LDACS Implementation and Validation

DLR was the first to build a functional LDACS demonstrator. After DLR's work on the LDACS demonstrator, industry has taken over the further LDACS prototype development. Frequentis AG and Leondarod SpA are building interoperable LDACS demonstrators within SESAR and Rohde & Schwarz within the German national projects ICONAV (Integrated COmmunication and NAVigation functionality for sustainable L-band use) and MICONAV (Migration towards Integrated COm/NAV Avionics). These industrial LDACS demonstrators will be used for future compatibility testing and flight trials.

LDACS Demonstrator Developed at DLR

In the development of LDACS, testing and verification plays a crucial role. Therefore, DLR has developed an experimental setup for the generation and processing of LDACS radio signals in the laboratory, which we call the LDACS demonstrator.

It consists of a transmitter and a receiver setup, both comprising an analogue frequency conversion and amplification unit, as well as a digital signal processing unit.

Figure LDACS Demonstrator

The signal processing system contains D/A and A/D converters to interface with the RF frontend at an intermediate frequency of 30 MHz, and uses FPGA technology for signal processing in the digital domain. With the FPGA design developed at DLR, any LDACS signal according to the published specification can be generated. The RF frontend handles analogue signal amplification and frequency conversion. It can be tuned to channels in the aeronautical L-band between 985 MHz and 1072 MHz with a 500 kHz channel spacing. Thanks to its duplexer filters, it permits full-duplex operation with frequency-division duplexing (FDD) as per the LDACS specification.

The LDACS demonstrator has already been used successfully in tests of the LDACS communication performance as well as in compatibility tests with legacy aeronautical radio equipment. The latter were conducted at the laboratories of DFS, the German ATC authority and investigated interference between LDACS and the navigational Distance Measurement Equipment (DME). As a first result, it could be confirmed that LDACS can operate under severe interference from DME. Secondly, important data has been collected for the establishment of the necessary separation criteria which warrant the correct operation of both systems.

Further technical details on the implementation of the LDACS demonstrator and on the compatibility experiments have been published in several papers presented at international conferences. .

Figure LDACS Demonstrator 2

LDACS Prototype Development in SESAR and SESAR2020

The development of LDACS has already made substantial progress in the Single European Sky ATM Research (SESAR) framework, and is currently being continued in the follow-up program, SESAR2020.

A key objective of the SESAR activities is to develop, implement and validate a modern aeronautical data link able to evolve with aviation needs over the long-term. To this end, an LDACS specification has been produced and updated; transmitter demonstrators were developed to test the spectrum compatibility of LDACS with legacy systems operating in the L-band; and the overall system performance was analyzed by computer simulations, indicating that LDACS can fulfil the identified requirements.

Building on the experience gained in the first phase of SESAR, SESAR2020 developed an integrated suite of CNS solutions to meet the operational requirements of the ATM system in the short, medium and long term, with technologies that are consistent with the European ATM Master Plan and the ICAO Global Air Navigation Plan.

Within SESAR2020 PJ14 the (sub-)project PJ.14-02-01 "FCI Future Terrestrial Data Link" had the objective to develop and standardize the future terrestrial data link system LDACS.

The goal of PJ.14-02-01 was to progress with the development and validation of the LDACS technology. This included security and digital voice concepts, and contributed to the development of a harmonized global standard.

To achieve these, the project developed and verified fully functional and interoperable LDACS prototypes built by Frequentis AG and Leonardo SpA and assessed the impact on other systems.


LDACS Prototype Development and Flight Trials in the MICONAV Project

MICONAV is a research project that received national funding from the research program LuFo (Luftfahrt-Forschungsprogramm) of the German Federal Ministry of Economy and Energy (BMWi). The project was led by Rohde & Schwarz GmbH & Co. KG. As project partners, DLR and two German SMEs, BPS (Bögl & Partner Systemtechnik GmbH) and iAd (Gesellschaft für Informatik, Automatisierung und Datenverarbeitung mbH), actively contributed to the project.

The goals of the project MICONAV (Migration towards Integrated COM/NAV Avionics) were twofold: First, a fully functional LDACS demonstrator as defined within the LDACS specification was developed and realized using industrial development methods. Second, LDACS ranging functionality was developed and implemented to support alternative positioning navigation and timing (APNT). Utilizing LDACS for navigation in addition to communication offers favorable synergy effects and minimizes costs: having deployed LDACS as a communications system, no additional infrastructure will be needed for APNT.

MICONAV built on the results of two previous projects: ICONAV and LDACS-NAV . It was conducted in liaison with SESAR2020. The results of the project were communicated to interested organizations such as EUROCONTROL and ICAO.

The LDACS prototypes built in the MICONAV project were integrated and tested in the DLR laboratories.

The image below shows the prototypes during the EMI tests and before flight certification.

Lab Testing of Maximum Data Rate

The maximum data rate of LDACS was measured in the lab before the prototypes were integrated into the aircraft. The expected maximum data rate was calculated from the LDACS specification. The expected data rate of the forward link (FL) was 291kbit/s and 238 kbit/s for the reverse link (RL).

The measured maximum data rate of LDACS in acknowledged data transmission mode (ACK) using QPSK with coding rate 1/2 (the most basic mode supported by LDACS) in the lab was approximately 298 kbit/s for the FL and 200 kbit/s for the RL. The data rate used for the flights, defined after the lab measurements, was set to 100 kbit/s with a duty cycle of 110 of 165 tiles.

The figure below shows the goodput performance measure, i.e., the successful received data packets of the FL and RL, in the ACK mode. The upper curve shows the sum of all packets while the lower curves split the packets into different priorities. It can be seen in the table, as well as in the graphics, that the high prioritized packets (priority 0 and 1) are preferred over the lower prioritized packets (priority 2; downward going green curve in the Figure). The MICONAV implementation used hard priorities. A more advanced implementation might use a more sophisticated scheduling mechanism.

Test Flight Routes

The MICONAV flight measurement campaign consisted of six flights with DLR's Dassault Falcon 20 aircraft, all of which started at Oberpfaffenhofen airport (EDMO). Each flight had a special focus, nevertheless, the communication and navigation experiments where conducted and recorded on all flights. The total time in the air was 12.9 hours excluding the times for taxi and on the apron. The total distance flown was 8105 km. One flight route is shown as an example below.


The handover tests pursued took the form of hard handovers with explicit disconnect and connect commands. In total, 99 handovers were demonstrated between AS to GS1 (OP) and AS to GS3 (SM) during the flights. The figure below shows 79 handovers in one sequence of flight 5. The figure shows the two return links from the Aircraft to the Oberpfaffenhofen ground station (GS1) and to the Schwabmünchen ground station (GS3).


The range measurements were evaluated with a long distance flight. An expected range of 99.3 km was calculated from the link budget of the LDACS specification with 40dBm EIRP TX power, as used for the flight trials.

The measurements showed 133.6 km between AS and GS3 (SM) and 137.5 km between AS and GS1 (OP) when employing a 40dBm EIRP transmit power from the ground stations. The figure below shows the link state of the long range flight at flight level 350 (altitude of approx. 10660m). The operational ranges of the two ground stations are visualized as the grey circles.

Latency and Prioritization of Large Packets under Heavy Load

The latency analysis was performed in several flights. First, latency was characterized in low altitude (5800m) and a constant distance to the GS, later it was characterized in high altitude (10660m). The data traffic contained high and low prioritized data packets. High priority traffic is scheduled first and is therefore expected to experience lower latency.

The evaluation showed that the LDACS scheduling prioritizes high priority traffic over low priority traffic during bursts. The figures below show the high priority and the low priority latency results for packets of constant size of 1,400 Bytes and constant data rate of 100 kbit/s in each direction. It can be seen that the high priority packets experience lower latency as the low priority packets. Note that LDACS offers more than the two priority levels used for illustration in the flight trials.

High priority latency for 1400B at 2 x 100 kbit/s

Low priority latency for 1400B at 2 x 100 kbit/s

LDACS automatically retransmits corrupted packets and there is almost no packet loss after retransmission. As a result, the packet loss rates for FL and RL are 0.31% and 0.82%, respectively, when the link is up. Retransmissions are included in the latency measurement.

Demonstration of ATM Applications over IP

In addition, LDACS capability to support existing and future ATM applications was demonstrated with experimental secure implementations of ADS-C, CPDLC, and GBAS running on top of IP over LDACS.

As a prerequisite for this we tested a post-quantum key agreement protocol via LDACS for data encryption on post-quantum secure levels. All of the demonstrations described below were thus secured in a (Post-Quantum) secure manner and without application specific security measures.

Secure CPDLC and Secure ADS-C

LDACS was used to demonstrate a secure implementation of Controller Pilot Datalink Communication (CPDLC). The application was used to send written instructions to the flight crew in the flight trials. In addition, it was also used to send regular position updates (in the spirit of a secure ADS-C implementation) to a ground based application displaying the position of the experimental aircraft on a moving map.

Secure GBAS

GBAS makes available correction data for satellite-based positioning systems, like GPS. The correction data is derived from the position estimates of a GPS receiver which location is known very accurately. By comparing the exact GPS receiver position with the position estimates, the correction data is obtained. GBAS transmits the correction data to incoming aircraft and, thus, allows localization and positioning in the order of decimeters.

Currently, GPS correction data is transmitted to aircraft via VHF Data Broadcast (VDB). The increased data capacity of LDACS compared to VDB enables transmission of additional correction data for other global navigation satellite services, like Galileo. In addition, GBAS correction data transmitted via LDACS is protected by security means, an important asset to secure aircraft positioning especially during landing. Secure GBAS via LDACS was demonstrated during the MICONAV flight trials. The actual achieved accuracy was 1.61m for VPL and 0.94m for LPL without any post-processing.

Secure Digital Voice

Digital voice may be an important service in the future air traffic management system. LDACS natively supports digital voice channels. In addition, LDACS is also able to transmit any form of digital data. It has therefore been demonstrated how digital voice can be transmitted via IP/LDACS. This has been accomplished by the use of commercial applications configured appropriately.

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