Surveillance Datalink Selection

Kim O'Neil
Advanced Aviation Technology Ltd.
September 2003

Abstract

When ICAO announced its CNS/ATM Concept, few would have anticipated the controversy that would follow what seemed like a widely held and intuitively acceptable vision for vast improvements in capacity, safety and operational efficiency. The CNS/ATM concept seemed a forgone conclusion - an irresistible drive to improvement and the transition from ATC to ATM. All we had to do was work together, put flesh on the bones of the concept and watch the future unfold.

Looking back over 10 years, it is clear this was a naive view of the world, for we had a grand idea, but no unifying process to make it happen - no way to reconcile the different views and to incorporate the fundamental realities of commercial aviation. Yet we still have the task of making it happen - for the problem has not gone away. So what is the way forward?

1. Introduction

In any debate, one expects that many opposing opinions exist. Civil aviation is a global enterprise, involving many thousands of organisations both large and small, with many diverse interests. This is normal, but it is essential that a consensus emerges from all these interests. Surprisingly, a consensus is emerging - due to the practical and commercial realities that face us. But consensus doesn't mean unanimous, and the eventual consensus will probably upset some very large organisations.

To quote from the ICAO World-wide CNS/ATM Systems Implementation Conference in Rio de Janeiro in 1998:

"the strategic vision of CNS/ATM systems was to foster implementation of a seamless, global ATM system that would enable aircraft operators to meet their planned times of departure and arrival and adhere to their preferred flight profiles with minimum constraints and without compromising agreed target levels of safety. To attain this goal………the vision, objectives and benefits of CNS/ATM systems had to be clearly established and the operational concept of a global ATM system had to be described in sufficient detail and clarity. Otherwise, different conceptions would continue to exist as to the benefits of the future system and how these benefits could be realized. A consensus on several issues therefore had to be reached to achieve the standardization necessary for future operation of ATM as a global service. These issues included, among others, autonomy of flight, separation assurance and situational awareness".

The broad thrust of this statement is still true today, but misses the need to ensure near-term commercial returns on all investments.

Picture: ICAO CNS/ATM concept.

2. Pre-Emptive Decision-making

Before this statement, several large institutions had already made their decisions. What followed was an attempt at pre-emptive decision making, whereby specific technologies were "pre-selected" as the components of this future system, despite obvious problems: the technologies did not meet known requirements, were entirely unproven and little consideration had been given to transition.

3. Operational Concepts and Operational Reality

Yet, the implementation of communications, navigation and surveillance (CNS) technologies should be based on clearly established ATM operational requirements, appropriate cost/benefits AND that the commercial commitment of all parties. Herein lies the problem: most "Users" cannot clearly see the benefits of new technology and in these uncertain economic times, they are unwilling to become guinea pigs.

Whilst making robust noises in ICAO and Eurocontrol panels, ATS providers have shown little real commitment to move toward ATM. We are still entrenched in simple ATC custom and practice. A widening gulf exists between the newly developed "operational requirements" put together in numerous committees and the real-world implementation plans of ATS providers.

There have also been many "magic bullets" to solve all our safety, capacity and efficiency problems: but introducing these services, and integrating them with existing systems and coordinating User upgrades has proved a mammoth task - especially when the costs are great, the benefits late and insubstantial.

Having a vision of the future is one thing, but getting there is quite another.

4. Commercial Realities

There have been amazing changes, particularly the rise of the low cost operator, flying brand new fleets attacking and undermining the markets of the "National Flag Carriers". Low cost airlines once operated old mixed fleets on routes that were not interesting to the larger carriers. Today, the large carriers are desperately hanging onto slots at their hubs against a possible further onslaught by the low cost carriers. The larger carriers now have old diverse fleets and little money to invest. Low cost carriers are cash rich, but prudent - every penny must show a return. The higher fares of the big carriers (once upon a time) made it easier for them to absorb delays - after all, where would the customer go if they didn't like it? Now, alternatives exist and it is the low cost carriers who express greatest concern about the impact of delays on their business.

Picture: Low Cost Carriers are changing the shape of air travel

4.1 Business Case for Datalink

Aviation economics have fundamentally changed, large carriers are financed out of debt, low cost carriers are financed out of revenue. Privatisation of ATS providers means a similar process is taking place on the ground. What this means is simple: If No Business Case (showing immediate returns) then No Go. There are no exceptions in the current economic environment, and any attempt to mandate technologies that don't generate immediate and large-scale benefits will fail.

The powers of the FAA, Eurocontrol and CAAs to mandate are significantly less substantial today. This is because aviation is seen as a purely commercial activity.

Not everyone has come to terms with these facts, many still believe that current events are simply an aberration of the market.

5. The Datalink Selection Process

Examining the surveillance datalink selection process illustrates the wider problems of CNS/ATM implementation and the difficulties of transitioning from ATC to ATM.

Datalinks have many applications in Communications, Navigation and Surveillance. One datalink may be able to serve more than one domain - or even all, it may depend on the roll out of applications. Equally, separate datalinks may be required for each domain or even for individual applications - if demand and safety merits it. Some "applications" stray over all three domains e.g. ADS-B. In fact ADS-B is not strictly an application in itself, but an enabling technology with many applications across CNS/ATM.

Nevertheless, the datalink technology must be available, functional, economically feasible and easily integrated in the aircraft. Some presume that one VDL will be fitted to all aircraft, effectively excluding all other VDL links. This is just wishful thinking by some, as no such decision has been made or given. The field is still wide open.

5.1 Standardisation

A standard is just a standard, nothing more. It is not a mandate - merely a tool to be taken off the shelf when required. Technologies need standards, for obvious reasons - but standardisation is only an enabler - it is not part of the decision process. Many years ago, standards were strongly associated with rule-making. Nowadays all products have standards.

Nevertheless, Standardisation covers airborne and ground equipment, performance, operating requirements, procedures, networking, installation requirements and various other aspects. Its hardly surprising that progress can be slow, as there are many hurdles and sometimes the obstacles have a habit of moving!

6. ADS-B Applications - Package 1

A group of ADS-B applications has been identified for early implementation (over a 5 to 10 year timeframe) - known as Package 1 and includes 7 airborne and 5 ground applications. Airborne applications include: airport and airborne traffic situation awareness, visual acquisition and visual approaches. Ground applications include: enhanced en-route, TMA and Airport ATC surveillance and derived data for ATC tools.

Support for these applications is substantial: Eurocontrol, the European Commission, FAA, IATA and AEA - even if these organisations cannot agree on technologies. Extensive work has already been carried out in many different projects to show these applications are feasible and beneficial. There is consensus that they are achievable within 5 to 10 years, delivering tangible commercial and operational benefits. Opinion is still divided on technology. For a definition of the package 1 applications, see Package 1

The link evaluation process will be complete around mid 2004, when the detailed Package 1 requirements are known. Any decision making before then is pre-emptive. Unfortunately, Airbus, Boeing, FAA and Eurocontrol have pre-empted this process by selecting 1090ES as their preferred link.

Picture: ADS-B Graphic

7. Surveillance Datalinks

The proposed surveillance datalinks - some of which still have yet to be demonstrated for Package 1 - include:

Figure 1: Functionality of Datalinks

These datalinks are very diverse and their technical characteristics and performance (e.g. range, capacity etc) are not strictly comparable:

7.1 High Frequency Datalink

High Frequency Datalink (HFDL) is the only available datalink for polar routes also deployed over remote regions. It has a low data rate, is shared among many users, but does support FANS1A and ACARs applications. Whilst it is likely to be replaced by Satcom in the future, it will, nevertheless, be around for some time.

7.2 Aeronautical Mobile Satellite Service (AMSS)

The AMSS is operated by Inmarsat, providing narrowband services to 1200 long-haul aircraft as part of the FANS1A package. A very expensive service (both in terms of avionics costs and communication charges) with significant delay times, it is also likely to be around for some time until replaced by more efficient Satcom services.

7.3 VHF Datalinks

VHF Datalinks modes 2, 3 and 4, have (to varying degrees) been subject to conventional standardisation at ICAO. In Europe, ETSI has published detailed standards for VDL mode 2 and VDL mode 4. Europe has not yet accepted VDL mode 3. It is unclear who controls the standards for VDL mode 3, other than the FAA. Currently, only "core SARPS" exist for VDL mode 3.

7.3.1 VDL Mode 2

VDL mode 2 is an air/ground D8PSK datalink, nominally operating at 31.5kbps, but with a net capacity estimated at less than 3 to 4kbps. Disputes over the actual capacity of VDL2 may lead to further simulations and capacity assessments - to see if improved performance is possible. VDL2 is a Carrier Sense Multiple Access (CSMA) link, in which the link is not available all the time - severely reducing its capacity. Significant frequency planning is required to protect its channels - due to low frequency re-use rates caused by CCI values of 22 - 26 dB (Co-Channel Interference) - from other VDL2 stations and VHF users and to prevent interference into neighbouring channels. VDL 2 does not support tactical and time critical ATC datalink requirements.

7.3.2 VDL Mode 3

VDL3 is a Time Division Multiple Access (TDMA) D8PSK service, operating at 32.5 kbps, presumably synchronised to GPS time and providing 4 channels per 25 KHz frequency (nominally 3 voice and 1 data). It has the potential to provide wide area datalink services with data rates up to 12.4 kbps. Intended as a digital replacement for analogue VHF voice, it is a key element of the FAA's planned ATC upgrade. Curiously, the main competitor to this service is the European proposed combination of additional 8.33KHz VHF channels and VDL 2.

7.3.3 VDL Mode 4

VDL4 is a Time Division Multiple Access (TDMA) GFSK service operating at 19.2 kbps with a net link capacity of 14kbps. VDL4 was specifically designed to be independent of ground infrastructure. It permits any number of Users to simultaneously share the link, enabling both air to air and air to ground communications. The FM modulation scheme also permits frequency re-use, due to a low CCI value of 10dB - meaning the loudest (and nearest) transmissions are always heard - and is very spectrum efficient. VDL4 has good range and capacity and is seen as a candidate for all long-term applications. It is also the datalink with greatest demonstrated capability in a wide range of intensive studies across many aircraft and ground ATM platforms.

States such as Russia and Mongolia have expressed strong support for VDL mode 4 as their preferred solution with implementation currently underway. Both countries have physically large territories that would be both difficult and extremely expensive to serve with SSR radar.

Website and links for VDLmode 4 activities can be found at www.nup.nu

7.4 Mode S Datalinks

Mode S Radar is the proposed replacement for Monopulse SSR, providing 24bit aircraft addresses and selective interrogation. Ground implementation is delayed and moving very slowly. Mode S cost/benefit analysis has been unconvincing, and simulations also indicate that the transition to Mode S may problematic - especially at the boundaries between Mode S and ModeA/C airspace (due to code changes). Delayed airborne equipage is proceeding - a long way ahead of any ground infrastructure - and far ahead of any commercial returns. Much work on ground infrastructure will be required - see the Eurocontrol Report Transition to Mode S

European implementation is proposed in two phases: Mode S Elementary Surveillance (ELS) followed shortly after by Mode S Enhanced Surveillance (EHS).

7.4.1 Mode S Enhanced Surveillance

Mode S Enhanced Surveillance (EHS) consists of Elementary Surveillance (ELS) with the addition of various Downlink Airborne Parameters (DAPs) including: Magnetic Heading, Airspeed (IAS and Mach No.), Selected Altitude, Vertical Rate, Track Angle Rate, Roll Angle, Ground Speed, True Track Angle.

ATC centers will require considerable investment to integrate, process and distribute this data with major changes to ATC procedures to make use of the data. It may be some time before this happens. Mode S EHS is not ADS-B. Although there are some overlaps, this technology does not resolve the air to air applications and so is not a competitor technology to, say, VDL mode 4.

Picture 2: Radar Datalinks

7.4.2 1090 Extended Squitter (ES)

1090ES is a derivative of Mode S technology, with messages being squittered at a high rate. The channel is shared with SSR Mode A/C, SSR mode S, Mode S EHS, TCAS/ACAS and military datalinks. The air to air range of 1090ES is insufficient, making it unsuitable for any long term purposes and it suffers from multi-path (e.g. reflections from buildings, aircraft etc.), making it unsuitable for airport applications. This is well documented in independent simulations and trials. 1090ES will saturate in Europe around 2010, effectively making it obsolete before any benefits are realised. There are also serious concerns about overloading 1090MHz with yet another application.

1090 Extended Squitter does not meet known Package 1 requirements. Despite this, Airbus, Boeing, IATA, Eurocontrol and FAA have expressed strong support for 1090ES - without any explanation of what the benefits are and how they will be achieved. This seems to be political support for Mode S, rather than any interest in ADS-B applications. The limited trials involve only a very small number of units and do not justify the position taken by these organisations - a decision taken ahead of the requirements process for Package 1 and the completion of any kind of proper validation process, with standards compliant equipment. Eurocontrol's position is particularly difficult to understand.

7.5 Universal Access Transceiver (UAT)

UAT was developed by Mitre Corporation, and is a random squitter broadcast technology operating at around 960MHz with a bandwidth of 2-3MHz. Two message types are supported: air/air and air/ground ADS-B messages and ground-station uplink messages (weather, Flight Information Services - Broadcast FIS-B). This technology is a direct competitor to 1090 ES, but has better range and capacity. It is supported by FAA - which does not clarify the FAA's position on ADS-B (i.e. what technology is the FAA proposing for future surveillance Mode S or UAT?).

Work on ICAO SARPs for UAT will start in the near future. Finding and protecting spectrum for UAT may be difficult. There is relatively little support outside the US.

The UAT Homepage with links can be found at: UAT Homepage

8. Recent Trials and Simulations

Many trials and simulations have been carried out over recent years. Comparison tests, results and simulations of the various technologies are accessible over the internet - See results of Heathrow Trials

There is a striking disparity between the demonstrated capabilities (e.g. performance and benefits) of the various technologies:

Limited trials for 1090ES have been carried out at Bretigny, Frankfurt and Los Angeles. These trials have not been carried out in sufficient depth to be in anyway conclusive.

Conversely, VDL mode 4 has been subject of the most intensive and most successful evaluations (since work began on early prototypes in 1990) providing much useful data for developing the benefits of ADS-B in airborne and ground applications. Indeed, VDLmode4 has achieved almost "Cinderella" status in the industry, with VDL2 and Mode S taking the parts of the ugly sisters.

VDL 4 Websites can be found at: NUP (Northern Europe) and MEDUP (Mediterranean trials), Mediterranean Free Flight and MA-AFAS (autonomous aircraft applications)

UAT has been the subject of an extended trial under the Capstone project in Alaska.

UAT was introduced as a low cost substitute for radar in order to reduce the high accident rates experienced in Alaska. Details of which can be found at: Capstone

9. Summary

If we concentrate on the consensus on datalink (i.e. with facts that all agree) rather than any individual organisation's technology preferences, we quickly centre upon a range of beneficial applications, on viable and cost effective technical solutions and a realistic implementation timeframe.

Decision-making will be a test of character for those organisations pre-empting the datalink decision process. Airlines are already spending substantial amounts, in very difficult economic times, on technologies that will yield few commercial returns. We must ensure that the best options are selected, that these rapidly deliver operational and commercial benefits the industry desperately needs.

In the end, it will be the customer that decides, not the vendor - and only solutions that speak directly to the bottom line will be considered. All else will fall on deaf ears. And the way ahead? The author believes the consensus will eventually be: Package 1 applications, implemented by VDL mode 4 over the next 5 to 10 years.

Advanced Aviation Technology Ltd.
The Old Post Office,
The Street, Compton,
Surrey GU3 1ED. ENGLAND.
Tel. +44 1483 811 311.
Email: kim.oneil@aatl.net