Compatibility and Standards Key to Introducing New Aviation Spectrum Applications in US

A FedEx aircraft lands at Memphis International Airport, which featured one of the first deployments of domestic CPDLC use in the US air traffic system. As aviation stakeholders develop next generation spectrum dependent aviation applications, how will the industry adjust? Experts attempted to answer this question during an Aug. 19 RTCA webcast.

The introduction of new wireless applications and services for communications, navigation and surveillance on aircraft flying within U.S. airspace will require some unique decision-making and collaborative spectrum sharing and re-purposing by government and industry stakeholders, according to experts featured on the Aug. 19 Radio Technical Commission for Aeronautics (RTCA) webcast "Spectrum: Generating Change in Aviation.”

As the commercial airline industry slowly continues to increase daily flying, the slowdown in air traffic has actually presented the opportunity for longer term considerations in establishing a strategy for ensuring aviation’s use of existing spectrum in the US is optimal and the introduction of near term applications like 5G and L-band Digital Aeronautical Communications System (LDACS) can be adopted with aircraft radio interoperability and spectrum system compatibility in mind. There is also a need to consider how the design and development of new navigation systems and radios can process and utilize signals from newer satellite navigation systems such as China’s Beidou, Russia’s GLONASS and eventually Europe’s Galileo system free of interference or spectrum compatibility issues.

The current state of aviation spectrum in the U.S. is fluid, and the Federal Aviation Administration (FAA) is already the nation’s second largest government agency user of spectrum. Prior to the impact of the COVID-19 pandemic on flight operations, the agency had introduced the domestic use of VDL Mode 2 CPDLC data communications to offload the growth in voice communications traffic brought 2 percent annually up until February 2020.

Among the most widely reported aviation spectrum issues regionally is the 5G network deployment plans put in place in April, when the Federal Communications Commission (FCC) allocated the use of C-band spectrum to Ligado, formerly known as Light Squared Corp. FCC officials are requiring the company to protect aircraft radio signals from interference by reporting the locations of their base stations and technical operating parameters.

However, an aviation industry coalition has raised concerns. In May, the coalition of industry groups raised concerns that the FAA’s admission of its testing of potential interference being flawed needs to be revisited before moving forward.

But the Ligado situation speaks to a broader problem that the air transportation industry in the U.S. and other regions will continue to face moving forward; Competition for sharing and repurposing spectrum to support growth in aviation and non-aviation spectrum use for new applications.

“There are no unused frequencies left,” said Andrew Roy, director of engineering services for Aviation Spectrum Resources Inc., whose organization was part of the coalition that filed a May 23 FCC petition against Ligado.

“All spectrum is currently used at the moment. If you have a new wireless application, and therefore you need new spectrum, it’s not actually new, you’re either repurposing it for another industry or sometimes convincing the regulator your priority is higher than someone else’s – or you can find a method of sharing with those existing industries to make sure it works. As a result of this a lot of industries are in competition with each other to ensure they can either protect their existing spectrum, or promote new wireless applications so they can continue to expand and grow as all industries do.”

An aviation industry coalition filed a petition with the FCC in May to advocate further consideration of Ligado's 5G network impact on aviation radio transmissions.

Efforts have already been undertaken by the FAA to repurpose some of the spectrum it has been assigned in support of safety of life transmissions for several decades. AeroMACS, a WiMAX system the enables wireless broadband connectivity use by aircraft terminals on airport surfaces, is one of the biggest current examples of how the industry is accomplishing this. It supplements the FAA’s aging cable loop system that supports airport vehicles and runway visual range among other airport operations applications to give airplanes the ability to use an airport-specific network for offloading maintenance data upon landing.

Introducing AeroMACS has required some repurposing of the 5 GHz band historically used in the U.S. in support of the microwave landing system (MLS).

“When regulators consider new allocations, the considerations should be, what will be the impact to incumbent systems, that are co-channeled or adjacent channeled,” said Michael Richmond, manager of the Spectrum International and Planning Team, Spectrum Engineering Services Group at the FAA. “What is the compatibility with incumbent systems? Is it feasible?”

Some of the key considerations for aviation spectrum use at the avionics manufacturing level are also becoming complicated as suppliers try to bring emerging spectrum-dependent technologies onboard airliners. In April 2019, the German Aerospace Center (DLR) tested the world’s first prototype LDACS system, using a Dassault Falcon jet modified with an LDACS transceiver. LDACS is enabled by the use of an L-band receiver that uses L-band spectrum to provide faster digital data transmissions exchanged between aircraft and ground stations than what the band has provided upon first being introduced as a cockpit safety services medium.

It is another communications medium that could offload voice communications between pilots and controllers in Europe, the U.S., and other regions. The challenge comes when developing new LDACS receivers for commercial use across a range of different aircraft types while ensuring the signals processed by that receiver do not impact other aviation L-band or 960-1215 MHz signals.

The 960-1215 MHz range is also used for; Automatic Dependent Surveillance-Broadcast (ADS-B)’ Tactical Air Navigation (TACAN); Secondary Surveillance Radar (SSR) and military Identification Friend or Foe (IFF) transmissions. This requires not only the considerations for potential signal interference, but also the need to establish what the standard criteria for that potential interference actually is and how suppliers can mitigate or eliminate it.

“In the 960-12815 MHz band, I keep coming back to that frequency slice because pretty much every system in that band today is a pulse system,” said Dr. Sai Kalyanaraman, a technical fellow at Collins Aerospace, where he oversees GNSS receiver design and development.

“If you throw a spectrum analyzer at it and you go across 960-1215, you see continuous peak above the noise floor there, there’s a reason that spectrum has been served that way, because new applications have to be competitive with all the other aviation sources in that spectrum,” he added.

Kalyanaraman believes that the introduction of next generation applications like LDACS will require additional spectrum needs by aircraft radio and transceiver manufacturers in the future. He referenced the advancement of Enhanced Flight Vision System (EFVS) for enabling lower landing minimums by restoring lost pilot visual cues in degraded visual condition.

According to Kalyanaraman, an STC for EFVS upgrades owned by Sierra Nevada Corp. applied for the use of the 94 GHz band in support of their upgrade’s spectrum utilization for fusing GPS information with high definition visual landing guidance for pilots on approach into airports. That is just one example where an applicant has already gone outside of the traditional aviation spectrum bands to 94 GHz, where no current aviation spectrum users exist, Kalyanaraman said.

Now that China has completed all satellite launches for Beidou, its GPS-like domestic satellite navigation system, a future focus for aircraft navigation systems makers will be the allowance of radio interoperability with Beidou, GPS, Galileo and GLONASS.

“If you look at Beidou for example, as long as that system is able to be added as an additional constellation to the existing set of GPS or Galileo and or GLONASS constellations, that would be relatively simpler and easier going forward to be able to harmonize it and have common standards and include that system as part of that common baseline from a radio perspective,” Kalyanaraman said.

Commercial airline pilots could also see major benefits as a result of the introduction of LDACs, 5G, AeroMACS and other next generation wireless aircraft communications applications. The Airlines Electronic Engineering Committee (AEEC) of the SAE Industry Technologies Consortia's ARINC Industry Activities (ARINC-IA) program recently approved data link standards changes to ease avionics interfaces with newer L-Band satellite and other Internet Protocol (IP) equipment for improved air-to-ground communications.

During the October 2019 AEEC mid-term session in Seattle, the committee approved updates to existing avionics standards that will enable future spectrum dependent use of new IP and satellite-based aircraft communication links.

David Sambrano, chief technical pilot communications for United Airlines said that as pilots become increasingly more dependent on connected electronic flight bag (EFB) applications, adopting 5G is a near term goal.

“We as the end user want to be able to take advantage of a lot of those technologies, 5G being one of them, Internet Protocol being another one, and we need to explore ways to get a lot of this data and communication onto and off of the airplane,” Sambrano said. “Looking to the future any new entrants into aviation or any new types of technologies that may come around really need to look to make sure those new technologies do not interfere with a lot of the equipment we have right now and as we look to the future i think one of the things we need to do is look at updating the standards related to each piece of equipment that we have onboard so that we as the end user want to be able to take advantage of a lot of those technologies.”

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