Striking a Balance for Spectrum Needs: 5G Communications vs. Aircraft Altimeter Operations

At this point, you've likely seen a slew of mainstream news articles about 5G causing safety concerns around air travel. In fact, ahead of the rollout of new 5G services from major US telecom companies including Verizon and AT&T on Jan. 19, 2022, many international airlines canceled or delayed flights to major US airports where they believed 5G signals could possibly interfere with the radar signals required to properly operate landing equipment on their planes.

To understand why these airlines took these actions, let's first look at the root of their concerns. In late 2020 into early 2021, the United States Federal Communications Committee (FCC) held Auction 107 to re-allocate bandwidth from 3.7GHz to 3.98GHz for mobile telecommunication operations. This portion of the spectrum is known as the C band by the FCC and the telecom industry, which can be confusing because the IEEE radar band known as the C band covers 4GHz to 8GHz. The 5G NR FR1 bands that fall in this range are n77 and n78.

Since airplane radio altimeters, and the systems that rely on them, operate from 4.2GHz to 4.4GHz, organizations ranging from the United States Federal Aviation Administration (FAA) to pilots' unions to individual airlines voiced concerns to the FCC that 5G telecom operations in the C band may cause interference with these devices. Prior to this auction, this was not a concern because the frequencies from 3.7GHz to 3.98GHz were dedicated to low-power satellite communications (SATCOM).

With this auction, only 220MHz of guard band was left between these new 5G operations and the bandwidth used by pilots to determine an aircraft’s height above the ground, which is extremely critical functionality for low-visibility landings. When evaluating this risk, the FAA determined that even with this guard band in place, there was still the potential for interference that could prevent engine and braking systems from transitioning to landing mode, making it difficult or impossible for an aircraft to stop on a runway. 

But just how likely is it that 5G signals in the C band will interfere with airplane altimeters? The answer to this question is proving to be complicated for the telecommunications and avionics industries as well as their respective regulatory bodies to determine. For starters, the spectrum emitted by an interferer, in this case the 5G base stations, is regulated in terms of emission spectrum. This means that a radio that passes certification for use in the US meets standards based on the 3GPP specifications, but further clarification would be required to determine if a base station within these emissions limits would interfere with radio altimeter functionality.

The power of that spectrum at a receiver is dependent on the distance from the transmitter to the receiver. In this instance, KNOWLES receiver is a radar-based altimeter on a quickly moving airplane, making it challenging to measure the sensitivity of the receiver to the spectrum in question. Another challenge with determining the potential for interference is that there are a variety of radar altimeters currently used in planes that have been built over the last three decades by various manufacturers. Since this portion of the C band was previously used for low-power SATCOM applications, altimeter manufacturers did not necessarily need to use high-performance filters. Therefore, depending on the level of filtering included in these devices, the altimeter may or may not pick up signals below 4.2GHz that would confuse the system.

Can 5G Communications and Avionics Coexist?

While these concerns were made known prior to the FCC auctions, as the deployment dates for these 5G networks neared, the aviation industry put pressure on the FCC to take action. The rollout of 5G at these frequencies was initially scheduled for December 2021, but the FCC worked with carriers to delay deployment to give altimeter equipment manufacturers more time to evaluate data from wireless companies regarding their signals. Having this data gave manufacturers time to determine if they believed each of their altimeter models could continue to receive signals safely without interference from signals emitted by the new 5G base stations. While this is still a work in progress, as of Jan. 26, 2022, 90% of US commercial aircrafts are using one of the 20 cleared altimeters and are approved for operation at airports near new 5G deployments.

Additionally, as mentioned above, the power level of the signals seen by a receiver depend on how close the receiver is to the transmitter. Therefore, wireless carriers also agreed to create temporary buffer zones for the next six months that limit the proximity of their broadcasts relative to specific airports that were identified as commonly having low-visibility approaches.

Working through this potential interference issue is critical because 5G is not going anywhere. In fact, the demand for 5G communications, and future generations of wireless communications, is only going continue to expand globally. So, what can be done to prevent these issues? The short answer is that altimeter manufacturers need to implement high-quality filtering in these devices. This will be a design change for many manufacturers because when this portion of the C band was used for satcom operations, the 200 MHz of guard band allocated by the FCC was sufficient. But, with ground operations of 5G signals now occurring in this portion of the spectrum, altimeter manufacturers should consider adding quality high-performance filters to their devices to ensure signals below 4GHz will not be detected. In this guide, the International Telecommunication Union (ITU) does include recommendations for protection levels for radio altimeters that can help prevent receiver front end overload, degradation of sensitivity, and false altitude readings.