Thomas Bögl, Rohde & Schwarz
Technical progress is not always about replacing analog technology with digital technology. Civil aviation provides an excellent example of how technology can advance. Voice communications between pilots and the tower still use double-sideband amplitude modulation (DSB-AM) that dates back to 1948. No one wants to replace this analog VHF technology (frequency range: 118 MHz to 137 MHz) with digital voice communications. Ultimately, analog radio has clear benefits that remain critical today, these include short delays, active monitoring by all participants and other features that digital radio still cannot replace.
Data transmission waveforms are a different story. Since the 1990s, aeronautical radio has involved more than just voice communications and has been expanded to include digital data transmission methods. VDL Mode 2 is the most common one.
Digitalization of aeronautical radio today means replacing older digital data transmission methods with new techniques offering better performance. Voice communications will continue to use analog technologies but digitalization will help to relieve some of its burden on the spectrum. This coexistence of analog voice communications and digital data technologies in air traffic control enables extremely high efficiency, reliability and operational safety that are both economical and future-proof.
VDL Mode 2 is provided by air navigation service providers (ANSP). ANSPs are similar to mobile network providers in the private sector. Because VDL Mode 2 also operates in the VHF frequency band, the available spectrum must be shared between data transmission and voice communications. The usable bandwidths are relatively small, which means VDL Mode 2 can only transmit data at a few kilobits per second.
This is where the new L band digital aeronautical communications system (LDACS) comes into play. LDACS offers data throughput that is up to 200 times faster than VDL Mode 2. As the name implies, the key innovation here is data transmission in the L band. LDACS specifically uses portions of the frequency band reserved for aeronautical radio. LDACS uses interference suppression algorithms and is optimized for minimal out-of-band emissions and trouble-free operation alongside other aeronautical equipment operating in the L band.
In the future, navigation data for flying around noise protection zones can be transmitted automatically via an LDACS data link. Up to now, this has been done via voice communications.
Modern aviation demands the secure exchange of data. Otherwise, organizing air traffic would be impossible. LDACS reliably enables high data throughput that is secured with encryption. It supports a wide variety of new applications, such as the data links airlines need for flight crews to efficiently plan fleet operations. There are far more aircraft flying nowadays and air traffic controllers need to be able to distribute new navigation data more quickly to adjust flight paths in time when the situation changes in their airspace.
One of the first new LDACS applications uses its data link capabilities to extend or even replace slow VHF radio links in existing ATC systems with fast LDACS links. This will quickly and noticeably speed up data transmission without requiring the purchase of new systems. Another factor in favor of LDACS is its very simple path forward to a market launch.
Reliable and secure LDACS data transmission will also be used for new environmentally friendly navigation methods. The plan here is to select aircraft routes using precise coordinates in three-dimensional space that must be passed within rigidly defined time limits. The 4D trajectory concept can be applied when aircraft need to be dynamically routed in the vicinity of airports to avoid noise protection zones.
The long-term goal is to manage the complete flight path from the departure airport to the destination exclusively with 4D trajectories, allowing for much more efficient and environmentally friendly use of airspace.
In the development of LDACS, four major research projects have played a key role. They are listed below the timeline and described in more detail in the info box further down the article.
Like all other systems on board aircraft, transceivers are also subject to international standards. Permits to fly are issued based on these standards to make sure that the technology on board is reliable and satisfies all air traffic safety criteria. The approval process is usually very onerous, taking anywhere from ten to twenty years. Since most companies cannot afford to wait this long, government research initiatives have been established to ensure industry involvement in the standardization process as early as possible.
The German Federal Ministry for Economic Affairs and Climate Action has launched the aviation research program (LuFo) for LDACS. Rohde & Schwarz is part of the program and is joining forces together with project partners such as the German Aerospace Center (DLR) to cooperatively develop LDACS technology.
Four successive research projects sponsored by the Federal Ministry for Economic Affairs and Climate Action (BMWK) have served to significantly advance the development of LDACS digital data transmission technology for civil aviation.
Along with the German Aerospace Center (DLR) and other project partners, Rohde & Schwarz worked within a consortium to develop a functional demonstrator for lab tests from 2012 to 2015.
The ensuing MICONAV project delivered proof of operational reliability dur- ing flight operation. Between 2016 and 2019, an airworthy demonstrator was developed that also included highly accurate navigation functions. In subsequent lab and flight tests, it was possible to successfully test logging on and off four LDACS base stations that were specially built for the project as well as the handover from one ground station to another. Communications between the aircraft and the ground station functioned just as reliably in various situations such as flying overhead at a high altitude and during landing, takeoff and taxiing at the airport.
This project (2019 to 2022) extended the capabilities of the waveform to include direct connectivity between aircraft. This means that LDACS is no longer strictly bound to terrestrial infrastructure and can also be used for flight paths over oceans.
The PaWaDACs project has been underway since 2022. It is focused on miniaturizing the equipment demonstrators and preparing a realistic test installation at two locations operated by the German air navigation service provider (DFS). Once this work is completed, the key technological steps needed for future LDACS systems will be securely in place. A solid basis will thus be available for developing new products for civil aviation. Rohde & Schwarz will not only be able to supply equipment for ground stations, but will also be well-positioned to offer solutions to its customers for use on board their aircraft.
Working groups are collaborating with the International Civil Aviation Organization (ICAO) to create and implement flight acceptance standards. Headquartered in Canada, ICAO is the highest standardization authority for civil aviation. It examines and integrates the most important documents in the standardization process.
As part of the recently launched LuFo project PaWaDACs (see information box), the technological cornerstone has been set, allowing product development of LDACS ground and onboard equipment starting in about 2025. The market launch is expected for 2028. Thanks to its early involvement in LuFo projects, Rohde & Schwarz has a competitive position in the race for market leadership among LDACS suppliers.
