43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004
WP No. 88
Investigate Operational Aspects of Automatic Dependent Surveillance – Broadcast
Presented by SC1
1.1 At last year’s conference Working Paper 85 on “ADS-B Operations” provided an overview Automatic Dependent Surveillance – Broadcast (ADS-B) and recommended the development of technical policy. This has been addressed in part by a separate working paper to this year’s conference on “the differences between dependent and independent surveillance in determining position reports”.
1.2 During discussions within Standing Committee 1, it was considered that an additional working paper addressing the question “is ADS-B reliable enough to be used for the provision of radar-type procedures and separation” was required. The answer is a qualified “yes”. This paper provides a brief explanation of the reasons but first discusses the various considerations for development on ADS-B policy. This paper concludes by recommending specific areas of future work.
2.1 Before discussing ADS-B, one should consider the problems of classifications and interrelations that are inherent in the technical analysis of systems. For example consider a basic position report provided by the pilot via radio. The concept of Communication, Navigation and Surveillance (CNS) systems has been applied for many years but there is still debate among experts as to whether a voice position report is communication, navigation or surveillance or a combination of these.
2.2 ADS-B has its genesis in datalink and has generated its own problems of classifications and interrelations. For example, ADS-B enables aircraft to be electronically aware of the position of other aircraft and so self-separation by pilots is considered as an ADS-B application. Is ADS-B just the “position information datalink” that enables any number of applications or is ADS-B the datalink and everything that can now be done (all applications) because the position information datalink now exists? The term “ADS-B applications” is often used in a generic sense, i.e. meaning all applications.
2.3 If an enabling technology and all its applications are considered as one, then there are problems with having overlapping and even contradictory requirements for similar systems. Consider radar and ADS-B surveillance systems for Air Traffic Control (ATC) – what are the similarities and differences? Are they significant differences to the services offered? The International Civil Aviation Organisation (ICAO) has been considering these types of questions in the rewrite of ICAO Doc 4444 Procedures for Air Navigation Services – Air Traffic Management (PANSATM) for initial non-radar only application, and has decided to have a separate section on ADS-B in the chapter on radar. One significant problem is the many references to items such as “radar displays” and “radar vectoring”, when it should perhaps be “air traffic display” and just “vectoring”.
2.4 It takes some effort to consider the application as separate to the enabling technology – but in the longer term this is what is needed. There is a need to identify the functions (or applications) that are required and then the operational requirements that make that application suitable for use. However, there are still considerable problems in this approach as defining the generic operational requirements is most challenging. Consider the application of controller-pilot direct voice contact. If the technology is not considered, only the application, how do we specify the requirements that deliver what we require? What technology can fulfil these requirements – Very High Frequency (VHF) radio, High Frequency (HF) radio, satellite phones, mobiles phones, digitally encoded voice data links between aircraft and ATC, etc? This issue is already causing problems, for example; IFATCA insists on ICAO Standards and Recommended Practices (SARP) compliant Aeronautical Telecommunications Network (ATN) for Controller Pilot Data Link Communication (CPDLC) but now that a HF SARP-compliant ATN system is available we have to reconsider performance factors.
2.5 ICAO avoids specific references to particular technology, and IFATCA wherever possible should do the same. IFATCA should concentrate on “functionality” or “operational requirement” and avoid statements on, and choices of, particular technology. For example ADS-B datalink can use three different types of datalink Secondary Surveillance Radar (SSR) Extended Squitter, VHF Datalink (VDL) Mode 4 and Universal Access Transceiver (UAT) – see last year’s paper for details. It can use other types too. There is often much commercial and political lobbying for the selection of a particular datalink and requests have been made to IFATCA to state its preferred datalink technology for ADS-B, but IFATCA has so far refrained from doing this. By specifying functionality or operational requirements IFATCA not only assists in selecting the balance between the strengths and weakness of candidate technologies (without entering the commercial debate) but also can assist in guiding future developments.
2.6 Technology can enable several different applications and each application will have its own requirements on the technology. For example it is possible to provide a radar separation service between aircraft without knowing the geographic location of the aircraft. As long as the relative positions of the aircraft are known and there is a means of measuring distance then radar separation can be applied between identified targets. However, if aircraft are to be separated from terrain or be provided a navigation assistance service, then the geographical location of the aircraft on the radar display is required. When developing operational requirements for applications, consideration needs to be given to exactly what is required.
2.7 To answer the question raised in this paper, another question must be asked, that is what constitutes “reliability” of existing radar systems? Some areas of the world have become accustomed to radar being many overlapping radar sources with electronic relay of aircraft identification whereas other areas of the world may only have a single radar source and require radar identification actions for each aircraft. Obviously if there is only one radar source then the loss of that source has a significant effect over a wide area, and it may not be possible to get any assistance from “adjacent” radar display systems. In a similar way, if all aircraft were using Global Navigation Satellite Systems (GNSS) for ADS-B, and GNSS becomes unavailable then many aircraft will be affected, both for surveillance and navigation.
2.8 The main issue about the reliability of ADS-B is that of accuracy of the position information, which is addressed in a separate working paper discussing dependent surveillance. Australia’s Burnett Basin ADS-B trial has indicated high accuracy of ADS-B information and is being compared to Monopulse Secondary Surveillance Radar (MSSR). This confirms trial results from other countries. Note this refers to absolute position information (location of the particular report) and not necessarily relative position information (that is between aircraft).
2.9 The update rate of ADS-B and the datalink specification indicate a reliable and timely provision of surveillance information equivalent to, or better than, the majority of radar systems.
2.10 The establishment and maintenance of the identity of the aircraft on the ADS-B display is superior to Mode A SSR identification used today.
2.11 A cautious approach to acceptance of new technology is appropriate and operational experience should be gained in an evolutionary way so that actual experience demonstrates the reliability.
2.12 Last year’s working paper on “ADS-B Operations” listed a number of issues to be addressed before ADS-B applications (note applications) can be implemented. Each of these is now reviewed in turn to see if additional IFATCA policy is required.
2.12.1 Safety Assessment
ICAO requires a safety assessment before the implementation of any new ATS system or procedure, including ADS-B applications. Additional IFATCA policy is therefore not required.
2.12.2 Use of Global Positioning Systems (GPS)/Global Navigational Satellite Systems (GNSS)
This is not a specific ADS-B application issue, but as GNSS is likely to be the source position information for many ADS-B applications, development of IFATCA policy in relation to alternative surveillance or aircraft backup systems may be required.
IFATCA has valid policy on ADS ratings.
IFATCA has valid policy on training requirements for new systems.
2.12.5 Equipment Certification
This is not just an ADS-B or even an application issue. Aircraft systems and their interfaces to Air Traffic Management (ATM) systems have been regulated and certified for many years. However ground ATM has been subject only to national regulation by the States without any harmonisation. With the move toward separation of service providers and regulators (and with the move towards multinational centres) there will be increasing pressure for the certification of ground ATM at a global level. This has many issues for ATM, not just ADS-B applications.
2.12.6 Operational Procedures
Operational procedures should in the first instance relate specifically to the application, except for procedures that relate to the ADS-B system itself. Policy will need to be developed as applications evolve.
2.12.7 Legal Liability and Responsibility
IFATCA has existing policy on “legal liability in automated systems” which is sufficient for ADS-B. IFATCA also has policy on the application of the “transfer of control functions – legal aspects” which covers some ADS-B applications.
2.12.8 Airborne Collision Avoidance Systems (ACAS)/ADS-B Interface
IFATCA has policy that the air traffic control system must be totally independent from ACAS. New policy is not required at this time. However, the implementation of ADS-B applications should continue to be monitored to ensure this independence is maintained.
3.1 Despite the generalised nature of this working paper, the need to move from general papers to specific policy is recognised. This is in part dealt with in the separate working paper on dependent surveillance and in specific actions arising from the recommendations of this paper.
3.2 A distinction should be made between the enabling technology and its applications. That is, policy on ADS-B technology should relate to “position information by datalink” and there should be separate policy on ADS-B applications.
3.3 ADS-B should be as reliable as radar, if used for radar type separation and procedures, a cautious evolutionary approach should be applied.
It is recommended that;
4.1 SC1 reviews IFATCA policy on surveillance and investigates generic surveillance requirements.
4.2 SC1 investigates the potential applications of ADS-B.
4.3 This paper is accepted as information material.
Last Update: September 29, 2020