- TECHNOLOGY
LDACS: The Futuristic Terrestrial Aeronautical Datalink System
By Adeyinka Olumuyiwa Osunwusi, PhD
With the increasing complexity of air navigation and air traffic operations coupled with the incremental growths in air traffic volumes across the vast swathes of the global airspace, the need for robust, digitalised and reliable air-ground connectivity in an attempt to promote seamless airspace management cannot possibly be overemphasized. With this overarching need, air navigation service providers (ANSPs) – faced with the operational limitations of existing systems such as VHF DSB-AM and HF SSB-AM communications systems, CPDLC (controller-pilot datalink communications), and VDL Mode 2 (VDLM2) particularly in continental airspace – are increasingly redirecting their attentions towards the multilink digital aeronautical communication sector. European ANSPs, for example, have all agreed to migrate to services provided by a single Datalink Service Provider by 2026 under Europe’s ATS Common Datalink Services (ACDLS) initiative, which strives to ensure service commonality in the context of the governance and deployment of datalink within the European airspace.
The industry is also moving steadily in the direction of forging strategic alliances with a view to re-engineering and harmonising aviation connectivity. One glaring example, of course, is the joint cooperative initiative involving the US’ Federal Aviation Administration (FAA), the European Union Aviation Safety Agency (EASA), Boeing, and Airbus, which focuses on defining a framework for the modernisation and harmonisation of the aeronautical data communication landscape by 2035. Expectedly, the increasing availability of innovative technologies coupled with the conceptualization of a growing number of innovative techno-operational concepts – from the 4D Trajectory-Based Operations (TBO) concept, User-Preferred-Routes (UPR) concept, single-pilot operations and sectorless flying concept to the XML-based system-wide information management (SWIM) concept – are opening up whole new vistas for the global air navigation ecosystem in general and the air traffic management realm in particular.
One solution that has caught the attention of industry stakeholders as a robust digital datalink technology capable of providing the level of service required for the full realisation of a globally harmonised, modernised, sustainable and interoperable air navigation/air traffic management system is the futuristic terrestrial-based distributed aeronautical datalink communication solution known as the L-Band Digital Aeronautical Communications System (LDACS). This futuristic broadband air-ground datalink technology is increasingly being recognised as the panacea for delivering secure, reliable and efficient datalink communications between the ground and the air as the aviation ecosystem strives to scale down voice-based legacy air-ground communications, increase the fortune of air-ground data communications, and promote smooth transitions from existing legacy datalink solutions such as VDL Mode 2 (VDLM2) to more robust, digitalised, reliable, high-throughput and spectrum-efficient broadband datalink technologies within the framework of the EUROCONTROL Future Communications Infrastructure (FCI) paradigm, the Single European Sky ATM Research’s (SESAR) ATM Master Plan and the ICAO Global Air Navigation Plan (GANP).
The LDACS is an ambitious initiative that is being promoted essentially not only as a supplement to VHF communication solution in the L-band but also as a broadband complement to VDLM2. It can also be seen as an integral part of a multilink wireless network architecture meant to provide air-ground connectivity for aircraft within the global aeronautical telecommunications network (ATN)/OSI network. It is also envisaged, in line with the GANP, that LDACS will be deployed alongside satellite communication systems to support aeronautical voice and data communications needs. Under the EUROCONTROL FCI initiative, LDACS is being developed to offer both air-ground and air-air communications using different radio channels with the adoption of different approaches to the definition of the physical and data link layers of the LDACS protocol stack.
THE TECHNOLOGY
LDACS is an air-ground broadband digital datalink technology designed to operate in the L-band (960 MHz to 1164 MHz). It is a terrestrial-based cellular broadband system with line-of-sight coverage and is based essentially on the multi-carrier transmission Orthogonal Frequency-Division Multiplexing (OFDM) modulation technique reminiscent of current mobile radio standards, especially the 4G communications systems, with a bandwidth of about 500 kHz per channel on both forward link (ground station to aircraft) and reverse link (aircraft to ground station). In terms of connectivity, LDACS features a full duplex radio link with a centralised communication architecture based on Frequency Division Duplex (FDD) with bi-directional forward and reverse links occurring at different frequency bands or simultaneously in time. As provided for in the proposed amendment to Annex 10 Volume III, forward link shall be assigned to the frequency band 1110-1156 MHz while reverse link shall be assigned to the frequency band 964-1010 MHz.
LDACS is designed for the Internet Protocol Suite (IPS) and the Open Systems Interconnection (OSI) stacks with the provision of the required mechanisms to transport communication services based on both ATN/IPS and ATN/OSI over IP, albeit the applicability of IPS rests squarely on the deployment of ground IPS infrastructure as well as the availability of IPS-compatible avionics. In this respect, two layers are defined for the LDACS protocol stack – the Physical Layer and the Data Link Layer.
The physical layer supports the transfer of data over the radio channel with the LDACS ground station supporting bi-directional links to multiple aircraft as well as the continuous transmission of a stream of OFDM symbols on the forward link. On the reverse links, multiple aircraft transmit discontinuously, sending a combination of orthogonal frequency-division multiple-access (OFDMA) and time-division multiple-access (TDMA) radio bursts in time and frequency according to the transmission slots assigned to different aircraft on demand by the ground station.
The data link layer, which supports concurrent and reliable transfer of data for different users, is organised in two sub-layers, namely: the Medium Access sub-layer, and the Logical Link Control sub-layer. The Medium Access sub-layer, which manages transmission sequences in slots of time and frequency, comprises the Medium Access Control (MAC) functional block. The Logical Link Control sub-layer, which manages the radio link between the aircraft and the ground, comprises the Data Link Services (DLS) and the Voice Interface (VI) functional blocks. The LDACS Management Entity (LME) functional block provides cross-layer management, while the Sub-Network Protocol (SNP) functional block, residing within the sub-network layer, provides higher-layer interface.
LDACS FUNCTIONALITIES AND BENEFITS
LDACS is essentially multi-functional, featuring a wide array of basic and extended functionalities. Although a largely IP-based digital datalink technology, LDACS is being designed to be able to provide optional support for IP-based (IPv6) digital voice communication as well as secure connectivity to onboard Electronics Flight Bag (EFB). Aside from communications capabilities, LDACS also supports alternative positioning navigation and timing (A-PNT) to supplement existing airborne Global Navigation Satellite System (GNSS), thanks to its robust ranging functionality. Tests and trials conducted by SESAR2020 and the German Aerospace Centre, DLR, within the framework of the German nationally funded ICONAV (Integrated COmmunication and NAVigation functionality for sustainable L-band use) and LDACS-NAV have confirmed LDACS communication and navigation capabilities.
Against the backdrop of the open standard and distributed characteristics of LDACS, the most striking feature of this futuristic broadband datalink technology is the technology’s capability to support global interoperability in the face of the multi-vendor product nature of both ground equipment and onboard avionics systems.
Bearing striking similarity with modern cellular telephony, LDACS presents a wide array of features, which are accentuated by the perfect propagation characteristics of the L-band spectrum. These features include LDACS’ scalability in terms of future datalink applications and the ease of moving to higher bandwidth without necessarily altering system characteristics, its capability to support expanded and scalable airspace management with its high data rate capacity, its capability to guarantee bandwidth re-use, its capability to guarantee high operational performance and spectrum-efficiency, its enhanced safety and cyber-security features, and its capability to guarantee quality-of-service through traffic prioritisation. Through the use of highly coordinated multi-channel access technique, LDACS also guarantees application low latency and robust data traffic prioritisation management; two of the features that the TSN (Time-Sensitive Networking) technology showcases. Aside from this, tests and trials have shown LDACS to be the truly modern aeronautical communication solution in terms of its enhanced resilience, and enhanced flight efficiency.
One feature that sets LDACS apart when juxtaposed against VDLM2 is LDACS’ robust data throughput, ranging from 550 kbps to 2.6 mbps and accounting for between 50 and 200 times the data throughput of VDLM2. This feature, coupled with LDACS’ use of an ACM (adaptive coding and modulation) scheme, further explains the robust spectrum-efficiency and high operational performance advantages of LDACS, albeit VDLM2 and LDACS are comparable in terms of coverage which is in range of 200 nautical miles (NM). The recommendations in the proposed amendment to ICAO Annex 10 Volume III is that LDACS should be capable of supporting communications between LDACS ground station and LDACS airborne station at distances of up to 200 NM when used in regular range mode and at distances of up to 400 NM when used in extended range mode. The value of the net data rates achievable for a given pair of forward and reverse link channels is actually a function of the strength of the forward error correction coding and the modulation scheme adopted with a strong coding coupled with robust modulation resulting in decreased net data rates and a weaker coding coupled with a higher order modulation delivering increased net data rates.
One particularly striking feature of LDACS is its ease of integration with other services, covering the communication, navigation, and surveillance spectrums and including all the ATM infrastructures for the future. In the communications realm, LDACS is capable of covering a wide array of communication platforms from air traffic control (ATC) communications to airline operational control (AOC) communications. From the navigation and surveillance perspectives, it can be deployed alongside legacy L-band Distance Measuring Equipment (DME) systems and can also co-exist with VDLM2 equipment in the avionics bay either as a stand-alone or within a combined radio equipment. From this perspective, LDACS experts believe that rather than encourage the proliferation of airborne equipment, LDACS actually promises to promote robust onboard equipment rationalisation as existing antenna and radios can be easily adapted, thus cutting down on the number of avionics systems required onboard aircraft.
Aside from the requirement relating to the capability to support automatic dependent surveillance – contract (ADS-C), the technology is also currently being explored from the perspective of integrating GNSS inertial and terrestrial ranging sources. Flight trials have also demonstrated the capability of LDACS to support ATM applications such as the exchange of flight plan, broadband weather maps applications and CPDLC. Owing to its use of a different frequency band, LDACS can also be co-located with existing radio installations, including VDLM2, without the risk of interference. This is aside from LDACS’ capability to co-exist with legacy systems such as TCAS, ADS-B, Mode-S and Mode-C.
LDACS VALIDATION AND STANDARDIZATION
Quite substantial progress has been made in relation to the development, validation, and standardisation of LDACS with SESAR – in liaison with Frequentis and Leonardo – and the German Aerospace Centre DLR – in liaison with Rohde & Schwarz GmbH – taking the lead in the development of LDACS prototypes or demonstrators. The success of verification and validation campaigns carried out so far, including series of laboratory tests, compatibility tests and flight trials, has provided the needed elixir for the continuing development of LDACS standard by ICAO and EUROCAE with the establishment of the EUROCAE standardisation process in early 2022 and the initiation of the ICAO LDACS standardisation process in December 2016 following the earlier establishment of the Project Team Terrestrial (PT-T) within the framework of the ICAO Communications Panel (CP) Data Communications Infrastructure Working Group (DCIWG).
The positive outcomes of these standardisation efforts have been the successful development of the required LDACS proposal for the amendment of ICAO Annex 10 Volume III (Communication Systems) and its expected insertion as the 13th Chapter (L-Band Digital Aeronautical Communications System – LDACS) of the revised Annex as well as the development of draft Guidance Material and Standards and Recommended Practices (SARPs) for LDACS. The trio of ICAO, EUROCAE and AEEC (Airlines Electronic Engineering Committee) have also adopted the IPv6 data/voice communication as the standard of choice for LDACS implementation. AEEC is responsible for the initiation of the standardisation process for the airborne integration of LDACS.
These milestones serve to confirm not only the growing acceptance of LDACS by the rank and file in the industry as the futuristic multilink aeronautical datalink communication solution but also the reality of an ICAO-endorsed LDACS standard. Given the procedures involved in the standardisation process within the ICAO organizational and operational structures, an LDACS standard can only be expected to become effective sometime in 2027.
LOOKING TO THE FUTURE
To ensure the ultimate deployment of LDACS on a global scale, quite a number of industry players are working round the clock to close whatever gaps require closing with stakeholders in Europe taking the lead in terms of the development LDACS prototypes and the conduct of tests. Leading the research and development sector is the German Aerospace Centre DLR (Institute of Communications and Navigation – Deutsches Zentrum fűr Luft-und Raumfaht, Institut fűr Kommunikation und Navigation), leveraging on research funds provided by the German Federal Ministry of Economics and Technology (BWMi).
On the OEM side are Frequentis, Leonardo and Rohde & Schwarz. Leading the organisational realm are EUROCONTROL, SESAR 3 Joint Undertaking, ENAIRE, SITA, NATS and German air navigation service provider DFS Deutsche Flugsicherung GmbH.
In the context of the SESAR 3 Joint Undertaking, projects such as the Making I-CNSS a Reality (MIAR) and the Future Connectivity and Digital Infrastructure (FCDI) have played a key role in the exploration of the airborne capabilities of LDACS. Under the nationally funded German project MICONAV (Migration towards Integrated COm/NAV Avionics) a number of campaigns were conducted in March 2019 involving tests on MICONAV LDACS prototypes in DLR laboratories as well as flight trials consisting of six flights using DLR’s Dassault Falcon 20 aircraft at Oberpfapfenhofen airport near Munich. The campaigns, which involved the participation of DLR, Rohde & Schwarz, Bögl & Partner Systemtechnik GmbH, and iAd mbH, validated a number of functionalities including communication and navigation, ranging, handover, latency and traffic prioritisation.
Whereas it is safe to say that LDACS has attained unto maturity to a reasonable extent, quite a lot remains to be done, particularly with respect to perfecting the airborne component of the technology and fine-tuning the ground infrastructure. Issues surrounding the cost implications of LDACS adoption also require special attention, particularly from the perspectives of ANSPs, Data Service Providers (DSPs, Communication Service Providers (CSPs) and airlines. Although ICAO is expected to complete the standardization process by 2027, LDACS is not expected to become operationally ready until sometime in 2030. ◙
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