46TH ANNUAL CONFERENCE, Istanbul, Turkey, 16-20 April 2007WP No. 94Surveillance Applications Policy – Operational Applications of ADS-BPresented by TOC |
Summary
Automatic Dependence Surveillance – Broadcast (ADS-B) is a new technology that has been forecast as the enabler of future Air Traffic Management (ATM). The Technical and Operations Committee (TOC) has been tasked to research the current issues with the use of ADS-B. This paper discusses the ICAO position as it was developed through the ANConf11 recommendations and the work of ICAO(s) various Panels. It focuses on ADS-B use in non- radar environments and details the issues faced in Australia in its initial implementation. It will also detail future work items on ADS-B for TOC.
Introduction
1.1 Much has been written on ADS-B and the potential uses in a modern Air Traffic Management (ATM) system. ICAO has placed enormous emphasis on this through the actions of its many panels and urgently promulgating the much needed guidance material and subsequent amendments.
1.2 One of these important issues is the interoperability of the systems and the ability for aircraft to use the many different ADS-B systems individualised by States. This is also of major importance to International Air Transport Association (IATA) in its representation of the worlds airlines. The cost to retro-fit an aircraft is significant and the possibility of reduced application in areas of known differences is a concern. With all technological change, workarounds to combat the differences are also possible. We have already seen how different ADS-B systems (Universal Access Transceiver or UAT) can be incorporated into an ICAO 1090 Extended Squitter (ES) standard system by receiving and transmitting (re-transmit) the signals in all formats (including radar feeds) to allow pilots to see all aircraft (both ADS-B and not) on its Cockpit Display of Traffic Info (CDTI).
1.3 ADS-B has proven already to have enormous benefits. In trials conducted in Alaska through the Capstone project a major benefits has been reduced Controlled Flight into Terrain (CFIT) and a substantial increase in ATC situational awareness. In Australia the Burnett Basin trial has proven how ADS-B can be integrated into an existing Air Traffic Services (ATS) system providing ATC with better surveillance coverage. This has then allowed ATC to provide a less restrictive service using a regulator approved 5NM surveillance standard.
1.4 While the benefits may be straightforward and easily recognised and unquestionably affordable (in comparison to the cost of installing radar) the more advanced applications of ADS-B (airborne) have significant implications on the Air Traffic Control environment. Not only is enormous research and development by both Air Traffic Service Providers (ATSPs) and State regulators required, but the work required by ICAO to produce guidance material and finally Standard and Recommended Practices (SARPs) is certainly an enormous task.
1.5 While most Air Traffic Controllers (ATCOs) will say that increased ‘radar-like’ surveillance coverage will always be welcomed, whether it is on the ground (surface movement applications) or in areas of non-radar, the issues may well be “but how will this change the way we operate?” What will be required (technically) to have the same functionality, safety systems and procedures that we would have now in a radar environment? And more importantly, what will be different?
1.6 The one philosophical argument to be made here is that procedural airspace (non- radar) that becomes surveillance airspace has to be more beneficial to both ATC and aircraft. Therefore, the differences and quite possibly greater complexity that ADS-B surveillance airspace may bring (compared to radar) it is still better than the alternative (both in safety, with short term/medium term conflict alerting, adherence monitoring and situational awareness). Where this argument terminates is where it’s introduced in areas of existing radar coverage. Even with the early implementation issues being faced by AirServices Australia (ASA) in its aggressive implementation of ADS-B there will be significant differences to the way that you interact with airspace and aircraft displayed using ADS-B.
1.7 It is worth mentioning here that IATA has put their full support behind the replacement of radar with ADS-B. This would certainly be expected given the possibilities and affordability of the technology. The foreseen cost benefit of future ATM without the high cost of installing and maintaining radar is significant, as well as the safety enhancements of improved ATM with multiple data downlinks and capacity improvement with airborne applications.
1.8 The impact on workload has not been fully proven yet, but the concept of introducing it in stages and in areas of ‘less reactive’ airspace (non-radar) is good methodology. This allows the availability of time and hindsight in securing technique and procedures before any attempt would be made to replace radar in high density airspace (let alone discussing the argument of mandated fitment of general aviation aircraft!!).
1.9 ICAO has made considerable effort in its development and promulgation of safety related material and initiatives. It has also through its safety oversight program provided accountability for States to meet the requirements of Annex 11 (Safety Management system (SMS)). Certainly amendment 44 to Annex 11, the introduction of ICAO Safety Management Manual Doc 9859 and the current information in ICAO PANS-ATM Doc 4444 wouldn’t tolerate any excuse for incorrect management of change to an ATS system through the necessary safety processes. The only safety concern that IFATCA should have is the possibility of States (due to the relative affordability of ADS-B technology) that may not have a robust Safety Management System (SMS), introducing this technology at the pressure of national airlines insistent on gaining efficiency and reducing their ever growing fuel bill. A process of monitoring this may be required.
1.10 This paper will cover some of the generically known issues of ADS-B and will also look at the issues of it being introduced in an existing automated ATC system. While there are many applications of ADS-B, we will mainly discuss the issues of it being used in areas of non-radar (as this can be argued as being well advanced in its implementation). Many of the other uses need further investigation and reporting on.
Discussion
2.1 ICAO
2.1.1 ICAO has prioritised the work on ADS-B due to the technology being seen as an enabler of future capacity and future Air Traffic Management. This includes the recommendations that came from the ANConf11. These recommendations are listed below.
2.1.2 ANConf11 Recommendation 1/6
2.1.2.1 Endorsement of the automatic dependent surveillance-broadcast (ADS-B) concept of use and recommendations for further work:
That ICAO:
a) follow research and development work in the area of ADS-B applications, and update/maintain the ADS-B concept of use as necessary;
b) work cooperatively with other international bodies to ensure that the ADS-B concept of use is properly aligned with existing operational and technical documents;
c) utilize the ADS-B concept of use, in its current form and as it matures, as a basis for development of SARPs and guidance material for air-to-air and air-to-ground surveillance applications; and
d) ensure that all future work on the ADS-B concept of use is aligned with the ATM operational concept and meets the emerging ATM requirements that emanate there from.
ICAO through its panels has ensured that recommendations b) and c) be met.
2.1.2.2 The completion of the work by Operational Datalink Panel (OPLINKP) included a recommendation to amend Annexes 2, 4, 11 and 15, the ICAO PANS-ATM Doc 4444, ICAO PANS-ABC Doc 9694 and ICAO Annex 10 (Volume III) concerning ADS- B. The report of OPLINKP/1 was reviewed by the Air Navigation Commission (ANC) working group for Panels and later by Air Navigation Commission (ANC) in February 2006. ANC made a preliminary review of recommendations for amendment of SARPs and Procedures (Rec 2/1, 3/1 and 4/1 concerning ADS-B, ATS Interfacility Data Communication (AIDC), Controller Pilot Data Link Communications (CPDLC), Required Communication Performance (RCP), etc) and agreed that they should be referred to States/international organizations for comment together with the Commission’s own comments and proposals therein. The Commission’s comments/proposals were largely editorial, consequential, and ensured consistency of terminology/procedures throughout ICAO documentation.
2.1.2.3 IFATCA responded to the State letter (SL2006/41) in relation to these amendments. While supporting the amendments, two issues were raised. Firstly on the definition of a ‘ATS Surveillance System’ and secondly the use of the term ‘ADS-B’ in the ICAO PANS-ATM Doc 4444 amendment. (Appendix A).
2.1.2.4 The ADS-B Concept of use document provides a description of the ADS-B system and its detailed role as an application enabling important changes to the future Communication, Navigation Surveillance/Air Traffic Management (CNS/ATM) system. This document it considered as a starting point for the multiple of changes that will affect ADS-B in its role to change the face of ATM. It was therefore important enough for a recommendation from the ANConf11 to make sure that the document changes as the beginnings of ADS-B evolve.
2.1.2.5 One significant proposal that has come through the ADS-B Study & Implementation Task force (ADS-B S&TIF) is the proposed amendment to the Global Plan with the immanent use of ADS-B:
“Worldwide deployment of ADS-B can be expected to have a profound impact on air traffic management and hence on most GPIs. Consequently all projects should consider the potential impact of ADS-B in the planning phase.”
2.1.2.6 For IFATCA, the link between ADS-B and the future is as important as the technological change itself. Because of an increased safety consciousness, brought about by many of the worlds leading safety experts and panels, implementation of any technology has the potential to be safe. This in no way dilutes the work that IFATCA or any individual ATC Association’s working with their ATSPs to manage change does. In fact it only improves it, as it increases transparency and allows additional voices to be heard in the correct management of change. The importance question is how will ADS-B affect the way that ATC is done and what can (and should) we do to affect these changes.
2.1.3 ANConf11 Recommendation 1/7
2.1.3.1 Ground and airborne automatic dependent surveillance-broadcast (ADS-B) applications for global interoperability:
That ICAO and States:
a) recognize ADS-B as an enabler of the global ATM operational concept bringing substantial safety and capacity benefits;
b) support the cost-effective early implementation of packages of ground and airborne ADS-B applications, noting the early achievable benefits from new ATM applications; and
c) ensure that implementation of ADS-B is harmonized, compatible and interoperable with respect to operational procedures, supporting data link and ATM applications.
2.1.3.2 This recommendation while generic in nature has the bigger concern for IFATCA. While IFATCA recognises the potential of ADS-B; including its future productivity gains and cost effective implementation possibilities. The global track record of unsuccessful change management (specially driven by perceived cost/staff reduction) by ATSPs only increases the concerns we have of managing the enormous changes that are required to quickly identify and implement potential cost saving strategies like ADS-B.
2.1.3.3 Part (c) of this recommendation says:
“ensure that implementation of ADS-B is harmonized, compatible and interoperable with respect to operational procedures, supporting data link and ATM applications.”
This is where the role of IFATCA can make such a difference. IFATCA understands that it can’t stop the development ADS-B (not that we would want to); neither do we have the resources/expertise to affect the technical development of the technology, but we do have the desire, greater than any other interested party to affect the harmonisation, compatibility and interoperability with respect to its operational incurrence and how it integrates with our existing technology.
2.1.3.4 The one concern that IFATCA will need to have is how the relevant ICAO panels are influenced. I would like to give you an example as how the changes affecting ADS-B need to be influenced by IFATCA.
2.1.3.5 AirServices Australia (ASA) has been highly motivated in its introduction of ADS-B. It has the perfect climate to manage the change, it has the perfect benefactor (lots of non-radar airspace); very little airspace sovereignty issues with few neighbours and lots of airspace to trial and implement it in. It also has an excellent working relationship with Thales and SITA who are all very commercially motivated to establish a working platform to sell this technology.
2.1.3.6 The Australian Regulator (Civil Aviation Safety Authority (CASA)) has been watching the development with obvious interest, noting the required changes to the regulations to allow ADS-B. CASA (as all regulators) carefully balance the position of high commercial interest, perceived fiscal benefit and an external will (ASA) to implement this new technology. CASA wants ICAO to lead this change with new SARP’s/documents supporting ADS-B. ASA is also aware of the required change and how they need to be influential at an ICAO level to make sure the change benefits them (or allows them to introduce it the way they want to).
2.1.3.7 Therefore the obvious choice is to drive the change through ICAO’s working groups and panels. Both SASP and OPLINKP have major contributors from both CASA and ASA. As the work was being completed by OPLINKP and handed to the ANC, CASA was using these amendments to edit its own Manual of Standards (MOS) allowing ASA to change its documents (Manual of Air Traffic Services) to allow the introduction of ADS-B (definitions, phraseology, generic ‘surveillance’ vice ‘radar’ issues etc).
2.1.3.8 While this may be the norm for most ICAO changes (as certainly ICAO cannot manage the magnitude of change internally) it is an important reminder that a pro- active approach on a ground level is required to influence change. IFATCA must also be as highly motivated as either of these two parties (ASA and CASA) to make sure that the change to procedures/documents/SARP is done with the interests of our profession at heart. What we don’t want to happen is to become reactive to the change, without knowing that we have done everything we can to influence the outcome.
2.1.4 ANConf11 Recommendation 7/1 and 7/2
2.1.4.1 Strategy for the near-term introduction of ADS-B:
That States:
a) note that a common element in most of the approaches currently adopted for early implementation of ADS-B is the selection of the SSR Mode S extended squitter as the initial data link; and
b) take into account this common element to the extent possible in their national and regional implementation choices in order to facilitate global interoperability for the initial introduction of ADS-B.
2.1.4.2 Support of longer term ADSB requirements:
That:
a) States recognize that in the longer term the current SSR Mode S extended squitter technology may not be able to fully satisfy all of the requirements for ADS-B services in all airspaces; and
b) ICAO continue development of technical standards for ADS-B link technologies, including SSR Mode S extended squitter, VHF Datalink (VDL) Mode 4 and UAT, with special attention being paid to ICAO ADS-B operational requirements, frequency spectrum availability and aircraft integration issues.
2.1.4.3 Recommendation 7/1 and 7/2 have also been approved by the ANC. Recommendation 7/1 has had action taken by the Secretary General. With 7/2 (b) the ANC has requested that Aeronautical Communications Panel (ACP) and Surveillance and Conflict Resolution Systems Panel (SCRSP) in coordination with other appropriate panels, to continue the development of provisions for ADS-B technologies as required.
2.1.4.4 ICAO has long been a supporter of interoperability and this is again seen by the re- enforcement of 1090ES and the emphasis on the regional and State choices to facilitate this. But it can also see that the benefits that VDL mode 4 and UAT also bring and that one technology strain of ADS-B may not realise the full potential of ADS-B. These other technologies include functionality such as Flight Information Broadcast (FIS-B) and Traffic Information Broadcast (TIS-B), as well as other data link possibilities using other data capable frequencies.
2.1.4.5 Like it was mentioned in the introduction, careful consideration must be given to the capability of aircraft and the cost to retro-fit an aircraft, further work is being done on this.
2.2 Using ADS-B
2.2.1 Navigational Uncertainty Category (NUC)
2.2.1.1 While it is easy to understand the importance of ADS-B data (based on GPS position) calculating and displaying its accuracy and integrity, what is complicated, is understanding how this information is calculated, what the data actually represents and how this will be incorporated into an existing ATC system. The Radio Technical Commission for Aeronautics (RTCA) has produced a standard (DO260) for how this information will be derived.
2.2.1.2 NUC is a generated by the GPS using either Horizontal Protection Limit (HPL) or by Horizontal figure of Merit (HFOM) it is then transmitted to the ADS-B ground station to allow the ground station to filter ADS-B tracks that don’t meet the required criteria to be used for separation.
2.2.1.3 The following extract from a paper written by Greg Dunstone provides more information on this:
“Positional data delivered by ADS-B typically depends on GNSS receiver data. The RTCA DO260 standards require the generation and transmission of a value called “Navigational uncertainty Category” (NUC) to all ADS-B receivers, so that receivers can determine if the data is “good enough” to use. NUC is to be based upon a Horizontal Protection Limit (HPL) if HPL is available on the aircraft. However, for aircraft without HPL, it unfortunately allows NUC to be derived from Horizontal Figure of Merit (HFOM).
HFOM is the result of a calculation of the EXPECTED accuracy of GNSS positional data assuming there is no satellite failure. HFOM provides a 95% bound on the horizontal position error. If HFOM is used to determine NUC, satellite range errors could allow incorrect positions to be transmitted.
HPL is the EXPECTED containment radius of the GNSS positional data within which a receiver autonomous integrity monitoring (RAIM) algorithm is able to assure that the aircraft is positioned with high certainty. Ie: It is the capability of the RAIM algorithm (to detect satellite range errors) that is the determinant of the HPL value rather than the simple satellite geometry used by the HFOM calculation. It is assumed that since the RAIM algorithm is capable of detecting satellite failures (satellite vehicle range errors of small magnitude) it will not provide such positional data based on erroneous satellite measurements.
If HPL is used to determine NUC, satellite range errors which could allow incorrect positions will be reflected in the value of NUC – and the controller can be presented the data accordingly.
For radar separation purposes, HPL based generation of NUC is desired. This has been recognised in DO260A which always transmits a derivative of HPL Navigation Integrity Category (NIC).”
2.2.1.4 While the DO260A Minimum Operational Performance Standards (MOPS) will go a long way to fix this issue, there are currently no DO260A compliant avionics in use. This will certainly become the future international norm but at this stage the possibility of HFOM derived NUC is very possible.
2.2.1.5 The Australian system manages this issue by individually approving every airframe that wishes to use ADS-B. It provides a filter to the controller to block transmissions of un-approved data. Australia may take a further step in establish law for ‘transmitting misleading data’ so that the responsibility for transmitting the required data (HPL derived NUC) is bestowed upon the companies that own the individual airframes.
2.2.1.6 Most modern air transport aircraft have Extended Squitter capability incorporated into their transponders. These are typically Traffic Collision Avoidance Systems (TCAS) capable transponders produced by ACSS, Honeywell, Garmin AT or Rockwell Collins. It is known which manufactures use HPL in lieu of HFOM and information has been circulated (for example ACSS transponder Models -10004 and -10005) to have them upgraded to HPL transmitted NUC.
2.2.1.7 For general aviation use in Australia, the regulator (CASA) has produced a Technical standing order (TSO) that details the requirement for SSR transponder that have minimum ADS-B capabilities in addition to operating as a Mode A/C transponder:
“a. RTCA/DO-260 states if HPL (Horizontal Protection Limit) information is not available from the navigation data source, then the transmitting ADS-B subsystem shall use HFOM, Vertical Figure of Merit (VFOM), and Altitude Type to determine the Type Code used in the Airborne Position Message in accordance with Table 2-11.
b. In lieu of this requirement, if HPL (Horizontal Protection Limit) information is not available from the navigation data source, then the transmitting ADS-B subsystem shall use HFOM, VFOM, Altitude Type and RAIM availability and integrity alarms applicable to the Non-Precision Approach phase of flight, to determine the ADS-B Type Code.”
2.2.1.8 Importantly is the requirement for the transponder to use HFOM and RAIM in lieu of HPL. Where this equivalent HPL can be created using the GPS RAIM flag from TSO129 compliant navigators, these solutions require that if a GPS RAIM Non precision approach (NPA) flag alert is generated, the NUC value transmitted is worse than NUC=5 and hence the data will not be used by ATC.
2.2.1.9 In other words, the horizontal error must be contained within 0.3Nm with a high degree of certainty as required by the GPS TSO129 RAIM requirement for non precision approach. In fact this is slightly more stringent than the HPL=0.5 Nm requirement expressed above.
CASA has also detailed another TSO (ATSO-C1005) for stand alone ADS-B out transmitters (not detailed here).
2.2.2 Flight ID
2.2.2.1 In a radar environment due to the ease of which transponder’s can be read and set, issues of incorrect input is rare. This may be because the technology is simple, been in use for many years and with only having certain numbers available (and selectable) means that human error is less likely to have an affect. One major difference is that the SSR code is assigned and readback, making sure that the setting is correct, flight ID is just set by the pilot.
2.2.2.2 In the equivalent ADS-B environment the flight ID transmitted by the ADS-B unit is responsible (primarily) as the means to correlate a Flight Data Record (FDR) to an ADS-B track. The process involved in entering this data (and indeed changing it) is a much more complex procedure and one that is proving an issue.
2.2.2.3 Airlines use different airline codes (both ICAO and IATA) at different stages to reflect different meanings. The codes that are used on the over-head boards above departure gates are different to the codes by which they flight plan (the numbers may not be). To further complex the issue systems such as Aircraft Communication Addressing and Reporting System (ACARS) also require inputting of a Flight ID, however this system requires that all the digits are filled in, which means that crews often substitute ‘0’ to allow the system to accept this ID.
2.2.2.4 The flight ID can be managed through the Flight Management System and depending on airframe may or may not be able to be changed airborne. Boeing NG 737 are able to change the flight ID airborne while Airbus A320 aircraft can not (some workarounds have been discovered recently, further detail about this workaround is unfortunately available). For ATC this will increase workload because if it can’t be changed then ATC may have to ask the pilot for the 24bit code (the Australian display allows a button to be selected to show the 24 bit code) and then enter this manually into the flight plan to allow coupling to occur. This is because it is not mandatory (ICAO) to flight plan the 24bit code and regulations in Australia only mandate it if an aircraft does not have the ability (either on or off the ground) to enter/change the flight ID.
2.2.2.5 These input errors have been highlighted in Australia, by numerous occurrences of incorrect flight ID. Cases of the previous flight ID still being in the system, extra ‘0’ being added and extra letters (Qantas might use QFA01 instead of QF01) are all examples of these are opportunities for incorrect flight ID leading to incorrect coupling.
2.2.2.6 A campaign to educate pilots as to the importance of flight ID has begun, with posters pinned up in crew rooms and information sent to the airlines there is hope that these incidents of incorrect flight ID will become less common.
2.2.2.7 Some B747-400 aircraft have been reported to have consistent problems with flight ID. This looks to be a hardware/software issue and not pilot error, these types of occurrences are to be expected as the technology develops and the integration into existing ATC systems commence.
2.2.2.8 These issues (and others) raise a significant point; as we can see there are a number of operational reasons that a pilot may need to activate or deactivate ADS-B equipment in flight. Examples include incorrect flight ID (without the capacity to change it) malfunctioning equipment, electrical fire and the conservative use of battery power. In-flight control of the equipment must be a safety requirement.
2.2.3 24 Bit Code
2.2.3.1 Some concern has been raised over the possibility of having two aircraft existing with the same 24 bit code. While the management of these codes is extremely important, some possibility exists if the proper procedures weren’t followed correctly. The management of these codes has been detailed in Annex 10 Volume III Chapter nine and the appendix to chapter nine and reads as follows:
APPENDIX TO CHAPTER 9.
A WORLD-WIDE SCHEME FOR THE ALLOCATION, ASSIGNMENT AND APPLICATION OF AIRCRAFT ADDRESSES.
5. Assignment of aircraft addresses
5.1 When required for use by suitably equipped aircraft entered on a national or international register, individual aircraft addresses within each block shall be assigned to aircraft by the State of Registry or common mark registering authority.
5.2 Aircraft addresses shall be assigned to aircraft in accordance with the following principles:
a) at any one time, no address shall be assigned to more than one aircraft;
b) only one address shall be assigned to an aircraft, irrespective of the composition of equipment on board;
c) the address shall not be changed except under exceptional circumstances and shall not be changed during flight;
d) when an aircraft changes its State of Registry, the previously assigned address shall be relinquished and a new address shall be assigned by the new registering authority;
e) the address shall serve only a technical role for addressing and identification of aircraft and shall not be used to convey any specific information; and
f) the addresses composed of 24 ZEROs or 24 ONEs shall not be assigned to aircraft.
2.2.4 Security
2.2.4.1 In Australia the security of ADS-B data being transmitted has gained extensive coverage both in the industry and the media. Most noticeably is the issue of ‘spoofing’.
2.2.4.2 Spoofing is the transmitting of electromagnetic data to deceive, block or reduce the capacity of surveillance/communication equipment to function accurately. This may take the form of sending false reports to an ADS-B receiver stating the position/height etc of an unknown aircraft; this would result in ATC then reacting to this information as if it was accurate.
2.2.4.3 While ‘spoofing’ has been around for some time, the ease at which it can be done (ADS-B as opposed to radar) has been highlighted as a possible security threat.
2.2.4.4 The one important note to make here is that ‘spoofing’ is illegal and that all air traffic control systems are cooperative efforts focused primarily on safety. It never was supposed to be a system encased in security. You would liken it to a navigation buoy, lighthouse, or even a traffic light. Is it possible to defeat these things? Sure. But that was never the point. This is a civilian system. So we are talking about a deliberate attempt to interfere with air traffic.
2.2.4.5 While it is has been noted by the experts that it is certainly easier (and less expensive) to spoof ADS-B than radar, it is (and has been) possible to do so with radar for many years. The reported number of ‘radar spoofing’ is negligible and even easier is the ability to spoof VHF. While obtaining a VHF transmitter and transmitting false information at crucial times (tower frequency; ‘clear to land’ instructions or ‘go round’) would be easy how often is this done?
2.2.4.6 Again using the Australian example, the security issue of this spoofing was identified as having the same criticality as both radar and VHF. Importantly, noting that the intention would be motivated illegally.
2.2.4.7 While some forms of ADS-B (UAT) do have the ability to receive GPS time stamped reports, allowing the ground station to determine the time sent/received and to confirm this with the reported GPS position. This will only be available with UAT (and possibly VDL mode 4) due to the ability to have extra capacity on the available data transmission. The ability to encrypt data for use of ADS-B has been researched. Again the issues of retro-fit and avionic costs is significant, and an international standard for its use is not in place.
2.2.4.8 Air traffic controllers are good at spotting abnormalities on their air situation displays; if a position of an aircraft doesn’t look right or a track appears that isn’t normal (ghost track or track jump/split) ATC has the ability to react accordingly and take the necessary actions to manage the situation.
2.2.4.9 One additional aspect to the security that does differ is the ability to purchase an ADS-B receiver that could accurately plot aircraft transmitting ADS-B in range of an aerial. An example of this technology is the SBS-1; for less than $1000 US in conjunction with a laptop you could easily track ADS-B tracks up to 100NM away. This plus the unique 24bit code assignment to aircraft would allow anyone to accurately find any aircraft they wished to. This is why some aircraft (Airforce 1) have the ability to change their 24 bit code (for national security reasons etc).
2.2.4.10 Aircraft transmitting their position will always raise some security concerns. After 9/11 the use of aircraft in any way for terrorist exploits has increased the security debate and certainly will continue to motivate measures to counter-act any threat. What have to be balanced are the proven benefits of the technology with the management of its security. TOC suggests that IFATCA remain vigilant to be a part of future correspondence/debate on this topic.
2.3 Introducing ADS-B in a non-radar environment
2.3.1 General
2.3.1.1 While this paper will begin to detail some of the issues that will be faced, the important point to make is that this paper won’t detail the more ‘generic’ issues associated with change management and managing implementation. In the current implementation climate within AirServices Australia the change process (including safety management) has been handled correctly and hazards mitigated successfully, but the management of the documentation and procedures wasn’t as successful and meant that some groups of sectors weren’t receiving finalised documents until 2 days before initial implementation. Externally, pilot training (developed and promulgated through CASA) within one of the national airlines also wasn’t completed on time resulting in pilots not able to use ADS-B (a hardware fix to block those aircraft’s ADS- B transmissions were made). Again, these internal issues could be different depending on the ATSP introducing the change.
2.3.1.2 As previously mentioned ASA has commenced the introduction of ADS-B into non- radar airspace. While they have had many technical hurdles to overcome; implementation in a staged approach with many of the required technical issues being resolved, (trials in the Burnett Basin) has proven very positive. While meeting the required ICAO requirements of duplication and integration, ADS-B procedure development has been quite immature (expected given the uniqueness of the change). While some this work has been done ‘on the run’ many of the issues that have been identified have been overcome by sound management and solid safety practises.
2.3.1.3 AirServices Australia has chosen the following reasons to have an interim stage in the introduction of ADS-B:
- It allows the opportunity for controllers to use the available equipment in a limited way until the system is fully operational with duplicated communications lines.
- It allows the controllers to become familiar with coverage and which aircraft have ADS-B equipment.
- It provides an opportunity to assess RAIM functionality – unable to be simulated.
- It allows controllers to use the equipment with limited capacity until all ground based equipment is duplicated.
2.3.1.4 The following information has been drawn from the real experiences that the implementation team have experienced. Many of the issues discussed herein may only effect the initial implementation, not its final use. Consideration must be given to the fact that the method of implementation and some of the issues discussed will not be universal, due to the many complexities of the particular technology system used and the cultural, geographical and local issues that Australia has.
2.3.1.5 Special thanks to AirServices Australia for the information that has been incorporated into this paper.
2.3.2 ADS-B co-existing with Radar
2.3.2.1 In its initial application, ADS-B co-existing with radar will not be allocated the display priority of radar. Where radar coverage is available, proven radar use and standards will be applied. Where this gets complex is in the areas where radar coverage ends and ADS-B is available. The hardware that is used to manage the integration of ADS-B into an existing ATC system needs to allow the cross flow of information between a radar system and an ADS-B system. AirServices Australia using the Thales Eurocat system is working towards a processor that will integrate ADS-B data and radar data, but in the initial stage has had two different processors presenting the data. This has meant some functionality won’t work across the two processors; importantly the Short Term Conflict Alert (STCA). It has been established through different strategies that the likelihood of an ADS-B track and radar track needing the use of the STCA due to the probability of both existing together less than 5 miles apart and in particular the class/density of the airspace, has allowed this to being mitigated, understanding that the end state will have this functionality.
2.3.2.2 System Architecture may vary within different automated systems, an example of the type of architecture that will be required can be taken from the Australian Eurocat system.
Initial Processor functionality to include:
- Message checking – receives the downlinked message from the aircraft and checks to ensure the mandatory fields have been received e.g. 24 Bit code. If not the track is rejected.
- Figure Of Merit (FOM) checking – FOM is a system calculation based on the Navigational Uncertainty (NUC) of the aircraft (integrity of data from aircraft). If it falls below a figure of confidence then the track is rejected.
- Site Monitor Management – end to end checking in the system to ensure the ADS-B sites are operating properly.
Further Processor functionality to include:
- QNH correction – compares pressure difference from 1013 sent from aircraft with pressure setting for the site to correct ADS-B Level below transition altitudes.
- System track creation – synthesises multi tracks, extrapolates position for the next screen update.
- Synchronisation with the Radar Data Processor (RDP) – receives timing info for next screen update from RDP so that Radar and ADS-B tracks are displayed simultaneously.
Additional Processor functionality to include:
- Track Coupling – associates eligible track with Flight Data Record (FDR) from Flight Data Processor (FDP).
- Auto position reports sent to FDP to update FDR.
- Predictive alerting (eg STCA, Safe altitude/danger area warnings).
- comparative alerting – route & level differences from FDP.
2.3.2.4 Degraded facility type operations are a major part of any automated ATC system. Redundancy and the ability to continue with safe operations, even though inefficiently, is a major part of our current safety systems. While this will be done differently throughout the many different automated systems used worldwide, ADS-B will need to be subject to the same back-up/redundant processes that normal radar does. This would include:
- The display of ADS-B tracks will need to be operable (even if without all of the mentioned capability) in a raw form should failure of the processor(s) happen.
- If radar data is available through a secondary/back-up system, then ADS-B data should also be available (again with limited functionality).
2.3.3 Symbology
2.3.3.1 The Symbology of an ADS-B track needs to be displayed differently from a radar track and indeed an ADS-C track and/or non-radar track. This could also be complicated by different ‘levels’ of ADS-B in differing stages of implementation. In the Australian Eurocat system an ADS-B track is a four bladed propeller type symbol, but prior to using ADS-B fully (radar like usage, with all the required training both for ATC and pilots), ADS-B is being used as a tool to ‘monitor procedural standards’ and allow familiarity of its use by controllers. This has required different symbology (a three bladed propeller). This is described as a ‘Class 2’ symbol. The end state will be a described as a ‘Class 1’ symbol.
Example of symbology in the Australian Eurocat system.
In the “Class 1 ADS-B (radar)” symbol, note the ‘b’ next to the 47
to indicate that it is receiving both ADS-B and Radar.
2.3.3.2 We can easily surmise that careful attention is needed to stop the possibility of incorrectly identifying the track and applying the wrong standard. If you also include non-radar symbology (all of which could be displayed on one Air Situation Display (ASD) at one time) we have the possibility of 6 different symbols, all requiring different requirements for separation and to be monitored differently.
2.3.4 Coupling
2.3.4.1 Track coupling will be different to radar, while radar uses discrete Mode A (SSR) and a ‘coupling style’ corridor (area) to define where an aircraft should be to allow coupling to happen. ADS-B uses flight ID transmitted from the aircraft as well as this coupling corridor. Additionally to Flight ID, the 24 bit unique identifier may be used for coupling, (the processor will check for correct flight ID and if not found look for the code, which needs to be in the flight plan) while many may argue that basing coupling on Flight ID has limitations (further detail later on this) to structure coupling solely on the hardwired 24 bit code isn’t viable until ICAO mandates flight planning of this code. If this was the sole means for coupling you would have to manually enter this code for every aircraft using ADS-B. With short notice aircraft changes coupling only to the 24 bit code, would again require detailed changes to the existing flight plan with the new registration, aircraft type and now 24 bit code manually edited into the system for coupling to occur.
2.3.4.2 For aircraft that may not be able to enter Flight ID into the ADS-B unit (aircraft without FMS/ACARS systems) and still wish to use flight number callsigns, the system of coupling with Flight ID will not work as a direct link between the two won’t establish a correlation. The way around this (as established by ASA within a Eurocat environment) is to have a secondary condition that allows coupling to the 24 bit code. This coupling will only occur if a specific phrase (code/******) is placed into Field 18 of the flight plan. The Eurocat system then looks at the 24bit code and the information in the Electronic flight plan and will couple if the same.
2.3.4.3 Further use of this function allows an aircraft to not be eligible for coupling by inserting code/000000. One of the limitations in the functionality of coupling in the Eurocat system is the capability to manually couple and decouple tracks. While this function may rarely be used in a radar environment, the only way to decouple a flight plan from an ADS-B track is to insert code/000000 in field 18. Manually coupling an ADS-B track is not possible at this stage. (Important to note that this is an ASA work around and should not be used with flights exiting the Australian FIR as this may cause issues with the messaging sent).
2.3.5 Automatic Position Reporting
2.3.5.1 ADS-B Tracks, like radar, create Automatic Position Reports (APRs); these are the same as radar and are sent from an ADS-B processor to a flight data processor to update the estimates etc for electronic strips. An example of this may be that:
A Coupled ADS-B track generating APRs:
– Cyclic – every 3min on climb and 5min in cruise;
– Event – waypoint, coupling, in/out Route Adherence Monitoring status, etc.
2.3.5.2 This may lead to aircraft not being required to make position reports while ADS-B identified. While this seems appropriate (especially if the electronic flight planning system is being updated) what is the effect on the controller, especially in the interim ‘ADS-B monitoring’ stages?
2.3.5.3 Some of the possible issues that could be faced especially in the interim stages of implementation with previous procedural (non-radar) airspace could include the following:
- Position reports previously used as a prompt for frequency changes are no longer there.
- Position reports that prompted coordination actions will no longer be available.
- As no voice or CPDLC position reporting may be required controllers are still required to ensure that they are the Current Data Authority for all CPDLC equipped aircraft in their airspace (position reporting prompted and checked for this).
- Controllers are required to ensure that relevant procedural standards are maintained at all times without the aid of position report prompts as detailed in Longitudinal Standards.
- Modified scanning behaviour.
2.3.6 Monitoring Procedural Standards
2.3.6.1 In normal procedural airspace, longitudinal time standards may be applied between two ADS-C capable aircraft by the ADS-C report updating the flight plan, providing accurate estimates of the position reports used to establish the standard. Several tools may be used to monitor this time standard provided that an alarm is set for the next position (either manually or automatically). In an ADS-B environment, ADS-B can be used to provide this estimate, but where ADS-C would have automatically set the position estimate alert (PETO) the controller must make sure that this is done to satisfy the requirements of the standard.
2.3.6.2 When using distance standards, however, the use of a bearing and range line (BRL) may be used to monitor the distance required by the standard reported by the ADS-B aircraft. Realising that even though ADS-B track display uses GPS info, it does not mean the aircraft is GPSRNAV capable. Navigation approval for aircraft determines the applicable standard.
2.3.6.3 The combination of ADS-B tracks and their use to monitor estimates and distance, with other procedural (ADS-C and Non-radar tracks) will need to be carefully established and trained for as the different combinations will be complex both in the establishment and continued monitoring of procedural standards. This also continues in the transition of aircraft from radar to non-radar. You may have the scenario where two aircraft on radar (procedural standard established by radar) enter ADS-B airspace and only one aircraft is ADS-B equipped. This will mean that a change in monitoring this standard will be required; with one track using an ATC monitored distance and one track pilot reported distance.
2.3.7 ADS-B Availability
2.3.7.1 The notification of ADS-B availability for a particular airframe/flight crew needs to be available to ATC. A draft Aeronautical Information Publication (AIP, pilot manual) has been done to establish this:
c. Flight Notification:
(i) Operators must provide an indication that the aircraft is ADS- B equipped and approved for ADS-B operations in Australia by inserting the letters “ADSB” as the first element following RMK/ in the other information field (Item 18) of the ATS flight plan.
Note: Inclusion of “RMK/ADSB” in a flight notification is an indication to ATC that the flight crew has undertaken the necessary training and the aircraft is approved for ADS-B operations in Australia.
2.3.8 Safety Nets/Alerts
2.3.8.1 As mentioned before, ADS-B tracks and radar tracks may initially have some limitations between them when they co-exist in the same piece of airspace (STCA). While this can be fixed with improved hardware what is most importance is that ADS- B tracks have all the same safety net functions as a normal radar track.
2.3.8.2 In the current implementation in Australia ‘adherence alerts’ (for both route and level) are fully functional, as well as Danger Area Infringement Warnings (DAIW) and Minimum Safe altitude Warning (MSAW). With future hardware/software upgrades all the functionality will be available across all mediums of surveillance.
2.3.8.3 Some emergency functionality when using ADS-B may have limitations. Most ADS-B aircraft will only have a general alert ‘EMG’ regardless of the code selected. Some aircraft (rare, A380) have avionics that will allow for display of EMG, HIJ or RAD. When an ADS-B aircraft is squawking emergency, this will override the SPI function. Consideration must be given to current ‘loss of radio communications procedures’, e.g. “ABC, if reading this transmission squawk ident”. Maybe phrases such as “ABC, if reading this transmission stop squawking emergency and squawk ident??” may be needed.
2.3.8.4 ICAO documentation on selection and reservation of the emergency codes is well contained within ICAO PANS-ATM Doc 4444. While this specifically details SSR code management, further work will be required to incorporate ADS-B. ICAO will need to establish procedures for the limitations associated with ‘single emergency code management’.
2.3.8.5 The current procedure for ADS-B Emergency in Australia is “If a controller has witnessed a generic Emergency (EMG) alert and has not received further information/confirmation from the pilot, the controller must default to current radar procedures for suspected unlawful interference.” This could have a major impact, as the scrambling of fighter jets responding to possible terrorism threats has become common place in some parts of Europe/U.S.
2.3.8.6 This is a major change in current procedure; in current non-radar airspace an aircraft will be required to report a position and the next waypoint. If that aircraft has had a radio failure enroute to that next waypoint, you will be unaware until your next position report is due. In this case the controller would carry out the required frequency checks, resulting in an emergency phase declaration. Using the same airspace example now with ADS-B, the controller would be notified immediately that an aircraft had some sort of issue (by the transmit of ‘EMG’) but upon trying to call the aircraft to ascertain the nature of the problem would be faced with a ‘radio failure’.
The resulting action, because communication couldn’t be established would have to treat the aircraft as subject to unlawful interference.
2.3.9 Phraseology
2.3.9.1 Phraseology changes are required to manage the new technology. OPLINKP has made these changes and subject to the normal ICAO processes will be promulgated for use and included in amendments to ICAO Annexes. Radar phraseologies must now reflect surveillance where applicable. Phraseologies specific to ADS-B will need to be established. All phraseologies need to be in accordance with ICAO PANS/ATM Amendments for all aircraft.
2.3.9.2 The following is an example of these phraseology changes:
2.3.10 Receiver Autonomous Integrity Monitoring (RAIM)
2.3.10.1 As we know ADS-B relies solely on GNSS/GPS data for its position information. While it has been proven that this data is highly accurate and if correctly linked to an ADS-B transmitter will provide a Navigational Uncertainty (NUC) value that will be filtered by the processors involved with ADS-B. It is integral for the system to provide the same certainty as radar, to be able to predict limitations (rather than react when the NUC value falls bellow a set level) is required so to alert controllers that certain areas of airspace will be in-capable of securing ADS-B NUC values equivalent to radar.
2.3.10.2 RAIM analyses system integrity and relative positions of all satellites in view and then picks the best for provision of information. Satellites too close together, or at a low angle of elevation are ignored. RAIM allows the GPS receiver to identify and exclude any satellite providing “rogue” data. It should be noted that on-screen RAIM outage predictions provided for ATC are based on different parameters to RAIM requirements for aircraft navigation and are not to be passed to aircraft.
2.3.10.3 A minimum of 5 satellites + barometric aiding are required for RAIM processing. RAIM unavailability for a location can be predicted based on constellation orientation and forecast satellite outages.
2.3.10.4 GPS RAIM prediction system processing is separate from an ATC system, it receives advisories from US Coastguard relating to satellite outages and changes and it will predict RAIM unavailability over a defined geographic area.
This information needs to be inputted into the ATC system to provide controllers with the necessary warnings to notify their behaviour in using ADS-B for separation.
2.3.10.5 The current proposal for the Australian ATC system relies on RAIM to be converted to lat and longs and a warning button to notify the ATC. Upon selection of this button the defined areas of RAIM are displayed by bordered coloured areas defining different times when RAIM isn’t available. (Displayed on the air situation display):
ES will define different times applicable to the area of RAIM.
2.3.11 Airspace design
2.3.11.1 Additional issues to be considered will be the size of the airspace compared to the size of the standards used. While procedural airspace can be very large with controllers working on ranges in-excess of 1000NM using ADS-B to either radar vector or to run ‘radar-like standards’ may be inappropriate. Running a 5NM standard on a large range would easily have the ADS-B tracks touching. This would then require re-sectorisation which could lead to inefficiencies as the traffic demand vs. staffing/sectorisation would most certainly be inefficient.
2.3.11.2 The human factor element involved with the introduction of ‘radar like services’ in non radar airspace needs to be clearly identified and mitigated for. Considerable development of the known issues and planning/training to compensate for existing methodology is required.
Conclusions
3.1 Introducing ADS-B is very complex and correct management of the issues both from an airframe and ATC engineering point of view are very detailed and require expertise.
3.2 ICAO is promoting the use of the technology both globally and regionally and considers it a fundamental part of future ATM. OPLINK panel has done considerable work on the required surveillance changes to the annex’s to allow the introduction of ADS-B and SASP has done the comparative studies (and others) to allow its use in a ‘radar-like’ manner including the approval of a 5NM standard.
3.3 Integrity of the GPS data is very important and has been recognised with the DO260A standard, even though avionics are yet to be introduced that meets this standard.
3.4 Local regulator’s play an enormous part in regulating the approval and the environment to keep safety and ‘financially driven outcomes’ in check and to provide accountability to the organisation introducing the technology.
3.5 Serious human factor issues are apparent with multiple track labels co-existing on the one display, especially if we have a ‘staged introduction’ of labels indicating it can’t be used for separation, to only then introduce a similar label indicating (and co- existing) that it can be. Added to this areas where ADS-B can be used for separation and areas where it can’t due to RAIM forecasts.
3.6 Understanding the importance of airlines to conform to international practise in the assignment of 24bit codes and to locally manage flight ID’s, imperative to the systems integrity in coupling and affecting the workload of individual controllers.
3.7 The ability to ‘turn-off’ ADS-B is important to manage the issues of incorrect flight ID and give pilots the capability to manage its use.
3.8 Security of ADS-B has been heavily debated and will be subject to individual State approval. The issue of ‘spoofing’ is real and contains the same threats associated with both radar and VHF spoofing.
3.9 Introducing ADS-B into an existing ATM system requires the continuance of all the relevant tools and safety nets that we currently operate with and should not allow for easy deterioration into degraded operations and areas on non-conformance monitoring. Relevant ‘fall-back’ or ‘secondary back-up’ systems need to be incorporated as would be available for radar.
3.10 Further studies will be needed to look at all the possible applications of ADS-B which could include, inter alia:
- ADS-B as a replacement for radar
- ADS-B used in the terminal area
- ADS-B used as part of a Precision Radar Monitoring (PRM) system
- ADS-B used to re-transmit data (including radar) to aircraft (ADS-B in, TIS-B)
- ADS-B used to provide WX and NOTAMS (FIS-B)
- ADS-B incorporated into Advanced Surface Movement Ground Control Systems (A-SMGCS)
- ADS-B used in Multilateration
- ADS-B used in self separated in trail climb procedures (ADS-B ITP)
Recommendations
It is recommended that;
4.1 This paper is accepted as information material.
References
ICAO
Report of ADS-B Seminar and the 5th Meeting of ADS-B S&ITF.
ADS-B S&ITF TF/2-WP/8 “Required ADS-B performance – radar like services”.
Assessment of ADS-B to support Air Traffic Services and Guidelines for Implementation.
ANConf11-WP006 ADS-B Concept of use.
ANConf11-WP006 ADS-B Concept of use Appendix.
Attachment to State letter ST 12/04-04/30.
ADS-B Implementation and Operations Guidance Document.
Annex 10 Volume 3.
Annex11.
IFATCA
Reply to State Letter 41/2006.
AIRSERVICES AUSTRALIA
V3 ADSB GEN Session 1.
V4 ADSB GEN Session 2.
V4 ADS-B Deg Modes.
ADS-B Procedures V9.1.
ADS-B Phrase card V2.
National Instruction ADS-B Draft V8.
Manual of Air Traffic Services.
The Australian Strategic Air Traffic Management Group (ASTRA) ADS-B Implementation Team (ASTRA ABIT)
ABIT06-IP006 “CAN HFOM BE TOLERATED IN SOME CIRCUMSTANCES?” Prepared by Greg Dunstone, Airservices Australia.
Last Update: September 29, 2020