Worldwide Aviation Digital Communications: An Overview

Worldwide Aviation Digital Communications: An Overview

Ken Elliott reviews worldwide digital communications in aviation. How is the efficiency of our shared airspace being improved? Find out here…

The transition to current and future aviation digital communications involves aircraft equipage that is ‘required’, based on operational need. While newbuild platforms are ready for datalink, however, pre-owned aircraft owners are forever in catch-up mode.

Aircraft have range and altitude limitations, based on their size and performance. Some are frequent users of oceanic airspace while others aren’t. Nevertheless, digital communication is not just for remote operations anymore. Significant reductions in dispatch and flight time, and fuel savings, represent a cost and time advantage to operators in domestic airspace.

As preferred tracks fill up across the oceans and remote regions, popular overland routes stretch the capacity of continental air traffic. ‘Best equipped and best served’ is the way forward, and operators who invest in the equipment, training, and approvals will continue to reap the cost and time savings.

Aircraft Communications, Addressing & Reporting System (ACARS) was introduced to commercial air traffic in the 1970s to satisfy a need to improve crew time-reporting. ACARS messages are short bursts of VHF data sent over the same VHF band as the radio voice on dedicated frequencies assigned to ACARS.

Messages were originally available as Telex printout, digital generated voice, and then – later – routed to Flight Management System (FMS) displays for visual display. Modern ACARS messages include Out-Off-On-In (OOOI) events, flight plans, weather data, and performance status of flights.

For Business Aviation, datacom has also been around for some time with its emergence in the VHF Data Link (VDL) capability engineered into aircraft VHF radios.

While ACARS is character-based data, Aeronautical Telecommunications Network (ATN) is bit-based. Early versions of ATN were Mode 0 and A. The current version is Mode 2, and the future version will be Mode 3. The Modes reflect the characteristics of the data transferred – for example, you may be familiar with the terms VDL Mode 2 and ATN-B1.

Note that among other characteristics (such as 4D trajectory data), the preferred ATN-B2 includes both ATN-B1 and the Oceanic/Remote Future Air Navigation System (FANS) as FANS 1/A+. Currently, ATN-B2 is the means of US FAA operating datalink services with the exception that Satcom datalink protocols are not currently in use domestically.

Digital communications are not restricted to the Very High Frequency communication radio bands. Understandably, because VHF is limited to a line-of-sight range, it is ineffective over oceans and in remote regions. Both High Frequency (HF) with Selcal and Satellite (Satcom) solve that problem, and while both are traditionally voice systems, today they are datalink capable.

Worldwide digital communications cannot be assessed in isolation. Indeed, the avionics suite of Communication, Navigation and Surveillance (CNS) are all integrated with respect to Air Traffic Management (ATM). For example, when

traversing intercontinental tracks, air traffic control must be assured of the capability of each aircraft to be tracked, with intentions known and guaranteed ability to fly within a specified 4D corridor. Data Link Services (DLS)

The umbrella term of DLS embraces a confusing array of technologies, flight performance criteria and flight monitoring tools. This can be overwhelming and daunting for any flight department to tackle.

While your flight department may only need partial equipage and possibly no crew authorization, it may still be beneficial to have a background understanding of Data Link. Figures 1 through 3 (overleaf) attempt to capture the full scope of DLS, as it exists today, and as it is being introduced.

In particular, the ATN-B2 Safety & Performance Requirements (SPR) is significant in its scope. It covers the provision of DLS across all phases of flight, with a keen focus on the future.

Required for operations in FANS oceanic & remote airspace: • Equipment (as shown in Figure 3, see page 94) • Approved training • Letter of Authorization • ICAO filed flight plan that includes correct equipage codes

Note: An additional equipment code [J6] exist for use of Japan’s MTSAT satellite system

PBCS the Assurance of CPDLC & ADS-C

Due to capacity limitations, the North Atlantic Airspace (NAT) has introduced Performance Based Communication and Surveillance (PBCS). Operators must meet the performance criteria and provide the assurance to Air Traffic Control (ATC) that they can safely fly within specified parameters that

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There are two parts of PBCS, communication and surveillance.

• RCP 240 – communications • RSP 180 – surveillance

Note: 240 and 180 refer to the maximum permissible transaction times in seconds.

Compliance with PBCS assures aircraft flying in selected NAT tracks that they can operate at 30nm or 50nm, and five minutes longitudinal separation minimum, and 23nm lateral separation minimum.

Each day the NAT issues Organized Track Systems (OTS), one east- and one west-bound. At the center of the OTS are those tracks that are available to qualified operators who meet PBCS monitoring requirements. To meet the requirements is a universal exercise, as all the following must be checked: • Aircraft operating systems meet performance criteria during flight • Aircraft operator approval • Crew qualification and approval • Onboard crew alerting of inoperative PBCS-related functions • Aircraft Master Minimum Equipment List (MMEL) related items • Aircraft Flight Manual PBCS procedures and limitations • Air Traffic System (ATS) able to monitor and manage

PBCS • Communication Service Provider (CSP) speed, capacity, and accuracy.

To operate within the OTS tracks, the aircraft and CSP must be able to meet the performance requirements for any data link communications. Performance criteria is based on transaction time, as the maximum time for a transmission/response. Also, performance is monitored as a percentage probability in three categories of: • Continuity as the probability that the transaction time

FIGURE 2: ATN-B2 Safety & Performance Requirements - Converged

FIGURE 4: PBN (RNP) Component to Complete the Monitoring Assurance Criteria for NAT Operations (add RVSM compliance)

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will be met • Availability as the probability that the communication can be initiated • Integrity as an acceptable failure rate per flight hour.

Regarding actual times and percentages…

• RCP 240: - Maximum transaction time – 240 seconds - Continuity – 99.9% - Availability – 99.99% - Integrity – 1 in 10-5

• RSP 180: - Maximum data delivery time - 180 seconds - Continuity - 99% - Availability - 99.99% - Integrity – 1 in 10-5

PBCS monitoring communication and surveillance is like the existing monitoring of altitude (RVSM) and navigation (PBN-RNP). NAT monitoring includes RVSM and PBN-RNP, and while PBCS is limited to certain OTS tracks in the HighLevel Airspace (HLA) of the NAT, in the future it will be required across other popular flight corridors.

Apart from the NAT regions, PBCS assurance is also monitored across the Pacific, including southern portions, and in a sector near Singapore. US DCL

Data Link, as automated Departure Clearance, is available across the US at 64 airports and is being introduced for enroute flight. The following are required for US DCL operations: • Equipment (as shown in Figure 3, above) • FANS protocol logon to US DCL tower using VDL Mode

A or 2 • No Letter of Authorization (LOA) required for US registered aircraft • Flight plan that includes correct codes • Pre-Departure Clearance (PDC) still in effect • Aural and visual warning.

Eurocontrol Data Link 2020

Europe’s data link evolution has gone through many stages with different titles, such as LINK 2000+, and frequent delays. More recently Eurocontrol has introduced another version, Data Link Services – Implementing Rule (DLS-IR).

As of February 5, 2020, flights undertaken above FL285, within the IFR General Air Traffic (GAT), were required to use ATN B1 VDL Mode 2 data link. But there are many exemptions.

This is a version of CPDLC, and those aircraft capable of CPDLC via their FANS 1/A+ may not use their CPDLC in place of the ATN B1 VDL Mode 2 version. Future Digital Communications and Data Link Overview

DLS are incrementally introduced, relying on regional and incountry specifics, and are subject to both delay and staggered implementation. When predicting the future of DLS, shifting trends and advancing technologies can skew outcomes.

Because DLS increases airspace efficiency, ATC can establish re-routes, tailored arrivals and ADS-B In Trail Procedures (ITP) more effectively. Both domestic and intercontinental ATC can be adaptable to the impacts of weather and traffic volume. Providing automated track changes to individual aircraft and knowing their unique equipage status, they can prioritize and optimize based on ‘best equipped, best served’.

Data communications will advance based (somewhat) on these technologies:

• Existing VDL Mode 2 services (US FANS 1/A+ with VDL

Mode 2) • Airport surface – AeroMACS - AeroMACS is a broadband wireless service operating in a protected aeronautical frequency band for use across the airport surface. • Terrestrial coverage – LDACS - LDACS (L-band Digital Aeronautical Communications System) is necessary to overcome the spectrum limitations of HF and VHF by introducing a new broadband service. Note its introduction is subject to no Radio Frequency Interference (RFI) across existing airborne, ATC and ground systems. • Oceanic + Remote coverage – Satellite - While satellite coverage is still limited to mostly Iridium and Inmarsat, it can be anticipated that the use of reduced latency, new Low Earth Orbit (LEO) satellite constellations will become popular.

The protocols and standards to apply present and future technologies are:

• ACARS/FANS – Legacy data and messaging communication standard • ATN-OSI a legacy IACO routing standard currently in use (ATN-B1 and B2 applications) • ATN-IPS (IPv6) Internet Protocol Suite, operating as an internet and allowing the volume of data link required for future 4D Trajectory Based Operations (TBO), such as Dynamic RNP plus D-TAXI, and other volume-sensitive

ATS services. (Note: ATN-IPS is planned to support technologies operating in the proposed ATN-B3 application.) DLS Aircraft Equipage

Operators should anticipate changes to existing, and acquisition of new, airborne equipment to accommodate ATN-B3 future applications. The equipment will need to cope with IPv6 internet protocol, providing advanced data link services, ranging from digital communications, through surveillance and to dynamic navigation.

Not only will aircraft need to function in this internet environment, but they will need to record activity and be capable of real-time flight monitoring, via their CSP to their flight departments and support services.

While cabin Wi-Fi systems will advance in cockpit capability, there will need to be a firewall between passenger and cockpit data link over IP services. Cockpit services will require safety, security, and reliability assurances way beyond those required for cabin applications.

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Recent Developments

In 2020, Europe’s SESAR Deployment Manager issued a very detailed and useful 36-page document, ‘Assumptions for a synchronised deployment towards Initial Trajectory Information Sharing’. For those seeking further clarification of Europe’s data link intentions that mostly applies anywhere, this document is highly recommended.

ICAO has taken a bold new stance on future aviation plans that includes DLS, via its ‘ICAO Draft Global Concept for Integrated Communications, Navigation, Surveillance and Spectrum - May 18, 2022’.

There are two major considerations introduced in this document:

• Other airspace users - Urban Air Mobility - Unmanned Aircraft/Remote Piloted Vehicles - Supersonic - High Altitude • Spectrum (use of frequency spectrum) - Airborne systems - Air traffic, satellite and service providers - Terrestrial systems (not aviation)

Although these are not new considerations, ICAO has brought them both into necessary focus in this draft proposal for future planning. Its busy roadmap takes the user through to 2050, in three strategic stages of short-, medium- and long-term. Apart from ‘airspace users’ and the ‘frequency spectrum’ concerns, the plan includes traditional topics, such as environment, airspace access, slow technology adoption, and regulatory processes.

Lastly, there is a possibility that greater use of 4G- and new 5G-based technology will further transform and disrupt the process of DLS advancement. While these internet capabilities are being explored and gradually implemented in airspace redesign, their true benefits are yet to be realized. ❚

KEN ELLIOTT

has more than 52 years of aviation experience focused on avionics in General and Business Aviation. Having a broad understanding after working in several countries on many aircraft types and avionics systems, he has contributed to several work groups and committees, including for NextGen, Airport Lighting, Human Factors, Unmanned Aircraft and Low Vision Technology. In retirement, he is striving to give back the knowledge gained with an eye on aviation’s future direction.

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