A-SMGCS Data Link Situation

Transcription

1 Contract No. TREN/04/FP6AE/SI / S. Gligorevic Document No: D1.7.1a Version No. 1.0 Classification: Public Number of pages: 31 Project Funded by European Commission, DG TREN The Sixth Framework Programme Strengthening the competitiveness Contract No. TREN/04/FP6AE/SI / Project Manager M. Röder Deutsches Zentrum für Luft und Raumfahrt Lilienthalplatz 7, D Braunschweig, Germany Phone: +49 (0) , Fax: +49 (0) Web page: , - All rights reserved - EMMA Project Partners The reproduction, distribution and utilization of this document as well as the communication of its contents to other without explicit authorization is prohibited. This document and the information contained herein is the property of Deutsches Zentrum für Luft- und Raumfahrt and the EMMA project partners. Offenders will be held liable for the payment of damages. All rights reserved in the event of the grant of a patent, utility model or design. The results and findings described in this document have been elaborated under a contract awarded by the European Commission.

2 Distribution List Member Type No. Name POC Distributed 1 Web Internet X Intranet X 1 Joern Jakobi X 2 AENA Mario Parra Martínez X 3 AIF Patrick Lelievre X 4 AMS Giuliano D'Auria X 5 ANS_CR Miroslav Tykal X 6 BAES Stephen Broatch X 7 STAR Jens Olthoff X 8 DSNA Nicolas Marcou X 9 ENAV Antonio Nuzzo X 10 NLR Juergen Teutsch X 11 PAS Alan Gilbert X Contractor 12 TATM Stéphane Paul X 13 THAV Alain Tabard X AUEB Konstantinos G.Zografos X 16 CSL Libor Kurzweil X 17 DAV Rolf Schroeder X 18 DFS Klaus-Ruediger Täglich X 19 EEC Stéphane Dubuisson X 20 ERA Jan Hrabanek X 21 ETG Thomas Wittig X 22 MD Julia Payne X 23 SICTA Salvatore Carotenuto X 24 TUD Christoph Vernaleken X Sub-Contractor CSA Karel Muendel X N.N. Customer EC Morten Jensen X Additional EUROCONTROL Paul Adamson X 1 Please insert an X, when the PoC of a company receives this document. Save date: Public Page 2

3 Document Control Sheet SP1 Project Manager Jörn Jakobi Responsible Author S. Gligorevic Additional Authors Subject / Title of Document: Related Task('s): WP1.7 Deliverable No. D1.7.1a Save Date of File: Document Version: 1.0 Reference / File Name D171a_INNO_V1.0.doc Number of Pages 31 Dissemination Level Public Target Date Change Control List (Change Log) Date Release Changed Items/Chapters Comment Initial Draft Final Draft (FF-FL) comments taken into account Formal Review SP1 manager Comments processed, Overview Table included Comments from European Commission and from SP1 Partners Comments processed Comments from European Commission Formal Review SP1 manager Version moved to 1.0 and is put on the public EMMA web: Approval by the European Commission Save date: Public Page 3

4 Table of Contents Distribution List... 2 Document Control Sheet... 3 Change Control List (Change Log)... 3 Table of Contents Scope of Document Introduction to A-SMGCS and current Data Link Situation Current Air / Ground Infrastructure Current Voice Systems DSB-AM with 25 khz and 8.33 khz Spacing Current Data Link Systems Data Link Currently Used for Air-Ground Communication Data Link Currently Used for Surveillance Additional Available Data Link Standards Summarize Surveillance and Communication Links Near-Term Solutions Near-Term Voice Solution Near-Term Data Link Solution Near-Term Data Services Communications Services Broadcast Services Long-Term Solutions Joint FAA & EUROCONTROL Future Communications Study Examples of Communications Developments Conclusion Annex I References Abbreviations Save date: Public Page 4

5 1 Scope of Document This document is an output of the EMMA sub-project 1 Concept and part of the workpackage 1.7, which is called Innovative It provides readers with a global view of both the current data link situation, linked to Air Traffic Management, and possible future data link realisations, with particular attention paid to A-SMGCS. 2 Introduction to A-SMGCS and current Data Link Situation The continuous increase in air traffic, which is expected over the next years, will lead to bottlenecks in air traffic handling en-route and on ground. According to the air traffic forecasts, the airports will be one of the most capacity restricting factors in the future. To cope with this challenge, modern communication, navigation, and surveillance tools have to be adopted for air traffic management both en-route and on ground. Especially, the A-SMGCS concept has to be further developed to guarantee the required improvements in overall throughput and safety on airports. Although existing communication links like VDL mode 2 are suitable for air traffic control en-route, they are less appropriate for additional air traffic management tasks on ground due to capacity restrictions and different physical communication environment on airports. For the choice of a prospective technology, a few important aspects of the communication link as well as of the physical channel should be taken into account: To handle the expected data traffic between controllers, airport authorities, and airline companies on the one side and the pilots on the other side, a new communication technology has to provide a high capacity data link to transmit high data rates. Since such high rate data link requires relatively large bandwidth, it can not be realised within the VHF band which is already extensively used. Thus, it is rather probable that a different frequency band will envisaged for A-SMGCS as for en-route communication. An on ground data link has to enable a simultaneous communication with a large number of users (aircraft and ground vehicles). This also implies an organised multiple access scheme, in order to prevent multi-user-interference. Moreover, a multiple access technique design is important to guarantee short end-to end delivery times crucial for secure communication. Delivery time guarantee and synchronisation requirements for different access schemes are indicated in the following table: Access CSMA (S)TDMA CDMA Delivery time guaranteed no yes yes Synchronisation required no yes no Due to the relatively high speeds of the mobile user at the airport, the wireless transmission channel is time-variant and introduces a significant Doppler shift. Moreover, severe signal fading is likely to occur due to the reflections from the ground and from buildings on the airport. Thus, the transmission channel characteristic is rather similar to the terrestrial mobile communications than to the air-ground and air-air communications en-route. However, different propagations condition in different frequency bands will also have a strong impact on the range of coverage, on the allocation and the number of the ground stations on an airport. Several data communication systems are already designed for aeronautical and terrestrial communication. They do not necessary present the end-solution, but can as well be used as prototypes for developing the best suitable solution for A-SMGCS. Therefore, the performance of different technologies should be considered in envisaged frequency band and regarding the above mentioned Save date: Public Page 5

6 requirements. The second step would be to design a frame design, comprising access scheme, coding, and encryption, for the chosen one or more technologies, in order to provide high security and authentication. However, looking at the achievements and rapid developments in digital communication, one of the most crucial aspects of the future aeronautical communication will be the determinedness of the aeronautical industry, airlines and airport authorities to support these developments. The following document provides an overview of existing data communications systems, which could be considered for the future aeronautical communication or as a prototype technology for a further development. A projection of different operational phrases on the data rate is not provided by this document. However, such a presentation containing message protocol, message heading, encryption, bit-stream, etc. would be expedient also for further A-SMGCS projects and would make this and similar documents better readable for all parties. An example of such estimation for future communication requirements also including the data rates for some traffic scenarios is given in Communications Operating Concept and Requirements for the Future Radio System by EUROCONTROL / FAA Future Communications Study Operational Concepts and Requirements Team. Due to the fact that airport targets need to be not only equipped with a means of communicating position and identity information to the A-SMGCS, but also able to receive and follow routing and guidance instructions, A-SMGCS data link will be more comprehensive than the current en-route surveillance including ADS-B. The proposed ADS-B data links can be extended to meet all the requirements of A-SMGCS or to be combined with other solutions, and will therefore also be considered in this document. Additional information about ADS-B technologies can be found in High Level Data Link Requirement Study of Co-operative Air Traffic Management Phase 1 project. Save date: Public Page 6

7 3 Current Air / Ground Infrastructure 3.1 Current Voice Systems The International Telecommunication Union (ITU) has assigned spectrum for use by aircraft for analogue voice in parts of the wider High Frequency (HF, 3-30 MHz) band and in the MHz section of the wider Very High Frequency (VHF) band. Aircraft can use radios operating in the HF radio band for long-range communications because the signals are reflected back to the ground by the ionosphere. The issue with using the HF band is that the link quality is very weak and that due to this long propagation there is limited reuse of the frequency channel. Aircraft can use radios operating in the VHF band for voice communication only with radios in line-of-sight coverage because the signals do not reflect off the ionosphere or penetrate obstacles such as mountains or buildings. The advantage of VHF over HF is that the link quality is much better and there is greater reuse of the frequency channel DSB-AM with 25 khz and 8.33 khz Spacing Analogue VHF voice communications systems are based on simplex user s access to the communications channel and on double sideband amplitude modulation (DSB-AM) with full carrier. The modulation index is 0.85; the frequency band of the modulating voice signal is khz. ATC controller and all pilots within his sector or area of responsibility represent a so called user group and share the same communications channel. Each member of the user group controller or pilot permanently monitors transmissions of all other group members ( party-line feature). Monitoring of the voice traffic is also required for the access to the communications channel applying push to talk communication. Today, the voice communications channel is identical with the VHF frequency assignment. Each sector is assigned a dedicated 25 khz or 8.33 khz frequency channel. The 8.33 khz channels are implemented in Europe above flight level 240 to increase the number of assignable voice channels. The coverage of 8.33 khz channels is to be extended in both horizontal and vertical direction. A spectral mask is applied to transmitted voice in 8.33 khz channels, reducing the bandwidth of the signal to 2.2 khz. 3.2 Current Data Link Systems An essential element of communications, navigation, and surveillance/air traffic management (CNS/ATM) systems is the use of air-ground data links as the main means of communications. It is envisaged that in the long-term the most routine air-ground communications in the en-route phase of flight will be via digital data interchange. Such data exchanges will greatly reduce the amount of voice communications and, therefore, reduce the work load of pilots and controllers Data Link Currently Used for Air-Ground Communication International Civil Aviation Organisation (ICAO) has been developing Standards and Recommended Practices (SARPs) for a number of data link systems, as it was recognized that a single system could not meet the wide variety of operational and technical requirements. First datalink using VHF radio was Aircraft Communications Addressing and Reporting System (ACARS) developed in the 70-ties and is now installed in approximately aircraft and used for some ATC messages (e.g. pre-departure clearance). While ACARS has not been subject to any ICAO standardization process, VDL Mode 1 has been specifically designed to permit the use of its radio and data modulation scheme and equipment. VDL mode 2 has been developed by the aeronautical industry as a higher capacity replacement for ACARS. The EUROCONTROL Link2000+ Programme, responsible for coordinating the implementation of the ICAO compliant Controller Pilot Data Link Communication (CPDLC) application in Europe, has selected VDL Mode 2 as the underlying communications infrastructure to be used. Save date: Public Page 7

8 ACARS / VDL mode 1 The ACARS communications sent via VHF radio or via satellite go through a network of ground stations linked via a terrestrial data network to a centralized data link service processor, which provides the interconnection to the ground systems of the airlines. The ACARS data link service processor routes messages automatically between the user aircraft and ground systems, using mostly a fixed configuration of delivery addresses by message type for downlink messages and by memorizing the ground station to be used for uplink messages. The main restriction on the ACARS system is that it uses the character codes defined by standards such as ASCII, and the user data field can only transport the codes representing printable characters. This limitation applied to all data communications systems when ACARS was first implemented in the late 1970s. Besides, the ACARS performance is far too unpredictable for general ATC use, especially in high density airspace. This is firstly because the overall bit rate at 2.4kbps (shared by all aircraft in a given airspace) is too low to support both Air Traffic Control (ATC) and Aeronautical Operational Control (AOC) communications but also because ACARS is poor at keeping track of aircraft. ACARS keeps track of aircraft by remembering which Ground Station (GS) in its worldwide network of VHF Ground Stations received the last downlink from a given aircraft. It then uses that GS for the next uplink for that aircraft. However, the aircraft could have gone out of range of that GS between the downlink and subsequent uplink. ACARS will retransmit several times before realising this and then hunt for the aircraft by attempting the uplink for nearby GS. The result is an unpredictable uplink delay. ACARS and VDL Mode 1 use the same modulation scheme Amplitude Modulated Minimum Shift Keying (AM-MSK) operating at a bit rate of 2.4 kbps. It is a constant-phase, frequency shift keying technique using two audio tones. The presence of the lower tone indicates that there is a bit change from the previous bit, and the presence of the higher tone indicates that there is no bit change. The phases of the two tones are chosen so that the phase discontinuity is minimised and the amplitude of each tone is zero at the bit transition. The audio tones are amplitude modulated onto the Radio Frequency (RF) carrier. A particular feature of this modulation scheme is that it is compatible with existing analogue voice radios. The audio tones may be fed into the radio in place of the line level audio signal conventionally connected to the aircraft intercom system. ACARS uses a technique known as Carrier Sense Multiple Access (CSMA). This technique is often known as listen before send ; transmission will take place only if the channel appears to be free. Otherwise, the sender will back off and try again later. Due to the very low data rate scheme and inefficient media access technique it is unlikely for ACARS and VDL mode 1 to satisfy requirements for improved capacity. However, ACARS is a robust system, and much experience has been gained through its deployment. VDL Mode 1 was specified in order to provide a fall back to utilise VDL Mode 2 protocols with a well validated physical layer. However, because of lack of interest by users and implementers, ICAO removed VDL mode 1 technology from Annex VDL Mode 2 VDL mode 2 has been developed by the aeronautical industry as a higher capacity replacement for ACARS. The nominal data rate of 31.5 kbps is compatible with the 25 khz channel spacing used for existing VHF analogue radios. VDL mode 2 uses Differentially Encoded 8-Phase Shift Keying (D8PSK) modulation scheme and the encoding of data for exchange over the VHF link. 8-Phase shift keying means that each transmitted symbol represents one of eight phase change states of the carrier (0, π/4, π/2, 3π/4 radians, etc). The availability of 8 states means that each symbol can represent 3 bits of data. The symbol rate is 10.5 kbps, giving a net bit rate of 31.5 kbps. Data is gathered into triples of bits before transmitting and padding is used to ensure that transmissions always include an integer number of 3-bit symbols. The phase encoding of the carrier is such that each symbol is represented by the difference between the previous phase and the current phase, i.e. the signal is differentially encoded. This is because the Save date: Public Page 8

9 receiver has no phase reference for the carrier signal, i.e. the receiver does not know what the signal should look like with a phase shift of 0 radians. Instead the receiver can measure the difference between two phases and use this to determine the symbol being transmitted. Following encoding of the phase changes the signal is filtered with a raised-cosine filter (with parameter α=0.6) prior to transmission. The raised-cosine filter reduces the spectral sidelobes of the D8PSK signal, reducing the amount of energy that spills outside of the channel. Scrambling is applied to data before being transmitted on the datalink by multiplication with a pseudo random binary sequence in order to aid clock recovery in the receiver and to stabilise the shape of the transmitted spectrum. As with ACARS, a single channel is shared by many aircraft and ground stations and a mechanism is needed to manage the competing requests for access to the channel and to allocate resources fairly. VDL Mode 2 uses improved CSMA algorithm. While ACARS uses non-persistent CSMA, VDL Mode 2 uses p-persistent CSMA. This approach reduces the probability of two stations both thinking that the channel is clear and corrupting each other s transmissions. If the channel is idle, one radio transmits with a certain random probability (designated p, 0<p<1) or stays silent (with probability = 1-p). If the radio does not transmit, it waits a random time before listening again to the channel and repeating the process. The typical value of probability of a station transmitting after sensing channel free is p=13/256. If a response is expected and one is not received after a certain number of attempts, the Media Access Control (MAC) sublayer reports a failure to the Data Link Sublayer (DLS) sublayer. CSMA is similar to the ethernet protocol used in local area networks. The CSMA algorithm offers equitable access to all stations, which means that there is no prioritisation of data traffic. For example, the protocol does not distinguish between tactical ATC messages and passenger communications. ATC messages may be delayed by non-critical data. The weakness of the CSMA media access protocol is that messages cannot be prioritised. This makes it harder to meet the levels of service required for time critical and safety critical ATM messages the particular problem is that end-to-end delivery times cannot be guaranteed. It is possible to improve the performance of VDL Mode 2 such that it might support ATC communications. This would be done by implementing it with specific channels dedicated to ATC communications and then to use low-levels of traffic loading on those channels. However, this is not a very efficient use of spectrum. The MAC sublayer in VDL Mode 2 does not add any data to user transmissions. VDL mode 2 makes the aircraft explicitly login to the network and then conduct an explicit handoff every time it goes out of range of one ground station and into the coverage of another. This handoff process allows the service provider to keep track of where each aircraft is and to affect an uplink in the shortest possible time. Thus, combined with the much higher bit rate, VDL mode 2 has a much faster and more predictable transit delay and is hence much more suitable for ATC than is ACARS Data Link Currently Used for Surveillance In contrast to primary radar, which determines the positions of aircraft by evaluating the reflected electromagnetic waves, Secondary Surveillance Radar (SSR) is able to send addressed inquiries. Targets with an active transponder can answer providing information about the identity and altitude. Automatic Dependent Surveillance (ADS) is a new precise navigation procedure based on aircraft self-location and the transmission of position data to the ground. Three candidate data link technologies are being considered by ICAO for use as the transmission media for ADS-broadcast (ADS-B) messages: Mode S 1090 MHz Extended Squitter (1090ES), Universal Access Transceiver (UAT), and VHF Data Link Mode 4. Of the three, 1090ES has emerged as the globally accepted ADS- B data link for all aircraft types. Whereas UAT and Mode S Extended Squitter can only broadcast Save date: Public Page 9

10 messages in uplink and downlink and between aircraft, VDL mode 4 can support not only surveillance but also air-ground and air-air data link SSR Mode S Secondary Surveillance Radar (SSR) mode S provides a surveillance capability and an air-ground data link which is specifically suitable for limited data messaging in high-density areas. It is also capable of operating in a mixed environment where different levels of data link capability exist among aircraft transponders. Secondary radar measures the range and bearing of an aircraft. The bearing is measured by the position of the rotating radar antenna when it receives a reply to its interrogation from the aircraft. The range is measured by the time it takes the radar to receive the reply. The beam of the antenna gets wider as the aircraft get farther from the antenna, thus making the measured position less accurate. Changes in the aircraft velocity can be detected only over a period of several position updates. Mode S surveillance protocols implicitly use the principle of selective addressing. Every aircraft will have been allocated with an ICAO aircraft address. This discrete identification code is known as an IC (or Interrogator Code). There are a number of ways of implementing this but the principle is that each airframe will have its own unique address. The IC field is included in all of its interrogations and in every reply that it sent to them. Targets that have been acquired in the all-call period (all-call address is ) are subsequently selectively interrogated for surveillance information in the Mode S period. Control information within the interrogation allows the ground sensor to apply lockout which means that the target will not reply to an all-call with that IC for a period of 18 seconds. This will be applied by the sensor for all acquired Mode S targets in all areas for which it has responsibility for maintaining lockout Mode S Extended Squitter (1090ES) The 1090 MHz Extended Squitter has been developed as an extension of Mode S technology widely used for aeronautical secondary surveillance radar applications. The squitters proposed for ADS-B are extended in the sense that existing Mode S acquisition squitters send 112 bits message containing 56-bit ADS message. The ES message is sent periodically by the transponder and provide IC code, position, velocity, heading, time and in future, intent. The added message information of 56 bit is inserted between the 24 bit aircraft address and the parity information. The data rate used within a message is 1 Mbps. Due to the fact that one ADS message is transmitted only at certain times, the net data rate allocated to one aircraft is less than 1 Mbps. Access to the 1090 MHz channel is randomised, and the channel is shared with current Air Traffic Control Radar Beacon System (ATCRBS) and Mode S responses to interrogations from ground-based radar and TCAS. Various 1090Es experiments show a range of approximately NM; 475 aircraft can be taken into account in an area like Frankfurt with improved reception techniques VDL Mode 4 VDL mode 4 has been designed by the Swedish CAA to support air-ground and air-air data link communications. VDL mode 4 SARPs specifies a general data communication system for a range of applications. The system supports navigation and surveillance applications. The VDL mode 4 standard specifies the message labels and the related application data format that can be entered in STDMA messages. There are specific labels to identify messages containing ADS-B or Global Navigation Satellite System (GNSS) Ground Based Augmentation Service (GBAS) messages. Furthermore, VDL mode 4 also supports full data communications compatibility. VDL Mode 4 uses a fundamentally different concept to Mode S Extended Squitter for communications. In Mode S, ADS-B messages are transmitted much more frequently than the required reception rate for most applications, allowing for many messages to be lost. In VDL Mode 4, Save date: Public Page 10

11 messages are transmitted at a lower rate on the assumption that fewer will be lost. Both approaches are intended to provide the appropriate rate for the application at the receiving aircraft. VDL Mode 4 uses STDMA algorithm on a 25 khz VHF channel. The STDMA algorithm divides access to the VHF channel into 4,500 time slots per minute and requires the mobile terminals to synchronize using GPS time signals. The algorithm is called self-organizing because mobile terminals reserve the use of a future slot in each transmission and do not rely on a centralized reservation system. The STDMA system allows for the use of the same frequency by multiple ground stations, like in VDL Mode 2 and VHF ACARS. However the VDL mode 4 ground stations would need to be connected to a central management system to avoid transmitting in the same slots. A set of reservation protocols is used to control access to the data link, such that, in general, only one station transmits in each slot. The protocols allow a station to reserve a slot for a future transmission i.e. pre-announce which slot it intends to use for transmitting information. Other nearby stations will respect this reservation and, in general, not transmit in the booked slot. In order to co-ordinate access onto the physical medium, all VDL Mode 4 stations maintain a slot map which identifies the status of each slot for the next four minutes ahead. For each slot in the map, the table identifies the station(s) which have reserved the slot, for each such reservation also the intended recipient (for a point-to-point transmission) as well as the reservation protocol used to make the reservation. Whenever a station wishes to find a slot for its own transmission, it consults the table to choose either a slot which is unreserved, or to re-use a slot under the prescribed rules for slot re-use. An important feature of VDL Mode 4 is the way in which slots are chosen for a new transmission or for placing reservations for future transmissions. When a channel is not busy, slot selection is straightforward since a previously unreserved slot can be found quite easily. However, as a channel becomes busier and unreserved slots are harder to find, the VDL Mode 4 system allows a station (mobile or ground station) to use a slot which has previously been reserved by another distant station. The rules that cover the re-use of previously reserved slots are based on two guiding principles, explained below: Robin Hood Principle This allows a station operating on a busy channel to use slots previously reserved for broadcast transmission by another station, as long as slots reserved by the most distant stations are chosen in preference to those of nearer stations. This results in a graceful reduction in the broadcast range of a station, as channels become busier. CCI protection This generalises the Robin Hood principle to allow slots previously reserved for point-to-point communication between two stations to be used by another station. CCI protection is based on relative aircraft distance and assumes that even though stations may be in radio range of each other, each station can discriminate the desired (stronger) signals over the undesired (weaker) ones. VDL Mode 4 defines a measure of co-channel interference (CCI), which is that the minimum range ratio between interfering sites is 3. Whilst ADS-B reports are designed to fit into a single timeslot, other transmissions may be longer than one timeslot and in this case they will run over several timeslots consecutively. There are 75 timeslots in each second. VDL mode 4 requires all users to be time synchronised so that their transmissions stay within their allocated slots and do not overlap unintentionally. The source of navigation data and precise time data is not specified in the SARPs. Five techniques have been proposed for obtaining Universal Time Coordinated (UTC) 2 : 1. GPS receiver 2. Synchronisation from ground station 3. Atomic clocks 2 Although in practice any method could be used since the source of time is not specified in the SARPs. Save date: Public Page 11

12 4. Synchronisation from other mobile users 5. Floating network 3 Each VDL Mode 4 station requires a source of precise timing to ensure that it transmits at the start of a time slot without causing interference to other users. The timing source is expected to be GNSS, but other sources may be used provided they can be related to UTC. There is a timing concept in VDL Mode 4 that is designed to satisfy stringent accuracy, availability, continuity, integrity requirements. It recognises several different accuracies of time: Primary time: In this state, each block of 75 slots aligned to UTC second with an accuracy of 400ns. In this state, a station s signals may be used as a source of secondary timing using received measured time, the signal known as certified. A GNSS receiver is likely to be required to achieve this primary time. Secondary time (failure mode): In this state, transmissions are aligned to within 15µs to the UTC second. This state should be achievable using receiver measured time, i.e. measuring the time of arrival of transmissions from a primary time station and deriving UTC time, or with stable quartz oscillators. Tertiary time (failure mode): In this state, transmissions are aligned to mean slot start times of 20µs. The receiving station measures the mean slot start times, so no UTC time source is required. This state should be achievable in the same way as secondary time. It is envisaged that stations should always be able to achieve at least tertiary time. The physical layer of VDL mode 4 is specified with two options stated in draft SARPs for the modulation scheme: 1. D8PSK operating at a bit rate of 31.5 kbps. 2. Gaussian-filtered Frequency Shift Keying (GFSK) operating at a bit rate of 19.2 kbps. GFSK is a form of Frequency Modulation (FM) compared to D8PSK which is a form of Phase Modulation (PM). GFSK uses two tones, alternating between them when a zero is transmitted. The change between the tones is smoothed using a Gaussian filter (Bandwidth-Time product = 0.28±0.03; the BT parameter defines the shape of the Gaussian filter). Gaussian Minimum Shift Keying is a special case of GFSK (with filter parameter BT = 0.5 and modulation index = 0.5) which is widely used for mobile communications. D8PSK has the advantage that it is the same modulation scheme as VDL Modes 2 and 3 and has a higher data rate. However, GFSK may be more suitable for navigation and surveillance applications because it operates at a lower Desired/Undesired Signal Ratio (DUR) than D8PSK. The DUR determines how much stronger a desired signal must be than an undesired signal in order for the desired signal to be correctly decoded (if the desired signal is not sufficiently stronger than the undesired signal, then neither can be decoded by the receiver). Flight trials have shown that the DUR of GFSK is approximately 7dB compared to approximately 16dB for D8PSK (with a single interfering source). Apart from the actual modulation schemes, the two options only differ in two significant areas: 1. The lengths of slots for data transmissions: D8PSK can use a shorter slot whilst transmitting the same information. Depending on the physical layer option selected GFSK time slots are 13.3 ms or 9.1 ms. D8PSK takes about 2/3 of this time. All transmissions are synchronised to the start of a timeslot. Transmissions can continue across many slots without a break, although a significant number of transmissions will be one slot long. 2. The transmitter ramp-up times: D8PSK ramp-up time is specified as 380 µs compared to 832 µs for GFSK. The shorter ramp time of D8PSK makes more efficient use of the available channel, but it is harder to implement while maintaining low levels of adjacent channel interference. 3 Similar to 4, with the difference that if all users have lost the GNSS or ground derived UTC time synchronisation, each user will tend to correct his own clock toward the "average drift rate" of the user population as a whole. Save date: Public Page 12

13 The core function of VDL Mode 4 is Automatic Dependant Surveillance Broadcast (ADS-B). ADS-B implemented by VDL mode 4 may improve all surveillance, including air-to-air surveillance. An independent position validation may be used to increase the integrity of the position contained in an ADS-B report. In the technique, a ground or airborne receiver of the ADS-B report makes an independent range measurement to validate the data contained in the report. In VDL Mode 4, transmitted ADS-B reports are closely aligned to a particular timeslot. A VDL Mode 4 station that receives messages from another station can deduce the distance of the transmitting station by the difference between the time of arrival of a message and the nominal start time of the slot in which it was transmitted. This technique relies on the transmitting station being accurately synchronised to the slot time ( primary timing ). If there is a discrepancy between the position information and timing delay of a station with primary timing then the ADS-B information will be treated as unreliable. This technique can be used for air-to-air or air-to-ground validation. A more sophisticated technique is to compare the arrival time of an ADS-B report at a number of ground stations. If arrival times from enough ground stations can be compared, then the position of the transmitting station can be estimated and the position information in the ADS-B report can be validated by the ground systems. This does not require the transmitting station to be using primary timing. VDL Mode 4 data broadcast can be received by all aircraft and vehicles. It provides end-to-end twoway communications e.g. air-to-air, without ground infrastructure. VDL Mode 4 is also characterized by low power, typically 5-25W, and long range coverage between 140 and 200 NM, nominally with no interference, which is the main advantage of this data link. One of the main limitations of the VDL mode 4 data link is the need for frequencies in high capacity areas where 75 slots per second will not be enough. In addition the cooperative slot access mechanism and various deployment/configuration possibilities of the VDL4 link means that it is more difficult to determine expected performance without extensive use of simulation and trials for the particular region/area concerned. VDL mode 4 also supports ATN and non-atn communication and offers a potential upgrade path for VDL mode 2 offering improved performance. However, as communications, navigation and surveillance are supposed to use different frequency bands, a VDL mode 4 avionics could not handle ADS-B and CPDLC on a single channel. Besides, the VDL mode 4 is already standardised for surveillance and the additional communication functions would raise difficulties of certifying avionics to provide specific functions without interfering with others UAT UAT is a remote mounted radio designed for transmission of airborne ADS-B reports and broadcast of ground based aeronautical information (i.e. is not a development of an existing design, but designed specifically for broadcast applications from the outset). Initial UAT operations have been conducted using the experimental frequency of 966 MHz. Operational demonstrations in Alaska are using 981 MHz as the UAT frequency. An UAT frame is one second long and begins at the start of each UTC second. Each frame is divided into two segments: the Ground Segment in which UAT Ground Uplink Messages, initially containing FIS-B products, are broadcast in one or more allocated time slots, and the ADS-B Segment in which UAT ADS-B Messages, and TIS-B, are broadcast. Guard times are incorporated between the segments to allow for signal propagation and timing drift. The pseudo-random selection of a Message Start Opportunities (MSO) within each UAT frame for the start of an aircraft s UAT ADS-B message is intended to prevent two aircraft from systematically interfering with each other s UAT ADS-B message transmissions. Adherence to the MSO based Save date: Public Page 13

14 timing scheme enables the receiving UAT equipment to determine the range to the transmitting UAT equipment. This information could be used in validity checks of the position data conveyed in the UAT ADS-B Message itself. UAT employs Time Division Multiple Access (TDMA) technique on a single wideband channel of 1 MHz at a frequency of transmission of 966 MHz (not definite). Transmissions from individual aircraft are composed of a single short burst, of duration 276 µs (basic message) or 420 µs (long message), that is transmitted each second. Ground uplink transmissions occur also once per second and last 4452 µs. The modulation employed is a binary Continuous Phase Frequency Shift Keying (CPFSK) at a Mbps rate and modulation index is not less than 0.6. Access to the UAT medium is timemultiplexed within a 1 second frame between ground-based broadcast services (the first 188 milliseconds of the frame) and an ADS-B segment. While the design presumes time synchronisation between ground-based broadcasts to reduce/eliminate message overlap, medium access within the ADS-B segment is randomised. In the downlink and air-to-air directions, ADS-B messages always start at one of 4000 MSOs in each 1 second UAT frame. In every frame, the MSO is selected randomly so that no two aircraft will repeatedly select the same MSO. The Type 0 Extended Length Message contains a 12 bit field which encodes the MSO in which the transmission began. A receiver can calculate the propagation time of the message and hence range to target, by knowing the MSO of the transmission and the time of arrival of the message. This time can be used by airborne receiving systems or applications to perform a validation check of the range to the target as encoded in the ADS-B positional information. From the ATC perspective, a single ground station can perform the same range validation check as an airborne transponder. Also networked ground station stations (in areas of overlapping coverage) can derive a position estimate independent of ADS-B using multilateration techniques Additional Available Data Link Standards While in Europe 8.33 khz channel spacing is introduced, FAA proposed to use VDL mode 3 to relieve congestion in VHF band. VDL mode 3 also provides digital voice communication, which has been planed to be used in USA for air traffic control in high-altitude sectors (above FL 240). However, no further statements confirming this plan have been given since Furthermore, in order to determine a solution for A-SMGCS communications, it may be interesting to take an example on data links currently used for vehicle, indoor and outdoor mobile communication, such as GSM and WLAN VDL Mode 3 While VDL mode 2 is only a data link, VDL mode 3 should provide both voice and data. The aim for VDL Mode 3 was a single radio capable to provide voice and data services simultaneously. VDL Mode 3 uses TDMA technique providing four logically independent channels in a 25 khz frequency assignment. Each channel can be used for voice or data transfer. There are seven configurations defined for VDL Mode 3 offering a range of static voice and data channel assignments, as well as standard or long-range operation. The VDL Mode 3 protocol divides time into super slots of 120 milliseconds, which subdivides it into either four 30-millisecond slots for normal range operations or three 40- millisecond slots for long (extended) range operation. Each slot is allocated to a logical circuit. In each 30-millisecond slot about 10 ms are used for channel management data and 20 ms for the exchange of user data, which gives time for about 600 bits 4. 4 In slots for extended range operation the length of data and management burst in the slot is the same as for normal range operations, but the guard bands between each burst are increased to allow longer range operation. Save date: Public Page 14

15 One of the configurations provides dynamic channel assignment, in which a channel can be switched dynamically between voice and data. The access technique is centrally managed from the ground station. As a consequence, if a mobile user wishes to transmit data it must request a transmission time from the ground station. The ground station will allocate the user a time based on the priority of the data and the other loading of the data link. The system applies a concept of user groups which is a small number of users (maximum 60) that share slot(s). A new mobile user to the system must join a user group before it can undertake data communications. VDL Mode 3 is based upon the D8PSK modulation scheme at a data rate of 31.5 kbps, as specified for VDL Mode 2. This high data rate is needed to be able to provide 4 digital voice channels in a 25 khz frequency allocation. Forward Error Correction (FEC) is applied to data bursts at the physical layer, but no interleaving as in Mode 2. FEC for voice should be applied in the digital voice coder (vocoder). The vocoder must operate at a rate of 4.8 kbps, including the coded voice and any error detection/correction coding that is applied GSM Global System for Mobile Communications (GSM) became a leading standard for mobile communication with networks established in most countries and an estimated one billion users as of early GSM is a digital system based on Time- and Frequency-Division Multiple Access (TDMA/FDMA). GSM operates with paired frequency allocations, one band being used for uplink from the Mobile Terminal (MT) to the Base Station (BS) and another for the BS to MT downlink. There are several ITU allocated frequency sets for GSM: GSM 400 system (Russia, Eastern Europe) or MHz uplink or MHz downlink GSM 850 system (USA and Canada) MHz uplink MHz downlink GSM 900 system (Europe, Asia, Australasia) MHz uplink MHz downlink GSM 1800 (Europe, Asia, Australasia) MHz uplink MHz downlink GSM 1900 (USA and Canada) MHz uplink MHz downlink Each allocated segment is divided into carriers spaced at 200 khz (e.g. 124 carriers per 25 MHz segment for GSM 900). These are then allocated between the base stations. Multiple carrier allocations can be made to a single BS. Each carrier is shared on a TDMA basis, the length of one time slot is ms. This is known as a burst period and forms the basis of a physical channel. Eight burst periods are grouped together to form a TDMA frame, within which logical channels are defined in terms of the number and position of their constituent burst periods. A group of 26 TDMA frames forms a Traffic Channel (TCH), of which 24 are available for user traffic the rest being used for signalling and control purposes. While the transmission rate of coded, interleaved and Gaussian Minimum Shift Keying (GMSK) modulated data is about 271 kbps, the effective (net) rate of Save date: Public Page 15

16 (uncoded) data is 13 kbps. Each burst includes a training sequence for equalisation, improving performance in multipath conditions. A slow frequency hopping system may be optionally implemented in order to further improve multipath performance. MTs pass received signal strength to the BS, which coordinates power control within the cell. This minimises co-channel interference and conserves the battery power of the MTs by ensuring that the minimum RF power levels necessary to maintain the network are used. The primary use of GSM is voice telephony. However, data services are also available, typically at 9.6 kbps. GSM cells are limited to approximately 35 km diameter WLAN The IEEE has defined a large number of standards for use in Personal Area Networks (PANs), Local Area Networks (LANs) and Metropolitan Area Networks (MANs). These standards form the 802.xx categories and define a broad range of wired and wireless networking protocols. The protocol has become the most widely recognized standard for commercial Wireless LAN (WLAN) applications. Wireless networks based on the standards are suitable for offices and homes, providing a range of up to 300 m depending on the environment operates in a similar way to common fixed Ethernet LANs (802.3), using Carrier-Sense Multiple Access (CSMA). Unlike Ethernet however, uses a Collision Avoidance (CA) protocol rather than Collision Detection (CD). In CSMA/CA, a station wishing to transmit data will first send a short frame to indicate to the network that it intends to transmit. This will be followed by the data frames if no initial collision is detected. In CSMA/CD, data is transmitted as soon as the network is seen to be available, and any collision is then resolved by conflicting stations backing off for a brief, random time delay. CSMA/CA performs better than CSMA/CD when there are medium to high levels of network traffic, as the overhead of collisions is smaller due to shorter frames colliding. The physical layer of the protocol can be implemented by infrared or radio, each supporting 1 Mbps and 2 Mbps bit rates. The radio implementation uses either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) techniques, and a range of different modulation methods. The protocol operates using packet data, with the number of active users determining the data throughput. Generally the system is suitable for voice, data and video transmissions as it is fairly low in the network layer model, allowing applications to operate on a higher level and use the network service provided. Standard a equipment has a maximum range of approximately 100 m b/g equipment typically has a maximum range of approximately 300 m. The data rate will be reduced at long range due to low signal strength, noise, interference and fading. Range extension is possible through the use of higher power and alternative antennas. However, there is a critical limitation of technologies. At a range of 5 km the propagation delay is of the same order as the short interframe spacing. As the range increases, the CSMA/CA system fails more often leading to more collisions. The system must then be operated in Point Coordination Function (PCF) mode, reducing efficiency. Save date: Public Page 16

17 3.3 Summarize Surveillance and Communication Links Surveillance 1090ES VDL Mode 4 UAT Status standardised standardised to be standardised Frequency Uses a single wideband channel at 1090 MHz. International spectrum allocation of the required 3 MHz channel exists. Initially developed as a multiple channel system in MHz band. It requires seven 25 KHz channels to operate in the high density scenarios. Operation in the VHF navigation band may require ITU coordination. Uses a single wideband channel (trials at 966 MHz). Operating frequencies (supporting the required 3 MHz channel) must be identified. Access non-coordinated CSMA coordinated S-TDMA uplink coordinated TDMA; downlink randomized TDMA Reporting rates fixed depending on operational fixed application Net data rate kbps 56 bits per message 282,6 / 430 bps 19.2 / 31.5 kbps 256 bits/slot per aircraft Performance requirements Requires time synchronisation 5 Limited performance due to multipath, random access and propagation constraints. 5 Limited performance if no line-of-sight conditions. Communication VDL 1/ACARS VDL Mode 2 VDL Mode 3 Status Not standardised standardised standardised Frequency 25 khz channels in VHF 25 khz channels in VHF 25 khz channels in VHF band ( MHz) band ( MHz) band ( MHz) Access non-coordinated CSMA non-coordinated CSMA coordinated TDMA Net data rate 2.4 kbps 31.5 kbps 31.5 kbps Table 3-1: Characteristics of current Data Links 5 The performance of 1090 ES, VDL Mode 4 and UAT was assessed in ADS-B Surface Surveillance Trials performed by NATS at London- Heathrow in 2002 using a single vehicle. Save date: Public Page 17

18 4 Near-Term Solutions In Europe, air traffic tripled over last 25 years and is predicted to double by The traffic grow requires additional communication capacity which can be achieved only by extending data communication. The aeronautical services up to year 2015 will be based on mixture of voice and data communication, whereby data link will be supplemented with voice backup. In the 2015 timeframe, the assumptions are that 60% of Air Traffic Services (ATS) are provided via voice in the Airport/TMA environments, 40% of ATS communications are provided via voice in the En Route airspace and 5% of all communications in Oceanic/Polar domains are conducted via voice. In general, the trend beyond 2015 will be a decreased use of voice and an increased use of data link. Beside expansion of 8.33 khz channel to extend the life time of VHF analogue voice, ICAO also proposed to use adjacent frequency bands and to relocate data services to other frequency bands ( MHz). 4.1 Near-Term Voice Solution 8, 33 khz channel spacing is now extensively used above FL 245 in the ICAO EUR region. As the demands for VHF assignment continue to grow, the coverage of 8.33 khz channels is to be extended in both horizontal and vertical direction. At ICAO EANPG 44 meeting, held in Paris on 2-5 December 2002, a decision was made to proceed with the implementation of 8.33 khz Vertical Expansion below FL245. EUROCONTROL is now focusing its efforts on an 8.33 khz implementation above FL195. Given the increasing demand for spectrum, 8.33 khz Vertical Expansion is seen as a mean of satisfying the need for new VHF assignments in the period leading up to the introduction of a new communication system around Actual information about 8.33 khz expansions can be found on EUROCONTROL 8.33 khz Programme website. In 2002 FAA planed to implement VDL mode 3 in the USA and to switch high-altitude sectors (above FL 240) to digital voice communication for air traffic control in However, this seems rather unlikely at the moment, and there is also no official statement about 8.33 khz channel separation in the USA. Still, VDL mode 3 remains an interesting candidate for the long-term data link solution. 4.2 Near-Term Data Link Solution The European Commission, supported by EUROCONTROL, is considering mandating the implementation of CPDLC/ATN/VDL2 for all aircraft operating in European airspace by the 2010 timeframe. An expansion of FANS CPDLC capability on aircrafts applying VDL mode 2 data link over the Aeronautical Telecommunication Network (ATN) is to be expected in the near-term time. The expansion of VDL mode 2 applications is also foreseen in the USA. Three link solutions, 1090 MHz Mode S extended squitter, UAT and VDL mode 4, are considered for the near-term surveillance MHz Mode S extended squitter is a mature and standardised technology that can be considered an extension of Mode S technology and is therefore chosen as the primary physical layer for ADS-B in the USA and in Europe. The main difference resides in that the squitter autonomously transmits at prescribed rates without being interrogated. Initially, the data to be transmitted is not expected to include aircraft intent, which means that it will not reach the full benefits ADS-B-Out can offer. At the Eleventh Air Navigational Conference in Montreal, 2003, EUROCONTROL, FAA and IATA agreed that 1090 Extended Squitter satisfied the requirements for the initial application. 1090ES is currently fitted in aircraft and is considered mature. This allows for early implementation. Boeing, Airbus and CANSO support the implementation using 1090ES. Save date: Public Page 18

19 Mode S Extended Squitter has been adopted as a transitory technology suitable for the short and medium term, but traffic density and interference levels could prevent its use beyond the 2010 timeframe. A foreseen lack of performance is foreseen especially due to increased interference levels. The evolution process to an enhanced surveillance service will continue during this period. Surveillance data links will be explored and as the need for additional airborne parameters will continue growing, the download of aircraft derived data supported by enhanced surveillance has the potential to enable a variety of ATM applications. The need for a second ADS-B link at some point around 2010 is generally accepted. The issue is rather whether the new link will be complementary to 1090 ES or a global single high performance link solution that would alone be capable of satisfying long-term global ADS-B requirements. EUROCONTROL, in its working paper to the ICAO AN Conference, discussed the need for an additional link developing VDL Mode 4 as the candidate technology. The United States, unlike Europe, are adopting UAT as an alternative to 1090ES. 4.3 Near-Term Data Services The deployment of operational mobile data link services for ATC and AOC, and the supporting infrastructure for the near and mid-term (indication ) are defined by the Link Programme. The programme will establish the operational, technical, and institutional basis for a scaleable implementation and operation on a regional basis in Europe. Link Programme supports Controller Pilot Data Link Communication (CPDLC) providing airground data communication for the ATC service. CPDLC started at EUROCONTROL Maastricht by Both ARINC and SITA deployed VDL2 networks with CPDLC service. An upgrade planned for 2007 should support data link clearances without voice readback. German Air Navigation Services (DFS) in Karlsruhe is expected to begin its operational service in 2007 using common equipment. To date a number of European airlines have committed to join the Link2000+ Programme using avionics from Rockwell Collins, Honeywell, and AIRBUS for VDL mode 2 data link. After controller pilot data link communications and down linking aircraft derived data are being implemented by the LINK and Mode S Enhanced Surveillance programmes the CASCADE programme addresses the next generation of data link applications and services to improve further the air traffic control sector productivity. By 2010, the objective of the CASCADE programme is to plan and co-ordinate the implementation of: A first set of Automatic Dependant Surveillance Broadcast (ADS-B) applications using the position broadcast by aircraft as a basis for surveillance More Controller Pilot Data Link Communications (CPDLC) services providing a robust menu based data communication channel to pilots and controllers Other data link services exploiting the existing data link to provide airborne data to the ground systems CASCADE will use to the maximum extent the Mode S and VDL Mode 2/ATN infrastructures deployed by the Mode S Enhanced Surveillance and Link respectively. It is envisaged that the above set of applications and services will be implemented in two streams. This will allow for early implementation of those elements that have reached the required level of maturity. Stream 1 contains ADS-B applications to provide ground surveillance in places where it did not exist before, or to enhance ground surveillance where this is necessary or cost beneficial. Stream 2 contains ADS-B applications to provide Airborne Situational Awareness and more advanced CPDLC services. Save date: Public Page 19

20 Figure 4-1: Link and Cascade Communications Services The CPDLC application includes a set of clearance/information/request message elements which correspond to voice phraseology employed by ATC procedures. The controller is provided with the capability to issue level assignments, crossing constraints, lateral deviations, route changes and clearances, speed assignments, radio frequency assignments, and various requests for information. The pilot is provided with the capability to respond to messages, to request clearances and information, to report information, and to declare/rescind an emergency. The pilot is, in addition, provided with the capability to request conditional clearances (downstream) and information from a downstream ATSU. A free text capability is also provided to exchange information not conforming to defined formats. An auxiliary capability is provided to allow a ground system to use data link to forward a CPDLC message to another ground system. The ground system must be capable of supporting communications procedures with minimal controller input. CPDLC will require some level of message processing that should be included in the ATC automation component 7. Error detection, correction and alerting mechanisms should be implemented. The air-ground data link will be connected to the ATC system through a terrestrial communications network. The required air-ground data link could be either satellite data link, VHF digital link (VDL), Mode S data link, or any other medium which meets the operational requirements. While satellite data links are most likely to be used in oceanic airspace, VDL is mainly chosen in domestic airspace. The resulting communications links will appear seamless from the user's perspective (i.e., independent of the communications systems in use). The data services defined by the Link are described in the following. Data Link Initiation Capability (DLIC) provides o necessary information to establish flight plan - address association in the ATC system; o necessary information to enable data link communications between ground and aircraft systems; 6 Source: [2] 7 An example of this is the NEXT DATA AUTHORITY message, which could be generated by the ground system when the aircraft reached a specific point en route, and which is noted in the aircraft system without informing the pilot, merely to allow the next data authority to set up a link to the aircraft without being rejected. Save date: Public Page 20

21 o Airframe identification, aircraft identification, available air applications, departure airport, destination airport, and estimated off block time (EOBT), when available. Flight Plan Consistency (FLIPCY) provides, on ground system demand, aircraft downlink of route points, destination and aircraft type, for comparison with the flight data held by the ground system. This will enable the generation of appropriate warnings in case of any detected inconsistencies. ATC Communications Management (ACM) provides o o Automated assistance to the aircrew and current and next sector controllers for the transfer of ATC communications from one sector/centre to another. The ACM Service encompasses the transfer of all controller/aircrew communications. Transfer of data communications, in synchronisation with the transfer of voice communications. It also retains the operational principle that there is only one controlling authority, and that the controlling authority is properly and unambiguously determined. ATC Clearances (ACL) service enables o the controller to issue ATC clearances for level, heading, speed, direct route, and rate of climb/descent ; o SSR code change instructions; o aircrew requests for the above clearances; o Acknowledgements in both directions. ATC Microphone Check (AMC) service provides controllers with the capability to uplink an instruction for aircraft to check that they are not inadvertently blocking a given voice channel. AMC will take two forms: o a broadcast version with the NEAN system, and o a multi-cast version with the connection-oriented ATN and FANS-1/A. Controller Access Parameter (CAP) service 8 provides aircraft system parameters such as o indicated heading; o current ROVC; o current MACH or IAS; o weather; o wind vector. Departure Clearance (DCL) service provides assistance for requesting and delivering departure information and clearance, with the objective of reducing aircrew and controller workload. The departure clearance contains all information in accordance with ICAO procedures. Downstream Clearance (DSC) service provides assistance for requesting and obtaining Downstream ATS unit s clearances or information. The DSC service can only be initiated by the aircrew. Data link ATIS (D-ATIS) service provides assistance in requesting and delivering compiled meteorological and operational ATIS information, specifically relevant to the departure, approach and landing phases of flight Broadcast Services The transition phase in the surveillance, from the system based on ground radar to one based on ADS- B, will be supported by the Traffic Information Service - Broadcast (TIS-B) application. TIS-B is a ground-based broadcast service that provides secondary surveillance radar (SSR)-derived traffic data. Messages are uplinked over the ADS-B data link system to the aircraft. This is a different technology than is used for Traffic Information Service (TIS), which relies solely on Mode S terminal radars. TIS-B allows any aircraft equipped with a conventional transponder to be made visible on a cockpit display in another aircraft. The tracked aircraft needs to be in radar coverage, while the receiving aircraft has to be within the broadcast region of the TIS-B ground station. The purpose of TIS-B is to 8 This is an enhanced surveillance application. Save date: Public Page 21

22 provide situation awareness of surrounding aircraft which are not equipped with ADS-B. Thus, TIS-B service is intended to provide ADS-B equipped aircraft with a more complete traffic picture in situations where all other nearby aircraft are not equipped with ADS-B. TIS-B can also be used to make aircraft operating on different ADS-B data links visible to each other. With the ADS-B surveillance technology an aircraft avionics broadcast the aircraft position, altitude, velocity and other parameters completely autonomously, without pilot involving in initiating broadcasts. The service is dependent on the aircraft position determination system. ADS-B can also receive reports from other suitably equipped aircraft within reception range. No ground infrastructure is necessary for ADS-B equipped aircraft to detect each other. These reports can be transmitted to ground based transceivers (GBTs) and used to provide air traffic surveillance services and fleet operator monitoring of aircraft. ADS-B ground stations listen to ADS-B transmissions from aircraft sent via the Mode S and Universal Access Transceiver (UAT) data links. The ground-based ADS-B listening stations, and the primary, en-route, and secondary surveillance radar sites feed their information to TIS-B ground stations which process the incoming data to remove redundant information and then uplink filtered data to aircraft via a ground-air data link or satellite network to provide a complete situational awareness picture to aircraft equipped to receive TIS-B information. Among the three data link technologies considered for surveillance and thus for ADS-B and TIS-B, the UAT and the VDL mode 4 link additionally support Flight Information Service Broadcast (FIS-B) service. FIS-B is the ground-to-air broadcast service of weather and other non-control, aeronautical information that allows pilots to operate more safely and efficiently. FIS-B products can be textually or graphically depicted. Typically, this includes information which is presently acquired by listening to VHF voice transmissions, which are frequently recorded messages. Save date: Public Page 22

23 5 Long-Term Solutions As the aeronautical air-to-ground VHF channel capacity for Air Traffic Management (ATM) is reaching saturation, most severe in Europe and parts of the United States, ICAO is seeking a common, global solution through the Aeronautical Communications Panel (ACP). EUROCONTROL and the FAA have initiated a joint activity under Action Plan (AP) 17 to identify potential future communications technologies to meet safety and regularity of flight communications requirements, i.e., those supporting ATS and AOC. In the 2030 timeframe, it is assumed that 85% of ATS communications are provided via data link in the Airport/TMA environments, 95% of ATS communications are provided via data link in the En Route airspace and 99% of all ATS communications in Oceanic/Polar domains are conducted via data link. 5.1 Joint FAA & EUROCONTROL Future Communications Study The Future Communications Study (FCS) addresses the need for globally harmonised planning of future aviation communications. A key output of the FCS is the recommendation of the most appropriate technologies to meet the communication requirements to support future ATM concepts. Various proposals to address this problem have been offered and approved independently, but none has achieved global endorsement. Technology candidates previously identified by the ICAO Aeronautical Communication Panel (ACP) Working Group C (WG-C) were assorted in following technology families Cellular Telephony Derivatives: TDMA (IS-136), CDMA (IS-95A), CDMAone (IS-95B), CDMA2000 1xRTT, W-CDMA, TD-CDMA, CDMA2000 3x, CDMA2000 1xEV, GSM/GPRS/EDGE, TD-SCDMA, DECT IEEE 802 Wireless Derivatives: IEEE , IEEE , IEEE , IEEE , ETSI HIPERPAN, ETSI HIPERLAN, ETSI HIPERMAN Public Safety and Specialized Mobile Radio: APCO P25 Phase 1, APCO P25 Phase 2, TETRA Release 1, TETRAPOL, IDRA, IDEN, EDACS, APCO P34, TETRA Release 2 (TAPS), TETRA Release 2 (TEDS), Project MESA Satellite and Other Over Horizon Communication: SDLS, Connexion by Boeing, Swift Broadband (Aero B-GAN), Iridium, GlobalStar, Thuraya, Integrated Global Surveillance and Guidance System (IGSAGS), HF Data Link Custom Narrowband VHF Solutions: VDL Mode 2, VDL Mode 3, VDL Mode 3 w/saic, VDL Mode E, VDL Mode 4, E-TDMA Custom Broadband: ADL, Flash-OFDM, UAT, Mode-S, B-VHF (MC-CDMA) Military: Link 16, SINCGARS, EPLRS, HAVEQUICK, JTRS Other: APC Phone (Airphone, AirCell, SkyWay) Save date: Public Page 23

24 Different evaluation criteria as communications capability, maturity for aeronautical environment, cost, etc. have been applied. By considering the matching with the spectrum options for the future infrastructure, following solutions are indicated: Technology that uses VHF more efficiently and are compatible with in-band transition Technology that uses DME or MLS spectrum Technology that uses AMS(R)S (Satellite) (Common candidate is Aero-BGAN, NASA/ITT also proposed Iridium). As a result of preliminary findings of technology pre-screening EUROCONTROL/QinetiQ provides following candidates for evaluation B-VHF in DME VDL-3 in another band P34 in DME; ADL in MLS Aero-BGAN and issues for further consideration Provision of Party-Line on 3G Aeronautical VoIP services Performance of 3G & WLAN at aeronautical velocities According to NASA/ITT technology pre-screening process technologies applicable for provision of communications over enroute, terminal and surface airspace domains are Primary: VDL3/VDLE in VHF; B-VHF in DME P34 in DME; VDL3 in DME (XDL3); Secondary: WCDMA in DME Technologies applicable for provision of communications over specific airspace domains Oceanic: Aero-BGAN; Iridium in AMS(R)S Surface: IEEE in MLS Because of the severe omni-omni antenna path loss in the MLS band, technologies in this band are recommended only for the airport surface and immediate terminal area. 5.2 Examples of Communications Developments In the following the evaluation candidates are shortly described. While B-VHF and WCDMA are considered mainly for the data link over enroute domain, ADL and are recommended for the airport surface and immediate terminal area communication. P34 is APCO Public Safety Wideband Data standard with the objective to establish a standard profile for the operation and functionality of new aeronautical and terrestrial wireless digital wideband public safety radio standard that can be used for the transmission and reception of voice, video, and high speed data in a ubiquitous, wide-area, multiple agency network. Data link using satellite communication, for example Aero-BGAN, is among the best solutions. Currently and upcoming satellite based data links offer non-safety services. Some initial discussion has started to extend the SwiftBroadband (aeronautical BGAN service) to support safety communications in the 2010 timeframe. Save date: Public Page 24

25 B-VHF Development and design of B-VHF started on January, 1 st 2004, within the FP6 European research project B-VHF ( Broadband VHF Aeronautical Communications System Based on MC-CDMA) with the aim of developing a broadband communications system which is capable to fulfil the expectations on a future ATC communications, offering increased capacity, high efficiency, increased robustness, and improved communications system safety. B-VHF is a broadband multi-carrier (MC) communications system proposed to be deployed for future ATC communications either by rearranging the VHF spectrum to free a certain continuous part of the VHF band for initial deployment or by introducing the new system initially in another frequency band and, if desired, moving the system later into the VHF band as soon as enough VHF band capacity is available. MC-CDMA is a combination of Orthogonal Frequency Division Multiplexing (OFDM) with the CDMA technique. Whereas OFDM enables a high rate data link and is standardized for DAB ( Digital Audio Broadcasting ), DVB-T ( Digital Video Broadcasting - Terrestrial ), and HIPERLAN ( High Performance Local Area Network ), the CDMA technique allows multiple-access with high capacity. The basic principle of MC-CDMA is to distribute the information of one user on several subcarriers by spreading. MC-CDMA also provides frequency diversity enabling reliable communication also in multipath channels without line of sight. By using orthogonal spreading sequences, a number of users can share subcarriers simultaneously without interfering each other. As a consequence, MC-CDMA provides very high spectral efficiency and is a flexible broadband transmission scheme. This technology is also a promising candidate technology for the next generation mobile communications systems, so the aeronautical communications system could profit from new hardware and software developments. A further advantage of the broadband aeronautical communications system based on multi-carrier technology is that the spectrum doesn t need to be continuous. Thus, the system can be deployed as an overlay system in the VHF band, co-existing with the legacy VHF systems and providing the possibility of an in-band transition. To establish such an overlay system in the VHF band B-VHF uses the feature of skipping subcarriers which are already occupied by transmissions of legacy VHF systems. Both forward/reverse link separation scheme, FDD and (currently used) TDD, are possible. The modulation types, QPSK or QAM, should be adjustable to channel conditions. System bandwidth allotted to one ATC sector is about 1 MHz. B-VHF supports not only data application but also combination of voice and data. Separate FEC schemes will be defined for each service type (voice, different data service classes) within the B-VHF project. Actual information about B-VHF project can be found on WCDMA While GMS and other 2G networks were designed primarily to carry voice services, the 3G systems like UMTS and cdma2000 provide high bandwidth mobile communications, offering also multimedia services at much higher data rates. WCDMA is a 3G cellular technology characterized by the use of Code Division Multiple Access (CDMA) techniques. With an occupied bandwidth of 2 x 5 MHz, bit rates of up to 384 kbps can be achieved. Data are modulated with BPSK in up- and QPSK in downlink, spreading is provided with QPSK in up- and downlink. CDMA/FDD uses down/uplink isolation in the frequency domain. Within an omni directional cell max. 98 users (voice channels) are supported. CDMA is an interference limited system. Its cell capacity limit is sooner reached by its interference level limit rather than by its code availability limit. Save date: Public Page 25

26 In 3G technologies mobile terminals (MTs) can move at higher velocities (typically 300km/h 500km/h) than in 2G networks and roaming capabilities extend globally. Like the 2G and 2.5G technologies they have a cellular structure relying on a network of base stations (BS) connected with wired and wireless links. MTs within a cell communicate with the local BS with calls routed either to another MT within the same cell or via the backhaul network to MTs or fixed terminals in other cells or on other networks. Messages are exchanged between MTs and BSs on a regular basis whether or not a call is in progress. This allows the network to maintain awareness of which cell a given MT is currently in. Various handoff methods may be implemented where an MT with a call in progress moves between cells. 3G systems implement power control algorithms in order to minimize interference between transmitters within the cell and between adjacent cells ADL Advanced Airport Data Link is a data link specially designed to fulfil the requirements of A-SMGCS with respect to communication. ADL provides a reliable wireless communication between tower and mobile users for exchange of all kind of data relevant within an airport environment. Thus, the system is designed to deal with different airport scenarios such as landing, taxi, and parking and also with mobile users travelling at speeds up to about km/h. It provides both voice and data channels within one transmission, as well as only data application. ADL uses the same broadband techniques as B-VHF, MC-CDMA for the tower-aircraft link and OFDMA ( Orthogonal Frequency Division Multiplexing Access ) for the aircraft-tower link. QPSK modulation and convolutional coding are applied. With the bandwidth of 8192 khz the system is able to provide bit data rates between 128 and 2048 kbps for up to 128 users, avoiding the Inter-Carrier- Interference (ICI). Due to the flexibility of the physical layer, ADL supports simple exchange between user capacity and transmission bit rate. Both duplex schemas TDD and FDD are possible. Inherent frequency diversity is provided due to spread-spectrum transmission based on multi-carrier technology. With the large coverage area of about 50 km around the airport ADL also supports communication during landing and take-off IEEE The 802 family have potential applications for airport surface networking was recommended by FCS for support of a broad scope of communication needs over the entire airport surface, particularly applicable if high data rate requirements on the airport surface cannot be met by a future system in the DME band. The group of standards define specifications for wireless Metropolitan Area Network (WMAN) systems, providing broadband wireless access (BWA) with performance equivalent to a digital subscriber line (DSL) or cable modem connection. The system allows fixed links with a range of up to 50 km to be established and data rates of 70 Mbps to be achieved. WMAN provides a much cheaper alternative to installing fiber optics or conventional copper cables to interconnect buildings and provide internet access. WMAN networks can connect together smaller network fragments, such as Ethernet or WLAN systems in separate buildings defines the lower levels of the network stack, allowing applications to operate on the higher network layers and use the system for voice, data and video services. The data rates obtainable are mainly determined by the quality of the radio links, which are largely influenced by the distance of a user from the base station and the presence of any interfering obstacles. The initial version of the standard caters for fixed point-to-multipoint (PMP) wireless communications with line-of-sight (LOS) radio links over a number of frequency bands in the GHz range. Individual devices utilizing only use a small band of frequencies in the range, but the standard is defined over a large spectrum to allow application for different channel requirements. Save date: Public Page 26

27 WMANs involve the use of a central base station which communicates to multiple users. Directional antennas are used to improve signal strength and achieve the long range high data rate communication uses dynamic adaptive modulation methods to achieve the best possible channel performance in the physical environment. This also allows the base station to trade-off data rate against range can use either FDD or TDD. FDD is used in most cellular phone systems and requires two carrier frequencies per channel, one for downstream data and one for upstream. TDD uses just one carrier frequency for both upstream and downstream data, providing time slots for each. TDD is very flexible allowing upstream and downstream bandwidth allocation to be altered depending on the level of traffic. The original standard does not perform well in non-los situations. This means that it is difficult to establish successful WMANs in built-up areas, due mainly to obstruction by buildings. Two years after the standard was first produced, the a amendment was introduced. This added bands in the 2-11GHz spectrum to the standard and made non-los WMAN communications possible. This, along with the falling cost of WMAN equipment, means that a large increase in the use of WMANs is expected over the next few years e is based on the earlier a standard and is aimed at mobile applications in the 2-6GHz bands. It is likely to be capable of serving terminals moving at up to 150 km/h. A great deal of work is still being done by the IEEE work groups on the protocol. The original standard has been superseded by a newer version released in 2004, labeled This standard includes work from other revisions of the original standard, namely a and c. The fixed nature of most systems makes them unsuitable for aeronautical use, with the possible exception of e. This may be applicable to aeronautical use, but it is still in the early stages of development so little information is available e is expected to work at ranges up to 50 km Aero-BGAN Inmarsat is currently in the process of defining the Broadband Global Area Network (BGAN), which is the generic name for the communication service available to mobile users on land, sea and air which will use the powerful new 4th generation Inmarsat satellites (I-4). The first of the I-4 satellites was successfully launched in March 2005 over the Indian Ocean at 64 degrees east providing coverage over Europe, the Middle East, Africa, the Indian sub-continent across through to Western Australia. INMARSAT plans to launch a second I-4 by the end of 2006 over the Atlantic Ocean at 53 degrees west and provide service for the America s. A third I-4 satellite is planned to be deployed in 2007 over the Pacific Ocean. Aero_BGAN uses FDMA / TDMA access schemas, thus there is no multiple access interference. Duplexing schema is FDD. Applicable modulation types are O-QPSK, π/4-qpsk, 16- QAM and turbo coding is provided. Data rates up to 432 kbps (average data throughput) per channel should be available for each user. Max. number of simultaneously supported users per sector depends on max. capacity per spot beam, i.e. approximate maximum of 10% of satellite capacity to a spot beam. System allows both shared and exclusive use of a nominal 432 kbps bearer. Transition between spot beams on the same satellite is seamless. Voice, also ISDN connection based, and data services are supported. It shares co-ordinated Inmarsat Spectrum in L band (1525 MHz-1559 MHz RX and MHz MHz TX), where for BGAN the spectrum is subdivided in 200 khz channels. SatCom delivers working latency to the passenger of between 600 and 800 ms. Save date: Public Page 27

28 6 Conclusion To handle the expected data traffic between controllers, airport authorities, and airline companies on the one side and the pilots on the other side, a new system for A-SGMCS should provide high transmission bit rate per user/aircraft and support a large number of active users (aircraft and ground vehicles) simultaneously. Such a high rate data link requires relatively large bandwidth but the VHF band is already extensively used. As communications, navigation and surveillance are supposed to use different frequency bands, a different frequency but also a technology different from those used for other aeronautical applications, such as the ATC data link, might be chosen for A-SMGCS. An additional issue of the communication channel in landing, taxiing and parking scenarios at the airport is the increased frequency selectivity due to the multipath and often inexistent LOS connection. Thus, techniques based on multi-carrier spread spectrum, such as ADL, which provide frequencydiversity on one side and may be designed also to cope with the large Doppler shift in landing and take-off phase on the other side, will be appropriate e standard also has a potential for airport surface networking. It provides the opportunity for a commercial system to support a broad scope of communications needs over the entire airport surface. It is also conceivable to combine two technologies, whereby e would be particularly applicable if high data rate requirements on the airport surface cannot be met by another future system. Save date: Public Page 28

29 7 Annex I 7.1 References [1] EUROCONTROL / FAA Future Communications Study Operational Concepts and Requirements Team; Communications Operating Concept and Requirements for the Future Radio System; 09/11/2005 [2] Co-operative Air Traffic Management Phase 1 project; High Level Data Link Requirement Study;09/12/2005 [3] Co-operative Air Traffic Management Phase 1 (6th Framework Programme); High Level Data Link Requirement Study (D2.1.5); 29/11/2005 [4] Dyer, ITT Industries; FCS Technology Assessment Status; ACAST/SBT Workshop, 08/2005 [5] Nelson, Rockwell Collins; VDL mode 3/NEXCOM; ICNS Conference, 05/2002 [6] EUROCONTROL; A-SMGCS Project Strategy; European Air Traffic Management Programme, 30/09/2003 [7] EUROCONTROL; ATC Data Link Manual for Link2000+ Services; 27/05/2005 [8] EUROCONTROL; Principles of Mode S Operation and Interrogator Codes, Project Strategy; European Air Traffic Management Programme, 18/03/2003 [9] EUROCONTROL; Future Communication Study Technology Pre-Screening, Draft; 12/04 [10] ICAO ACP; Report of Technology Assessment Group, WG C7 Meeting; 09/2004 [11] ICAO ACP; Working Group C-9 Meeting Report; 04/2005 [12] ICAO ACP; Final Communications Operating Concept and Requirements for the Future Radio System; DRAFT 0.1; WG-C9 Meeting, 04/2005 [13] Nelson, Rockwell Collins; VDL mode 3/NEXCOM; ICNS Conference, 05/2002 [14] Pouzet, EUROCONTROL; Future Comms; ATN Conference, 09/2005 [15] SITA; Aircraft Communication Link Selection; Document issued in 06/2005 on [16] [17] Save date: Public Page 29

30 7.2 Abbreviations ACARS Aircraft Communications Addressing and Reporting System ACAS Airborne Collision Avoidance System ACL ATC Clearances ACM ATC Communications Management ACP Aeronautical Communication Panel ADL Advanced Airport Data Link ADS Automatic Dependent Surveillance AMC ATC Microphone Check AMCP Aeronautical Mobile Communications Panel AM-MSK Amplitude Modulated Minimum Shift Keying AOC Aeronautical Operational Control A-SMGCS Advanced Surface Movement, Guidance and Control System ATC Air Traffic Control ATM Air Traffic Management ATN Aeronautical Telecommunication Network ATS Air Traffic Services ATSU Air Traffic Service Unit BGAN Broadband Global Area Network BPSK Binary Phase-Shift Keying BS Base Station CAP Controller Access Parameter CNS Communication, Navigation, Surveillance CPDLC Controller Pilot Data Link Communication CPFSK Continuous Phase Frequency Shift Keying CSMA Carrier Sense Multiple Access D-ATIS Datalink Automatic Terminal Information Service DCL Departure Clearance DFS Deutsche Flugsicherung (German Air Navigation Services) DLIC Data Link Initiation Capability DLS Data Link Sublayer DME Distance Measuring Equipment DSB-AM Double Sideband Amplitude Modulation DSC Downstream Clearance DUR Desired / Undesired Signal Ratio D8PSK Differentially Encoded 8-Phase Shift Keying FAA Federal Aviation Administration FANS Future Air Navigation Systems FDD Frequency Division Duplex FDMA Frequency Division Multiple Access FEC Forward Error Correction FIS-B Flight Information Service Broadcast FLIPCY Flight Plan Consistency GBT Ground Based Transceivers GFSK Gaussian-filtered Frequency Shift Keying GMSK Gaussian Minimum Shift Keying GNSS Global Navigation Satellite System GPS Global Positioning System GSM Global System for Mobile Communications HFDL High Frequency Data Link ICAO International Civil Aviation Organisation ITU International Telecommunication Union LOS Line-Of-Sight Save date: Public Page 30

31 MAC MC-CDMA MLS MT OFDM OFDMA QAM QPSK RF RTCA SARPs SSR STDMA TDD TDMA TIS TIS-B UAT UTC VDL VHF WCDMA WLAN WMAN Media Access Control Multi Carrier CDMA Microwave Landing System Mobile Terminal Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing Access Quadrature Amplitude Modulation Quadrature Phase Shift Keying Radio Frequency Radio Technical Commission for Aeronautics Standards and Recommended Practices Secondary Surveillance Radar Self-organizing Time Division Multiple Access Time Division Duplex Time Division Multiple Access Traffic Information Service TIS Broadcast Universal Access Transceiver Universal Time Coordinated VHF Data Link Very High Frequency Wideband CDMA Wireless Local Area Network Wireless Metropolitan Area Network Save date: Public Page 31