoding,Reserved access

Reservation table update, Burstdecoding,Data error detection

VSS

DLSsublayerRe-transmission,Frame error detection,

DLS burst formatting,Mode 4 data transfer protocol,(ground-air) and (air-air),Mode 2 data transfer protocol

Synchronization bursts,Peer entity contact table, Netentry

Link establishment andmaintenance (exchangeidentity protocols), Directory ofservices

LME

Positionsource

Figure 2-3 Layered Structure of VDL Mode 4

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Layer 3: The VDL SARPs defines only the lowest network sublayer of layer 3 (SNAcP). It iscompliant with the subnetwork sublayer requirements defined in the ATN SARPs andconforms to the ISO 8208 (or network layer of CCITT X.25). It provides packet exchangesover a virtual circuit, error recovery, connection flow control, packet fragmentation, andsubnetwork connection management functions.

VDL Mode 4 operation is built up from the following fundamental features, which supportvarious CNS operations:a. A robust modulation scheme for encoding data in each slot. VDL Mode 4 supports

Gaussian Filtered Frequency Shift Keying (GFSK) with a modulation rate of 19,200bits/sec.

b. A self organising time division multiplex access (STDMA) frame structure. In VDLMode 4, channel time is divided into fixed length time slots. A superframe consists ofa group of slots that span a period of 60 seconds and contains 4500 slots (equivalentto 75 slots per second).

c. A timing reference providing a unique marker for the start of each communicationsslot. The timing concept used in VDL Mode 4 is based upon Universal Co-ordinatedTime (UTC). The primary source during normal operation is typically GNSS whichincludes Galileo, but other sources may be used as long as they can be related to UTCand satisfy the performance requirements. Hence VDL Mode 4 derives the benefit ofhigh precision timing source from GNSS including Galileo

In the event that a station loses its primary source of UTC time, it may resort to afailure mode known as secondary timing of reduced precision, which is a backupmode. A possible source of secondary time may be derived from the time of arrival ofsynchronisation bursts received from another station declaring primary time. Afurther failure mode (known as tertiary timing) allows a station to transmit when it isunable to derive time from primary or secondary time sources. In tertiary time, astation maintains synchronisation to an estimate of the mean slot start time of a set ofstations. It should be noted that secondary timing is only used as backup modes incase of a possible failure of all the GNSS satellite systems.

d. Position information from the aircraft’s navigation system, which could be GNSS,including Galileo derived is used to organise access to the slots. If a station loses itssource of position information it may continue to derive position fromsynchronisation bursts received from other stations (known as secondary navigation)shown in Figure 2-6. Stations operating on secondary or tertiary timing do not offercertified data quality and thus cannot be used for derivation of secondary navigation.

e. A flexible message structure that can support a wide range of broadcast and datatransfer protocols such as ADS-B and point-to-point communication.

f. A slot selection function that determines when a station can access the channel andmaintains information on the current and planned slot assignments.

g. A slot access management function, controlling the use of each slot.h. A number of link management functions that support access to data link services on a

wide range of channels.

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2.2 LOCAL FUNCTIONS AND SERVICES

VDL Mode 4 local functions and services described in this section are based on a number ofEC sponsored programs. These projects are today exploring the benefits of such applicationswith VDL Mode 4 as a key component to enable the different applications through services.

The most recognised project within this field is the NUP II (North European ADS-B NetworkUpdate Programme) Phase II which is a direct follow up on NUP phase I and the NEANprojects. The main objective in NUP II is to “Establish a European ADS-B network based onglobal standards supporting certified applications and equipment in synergy with theEuropean ATM concepts providing benefits to ATM stakeholders”. The development ofdifferent ADS-B applications is the core process in NUP phase II, such as ADS-B in NonRadar Environment, off-shore operations, surface movement operations (SMGCS), air-to-airoperations and ADS-B in ATC. Although the main focus is on ADS-B the scope is not limitedto this. The use of complementary services like TIS-B (Traffic Information Service-Broadcast), FIS-B (Flight Information Services-Broadcast) and services in the point-to-pointarea will also be investigated during the projects life cycle.

Other Projects are the MFF (Mediterranean Free Flight Programme) and MEDUP(Mediterranean Update Programme). The main goal of ADS-MEDUP is the construction of apre-operational infrastructure serving a large portion of the Mediterranean airspace, whichincludes key Ground and Airborne CNS/ATM elements based on satellite navigation andVDL Mode 4 data link as enabling technologies. The MFF programme is investigating FreeRouting and Free Flight providing technical and operational evaluation of integrationinteroperability and safe use of CNS/ATM technologies and applications (e.g. operationalrequirements and procedures based on the use of new CNS/ATM technologies enabling theintroduction of free flight operations in the Mediterranean area). Also to verify appropriatenew operative procedures for ATM staff and crew in free routing and free flight scenariossuch as delegation of separation responsibility from ATC to aircraft and vice versa, throughsimulations and flight trials using specially equipped aircraft and controller workingpositions.A close coordination among these and other programmes in the same field are conducted inorder to obtain interoperability.

VDL Mode 4 provides a platform of services on which to develop new applications. Suchapplications operate in a wide range of operational scenarios from worldwide civil aviation tothe local airport environment. Most of these applications that are imbedded in the servicesprovided by VDL Mode 4 are complemented by GNSS, which includes Galileo.

Example applications include:1. Automatic Dependent Surveillance-broadcast (ADS-B);2. Differential GNSS; utilised for augmentation as shown in Figure 2-43. Surface Movement Guidance and Control (SMGCS);4. Uplink information such as traffic, meteorological, etc.

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End-to-endcommunications

ground-air air-airVDL Mode 4

Services

ATN services VDL Mode 4 specific services

Cockpit Display of TrafficInformation ( CDTI)

Airborne separationassurance (ASAS)Precision RunwayMonitoring (PRM)Surface movement

surveillanceSecondary navigation

Flight InformationService- broadcast

(FIS-B)Traffic InformationService - broadcast

(TIS-B)

Non- ATNground-air

communication

Controller-Pilot DataLink Communication

(CPDLC)ADS contract

(ADS-C)PrecisionnavigationNon PrecisionApproach

Core functionssupportedVDL Mode 4

Userfunctions &applications

Air-aircommunications

Air-airtrajectorynegotiation

ADS-BGNSS

AugmentationATN

Databroadcast

ground-air air-air

Positionbroadcast

ground-air air-air

End-to-endcommunications

ground-air

ground-airdata broadcast

Figure 2-4 Services Supported by VDL Mode 4

VDL Mode 4 provides a platform on which to develop new, yet not fully defined applications.Such applications operate in a wide range of operational scenarios from world-wide civilaviation to the local airport environment. Figure 2-4 illustrates VDL Mode 4 user functionsand applications and their relationship to the core functions and communication servicessupported by the system. The description in this section expands on the diagram.

2.2.1 COMMUNICATION SERVICE

VDL Mode 4 supports two different types of communication services:• VDL Mode 4 specific services (VSS);• VDL Mode 4 ATN data link services (DLS).The VDL Mode 4 specific services include broadcast and point-to-point (addressed)communications with a minimum of overhead information for exchange of time-critical data.VDL Mode 4 constitutes an ATN sub-network and thus provides fully ATN compliantcommunication services. Together these services support several broadcast and end-to-endcommunication functions that supporting a range of air-ground and air-air ATM applications.VDL Mode 4 services are accommodated on multiple VHF channels. While DLS channelsmust be separated from those supporting VSS, various broadcast functions and applicationscould share a channel. The possibilities for channel sharing depends on various constraintssuch as channel availability, certification requirements and ATS regulations and may differbetween states and regions.

2.2.2 FUNCTIONS AND APPLICATIONS

This section expands on the VDL Mode 4 ATM functions and applications identified inFigure 2-4 following the traditional subdivision into Communications, Navigation and

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Surveillance (CNS). As ADS-B is the fundamental service provided by VDL Mode 4,Broadcast of data is the fundamental VDL Mode 4 technique, while point-to-pointcommunications could be seen as a complement necessary for realising specific needs in thefuture ATM concept. As an enabler of important applications and services such as ATSsurveillance, cockpit display of traffic, surface movement surveillance and airborneseparation assurance, ADS-B is the key VDL Mode 4 service. Surveillance applications aredescribed first, followed by communications and navigation applications. It should be notedthat the boundaries between C, N and S are somewhat blurred, as all functions andapplications essentially are based on (data link) communications. In this section, the intendeduse of an individual application has been used to determine the group in which it is described.

Use of multiple channelsThe number of channels required to support VDL Mode 4 C, N and S services in a certainarea will depend on local and regional conditions such as the traffic density (affecting thechannel load), certification requirements, ATS regulations and spectrum availability. Whereasa single channel may be acceptable to support ADS-B, GNSS augmentation, TIS-B and FIS-Bin one area, multiple channels may be required to support ADS-B and TIS-B alone in a high-density terminal area, supplemented by separate channels that support any augmented GNSSnavigation and FIS-B.

2.2.2.1 Surveillance applications

ADS-BThe ADS-B function is an evolving technology that uses the VDL Mode 4 synchronisationburst message formats to broadcast regularly an aircraft or vehicle’s identity, position,altitude, time, intent and vector information for use by other users, both mobiles and groundstations. Because position reporting is an integral part of communications management inVDL Mode 4, the core elements of ADS-B are already present on the link.ADS-B supports many mobile-mobile surveillance applications such as cockpit display oftraffic information (CDTI), airborne situational awareness (AIRSAW) and station keeping.When the VDL Mode 4 system also includes ground stations it is also able to supportapplications such as Advanced Surface Movement Guidance and Control Systems (A-SMGCS), enhanced ATC, Search And Rescue (SAR) coordination, etc.

Cockpit Display of Traffic Information (CDTI)One of the greatest benefits of VDL Mode 4, and a natural extension of its ADS-B capability,is that it provides a pilot with situation awareness using CDTI. This means that a display inthe cockpit can show the pilot the positions of all other aircraft in the vicinity with a range ofup to 200 nautical miles.Traffic Information Service (TIS)TIS is an ATM function that uses a data link to upload radar surveillance data from theground to aircraft to supplement ADS-B reports in airborne surveillance. VDL Mode 4supported broadcast TIS (TIS-B) will provide ADS-B equipped aircraft with positioninformation from non-equipped aircraft to provide situational awareness of all nearby traffic.Thus TIS-B is an important function to deliver benefits from ADS-B in a partially equippedenvironment and during the transition from a radar-based to ADS-B surveillanceenvironment. TIS-B reports are typically restricted to position information on aircraft notequipped with ADS-B.

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Air-to-air surveillanceBasic air-to-air surveillance is provided by ADS-B. The direct air-to-air communication(addressed) capability of VDL Mode 4 can be used to implement a temporary pair wise“crosslink” for trajectory negotiations between two aircraft in the Free Flight concept. Such acrosslink is used to ensure that action taken by one aircraft in a conflict situation does notconflict with the other aircraft’s intentions in response to the same conflict5.ATS surveillance (air)The ADS-B application of VDL Mode 4 can be used with ground stations to provide ATSsurveillance either as an alternative to radar or working in conjunction with existing radarsystems. During a transition phase, there will be mixed coverage of ADS-B and radarsurveillance.Track data based on ADS-B; radar or merged (fused) ADS-B/radar data is presented on ATSdisplays. The quality of ADS-B data (based on precise GNSS position and courseinformation) information is superior to radar-only data and therefore provides a better basisfor predictions made by the ground system. This capability is further enhanced by thesupplementary intent information in ADS-B reports.Surface movement surveillance (& Navigation)Advanced Surface Movement Guidance and Control System (A-SMGCS) will become anessential means for maintaining maximum capacity and safety in low visibility conditions athigh-density airports.VDL Mode 4 provides a flexible communication, surveillance and navigation backbonewhich supports the creation and operation of A-SMGCS, providing for example:• ADS-B data to support the ground movement surveillance system;• ADS-B combined with CDTI for support of guidance on the ground, surface navigation,

situational awareness, and collision avoidance;• a two-way data link to support automated controller-pilot communications;• up linked GNSS augmentation to support aircraft navigation in poor visibility;• a communication link to assist airline operators in the surveillance and control of support

vehicles.Search and Rescue (SAR)In Search And Rescue (SAR) operations, VDL Mode 4 services could be used to providesurveillance services to support, for instance:• provision of an overall situation display to support SAR activities and coordination of

resources including participating vessels;• point out last known position from disabled aircraft or vessel;• aid to visual acquisition;• separation assurance.

2.2.2.2 Communication applications

Standardisation effort is ongoing within the industry for the applications described below:

Controller-pilot data link communications (CPDLC)

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Controller Pilot Data Link Communication (CPDLC) illustrated in Figure 3-1 is an ATNfunction providing point-to-point communications of time-critical data between pilots andcontrollers.Departure clearance (DCL)DLC could be described as a variant of CPDLC for semi-automated data link exchange ofmessages between an aircraft and the control tower (TWR) prior to commencement of taxiingfor take-off.Flight Information Services (FIS)FIS is a ground-generated communication function that provides time sensitive weather andsupporting information to the aircraft. Although the information can be carried via the point-to- point services provided by the ATN services of VDL Mode 4, an alternative and moreefficient method is to broadcast such information to all users using the broadcast services ofVDL Mode 4.

2.2.2.3 Navigation applications

GNSS augmentationWhen using GNSS data for navigation or surveillance, a GNSS augmentation system isrequired to ensure the quality of the position data. Combined with a GNSS reference systemfor computing differential corrections and integrity data for satellites in view of the groundstation, VDL Mode 4 can be used to broadcast a differential GNSS signal using the GNSSaugmentation. Air and ground mobile users can receive differential GNSS augmentationsignals when a VDL Mode 4 ground station is within line-of-sight. This augmentationprovides navigation application, which is complementary to GBAS.

Service for wider geographical area can be provided through a network of ground stations,which are connected through a ground network, typically located at airports would extend thecoverage of the augmented system. The network could be used for monitoring of theoperation and possibly for exchange of information between individual stations in order tofurther enhance the integrity of the broadcast augmentation data. The augmented networkcould be expanded indefinitely.Overall objective is to achieve complementary systems that can support navigation in allphases of flight for gate-to-gate operation including A-SMGCS. Galileo is a component ofGNSS for augmentation and therefore would be used as a complementary component toprovide navigation during the various phases of flight. In order to achieve this functionalityand implement it on a large scale, standardisation of this architecture and generation ofstandards would be required. This approach is currently not under consideration within thestandardisation organisations such as Eurocae, ICAO etc.

2.2.2.4 Secondary Navigation in VDL Mode 4 based on triangulation

VDL Mode 4 is capable of providing navigation without GNSS signals as a backup orfallback function in case of primary navigation failure in the GNSS receiver or GNSS signal.The secondary navigation function, which is defined as an optional element within the SARPsfor VDL Mode 4, provides for navigation to en route accuracies (RNP 1) without reliance on

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GNSS. In remote/oceanic airspace, a user with a failed GNSS receiver can navigatesuccessfully by relying on the UTC-synchronised Mode 4 synchronisation bursts of otherusers in the airspace (who determine their own position and accurate time either via GNSS orother system with similar performance). In domestic airspace, the secondary navigationfunction allows a civil aviation authority to use its VDL Mode 4 ground infrastructure as abackup to GNSS. This assumes a stable source of time is available to the associated ground-based radios.

The secondary navigation concept is similar to that of GNSS, illustrated in Figure 2-5. InGNSS, pseudo-noise (PN) ranging signals transmitted by the GNSS spacecraft are slaved toUniversal Co-ordinated Time (UTC). These signals emanate from known points in spacesince the ephemeredes of the spacecraft are known. The arrival times of the signals at a GNSSreceiver are measured relative to the local clock of the receiver. If the GNSS receiver issynchronised to UTC, the signal flight time (propagation time) divided by the speed of lightyields the range to spacecraft, which generated the signal. If the GNSS receiver is notsynchronised to UTC, the apparent propagation time yields a so-called "pseudorange". In thegeneral case the receiver is not synchronised, and must solve for its (x, y, z) position as wellas its clock offset. These four unknowns can be determined if four measurements with goodrelative geometry are available. If more than four measurements are available, the system isover determined and can be solved with reduced error and redundancy.

The key elements of GNSS are:

• a set of transmitting platforms at known locations which transmit time-synchronisedranging signals, in sufficient numbers to provide a minimum of four ranging signals withgood geometry to the user community with high probability;

• user equipment able to receive and measure the arrival times of these signals, andperforms the necessary computations to triangulate its own position.

The fact that the transmitting platforms are moving at orbital speeds is not significant, as longas their paths of motion are known with high accuracy and their locations at any instant oftime can be determined to within a few meters. GNSS performs well because the signals aresynchronised with great accuracy; the positions of the transmitting satellites are measured andreported with great accuracy, and the ranging signals have sufficient bandwidth to allowhighly accurate measurement of arrival time.

The key elements of a position determination system are available through the VDL Mode 4as well: (1) numerous transmitting stations reporting their position with fair accuracy; and (2)time-synchronised signals from these same platforms. In the case of VDL Mode 4, thetransmitting stations of interest for secondary navigation are those aircraft or ground stationsthat report that they are time-synchronised, and whose reported positioning figure of merit isgood. Figure 2-6 illustrates the basic concept, which parallels that of GNSS.

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G N S S S 1(P 1 , d t1)

G N S S S 2(P 2, d t2)

G N S S S 3(P 3 , d t3)

G N S S S 4(P 4, d t4)

R 1 R 2 R 3R 4

• S a te llite p o s itio n s d e te rm in ed fro m ep h em erid es• S a te llite U T C c lo ck o ffse ts a lso read fro m m sg s• P seu d o ra n g es (R i) fo u n d re la tiv e to lo c a l c lo ck• U se r p o s it io n (P 0) an d c lo ck (d t0) a re ca lc u la te d

(P 0, d t0)

• P = (x , y , z )• t = tU T C = t i + d t i• s ig n a l flig h t tim e = m i + d t0 - t i - d ti

Figure 2-5 Overview of GNSS Navigation Concept

The position solution determined through the VDL Mode 4 that will be less accurate than thatof GNSS for the following reasons:

1. In most cases, the uncertainty in the reported position of a user is larger than theuncertainty in the reported position of a GNSS satellite;

2. The transmissions are emitted with greater time jitter and uncertainty than thetransmissions of a GNSS satellite; and

3. The arrival time measurements at a receiving station are not as precise as the arrival timemeasurements performed in a GNSS receiver.

Nevertheless, a position solution can be generated. The draft SARPs for VDL Mode 4 specifytransmit timing jitter ≤ 1 usec (2σ). This corresponds to a range uncertainty of 300 m (~1000ft). If signal arrival times can be measured to equivalent accuracies (approximately 1 usec(2σ)), ranging errors on each signal would be bounded by 500 m (2σ) and users experiencinggood geometry would achieve positioning accuracies on the order of 1 km or better. Thiswould be the typical performance for users in active airspace (where many suitably-equippedusers would be located in all directions about the user), or users in airspace where the CAAhad explicitly designed the ground infrastructure to provide acceptable geometry (a networkof ground stations could be completely independent of GNSS, and could provide enhancedperformance2 due to their highly accurate position reports and possibly their more accuratetransmit timing).

2 Relative to that achieved via ranging from mobile units.

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• Requires measurement of syncburst arrival time to a smallfraction of a symbol period

– achievable with careful design• User position and time derived

from multiple external stations(3 or more) + altimeter

– expected accuracy ~ 1-2 km• System time (w/o position)

derived from autonomous nav(Loran, INS, ...) plus one externalstation

Full backup for GNSS sufficient to maintain communications, and navigate in en route airspace.

Similar toGNSS

Figure 2-6 Secondary Navigation in VDL Mode 4

2.2.2.5 Operational benefits of secondary navigation

The operational benefits of secondary navigation are:

• A backup navigation source for Galileo for conditions where Galileo signals are notavailable for enroute navigation due to failure, masking or other reasons.

• A robust means to maintain efficient communications and resource sharing in the VDLMode 4 protocol.

• Ability to generate an independent range estimate to a user (i.e., by estimating thepropagation time of the received message) supports enhanced integrity features at theMAC layer and above.

• In domestic airspace, secondary navigation in combination with a ground network whichtransmits synchronisation bursts on the Global Signalling Channels (GSC) can provide afull GNSS backup suitable for en route navigation.3 This robust, locally controlled backupmay ease the global introduction of GNSS as a primary means of navigation and allowsfor an early introduction of GNSS navigation using Galileo satellites.

• Airborne and/or ground stations provide a backup to isolated users with failed GNSSequipment.

3 This requires an independent source of accurate time at the ground stations, and sufficientredundancy of ground stations to provide coverage by 3 or more stations, with high probability, tousers at or above a designated altitude

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2.3 VDL MODE 4 LOCAL PERFORMANCES

The standardisation work at ICAO and other standardisation institutions for CNS services isunderway; Table 2-1 shows the expected target performance for the Communication,Navigation and Surveillance services. Only Terminal and APV-I, navigation operation usingdifferential GNSS correction broadcast via VDL Mode 4 data link is considered here. Theperformance shown here is complementary to the performance provided by GBAS for theservices shown in the table.

Table 2-1 VDL Mode 4 Target Performance for CNS Services

VDL Mode 4Performance

Communication(FIS-B; Point-to-point)

Navigation(APV-I)

Surveillance(ADS-B; TIS-B)

Integrity 1 - 1 x 10-5

per message1 - 1 x 10-7

per hour1 - 1 x 10-6

per target

Continuityof Service

1 – 1.6 x 10-4

per hour1 –5 x 10-5

per hour1 – 1.6x 10-4

per hour

Availability 0.999 0.9998 0.999

MTBO 20.000 hours 20.000 hours 20.000 hours

These performance figures are still under development but it can be seen from the open accessservices of Galileo and VDL Mode 4 that these figures can be achieved easily. Thenavigation performance would be achieved using differential Galileo signals generated via anetwork of ground stations as described in section 3.3.1. Currently ICAO GBAS SARPs doesnot specify the use of VDL Mode 4 data link for GBAS for Category I precision approach,however using Galileo and differential Galileo signals the navigation performance shown inTable 2-1 could be achieved. Spectrum planning which is an ongoing process at ICAOcurrently supports the use of VDL Mode 4 in the aeronautical navigation band forsurveillance only; hence additional institutional issues are required to be addressed for thisconcept to be implemented.

2.4 VDL MODE 4 COVERAGE

The planned service volume (range) is 200 NM air-to-air, air-to-ground and ground-to- air.When the load on the channel exceeds 90% then the service volume (or “cell”) around a usershrinks as described in Figure 2-8. (The “Robin Hood” principle). However, the minimumcell size is 150 NM, and the transmission rate is always kept constant out to the edge of thecell.The Robin Hood principle allows a station operating on a busy channel to use slots previouslyreserved for broadcast transmission by another station as long as slots reserved by the mostdistant stations are chosen in preference to those of nearer stations. This results in a gracefulreduction in the broadcast range of a station on busy channels as illustrated in Figure 2-8

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Figure 2-7 shows the 200 nm coverage distance in actual flight tests conducted at theStockholm/Arlanda airport. The results were similar for both air to air and air to groundbroadcasts.

Figure 2-7 VDL Mode 4 Coverage from Flight Test at Stockholm/Arlanda Airport

Figure 2-8 Illustration of Cell Shrinkage using Robin Hood principle

2.5 VDL MODE 4 TRANSCEIVER ASPECTS

2.5.1 VDL MODE 4 TRANSCEIVER

The VDL Mode 4 system is a transponder that provides for a number of transmitters andreceivers capable of transmitting at any 25 kHz channel in the 112 – 136.975 MHz band andreceiving at any 25 kHz channel in the 108 – 136.975 MHz band. Typically the systemconsists of one transmitter and two receivers where the signal transmission and reception is

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via one transmit and one receive VHF antenna. It is possible to use one antenna interface fortransmitting and receiving. The GNSS subsystem of a VDL Mode 4 transponder alsoaccommodates one GNSS antenna interface that functions as interface to the GNSS Signal inSpace and a GNSS subsystem.

The GNSS system could be based on Galileo or GPS or a combination of GPS and Galileo.The GPS component of the hybrid transreceiver would comply with the existing ICAO,Eurocae and other international aviation organisation’s specifications and standards.

The GNSS subsystem is used for both Navigation and Surveillance. Enroute and terminalnavigation operations are performed using GNSS based positioning data. Precision approachis performed using differential GNSS signals. GNSS based position reporting is used forADS broadcast for surveillance functions. The transponder has the capability to receive dataon multiple VHF channels and transmit on one channel simultaneously. A single transmittercan typically report on several VHF channels by changing channels between transmissions.This feature provides full ADS-B reporting and monitoring capability.

Figure 2-9 shows a block diagram of a typical VDL Mode 4 transponder. The transpondertypically contains one transmitter and two receivers. The embedded computation subsystemperforms all the software necessary to perform the necessary STDMA, slot allocation andother control of the transmitters and receivers to perform the VDL Mode 4 functions asdescribed in section 2. The embedded computer also process the GNSS data for interface withthe external equipment for Navigation and Surveillance functions. The computation blockwould also perform the input/output interface and control with the external equipment. Itshould be noted that within the computation block the processing of the Communication,Navigation and Surveillance functions are partitioned such that the failure of one functiondoes not affect the operation of the other function. Brick wall partitioning is implementedalong with extensive safety analysis to ensure that the partitioning is robust and no commonmode failures exist.

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Figure 2-9 Block Diagram of a Typical VDL Mode 4 Transceiver for CNS applications

The number of transmitters and receivers in a VDL Mode 4 transponder and for an aircraftdepend on:• the services required, e.g. ADS-B, FIS-B, ATN and non-ATN point-to-point

communications; Navigation functions for enroute and terminal and augmentation withGalileo for precision approach capability

• the quality of service (QoS), including possible sharing of channels for different services;• the number of aircraft to be supported in a given environment at a given reporting rate.

Multiple channels may be needed to accommodate a large number of aircraft. Thisimpacts on the required transceiver configuration.

• the redundancy and availability of services required

In most applications a single VHF antenna for transmitting and receiving is sufficient.

2.5.2 HYBRID TRANSCEIVER

As described in section 2.5.1 a VDL Mode 4 system consists of a number of transmitters andreceivers, which operate, in the VHF band. Also embedded in the VHF Transponder is aGNSS receiver. This receiver is typically a GPS receiver. As a complimentary system toGalileo the GNSS receiver would be a hybrid receiver. The GPS component of the hybridreceiver would comply with the existing ICAO, Eurocae and other international aviationorganisation’s specifications and standards. The hybrid GNSS receiver would also complywith the ICAO, Eurocae and other international specifications and standards for aviation thatwould be developed.

GNSS Receiver

VHF Transmitter

VHR Receiver

VHF Receiver

ComputationSubsystem

External Interface Input

External InterfaceOutput

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The hybrid receive would contain the functionality of a Galileo receiver. This receiver wouldbe able to track and decode the navigational ranging signals. These signals would be used toprimarily compute Position, Velocity and Time and other navigational parameters. A GPSreceiver and an EGNOS receiver will also augment the Galileo receiver and other navigationGNSS signals such as WAAS etc. could also be included in the Hybrid receiver, referencefigures Figure 2-10and Figure 2-11. Within the VDL Mode 4 transponder there iscomputational capability provided to compute the best possible navigation solution based onthe characteristics of the raw ranging signals. Because of the multiplicity and redundancy ofnavigational signals which include VDL Mode 4 (described in section 2.1) as a backupnavigational source a navigation solution which has high integrity and accuracy better thanthat of standalone GPS with virtually 100% availability and a high continuity of function isprovided. Improvement in performance due to the different components of the hybridreceiver is shown in Table 3-1. This table shows how the VDL Mode 4 and Galileocomplement each other and bringing benefits to each other along with a method for earlyimplementation of Galileo.

Figure 2-10 Detailed Hybrid GNSS Transceiver

VHF Transmitter

VHF Receiver

ComputationSubsystem

External Interface Input

External InterfaceOutput

VHF Receiver

GPS Receiver

GalileoReceiver

EGNOSReceiver

Other GNSSReceivers

Best NavigationSolution

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2.5.2.1 Spectrum for Hybrid Transceiver

As shown in Error! Reference source not found. VDL Mode 4 for aviation use operates inthe aeronautical VHF spectrum band, i.e. 108-136.975 MHz, Galileo ranging signals for openaccess are broadcast on L1, E5a and E5b band. The masks defined for VDL Mode 4equipment should prevent any harmonics or interference signals being generated that wouldcause problems to the Galileo or Hybrid receiver. Conversely the Galileo and Hybridreceivers should be so designed to have immunity similar to that defined currently for GPSreceivers.

Figure 2-11 VDL Mode 4 and Galileo Spectrum Allocation

E5A/E5B E6 E2 E1L1

1164

MH

z

1215

MH

z

1260

MH

z

1300

MH

z

1559

MH

z

1591

MH

z

108

MH

z

136.

975

MH

z

VDL Mode 4 Galileo Signals

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3 COMBINATION IN CRITICAL ENVIRONMENT

3.1 CRITICAL ENVIRONMENT

Critical environment considered for CNS applications are areas where the CNS functions arelimited. These limitations are due to various reasons, such as terrain, limited space, hightraffic volume, interference, lack of radar or radar coverage etc.

3.2 VDL MODE 4 AND GNSS IN CRITICAL ENVIRONMENT

The Surveillance function of ADS-B service and Navigation are interdependent for whichVDL Mode 4 and Galileo form a complementary system. ADS-B- as defined, developed andtested within several EC projects such as NEAN, NEAP, NUP, MEDUP, MFF, comes as aservice bundled with VDL Mode 4 when augmented with Galileo as described here builds onthis inter-dependence and bundling. These programs demonstrate that the VDL Mode 4 andGNSS form complimentary systems where timing and navigation redundancy is provided byeach system allows for continuos operation in areas where signals are interrupted due toterrain, interference and other critical environmental situations.

It should be noted that the overall objective of airport authorities and service providers is tobe able to provide navigation capability for a full up gate to gate operation. New technologiesare investigated and exploited to ensure that these operations can be achieved to the extentpossible as well as to provide full performance in Critical Environment.

The sections below describe VDL Mode 4 and augmentation components in the Galileocontext. The objective of this is to propose a comprehensive and homogenous approach ratherthan dealing with the VDL Mode 4 and augmentation components as independent LocalComponents. The systems being interoperable combine assets of different Europeandevelopment programs into an integrated approach for Aviation Users. Some of theseprograms are the Mediterranean Update Programme (MEDUP) and Mediterranean Free Flight(MFF) programme.

3.3 EXTENSION TO GALILEO

3.3.1 BRIEF DESCRIPTION OF VDL4- AUGMENTATION WITH GNSS

VDL Mode 4 can be augmented with GNSS including Galileo to provide a service thatprovides GNSS augmentation data to mobiles in all user groups of the aeronautical transportsector. This augmentation can be extended by a network of ground stations that provide thisaugmentation. The overall long term objective of this augmentation is toto support all usergroups and all phases of operation. The requirements on accuracy, integrity, alert limit, etc.vary for different operations. In order to use the VDL Mode 4 efficiently, differentoperational service levels have been considered each intended for different operations and/orapplications. Such a service could provide an enhancement of performance whereby VDL

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Mode 4 and Galileo complement each other. This augmentation concept has been extensivelytested in EC projects such as NEAN, NEAP and NUP. Tests were led by Sweden and arecurrently being also tested in the MEDUP and MFF programs.

A VDL Mode 4- system , which provides GNSS augmentation service by which the userreceives information directly from ground-based transmitters, allows continuous reception ofthe service over a large geographical area (200NM+). The ground components may beinterconnected in a network. This network is basically made up of multiple ground stationswith overlapping coverage. The stations are networked together in order to provide coverageover a larger area than that provided by a single ground station. Such a network can be usedin:• High latitude locations• Gate-to-gate surveillance and navigation operation for aviation• Global Coverage• Is unencumbered by Institutional issues• Provides Redundancy• Reduction of Liability due to Redundancy

VDL-4 is a particularly effective communication link for multiple CNS (Communication,Navigation, and Surveillance) applications, such as ADS-B, FIS-B, TIS-B, A-SMGCS, andCPDLC. Therefore, it is expected that most high value air traffic will be within reach of oneor more VDL-4 Ground Stations whenever these aircraft are near land. As a result, VDL4-augmentation will be able to provide high accuracy and high integrity GNSS for all suchtraffic. In addition, the same accuracy and integrity will be available for Surface MovementGuidance and Control of aircraft and of support vehicles at any airport equipped with aVDL-4 Ground Station. When integrating multiple services such as Communication,Navigation and Surveillance in a single system, care must be taken in partitioning andappropriate system and safety analysis should be performed to ensure no single point orcommon cause failures exist and the reliability and integrity of each service meets thespecification.

The liability issue, which is associated with loss of service due to the failure of the primarysystem, without a backup system of equivalent performance. VDL Mode 4 and Galileoprovide a reduction in liability by providing complimentary backup to secondary timing andnavigation for each other.

3.3.2 VDL4-AUGMENTATION WITH GALILEO FOR AVIATION

Galileo and its’ potential Local Components provides the navigation service delivering highprecision and integrity service to the users. In aviation, this service will be utilised by thesurveillance function to provide an accurate position with high integrity. This inter-relationbetween the navigation and surveillance services and systems is critical and is addressed inthis section.

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Automatic Dependant Surveillance Broadcast (ADS-B) is a surveillance function that will useGalileo and its’ Local Components and is generally seen as an enabler for true growth ofcapacity and increased safety in European Airspace.

The purpose of VDL4-augmentation is to provide greatly enhanced GNSS surveillance andnavigation accuracy and integrity throughout any region served by VDL-4. As a direct result,the performance of ADS-B over the VDL-4 data link will also be enhanced because of theimproved GNSS accuracy and integrity provided by the Galileo component.

Galileo augmentation of VDL4 provides improved GNSS accuracy, integrity and availability.The multiple applications inherently requires different service levels, since the requirementsfor en-route navigation is very different from approach or surface movement applications.The different service levels are discussed in the next section.

Enhanced VDL4-augmentation using GPS and Galileo satellite navigation system will bedesigned to allow support for:• All phases of flight (seamless gate-to-gate) some of the gate-to-gate operations may need

additional augmentation;• Multiple service levels to support the above;• Multiple applications down to non-precision approach with vertical guidance (APV-I and

II) and Advanced Surface Movement Guidance and Control (A-SMGCS) and;• For all user groups.Table 3-1 shows the improvement in performance when VDL 4- is augmented by GNSS as aprogression when the GNSS complement is changed from GPS to Galileo and GPS + Galileohybrid. Since the specifications are still under development, the exact quantification ofperformance figures is not feasible currently. However, it can be easily seen that there is animprovement in accuracy by the use of Galileo instead of GPS and there is a considerableimprovement in availability by the combined use of GPS and Galileo. VDL Mode 4 beingable to be used as a secondary navigation source as described in section 2.2 additionallycompliments this improvement.

Figure 3-2 shows an actual VDL Mode 4 Airborne Transceiver and Ground Station used forCommunication, Navigation and Surveillance as part of NEAN Update Program. Thesesystems are currently operational in Sweden, Germany, France and Denmark.

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Figure 3-1 Basic VDL4-Augmentation with Galileo Concept

Table 3-1 VDL4-Augmentation Performance Improvement progression

PerformanceParameters

Augmentationusing GPS

Augmentation usingGalileo

Augmentation using GPS +Galileo

Accuracy (2dRMS) <10 m + =

Availability (%) 99 – 99.7 + ++IntegrityTime to Alarm

6 sec = =

SecondaryNavigation

+ + ++

Reduction ofLiability

+ + ++

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VDL4 Airline transponder

VDL4 General Aviation transponder

VDL4 CNS Ground station

Figure 3-2 VDL Mode 4 Airborne and Ground Subsystem for CNS/ATM

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3.3.3 VDL4-AUGMENTATION WITH GALILEO FOR MULTIMODAL APPLICATIONS

VDL Mode 4 and GALILEO play an important role in multimodal applications involvingAviation, Maritime and Land applications. Search and rescue is an example of a multimodalapplication, which could be envisaged through VDL Mode 4.Intermodal is defined as transportation using at least two modes of conveyance withoutchange of container.Multimodal is defined as transportation using at least two modes of conveyance, allowing ashipment to change between conveyance container (e.g. Air Cargo).

Examples of Applications are:• Real time origin - destination tracking of containers and swap bodies• Remote monitoring of container seal integrity, esp. for high value goods• Monitoring of shipment condition (perishables and dangerous goods)• Telematics & fleet management• Search and Rescue

Benefits include:• More accurate and timely dispatch of resources• Increased transparency of Supply Chain• Less production disruptions in JIT / sequenced deliveries• Higher Security => lower insurance premiums• More effective and efficient tracking & tracing

System requirements for intermodal and multimodal applications include:• Easy to use, easy data access• Protection of sensitive / proprietary information• Robust & reliable• Tamper proof• Low investment• Low operating costs• Global coverage• Integration of mode specific information (e.g. UIC)• Open system architecture• More than tracking capability

3.3.3.1 Universal Automatic Identification System for Maritime

The emerging ship and shore-based broadcast system within Maritime is called Universal AIS(or AIS, as it is commonly known).

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AIS uses the STDMA technology which is also used by VDL Mode 4 although it is simpler ingeneral. It uses a lower baud rate, less protocols etc. VDL Mode 4 is the more advancedgeneration of STDMA/AIS. Clearly, the close relationship facilitates multimodal solutions.AIS includes the Navigation and surveillance piece, i.e. GNSS Augmentation and Positionbroadcast (including identification), a variant of “ADS-B for ship”. In the maritime world it isusually referred to as “Ship to Ship” and “Ship to Shore”. AIS also supports communicationof short messages between ships. A user interface with a digital moving map providesimproved navigation capability as well as situational awareness by showing other equippedships.

An AIS station is a VHF radio transceiver capable of sending ship information such asidentity, position, course, speed, length, ship type and cargo information etc., to other shipsand to suitable receivers ashore.Information from an operational shipboard AIS unit is transmitted continuously andautomatically without any intervention of the ship’s staff.When used with an appropriate graphical display, shipboard AIS enables provision of fast,automatic and accurate information regarding risk of collision by calculating Closest Point ofApproach (CPA) & Time to Closest Point of Approach (TCPA) from the positionalinformation transmitted by target vessels.Therefore, AIS will become an important supplement to existing navigational systems,including radar. In general, data received via AIS will enhance the quality of informationavailable to the ship's staff. AIS is an important tool to enhance the awareness of the trafficsituation for all users.

PurposeThe purpose of AIS is to:

• identify vessels,• assist in target tracking,• simplify and promote information exchange,

Use of AISThe International Maritime Organisation (IMO) specifies three main applications for AIS:

1. Ship to ship, for collision avoidance.2. For littoral states, in order to obtain information about ships and their cargoes.3. As a VTS tool, for traffic management.

AIS is an additional source of navigational information. AIS supports, but does not replacenavigational systems such as radar target tracking and VTS.

In general, AIS tracking offers the following significant benefits:

• highly accurate information,• provided in near real-time,• capable of instantaneously presenting target course alterations,

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• not subject to target swap,• not subject to target loss in clutter,

• not subject to target loss due to fast manoeuvres, and

• ability to ‘look’ around bends and behind islands.

Furthermore, AIS can:• ‘look’ behind the bend in a channel or behind an island in an archipelago, to detect the

presence of other ships and identify them.

• predict the exact position of a meeting with other ships in a river or in an archipelago.

• know which port and which harbour a ship is bound for

• know the size and the draft of ships in the vicinity.

• detect a change in a ship’s heading almost in real time

• identify a ferry leaving the shore bank in a river.

Figure 3-3 Overview of the AIS

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4 RECOMMENDATIONS

4.1 INTEROPERABILITY

Spectrum InteroperabilityVDL Mode 4 operates in the aeronautical VHF spectrum band, i.e. 108-136.975 MHz andtransmits digital data in a standard 25 kHz channels and divides these channels into a largenumber of time slots. The start of each slot is an opportunity for a station to transmit. VDLMode 4 is built on the Self-organising Time Division Multiple Access (STDMA) concepts, inwhich the time-slots are synchronised, to UTC, and stations advertise their intention totransmit in a specified time-slot by means of a reservation protocol carried in a priortransmission.

Galileo ranging signals for open access are broadcast on L1, E5a and E5b band. Four signalsare transmitted in the frequency range 1164 – 1215 MHz (E5a – E5b) and three signals aretransmitted in the frequency range 1559-1591 MHz (L1). Each navigation signal consists of aranging code and data. There are different types of ranging codes and different types of datathat can be used for Galileo signals. The ranging code is a sequence of –1 and +1 withspecific characteristics in the time (code length) and frequency (chip rate) domains. There isone unique sequence for each signal coming from a unique satellite. The actual sequencegives an identity of the satellite from which the signal is coming.

Hence it can be seen that VDL Mode 4 and Galileo operate in two different frequency bandswith different modulation schemes and hence there is very low probability of either systemsprimary signal or the harmonics interfering with the other system. Further VDL Mode 4 hasbeen coexisting with GPS receivers in ADS-B applications with no interoperability issues.ICAO supports the use of VDL Mode 4 in the aeronautical navigation band for surveillance.

Operational InteroperabilityVDL Mode 4 is a bi-directional datalink designed to provide Communication, Navigation andSurveillance data on its assigned frequency. Some of the functions of these services are listedbelow:• Communication

− Controller-pilot datalink communications− Departure Clearance− Flight Information Services

• Navigation− GNSS Augmentation− Surface Movement Guidance and Control System• Surveillance

− Automatic Dependent Surveillance Broadcast− Cockpit Display of Traffic Information

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− Traffic Information Service− Air to Air Surveillance− Surveillance of Surface movement vehicles

Galileo provides navigation ranging signal in its assigned spectrum. Hence no operationalinteroperability issues exist between VDL Mode 4 and Galileo signals. The two systems arecomplimentary and co-exist and provide mutual benefits and it is recommended that furthertechniques and compliments between these two systems be continuously evaluated.

4.2 CERTIFICATION AND STANDARDISATION

4.2.1 CERTIFICATION OBJECTIVE

The certification4 objective is to certify VDL4-augmented with Galileo extension as a primarymeans of navigation for gate-to-gate operation. There are regulations that exist for thevarious different segments of gate-to-gate operation such as, en-route navigation, precisionapproach and landing etc. However, regulations have not yet been developed for segmentssuch as taxiing and ground surface movement operations. The guidelines and certificationrequirements for these segments need to be developed so that full benefit of VDL4-augmentation with Galileo extension can be realised through all phases of flight.

Certification Guidelines and performance requirements are being developed by internationalorganisations such as ICAO, EUROCAE etc. These guidelines/requirements are the basis ofmost certification criteria. Individual States and regions may apply additional requirementsand regulations depending upon the usage and operation.

Overall certification objectives should be toward certifying VDL Mode 4 and Galileo ascomplimentary systems to be able to be used for operations to support full gate-to-gatescenarios.

4.2.2 CERTIFICATION PROCESS

Figure 4-1 shows a typical Certification/Development process used by most Regulatory andIndustrial organisations. The process shows how the regulatory authorities interact with theindustry in the development of architecture and design so that the system development anddesign is robust and the certification is achieved in an efficient manner.

Key elements of system design and certification are system performance and safety. Figure4-2 shows a typical safety allocation process. This certification process is intended to be usedfor certifying the airborne and ground segments of VDL4-augmentation with Galileo 4 Please note that the regulatory authorities use the term “Certification” for Aircraft and Aircraftsubsystems. The term “Type Approval” is used in the context of Ground Systems such as ILS, GBASetc. In this section the term “Certification” is used for Airborne and Ground Systems.

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extension will be based on the existing process used for similar operations. Currently thereare European Joint Airworthiness Authority and Local/State and Regional Authorityguidelines and certification process defined for certifying equipment to be used for differentphases of flight, a guideline on this can be seen in Table 2-1. These guidelines show howcompliance to requirements can be demonstrated to achieve certification. The guidelinesinclude system methodology to be used for system safety, software development and otherdesign practices.

Figure 4-1 Typical Development/Certification Process used in Civil Aviation

Conceptual Design andArchitecture Defined

Preliminary DesignCompleted

This is the appropriate time toinitiate certification project

Detailed Design Complete

Certification Plan is prepared andsubmitted to the Certification

Authority for review and approval.Plan will address the System Safety

Assessment and The SoftwareAspects of Certification

System Testing Complete

Installation in Aircraft andCertification Testing

Completed

Testing Plans and System SafetyAssessment prepared and

submitted to the CertificationAuthority for Review and Approval

Flight Test Plan and remainingDesign Approval documents

submitted to Certification Authorityfor Review and Approval

Regulatory/CertificationAuthority issues Certificateand Operational Approval

Close consultation with RegulatoryAuthority's Engineering Personnel isrecommended during the DesignProcess

Regulatory Authority witnesses manyof the System Tests for Certification

Regulatory Authority witnesses all of theFlight and Ground Tests conducted on anAircraft for Certification

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Figure 4-2 Top Level Safety Allocation Process

The main objectives of the certification process is to demonstrate that the aircraft and relatedsystems perform their intended functions with a target level of safety. A certification processmay be entirely performed by a single National Airworthiness Authority; it may involvespecific bilateral agreements between national authorities, or may be a collaborative jointJAA effort.

Table 4-1 Certification Authorities for Various Segments

Segment Applicant Authority Type of CertificateA/C Airline The Airworthiness Authority

National CAA - JAAOperational ApprovalSupplemental TypeCertificate (STC)

A/C Aircraft Manufacturer The Airworthiness AuthorityNational CAA - JAA

Type Certificate (TC)Amendment to the TC

A/C Installation supplier The Airworthiness AuthorityNational CAA - JAA

Supplemental TypeCertificate (STC)Amendment to the TC

Ground ATS Service Provider Regulation Commission of NationalATS ProviderSRC

Operational approval

Operation Environment DefinitionOED

Operational Hazard AssessmentOHA

Allocation of Safety Objectives and RequirementsASOR

Airborne Segment Ground Segment

Operational Safety Assessment

OSA

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Figure 4-3 Typical Safety Assessment Process Model

Requirements that are defined to prevent failure conditions or to provide safety relatedfunctions should be traceable through the levels of development at least to the point ofallocation to hardware and software. This will ensure visibility of the safety requirements atthe software and hardware design level.

CommonCause

Analysis(CCA)

Aircraft LevelRequirements

Allocation ofAircraft Funtions

to Systems

Development ofSystem

Architecture

Allocation ofRequirements toHardware and

Software

SystemImplementation

Aircraft levelFunctional Hazard

Assessment(FHA)

System LevelFunctional Hazard

Assessment

PreliminarySystem SafetyAssessment

(PSSA)

System SafetyAssessment

(SSA)

Certification

AircraftFunctions

Failure Condition, EffectsClassifications, SafetyRequirements

SystemFunctions

Failure Condition, EffectsClassifications, SafetyRequirements

ArchitecuralRequirements

SystemArchitecture

Implementation

Results

Separation &Verification

SeparationRequirements

FailureConditions &

Effects

Item Requirements

Physical System

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Figure 4-4 Software Life Cycle Process

In order to ensure the correct performance of software and its development to the criticalitylevel determined during the Safety Assessment process, a software life cycle and developmentprocess is to be used as defined in Eurocae document ED-12B. This process is shown inFigure 4-4.

4.2.3 DEVELOPMENT AND CERTIFICATION STANDARDS

VDL Mode 4 with Galileo and other satellite systems (EGNOS, GPS, etc.) supports the fullCNS (Communication, Navigation and Surveillance) functions, details of these functions andservices are provided in section 3.3. The development and certification standards for variousoperations and their performances to support these services are being generated by a numberof international organisations. ICAO (International Civil Aviation Organisation) is one of themain bodies, which provides the basis for the performance and certification of aircraft and

Software Configuration Control

Software Quality Assurance

SoftwarePlanningProcess

� SoftwarePlans

� SoftwareStandards

Requirements Design Coding Integration

RequirementsVerification

DesignVerification

CodeVerification Testing

Software Life Cycle

System Life Cycle Process

System Safety Assessment ProcessSy

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ground systems. These standardisation activities are on going activities and should continueto develop standards to support full up gate to gate operation using VDL Mode 4 and Galileoas complimentary systems.

Currently there are ICAO standards specifying the use of VDL Mode 4 for surveillance. Useof VDL Mode 4 for FIS-B, TIS-B are ongoing activities. There are currently no standards forthe use of VDL Mode 4 for navigation. Use of Galileo as a component of GNSS and use forprecision approach is an ongoing activity at ICAO.

The documents generated by ICAO are the SARPs (Standards and RecommendedProcedures). A number of regional organisations such as Eurocae, JAA, etc., providesupporting material for the SARPs prepared by ICAO. These supporting material aretypically MASPS (Minimum Aviation System Performance Specifications), MOPS(Minimum Operational Performance Specifications) etc. produced by Eurocae which are thebasis for JTSO (Joint Technical Standards Order), JARs (Joint Airworthiness Regulations)and other regulatory and guidance standards generated by JAA.A number of the specific documents mentioned above are listed in the list of referencedocuments in section 1.1.2

4.3 LEGAL ASPECTS

The Individual State or region where the equipment would be operated governs legal issues.These legal issues would be based on the purpose for which the equipment is being used.For example for the purpose of civil aviation legal requirements exist on equipment which areoperated for various operational modes. Individual states at minimum require thedemonstration that the system meets and performs all the functions its intended to perform.These issues also cover interoperability, interference, susceptibility and other local andregional requirements. Depending upon the criticality of the operation for which theequipment is intended to be used the severity of integrity and performance requirementslevied are more stringent.

One of the legal aspects is the liability issue, which is associated with loss of service due tothe failure of the primary system, without a backup system of equivalent performance.Complimentary systems examined in this document could help reduce liability issues.Reduction of liability is also a benefit for individual states and reduces the exposure byproviding back up systems for primary functions.

4.4 SECURITY ISSUES

The Ground infrastructure of the VDL Mode 4 CNS system would require Security issues tobe addressed. State and regional authorities do have general guidelines on the security issuesto be addressed during the design and implementation of ground subsystems of avionicssystems providing critical functions and data. CNS Ground Station and each of its sub-systems should be designed and installed so as to prevent unintentional as well as

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unauthorised alteration of system operation. Following are some of the securityconsiderations for such systems:

• It is required that IT Security be considered during the concept and design phase of theequipment and that Risk Analysis be the basis of the Security designs.

• Examination of the IT Security Requirements has to be part of the development checklistprocess.

• Security configuration of Personal Computers and other appliances need to be considered.• Least-Privileged-Principle should be considered as part of IT security design.• Sufficient Training of Security would be required for System Administrator and users.

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5 CONCLUSION

• VDL Mode 4 is an existing ICAO standardised5 datalink system providing acombination of Communication and Surveillance services and capable of being used forNavigation service.

• The system integrates GNSS and datalink communications• Secondary navigation provided by VDL Mode 4 provides a backup for Galileo based

navigation and facilitates the early implementation of Galileo.• Local GNSS augmentation are made widely available over VDL Mode 4 for aviation

user groups• The VDL Mode 4 airborne system can integrate Galileo in its architecture to provide a

“Fast track” to Galileo deployment in aviation.• The VDL Mode 4 ground infrastructure may serve as an independent verification of the

aircraft position with triangulation, thus improving the overall system integrity at anextremely low cost.

• An integrated development of VDL mode 4 / Galileo may provide an attractive and costefficient option for the aviation community, providing a wide range of CNS services.

Hence it can be seen that Galileo and VDL Mode 4 are both robust systems that providecomplimentary benefits to the aviation and other multimodal users for Communication,Navigation and Surveillance. Galileo with its enhanced navigation capabilities and VDLMode 4 as an efficient datalink with a high throughput and backup navigation capabilities willprovide high accuracy navigation with high integrity on a continuous basis globally andfacilitate the early implementation and use of. Galileo with reduced liability.

5 VDL Mode 4 is currently standardised within ICAO for Surveillance applications.

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END OF DOCUMENT

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Complementary Systems: VDL Mode 4 ISSUE : 1.1 PAGE: i

DOCUMENT DISTRIBUTION

From: Jens Redeborn, Abdul TahirProject Acronym : GALILEIProject Name : Galileo System AnalysisTitle : Complementary Systems: VDL Mode 4Issue : Issue 1.1Reference/identification : GALI-SWED-DD045Date : 06.11.2002Page Number : 49File : GALI_SWED_DD045_20.docClassification : Internal DraftWBS : C.2.B.8Contract : GMA1-42031-2001-SI2.324077Type of Document : DeliverableTemplate Name : GALI-SWED-TN21G-issue 1[1].0_1.dot0

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DistributionCompany Name N° Ex. Company Name N° Ex.