Informal paper. An Overview of ADS. - Principles, Drivers, Activities, Technology and Standards - 1 June 1999, v1.0 EUROCONTROL

Transcription

1 Informal paper EUROCONTROL An Overview of ADS - Principles, Drivers, Activities, Technology and Standards - 1 June 1999, v1.0

2 Table of Contents 1. Introduction General How to get more information Principles of ADS Overview Information content of messages Operation in different phases of flight ADS in relation to other surveillance approaches ADS-Contract and ADS-Broadcast The ADS System Drivers for ADS The Historical Development of ADS ADS Requirements New Applications Enabled by ADS-B Use of ADS in ATM and Non-ATM Functions ADS-C Data Link Technologies Introduction FANS-1/A ADS-C ICAO ADS-C The ADS-C protocol Subnetwork issues ADS-B Data Link Technologies Introduction Mode S Extended Squitter VDL Mode UAT Supporting Technologies Introduction Navigation and other onboard data sources June 1999, v1.0 An Overview of ADS Page 2

3 6.3. Cockpit Display of Traffic Information (ADS-B) Changes to SDPD and ground functions A Selection of ADS Activities Introduction EUROCONTROL ADS Programme Operational use of FANS-1/A in the South Pacific NEAN/NEAP/NUP Safeflight Standards Development Introduction ICAO AEEC EUROCAE/RTCA Summary of ADS Standards Abbreviations June 1999, v1.0 An Overview of ADS Page 3

4 1. Introduction 1.1. General This document gives an overview of ADS and in particular covers these topics: The principles behind ADS, The drivers for ADS, ADS technologies, A selection of ADS activities, ADS standards. In this document, the term ADS embraces both ADS Contract and ADS Broadcast. The terms ADS-B / ADS Broadcast or ADS-C / ADS Contract are used explicitly where only one concept is being discussed. This differs from ICAO usage where ADS refers to ADS Contract, ADS-B refers to ADS Broadcast and there is no general term including both concepts How to get more information For more information on EUROCONTROL s ADS Programme, contact Pieter van der Kraan at EUROCONTROL on: Telephone: Fax: Or visit the EUROCONTROL ADS website: 1 June 1999, v1.0 An Overview of ADS Page 4

5 2. Principles of ADS 2.1. Overview ADS is defined by ICAO as: A surveillance technique in which aircraft automatically provide, via a datalink, data derived from on-board navigation and position-fixing systems, including aircraft identification, fourdimensional position, and additional data as appropriate. (ICAO Circular 256-AN/152) Although the ICAO definition refers explicitly to aircraft, ADS may also be used by ground vehicles, although this is less mature than aircraft use of ADS. Together, aircraft and ground vehicles may be referred to collectively as mobile platforms. Using ADS, a mobile platform will send information in a surveillance message (an ADS report ) to other systems via the datalink. ADS-C and ADS-B are different techniques built on this principle. Figure 1 illustrates data transfer between the mobile platforms (aircraft and ground vehicles) and ground systems that may occur using ADS. Figure 1: ADS data transfer ATM/Non-ATM functions ADS requires a data link and, reliable and accurate navigation system to be available on board of the mobile platform. As a result of these requirements, ADS is highly dependent on sophisticated airborne systems. This is different from conventional surveillance systems which require less sophisticated aircraft equipment, if any. Another difference is that aircraft position measurements are made at the ground sensor with conventional surveillance radar, but with 1 June 1999, v1.0 An Overview of ADS Page 5

6 ADS the position measurements are made on the aircraft itself and are based on the information used to navigate the aircraft. On the ground ADS requires ground communications equipment (known as data acquisition units ) to receive ADS information and to pass it to the appropriate surveillance systems. ADS does not usually require any input or co-operation from the aircrew or vehicle driver Information content of messages A mobile platform s ADS surveillance message is usually known as an ADS report and contains information such as the following: mobile platform identity; 3-D position; a timestamp; indication of navigation figure of merit (this is a measure of the accuracy of the onboard navigation system). Other information may also be contained in the ADS report, such as: ground track and speed; airspeed and heading; vertical rate; next waypoint; meteorological information Operation in different phases of flight ADS has the potential to be used for: Air-to-ground surveillance, i.e. surveillance of aircraft in flight by ground systems. This is a conventional ATC role and would complement conventional techniques. Air-to-air surveillance, i.e. surveillance between two aircraft, potentially without the presence of a ground infrastructure. This is a unique feature of ADS-B and also gives mobile platform-to-mobile platform surveillance. Ground movement surveillance, i.e. surveillance of aircraft or ground vehicles when on the ground by ground systems. This would provide for surveillance when on the airport surface and again would complement conventional techniques. ADS can potentially provide surveillance coverage from gate-to-gate, i.e. from the aircraft s first movement on the ground through all phases of flight and back onto the ground ADS in relation to other surveillance approaches The following list describes the different approaches that presently used for civil aviation: Primary radar (PSR). Primary radar provides surveillance of all aircraft, independent of their equipage. With primary radar, only aircraft 2-D position is known and other information such as identity and altitude is not available. It is used largely by the military and as a back-up in case SSR fails. Primary radar coverage is required for high-complexity TMAs by the 1 June 1999, v1.0 An Overview of ADS Page 6

7 EUROCONTROL surveillance standard. Primary radar is also used to provide surveillance used on the airport surface. Secondary radar (SSR) Mode A/C. Secondary radar provides surveillance of all aircraft equipped with a functioning SSR transponder. The SSR transponder is used to report the aircraft s measured barometric altitude and an identity code via a simple data link, thus the aircraft 3-D position and identity is known. Dual SSR coverage is required for use in enroute airspace and major terminal areas by the EUROCONTROL surveillance standard. Mode S. This is an enhancement to SSR Mode A/C in which more data transfer capabilities are added to the aircraft. It also overcomes certain limitations of SSR Mode A/C, for example, increasing the number of aircraft identity codes available and providing greater altitude precision. Mode S is not presently operational in ECAC airspace. Mode S includes several capabilities: Elementary Surveillance: Basic identity and altitude reporting. The aircraft s callsign can also be made available to the ground via the data link. Enhanced Surveillance Reporting of intent, velocity and other information through the use of the downlink of airborne parameters (DAPs). This makes information from the avionics systems available to ground systems and in this respect provides a similar function to ADS. The implementation of enhanced surveillance in the core area of ECAC is the target of the EUROCONTROL IIMSES program. Data link: Mode S can potentially act as a communications subnetwork of the Aeronautical Telecommunications Network (ATN). It also can also act as a non-atn data link with broadcast and other functions. Extended Squitter ADS-B: This is a potential ADS-B data link and is described in Section 5.2. ACAS/TCAS is also related to Mode S/SSR operation. This airborne collision avoidance system operates in the same frequency bands as Mode S/SSR and with compatible protocols. Manual position reporting. Aircrew reports data from navigation system via a voice communication link (usually HF radio). It is used in the North Atlantic and other areas where there is communications coverage but no surveillance infrastructure. It may also be used in the event of a failure of surveillance infrastructure in continental areas. Manual position reporting is not generally used in ECAC airspace. In the long term, a multi-surveillance environment could evolve in ECAC airspace. This would see different types of surveillance system, including ADS, in operation in overlapping and complementary coverage. Figure 2 illustrates the concept of the multi-surveillance environment. 1 June 1999, v1.0 An Overview of ADS Page 7

8 Figure 2: Multi-surveillance environment ADS-C PSR SSR/Mode S ADS-B 2.5. ADS-Contract and ADS-Broadcast There are two types of ADS: ADS-Contract and ADS-Broadcast, described below ADS-Contract ADS-Contract (ADS-C, also known as ADS-Addressed, or ADS-A) involves the transmission of position, identity and other information from a mobile platform (aircraft or ground vehicle) to a single ground function. All ADS-C communications are point-to-point communications between systems on the aircraft and the ground. ADS-C relies on a two-way communications flow from ground functions to and from the mobile platform. Figure 3 illustrates ADS-C information being passed from an aircraft to a surveillance function on the ground. 1 June 1999, v1.0 An Overview of ADS Page 8

9 Figure 3: ADS-Contract ATM/Non-ATM functions The rate at which the mobile platform transmits information, the information sent and the conditions under which the mobile platform transmits data are all controlled by the ground function using a contract with the aircraft which specifies the reporting conditions. The contract is initiated by the ground function and must be agreed between the ground function and the ADS application on the mobile platform. An aircraft may have several different contracts with different ground functions at the same time and they may be changed during the course of the flight. The ground function that establishes the contract and uses the data can include ATM and non- ATM functions, and these are described in Section 3.4. Because a point-to-point (connection oriented) data link is used, the reception of each ADS-C message is guaranteed and reliable unless there is a total loss of data link communications in which case the sender would be notified of this fact. ADS-C users may be confident that either data will be delivered to the receiving party or they will be notified if communications have failed. Early implementations of ADS-C have focussed on satellite communications as the data link because the greatest initial benefits of ADS are in areas where there is no surveillance infrastructure at present, e.g. oceanic or continental areas with low traffic density. Although it is technically possible for ADS-C to be used to provide position reporting in busy TMAs and on the airport surface, the presently available data links are not suitable for this. The EUROCONTROL ST-15 study, which investigated data link suitability for ATS applications, 1 June 1999, v1.0 An Overview of ADS Page 9

10 determined that there was insufficient capacity from presently available data links to meet the high reporting rate requirements that are defined for TMAs and surface movement. Hence ADS- C will probably not be used by ground vehicles in the intermediate term and will be restricted to aircraft applications where low reporting rates are required (eg low density areas). ADS-C may be implemented at the same time as other data link applications, in particular Controller-Pilot Data Link Communications (CPDLC). In fact, it is foreseen by ICAO that the ADS[-C] application should be supported by direct two-way controller pilot data link and voice communication, i.e. there will be CPDLC and voice communications also available ADS-Broadcast ADS-Broadcast (ADS-B) is defined by ICAO as: ADS-B is a surveillance application transmitting parameters, such as position, track and ground speed, via a broadcast mode data link, and at specified intervals, for utilisation by any air and/or ground users requiring it. Figure 4 illustrates the transfer of ADS-B information from an aircraft to other users in the vicinity. 1 June 1999, v1.0 An Overview of ADS Page 10

11 Figure 4: ADS-Broadcast ATM/Non- ATM functions For example, cockpit display ATM/Non-ATM functions ADS-B relies on the regular and frequent transmission of ADS reports via a broadcast data link. The ADS-B reports are sent periodically by the mobile platform with no intervention from the ground function. ADS-B reports may be received and processed by any recipient in range of the transmitting mobile platform. If received by a ground data acquisition unit, the ADS-B report will be processed with other surveillance data and used by ATM or non-atm functions. ADS-B offers surveillance data delivery from air-to-air or from air-to-ground. Transmitting data directly from air-to-air means that there is no need for a ground infrastructure to be present for airborne surveillance to be performed. Using ADS-B reports received from surrounding aircraft, a traffic surveillance picture can be generated in the cockpit of all aircraft. Direct air-to-air surveillance, as offered in ADS-B, is not available from ADS-C. ADS-B reports are not acknowledged. Therefore, the transmitting mobile platform does not know which, if any, recipients are receiving and processing the ADS-B reports. There will be an occasional loss of messages (ie ADS reports transmitted but not received correctly) due to the nature of this broadcast approach. ADS-B transmissions lie outside of the ATN or ACARS, which only provide point-to-point (connection oriented) communications. 1 June 1999, v1.0 An Overview of ADS Page 11

12 2.6. The ADS System Figure 5 illustrates the components in the overall ADS System on an aircraft and ground systems. Note that the ground components will always be present when using ADS-C, but they may not be present in an ADS-B environment. Some of the components may have multiple roles, for example the crew display may support other systems in addition to ADS. Figure 5: ADS system components Crew interface/ display * Data link equipment/ Data acquisition unit ADS application Navigation system ATM/Non-ATM functions Data link equipment/ Data acquisition unit ADS application Surveillance data processing and distribution (SDPD) * = Depends on implementation The components are described below: Navigation/avionics systems (mobile): These are the systems that provide the necessary data to the ADS system, including user s identity, position and other data. ADS Application (mobile): This manages the processing, encoding and transmission ADS data. It may also manage the reception of ADS-B information from other mobile units. In the case of ADS-C, it must manage the contract that has been established with the ground ADS application. SDPD (mobile): This processes surveillance data, if it is received on the mobile unit, for use by on-board systems. Data Link Equipment/ Data acquisition unit (mobile): For ADS-B, this transmits the ADS-B reports to other users and, possibly, receives ADS-B reports from other mobile units. In the 1 June 1999, v1.0 An Overview of ADS Page 12

13 case of ADS-C, it provides a two-way communications functions with the ground systems, including the transmission of the ADS reports to the ground. Crew interface/display (mobile): This may have several functions, including the display of other ADS information received from an ADS-B system, or the input of ADS data (for example, aircraft callsign or next waypoint). Data Link Equipment/ Data acquisition unit (ground): For ADS-B, this receives ADS-B reports from mobile platforms and possibly transmits broadcast information to those users. In the case of ADS-C, it provides a two-way communications functions with the mobile systems. ADS Application (ground): This manages the reception of ADS information from mobile platforms. In the case of ADS-C, it manages the establishment and change of the ADS contracts. SDPD (ground): This processes the received surveillance data for use by ground systems. 1 June 1999, v1.0 An Overview of ADS Page 13

14 3. Drivers for ADS 3.1. The Historical Development of ADS ADS has been driven, to a greater or lessor extent, by the availability of new technology. Through the new technology, the possibilities of dependent surveillance have become visible. The main two technological developments that have led to ADS are: Data links which are used to transmit digital information from an aircraft to a ground system (possibly via a satellite) or to another aircraft. Navigation systems which can provide the aircraft location in terms of latitude/longitude coordinates. Such systems have been around for some time (inertial navigation systems are used for navigation in remote and oceanic areas) but low costs systems based on the Global Positioning System (GPS) have only recently become available. In ADS, these two technologies are integrated with appropriate protocols to provide the ADS application. The rapid pace of technology development, combined with the relatively slow pace of international standardisation, has meant that the first systems to be available are based on airline industry standards ADS Requirements Development of operational requirements for ADS-C and ADS-B is undertaken at the highest level by ICAO. The ICAO ADSP has produced a manual of ATS data link applications which provides guidance on the implementation of data link applications for aviation authorities, airspace users and service providers. It defines requirements for ADS-C and ADS-B performance for air-to-ground purposes (not air-to-air). The RTCA, an industrial organisation in the US, has also defined a MASPS for ADS-B, including requirements for air-to-air range for different applications New Applications Enabled by ADS-B The unique feature of ADS-B is the capability that it provides for air-to-air (or mobile-to-mobile, eg aircraft-to-ground vehicle) situation awareness without a requirement for a ground infrastructure. This will enable new applications and the RTCA ADS-B MASPS identify such applications. Some of the RTCA near-term airborne applications are summarised in Table 1, although the operational details of these applications are not yet internationally agreed. 1 June 1999, v1.0 An Overview of ADS Page 14

15 Table 1: New applications enabled by ADS-B Application In-trail climb/descent or lateral passing manoeuvres Station keeping Final approach spacing Enhanced visual acquisition Description Limited autonomous passing manoeuvres. Likely to be first introduced in remote/oceanic areas. Airborne separation maintained by one aircraft following another. Separations may be set by a ground controller who maintains control. Pilot takes some responsibility for achieving optimum final approach spacing. Low-criticality application. ADS-B is used to help pilot identify other aircraft in the vicinity. Note that ADS-B is not necessarily the only enabling technology that could support these applications Use of ADS in ATM and Non-ATM Functions The EATCHIP operational concept document defines the following ground ATM functions, all of which could potentially benefit from ADS data: Controller situation display which presents an accurate and timely indication of aircraft position, identification and associated data. Safety nets (such as short term conflict alert, STCA, or minimum safe altitude warning, MSAW) which alerts controllers of potential conflicts time, while minimising the number of nuisance alerts. Medium term conflict detection (MTCD) which detects conflicts between approximately 2 and 20 minutes ahead of current time of the aircraft and makes them available for display or to other functions Trajectory prediction tool which predicts the relevant trajectory data items of an aircraft over a given time horizon ahead of current time. Flight data processing and distribution (FDPD) which checks and fuses all incoming flight data, determines the route that each flight will follow and the sectors/terminal airspace through which the flight will pass. FDPD also calculates and updates the estimated times at which flights will be overhead points on the route, on the basis of Surveillance Data. Monitoring aids which has two functions: Conformance monitoring functions which obtain trajectory prediction data and compare it with current Surveillance data. If the current data deviates from the predicted data by more than a parameter then a deviation is declared and the controller warned. Automatic reminder functions which reminds the controller of actions he should perform at a certain time or position. Departure and arrival management (DMAN/AMAN) which assists a controller in optimising the sequence of arrivals and departures from one, or more, runways). Meteorological data management which collects the latest and forecast meteorological data, and provide integrated meteorological information to functions which require it. Air traffic flow management which balances demand against capacity for the ATM system. Non-ATM functions could also make use of ADS data. These functions include: international airports, 1 June 1999, v1.0 An Overview of ADS Page 15

16 aerodromes and heliports, weather service providers to aviation, air defence systems, search and rescue, law enforcement, and accident investigation authorities. 1 June 1999, v1.0 An Overview of ADS Page 16

17 4. ADS-C Data Link Technologies 4.1. Introduction There are two implementations of ADS-C: FANS-1/A and ICAO ADS-C. They are described in detail in the following sections, but the primary characteristics are summarised in Table 2. Table 2: Primary characteristics of ADS-C technologies FANS-1/A ATN Standardised by Industry ICAO Communications network used Use of subnetworks Availability and operation ACARS Any ACARS subnetwork (Satellite, VHF, HF) Available and operational now ATN Any ATN subnetwork (Satellite, VHF, Mode S, HF) including those defined in the future Available and operational in the future Although it is technically feasible for a ground vehicle to support ADS-C, it is not widely foreseen that this would happen due to the relatively high costs of ADS-C equipment and the lack of a suitable subnetwork at present. Therefore, the following ADS-C description is restricted to aircraft FANS-1/A ADS-C This is a general name for the industry-standardised implementations of ADS-C. FANS-1 is the Boeing implementation and AIM-FANS is the Airbus implementation (commonly known as FANS-A). Generically, they are known as FANS-1/A. Implementations by other airframe manufacturers are also under development. FANS-1/A operates over the ACARS data link and can use any of the ACARS mobile subnetworks, i.e. satellite, VHF or HF. The ACARS (Aircraft Communication Addressing and Reporting System) is a data link that is operated mainly for airline purposes. It provides air-toground and ground-to-ground communications and has been operational for more than 20 years. There are two service providers that provide ACARS services: ARINC, which operates principally in the United States and China, and SITA, which operates mainly in Europe and the rest of the world. FANS-1/A has been widely trialled by many states and is presently operational in the South Pacific. FANS-1/A implementations are based on the ARINC 622 and ARINC 745 standards defined by the AEEC (Airlines Electronic Engineering Committee), which is an airline industry standards organisation. 1 June 1999, v1.0 An Overview of ADS Page 17

18 4.3. ICAO ADS-C This is ADS-C implemented to ICAO standards and operating over the Aeronautical Telecommunications Network (ATN). ICAO ADS-C can use any of the mobile ATN subnetworks that are available to the aircraft and ground. It may also be known as ATN ADS-C. At present the following ATN mobile subnetworks are defined by ICAO: satellite (AMSS), VHF (VDL Modes 1 and 2), Mode S and HF. More subnetworks may be defined in the future. ATN ADS-C is not used operationally and is presently the subject of trials and development. The ICAO ADS-C standards are defined in SARPs (Standards and Recommended Practices) of Annex 10. Various SARPs are defined, including those that define the ADS application and those that specify the ATN communications systems and its subnetworks. The SARPs are at different levels of maturity not all are finalised yet. They are also supported by relevant industry standards, such as MOPS. The ICAO ADS-C application is similar, but not identical, to the FANS-1/A application. Whilst ICAO ADS-C is generally accepted as the long term solution for ADS-C, there will need to be a transition for those aircraft that are already equipped with FANS-1/A to ATN. The timescales for such a transition are not clear and may take many years The ADS-C protocol ADS-C relies on the establishment of contracts between an aircraft and the ground system. A contract is an agreement between ground and aircraft systems as to how and when the aircraft will transmit ADS reports to the ground. A contract is used in ADS-C to allow a ground system to specify what information an aircraft reports and when it reports it. Contracts can be established, modified and cancelled at any time during the flight. There are three types of contract: Periodic: The aircraft transmits ADS-C reports at a regular rate. Event: The aircraft transmits ADS-C reports when certain events occur. An event could be, for example, a change in altitude or speed for the aircraft or a deviation from the route. On-demand: The aircraft transmits a single ADS-C report when requested by the ground ATCC (a one-shot ). An aircraft may also report in emergency mode. This is initiated by the airborne systems and is not a contract. In this mode, the aircraft transmits ADS-C reports at a high rate to all ground functions that it has a contract established with. All ADS-C reports consist of a basic ADS group to which optional groups may be added (Note that there are small differences between FANS-1/A and ATN messages.) The basic ADS group consists of: Latitude/Longitude, Altitude, Time, Figure of Merit. (Identity is also provided, although it is not part of the ADS group.) 1 June 1999, v1.0 An Overview of ADS Page 18

19 The optional data can include: the projected profile, ground vector, air vector, weather information, short term intent and extended projected profile Subnetwork issues The performance of ADS-C is critically dependent on the performance of the mobile subnetwork that provides the air-to-ground communications because this is often performance or capacity limited. Therefore, subnetwork performance can place a practical limit on the overall performance of the ADS-C application. The primary characteristics of each ATN subnetwork are summarised in Table 3. Capacity (relative) Latency (relative end-to-end delays) Table 3: Primary characteristics of ATN subnetworks Satellite (AMSS) VHF * (VDL) Mode S * HF * Low High High Low High Low Medium Low Cost (relative) High Low Medium Low Coverage * = Expected or predicted Almost global for geostationaries: up to lat excluding poles Within line of sight of ground station Within line of sight of ground station Hundreds of NM from ground station 1 June 1999, v1.0 An Overview of ADS Page 19

20 5. ADS-B Data Link Technologies 5.1. Introduction There are two primary implementations of ADS-B under discussion: Mode S extended squitter and VDL Mode 4. A third implementation, UAT, is at a less mature stage of development. All three technologies are discussed in the following sections and a summary of their primary characteristics is given in Table 4. Table 4: Primary characteristics of ADS-B technologies System Communications band used Communications protocol Communications types offered Mode S Extended Squitter Development of Mode S system VDL Mode 4 New VHF data link 1090 MHz (SSR band) MHz (VHF band) Random squitter Broadcast and point-topoint Co-ordinated use of timeslots Broadcast and point-topoint UAT New data link around 960 MHz (proposed) Squitter in a random timeslot Broadcast 5.2. Mode S Extended Squitter Mode S Extended Squitter is an enhancement to the Mode S SSR in which aircraft regularly transmit extended squitter messages containing aircraft position, identity and other information. The extended squitters are transmitted on the SSR frequency of 1090 MHz. They can be received by any suitably equipped mobile platform or ground station. In theory, ground vehicles could also transmit Mode S Extended Squitter, but this application is much less mature than aircraft equipage. Transmission of extended squitters should not be confused with the other functions of the Mode S system: elementary surveillance, enhanced surveillance, and datalink, or with ACAS/TCAS. All of the Mode S functions, and also ACAS/TCAS, operate using the same protocols and message formats and on the same frequencies (1030 MHz for interrogations and 1090 MHz for replies.) Extended squitters are transmitted at random intervals with certain average transmission rates. The transmission of extended squitters is not synchronised with the transmissions from any other user. A summary of the extended squitters contents and transmission rates is given in Table 5. 1 June 1999, v1.0 An Overview of ADS Page 20

21 Table 5: Mode S extended squitters Extended squitter type Message contents Average transmission rate Airborne position (transmitted when airborne) Surface position (transmitted on surface) Status Identification Velocity latitude longitude altitude status (i.e. alert conditions) CPR format/time marker * latitude/longitude ground speed ground track CPR format/time marker * altitude type (barometric/gnss) transmission rate aircraft type accuracy of position information callsign (8 character ICAO code) east-west velocity north-south velocity vertical rate intent change flag 0.5 s (when moving) 0.5 s (when stationary) 5.0 s Only transmitted on demand from a ground station interrogator (when moving) 5.0 s (when stationary)10.0 s 0.5 s Event driven variable Only transmitted as result of certain data changes * CPR = Compressed Position Reporting, which is the position encoding system used. The CPR format/time bit provides information for the CPR algorithm and time information. A transponder capable of transmitting extended squitters may retain its original functionality associated with Mode S SSR. Alternatively, an equipment might transmit/receive only extended squitters, and not perform any other Mode S functions VDL Mode 4 VDL Mode 4 is being developed from a system known as STDMA (Self-Organising Time Division Multiple Access) which was invented and developed in Sweden. VDL Mode 4 is intended to support a range of broadcast and point-to-point applications of which ADS-B is the most important. Like Mode S SARPs, VDL Mode 4 SARPs also include ATN compatibility, so that VDL Mode 4 can act as an ATN subnetwork at the same time as providing ADS-B (and other applications). VDL Mode 4 operates in the Aeronautical Mobile (Route) Service (AMRS) band that is a VHF band extending from MHz to MHz. Each VDL Mode 4 channel occupies one 25 khz channel and the system is intended to operate with a minimum of two channels simultaneously. More channels may be used locally to provide sufficient capacity for the applications being supported by VDL Mode 4. VDL Mode 4 is based on a VHF data link that uses a timeslot structure for data link communications. All transmissions are synchronised to the start of a timeslot and each timeslot is a nominal period in which one user transmits a single ADS-B report. Timeslots are used in turn by different mobile or fixed platforms. 1 June 1999, v1.0 An Overview of ADS Page 21

22 The data link uses a set of reservation protocols for managing media access which allow a transponder to reserve a later timeslot for another message. They are designed to minimise occurrences of two transponder s transmissions interfering with each other and they also support the basic ADS-B function. A VDL Mode 4 transponder requires a source of precise time to maintain synchronisation to the timeslots. The source of this data is not specified in the SARPs. A GNSS receiver can be used to provide the necessary synchronisation, or another source might be suitable, e.g. an accurate clock. Accurate time sources are presently under investigate through EUROCONTROL UAT The Universal Access Transceiver (UAT) is an experimental data link system being developed as a research and development project at The MITRE Corporation's Centre for Advanced Aviation System Development in the US. The aim of the project is to illustrate and evaluate the concept of a multipurpose broadcast data link architecture to meet aviation requirements. UAT is a single-channel system, using a channel bandwidth of approximately 2 MHz. It operates on the same frequency for transmit and receive in order to allow full air-air connectivity for ADS-B with a minimum of new hardware. All UAT stations access the channel autonomously, using random transmissions in a timeslot structure. UAT trials have been conducted at the frequency of 960 MHz, which is a frequency in the DME band. UAT supports only broadcast communications and does not provide point-to-point communications or operate as an ATN subnetwork. There are no draft SARPs available for UAT and it is not under development in any ICAO panel. 1 June 1999, v1.0 An Overview of ADS Page 22

23 6. Supporting Technologies 6.1. Introduction This section describes some of the supporting technologies that, in addition to the data links described in the previous section, are required to implement ADS Navigation and other onboard data sources Suitable navigation systems (and other onboard systems) are required to provide the necessary data for ADS. They must also have the necessary integrity, availability and other performance parameters required for the functions/applications which are using ADS. However, existing navigation systems were not designed with the requirements of ADS in mind and therefore may not the requirements of some ADS surveillance applications. Possible navigation systems that can provide ADS data on an aircraft include: GPS/GNSS, Inertial reference systems (IRS), Multi-DME systems. Combinations of these systems may also be used to improve overall performance. For a ground vehicle, the most pragmatic system in the near-term is GPS. This can give high accuracy when used with differential corrections and is a low cost system Cockpit Display of Traffic Information (ADS-B) Some applications may require a cockpit display of traffic information (CDTI) that gives an aircrew a surveillance picture of surrounding traffic. The data for a CDTI can be obtain by receiving and processing ADS-B reports from surrounding mobile platforms or information received from a ground station. The range of traffic on the display may be up to 200 NM. Using a CDTI, an aircrew may be able to undertake new applications. These include airborne self-separation and passing manoeuvres. Another benefit of CDTI is the increased situation awareness for aircrew. This may be a particular benefit in the future as data link communications replace the voice radio/telephony (R/T) that presently provides situation awareness through the party line. Thus ADS-B may assist the introduction of data link communications. However, the introduction of these applications is not without difficulty since it raises questions of integrity/availability of data and also of control/responsibility residing in the cockpit or with the ground controller. The NEAN project, and other VDL Mode 4/STDMA projects, are testing prototype CDTI s (as shown below) in various commercial aircraft and also in ground vehicles. The NLR has undertaken flight simulations of a free flight environment in which all aircraft perform self separation using a CDTI. 1 June 1999, v1.0 An Overview of ADS Page 23

24 Figure 6: Prototype CDTI s tested in the NEAN project In order to use surveillance data on the aircraft, some surveillance data processing (SDP) will be required. This is largely undefined at present. CDTI could also be provided using the Traffic Information Service (TIS or broadcast TIS, TIS-B) data link application. This involves transmitting surveillance information from the ground to the aircraft for display in the cockpit. The transmitted surveillance picture is derived from SSR Changes to SDPD and ground functions Figure 7 shows how ground surveillance data processing and distribution (SDPD) systems will be required to cope with more surveillance data sources in a multi-surveillance environment that include ADS. In addition to previous activities, the SDPD system(s) will be required to: fuse data from very diverse surveillance sources, undertake integrity checks between different surveillance systems, manage ADS-C contracts, process ADS-B data, cope with the new information (e.g. next waypoint) that will become available through ADS and other similar systems. 1 June 1999, v1.0 An Overview of ADS Page 24

25 Figure 7: Multi-surveillance environment ADS-C ADS-B PSR/SSR/ Mode S ATN subnetwork Surveillance Data Processing and Distribution (SDPD) ATM Functions Non-ATM Functions In addition to the changes to SDPD, changes to the downstream functions relying on SDPD data may be required. Examples of these changes include: Enhancements to controller displays to show additional information, eg callsign, next waypoint. Enhancements to the performance of ground functions, eg improvements to trajectory prediction using next waypoint and weather forecasts using aircraft wind measurements. The development of new functions making use of newly available information. 1 June 1999, v1.0 An Overview of ADS Page 25

26 7. A Selection of ADS Activities 7.1. Introduction This selection describes a selection of ongoing ADS activities. There are too many to describe in full, so a small number of influential activities have been selected EUROCONTROL ADS Programme At the end of 1998, the first stage of the EUROCONTROL ADS Programme was formally approved. The ADS Programme includes all necessary actions to achieve the initial implementation and operational use of ADS in Europe. The programme relies on considerable co-ordination and sharing of resources with States. The following text describes the programme objectives: The first objective of the ADS Programme is to determine if ADS, as either sole means or in conjunction with other surveillance sources, can meet the operational requirements for surveillance for the medium or long-term. In parallel with this it has to be assessed whether ADS as a concept and the various corresponding candidate ADS-B and ADS-C technologies are safe and cost-beneficial in ECAC airspace. The second objective of the Programme, subject to a positive outcome of a corresponding business case, is the local implementation of operational systems (i.e. deployment of ADS infrastructure in ECAC) and the maintenance and support activities for those systems. The programme is in 4 stages, of which the first (Stage 0) will last for Programme activities are foreseen up to The activities in the Stage 0 include: Development of an ADS strategy, Initial safety analysis, Initial cost-benefit analysis, Development of ADS initial technical requirements, Development of ADS initial specifications Operational use of FANS-1/A in the South Pacific Of the ADS technologies described here, only FANS-1/A is operational. As of March 1999, FANS-1/A systems are operational in the South Pacific, between the US west coast and Sydney and Auckland. Much of the trials of FANS-1/A systems has been in the Pacific area. Three airlines are flying the FANS routes that have been established: United, Qantas and Air New Zealand. Of the bordering control authorities, only the New Zealand authority has an operational ADS ground system. Air Services Australia are presently testing an ADS system. The primary certification of FANS-1 took place in June 1995 in conjunction with Qantas. To meet the requirements for Pacific operation an aircraft must equip with the FANS-1 package consisting of GPS, CPDLC, ADS and ACARS upgrades. 1 June 1999, v1.0 An Overview of ADS Page 26

27 In the area of FANS-1 operations, two new operations have been built on the ADS-C/CPDLC infrastructure: flex tracks and Dynamic Aircraft Re-route Planning (DARP). Flex tracks are tracks that can move based on changing wind and temperature conditions, instead of the fixed tracks previously in place. DARP allows a new track to be uplinked to an aircraft in flight. The aircrew can choose whether to adopt the new track. United Airlines have reported that flying FANS-1/A routes over the South Pacific has given them typical fuel savings of 1,500 to 1,800 kg, and flight time reductions of 15 minutes per flight. One of the aims of the FANS-1/A Pacific work is to reduce separation standards. Current separation standards in the Pacific are generally 100NM, although 50NM is used in some areas. One quoted target is to reduce the standards to 30NM lateral separation NEAN/NEAP/NUP Prototype VDL Mode 4 equipment, known as STDMA, has been trialled in several projects such as the EC-sponsored North European ADS-B Network (NEAN) and its sister project NEAN Applications Project (NEAP). The NEAN project involved the implementation of a trials infrastructure for STDMA equipment involving ground stations, aircraft and ground vehicles on the airport surface. The trials infrastructure spreads across Denmark, Germany and Sweden. Participants include Lufthansa, Scandinavian Airways System (SAS), German CAA, Swedish CAA and Danish CAA. In the project, STDMA equipment and cockpit displays (as shown in Figure 6) were installed on revenue aircraft including: SAS Fokker 28s, SAS DC9s and Lufthansa 747s. The NEAP project testing applications of the NEAN infrastructure. The applications included: air and ground situation awareness, surface movement surveillance and runway incursion, ATIS-broadcast, TIS-broadcast, DGNSS-supported approaches. Although the STMDA trials are widespread and fairly extensive (they are the largest ADS-B activities presently underway), the equipment used is simpler than that described in the draft SARPs. An extension to NEAN/NEAP has recently started. This is known as NUP (NEAN Update Programme) and it will include a transition of the NEAN infrastructure to equipment that is compatible with the VDL Mode 4 draft SARPs Safeflight 21 Safeflight 21 is an FAA programme which will conduct and operational evaluation of nine operational enhancements that ADS-B and other systems could deliver. The nine enhancements have been defined by the RTCA: 1 Provide weather and other information to the cockpit, 1 Joint government/industry roadmap for free flight operational enhancements, RTCA, August June 1999, v1.0 An Overview of ADS Page 27

28 Affordable means to reduce controlled flight into terrain, Improved capability for approaches in low visibility conditions, Enhanced capability to see and avoid adjacent traffic, Enhanced capability to delegate aircraft separation authority to the pilot, Improved capability for pilots to navigate airport taxiways, Enhances capability for controller to manage aircraft and vehicular traffic on airport surface, Provides surveillance coverage in non-radar airspace, Provides improved separation standards. The Safeflight 21 programme involves the trial of ADS-B (and associated) technologies to evaluate the delivery of the above operational enhancements. 1 June 1999, v1.0 An Overview of ADS Page 28

29 8. Standards Development 8.1. Introduction The section summarises the main ADS standards and activities of organisations working on ADS standards ICAO There following list describes the main ADS activities undertaken by ICAO Panels: Automatic Dependent Surveillance Panel: The ADSP deals with the operational aspects of ADS and Air Traffic Management data link services in general. The panel is investigating the implementation of ADS-C and ADS-B with particular reference to requirements and procedures. It has produced a Manual of ATS Data Link Applications. Aeronautical Telecommunications Network Panel: The ATNP is responsible for developing the ATN SARPs and the ICAO ADS-C application. These SARPs (known as CNS-ATM/1 SARPs) are already published. Airborne Mobile Communications Panel: The AMCP is developing SARPs for the mobile subnetworks of the ATN. SARPs have already been developed for the satellite subnetwork (AMSS), VHF data links 1 and 2 and the HF data link. The panel is also developing SARPs for VDL Mode 4 for surveillance applications. SSR Improvements and Collision Avoidance Panel: The SICASP has developed SARPs for Mode S as an ATN subnetwork and also for extended squitter AEEC FANS-1/A ADS-C is defined by a set of ARINC standards, which are industry standards defined by the AEEC (the Airlines Electronic Engineering Committee). The following standards define FANS-1/A ADS-C: ARINC 745: This defines the ADS application, airborne equipment requirements and interfaces to other airborne systems (e.g. navigation). ARINC 622: This is used to provide a suitable interface for the ADS application and to enhance communications integrity of the ACARS data link EUROCAE/RTCA EUROCAE (The European organisation for Civil Aviation Equipment) and RTCA (Requirements and Technical Concepts for Aviation) are developing MASPS (Minimum Aviation System Performance Standards) and (Minimum Operational Performance Standards) related to ADS. EUROCAE is a European standards organisation and RTCA is the US equivalent. The following list summarises the main EUROCAE and RTCA activities in the area of ADS: EUROCAE Working Group 51: EUROCAE WG-51 is developing MASPS for ADS-B and for MOPS for both Mode S Extended Squitter and VDL Mode 4. 1 June 1999, v1.0 An Overview of ADS Page 29

30 RTCA Special Committee 186: RTCA SC-186 is developing ADS-B MASPS and ADS-B MOPS for Mode S Extended Squitter. It is also developing CDTI MOPS for ADS-B and Airborne Surveillance Processing MOPS (ASSAP). EUROCAE WG-49: EUROCAE WG-49 is responsible for the specifications for Mode S data link transponders and processors. The ED-73 equipment characteristics document published in 1998 includes the extended squitter functionality and is currently being updated by the group. EUROCAE WG-55: EUROCAE WG-55 is analysing the capabilities of next generation satellite systems communications (NGSS). Part of its considerations include the use of NGSS for ADS-C applications. 1 June 1999, v1.0 An Overview of ADS Page 30

31 8.5. Summary of ADS Standards Document Developed/Published by Status General ADS Standards (not data link technology specific) ICAO Manual of ATS Data Link Applications ADS and Air Traffic Services Data Link Applications Circular ICAO ADSP Published 1999 ICAO ADSP Published 1995 RTCA ADS-B MASPS RTCA SC186 Published Update expected early 2000 ADS-B CDTI MOPS RTCA SC186 WG-1 Being drafted, expected in Airborne Surveillance Processing MOPS (ASSAP) ATN ADS-C Standards ATN SARPs, Sub-Volume II Air- Ground Applications ( CNS/ATM-1 SARPs) ATN Subnetwork SARPs and MOPS (Satellite, VDL, Mode S, HF and others) FANS-1/A ADS-C Standards RTCA SC186 WG-4 ICAO ATNP Various, including EUROCAE, RTCA, and ICAO AMCP and SICASP Draft expected late 1999/early Published ARINC 745 AEEC Published ARINC 622 AEEC Published Mode S (1090MHz) Extended Squitter (ADS-B) Standards Various, including some published and some in draft stage Mode S Extended Squitter SARPs ICAO SICASP Published July 1998 Manual on Mode S Specific Services ICAO SICASP Issue 1 published Updates and modifications continue. Mode S Extended Squitter ADS-B MOPS VDL Mode 4 (ADS-B) Standards WG51 Sg1 & SC186 Wg3 Being drafted, expected early VDL4 SARPs ICAO AMCP Under validation. Expected 2000 VDL4 ADS-B MOPS EUROCAE WG51 SG2 Being drafted, expected early 2000 (dependent upon VDL4 SARPs) 1 June 1999, v1.0 An Overview of ADS Page 31

32 9. Abbreviations ADS Automatic Dependent Surveillance ADS-B Automatic Dependent Surveillance - Broadcast ADS-C Automatic Dependent Surveillance Contract ADSP Automatic Dependent Surveillance Panel AEEC Airlines Electronic Engineering Committee AMAN Arrival Manager AMCP Aeronautical Mobile Communications Panel ARINC Aeronautical Radio Incorporated ATN Aeronautical Telecommunications Network ATNP Aeronautical Telecommunications Network Panel CDTI Cockpit Display of Traffic Information DGNSS Differential Global Navigation Satellite Systems DMAN Departure Manager EUROCAE European Organisation for Civil Aviation Equipment FAA Federal Aviation Administration FDPD Flight data processing and distribution GNSS Global Navigation Satellite Systems GPS Global Positioning System MASPS Minimum Aviation System Performance Standards MOPS Minimum Operational Performance Specification MSAW Minimum Safe Altitude Warning MTCD Medium Term Conflict Detection NEAN North European ADS-B Network NEAP North European ADS Applications PSR (or PR) Primary Surveillance Radar RTCA Requirements and Technical Concepts for Aviation SARPs Standards and Recommended Practices SICASP Surveillance Improvements and Collision Avoidance Panel SSR Secondary Surveillance Radar STCA Short Term Conflict Alert STDMA Self-Organising Time Division Multiple Access TIS-B Traffic Information Service - Broadcast UAT Universal Access Transceiver VDL VHF Digital Link 1 June 1999, v1.0 An Overview of ADS Page 32