Note: The above explainer video from Alex Scerri was added on 12/2/20
Is the impeccable safety record of aviation threatened by the use of C-Band frequencies (specifically 3.7-3.98 GHz in the U.S.) for 5G? Yes, would be the conclusion one could draw from RTCA’s November 30th, 2020 webinar, Interference Risk to Radar Altimeters from Planned 5G Telecommunication Systems.
The RTCA is recommending that the mobile wireless and aviation industries work with their respective regulators to take appropriate steps to mitigate the risk associated with the deployment of 5G in the C-Band. The question is what impact will this interference risk have on the rollout of 5G in the C-band?
As background, the RTCA, founded in 1935 as the Radio Technical Commission for Aeronautics, is a Standards Development Organization that works closely with the FAA “to develop comprehensive, industry-vetted and endorsed standards that can be used as means of compliance with FAA regulations.” The FAA’s Technical Standard Order TSO-C87a specifies requirements for radar altimeters. In turn, this TSO references the RTCA DO-155 Minimum Operational Performance Standards (MOPS) and the EUROCAE ED-30 Minimum Performance Specification (MPS) .
The Only Above Ground Level Sensor #
The radar altimeter is an important safety feature as it is the only sensor on an aircraft that measures Above Ground Level (AGL) altitude. The widespread use of these sensors in civil aviation began in the 1970s, following several deadly Controlled Flight into Terrain (CFIT) accidents.
It works by transmitting either a pulsed or frequency-modulated, continuous-wave radio frequency carrier and listens to the reflection of that signal from the ground. Operating in the Aeronautical Radionavigation Service (ARNS) from 4.2–4.4 GHz, it is just above the soon-to-be auctioned 3.7-3.98 GHz frequencies that will be repurposed for 5G.
The last update to the aforementioned radar altimeter requirements was in 1980 when the use-cases for the 3.7 to 4.2 GHz band were Fixed-Satellite Service (e.g. distribution of cable television programming from programmer to cable headend) and Fixed Service (e.g. point-to-point terrestrial communication links).* Due to the sparsely distributed, highly-directional, and relatively low-power nature of these services, interference with radar altimeters was not a problem.
RTCA’s Study – 5G Interference Risk to Radar Altimeters #
Enter 5G and the potential for interference with existing radar altimeters. In response to the March 2020 FCC Report and Order regarding opening up the 3.7 to 4.2 GHz for 5G use, RTCA established a 5G Task Force within a Special Committee 239 to investigate potential coexistence issues between 5G and radar altimeters. This task force consisted of representatives from industry, the Air Line Pilots Association, airlines, and the FAA. On October 7th, RTCA released their report (PDF) and submitted it to the FCC.
The core issue deals with potential interference either from fundamental emissions or spurious emissions. Fundamental emissions have to do with potential receiver overload due to lack of adequate out-of-band rejection of the 5G signals. The spurious emissions are potential frequencies emitted by 5G transmitters that would land in the 4.2-4.4 GHz range.
Three different radar altimeter use-cases were studied.
- Commercial airplanes used for passenger travel and cargo transport
- Business aviation, general aviation, and regional transport airplanes
- Both transport and general aviation helicopters
Within those use-cases, they study how the lateral distance and altitude between base stations and aircraft impact interference. Their investigation also examines the impact of User Equipment (e.g. devices with 5G wireless) both on the ground and onboard the aircraft.
Real-World Examples – Houston, We Have a Problem #
What is most interesting are the real-world cases they model. One of their models looks at the precision approach path to Chicago O’Hare International’s Runway 27L. They assume 5G upgrades of the existing five LTE base stations that are near this path. For all usage categories, there is the potential for radar altimeter failure due to interference (see page 76).
“In all cases, possibility of harmful interference in this instrument approach scenario is particularly dangerous given that up to the present time, radar altimeter failures during this phase of flight have been extremely uncommon, especially on Usage Category 1 aircraft.”
To mitigate its impact, why not just be more selective about where to deploy 3.7 to 3.98 GHz?
Well, that might work well for the O’Hare use-case, but it wouldn’t work so well for the Helicopter Air Ambulance (HAA) scenario. Flight paths are lower and more broadly distributed throughout a city with HAA, as compared to the O’Hare example. The flight paths for four Houston hospitals were studied and fundamental emissions were exceeded in all cases. In one case the interference is almost 40 dB above threshold,
“the radar altimeter(s) on the HAA aircraft would be completely inoperable, limiting the capabilities of these aircraft to operate safely and dispatch quickly to those in urgent need of medical attention.”
Fundamental emissions and spurious emissions from user equipment on the ground is not anticipated to result in significant operational impacts on civil and commercial aircraft. User equipment onboard aircraft in Usage Categories 2 and 3 aircraft, however, is anticipated to “introduce a significant risk of harmful interference to the radar altimeters used on these aircraft [as much as 47 dB – see page 87].”
An Uncertain Flight Plan #
Not discussed in the webinar or report is the potential impact on the nascent urban air mobility industry, which promises to greatly multiply the approximately 180k+ aircraft in the U.S. skies today. There may be a solution for future aircraft, as RTCA SC-239 is working jointly with the EUROCAE Working Group 119 to develop new radar altimeter Minimum Operational Performance Standards (MOPS). Still, it will take years for new designs to be in place as the new MOPS isn’t expected until October 2022. The FAA would then have to approve new equipment-level designs.
RTCA points out that new designs will not fix existing radar altimeters. It is not clear what possible retrofits, such as the addition of external bandpass filters, are possible. RTCA suggests that a retrofit, if possible, would take several years to properly validate and deploy.
RTCA recommends further research into risks and mitigations and offers to help industry and regulators (page 88 & page 89 PDF)
“Therefore, it is critical that the performance of radar altimeters which are currently in service across tens of thousands of civil aircraft be understood and the risks and operational impacts due to interference be appreciated based on the characterization provided in this report. Given the planned timeline for deployment of 5G systems in the 3.7–3.98 GHz band, these radar altimeters will be exposed to such risks and operational impacts if proper mitigations are not put in place…..The SC-239 membership will work with interested parties, both regulatory authorities and industry representatives, to develop any further analysis efforts or discussion of interference mitigation approaches as needed.”
It will be interesting to see how the potential interference risks raised by RTCA impact the rollout of 5G in the C-Band.
*In 2012 the FAA created TSO-C87a, which supersedes the 1966-released TSO-C87 and references the 1980-released EUROCAE ED-30 MPS.
Disclosure: In addition to being Managing Editor of the Viodi View, Ken Pyle is an Airport Commissioner for SJC. This article is attributed solely to his role as Managing Editor of the Viodi View and is not reflective of his role as Airport Commissioner.
Leave a Reply