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LDACS-NAV could guide global aviation

January 4, 2021  - By

GNSS is a critical single point of failure for navigation in the aviation industry. A new white paper published by Egis says it’s time for the industry to get rid of legacy navigation aids (NavAids) and catch up technologically with the rest of the communications industry.

The following is summarized from “Is This the Time and Place to Finally Back up GNSS?” published by Egis.

Current navigation backups are ground-based navigation aids such as distance measuring equipment (DME). These use post-World War II technologies, with very low spectrum efficiency. Some might find it surprising to learn that they are still using Morse code.

While difficult to jam due to their strong signal, current navigation aids are not cyber secure. Due to their spatial distribution, they can be limited in their support to PBN (performance-based navigation) or any new concept of operations.

The horizontal positioning error was measured under 10 m, so the LDACS-NAV would easily meet RNP 0.3 requirements.

Legacy NavAids — NDB (non-directional beacon), ILS (instrument landing system), VOR (VHF omnidirectional range) and DME — all require a specific frequency band, various equipment, and airborne and ground antennas.

As a result, the average commercial airliner can carry around seven specialized navigation antennas, and as many as 20 when accounting for all the other communication, navigation and surveillance (CNS)  functions. Having different radio systems is adding redundancy but makes the aircraft and the ground equipment very costly, as well as difficult to engineer and to maintain.

Two major problems could affect the aircraft industry. First, software-defined radio, and powerful low-cost radio systems are available to the public and any ill-intended person could interfere, deactivate or worse, divert these vulnerable systems from their purposes. Second, spectrum is a finite and fixed asset (aviation uses 14% of the total available spectrum).

Why hasn’t this problem been solved already?

There are no market incentives for air navigation service providers (ANSPs) and airlines to make expensive investments in ground infrastructures and aircraft retrofits. With an average lifetime of 25 years per plane, commercial fleets take a long time to be renewed.

Also, the aviation spectrum is protected, which has led to complacency and a lack of pressure to use the latest technologies to improve spectrum efficiency.

Stakeholder Coordination. States, ANSPs, airlines, airports, aircraft manufacturers, communications providers and system providers all have their own interests and perspectives, which increases the difficulty in developing and maintaining a global CNS roadmap.

Deployment. Once a roadmap is agreed on, the deployment challenge remains. For instance, the retrofit compliance date for ADS-B was pushed back from June 2020 to to June 2023 due to the pandemic. The capacity of aviation to evolve depends on when the operational and commercial benefits are clear, such as when GNSS was implemented for navigation.

The Human Factor. Human factors have to be considered for any critical change in aviation. Pilots are trained on navigation aids and GPS, and used to communicating by VHF voice with air traffic control officers. This is the reason why the evolution of navigation and communication systems must be seamless with current systems or require an in-depth human-factor risk assessment.

Potential Solutions

To future-proof aviation and performance-based operating procedures, aircraft need both a broadband, IP-based datalink capable of VoIP and a secure, cost-effective alternative positioning, navigation and timing (A-PNT) system as a back-up to GNSS. Today, GNSS backup is the 70-year-old DME — using the signals from multiple DMEs, aircraft can locate themselves with reasonable accuracy.

The main choices to replace the DME are either an enhancement of DME systems (Multi-DME RAIM, eDME, Mosaic DME) or an A-PNT solution (LDACS-NAV, WAM-TISb, SSR mode N, eLoran).

If we look at the most mature solutions, the DME/eDME and the LDACS-NAV are the main options, and they represent a real dilemma.

DME/DME. This solution represents the best GNSS backup currently available. One possibility is to improve the signal to improve accuracy. Other improvements would allow the detection of more than two ground stations, or even receiver autonomous integrity monitoring (RAIM) capability. Only small improvements need to be made to the signal and to the FMS (Flight Management System), making it the option requiring the least effort and expense.

However, to reach a reliable Required Navigation Performance (RNP) standard of RNP 0.3, additional distance measurements are required, especially at low altitudes, and more DME facilities might be needed. Plus, this solution does not provide a secure, integrated communication and navigation solution and does not improve spectrum efficiency.

Photo: AlexeyPetrov/iStock/Getty Images Plus/Getty Images

Photo: AlexeyPetrov/iStock/Getty Images Plus/Getty Images

LDACS-NAV. The L-band digital aeronautical communication system (LDACS) for continental ground communication is an IP-based data-link solution with a built-in navigation capability. It uses orthogonal frequency-division multiplexing, organized as a cellular network and sharing features with 3G and 4G. It works by detecting signals of opportunity within the communication exchange, and then multi-laterating the signals from at least four ground transmitters to calculate an airborne position. The frequency is ingeniously placed in the L-band between each DME frequency. It is built with interference mitigation algorithms and minimizes out-of-band radiation to protect DME.

This solution is spectrum efficient, cybersecure, doesn’t require additional frequency assignment, and is scalable and adaptable to local needs. Given LDACS is almost certain to be implemented in communications to replace VDLM2, using this capability would be an easy choice for navigation.

Features like air-to-air ranging, surveillance or enhancements to DFMC GBAS are possible. Also, additional navigation information can be transmitted, such as trajectory-based operations and 4D trajectories.

Both Frequentis AG and Leonardo SpA have built fully functional and interoperable prototypes. In March 2019, the German Aerospace Centre (DLR) tested LDACS. The flight campaign showed its capabilities in practical scenarios with industrial demonstration equipment. The horizontal positioning error was measured under 10 m, so the LDACS-NAV would easily meet RNP 0.3 requirements.

ICAO Recognition. An International Civil Aviation Organization (ICAO) standardization group has started work on LDACS for both communication and navigation. The LDACS-NAV will first be used to augment the DME system.

More Study Needed

To fully validate the LDACS-NAV concept, further studies and large-scale demonstrations must be conducted, and a cell-planning study needs to determine the number of necessary ground stations.

Also, a detailed cost/benefit analysis must be undertaken to evaluate the cost of an EU-wide LDACS-NAV network. It would take into account the manufacturing and deployment costs of ground stations as well as equipment costs of multi-mode LDACS/VDL avionics, identifying whether it can support navigation functions.

Also studied should be the benefits of having a GNSS back-up system, equipage costs of a dedicated avionic system and the direct operational benefits of providing a reliable, low latency and cost-efficient communication and navigation network for all aviation stakeholders, including secured proprietary information for airlines and aircraft manufacturers, and including full 4D trajectory-based operations and flight-centric air traffic management for ANSPs.

If both the cell-planning study and the cost/benefit analysis suggest a positive economic advantage to implement the LDACS system compared to the current system or the other potential A-PNT solutions, then European institutions could select LDACS as the official long-term A-PNT solution in the CNS Roadmap & Strategy and enable the SESAR Operational Concept high-level goals. This would help accelerate the standardization and industrialization activities to resolve the current lack of redundancy in our CNS systems.

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