If you have been following the 5G rollout in the US (real 5G, not just whatever your cellular carrier is trying to market), you have likely heard about the conflict between 5G devices and radio altimeters on airplanes. This post is meant to provide some working knowledge on the tech specifics, and while I am trying to be accurate and precise, it is geared more to helping understand the core issues without getting overly deep into technical and regulatory particulars.

To understand the issues causing the conflicts it helps to first understand the radio altimeters in airplanes that are the primary item of concern. As the name implies, a radio altimeter uses radio waves to determine the altitude of an airplane in flight. Fundamentally, it works like radar, broadcasting a signal and measuring the time for the signal to hit the earth and bounce back to the airplane.

A frequency range of 4200-4400 MHz (4.2-4.4 GHz) was defined as a shared band, recognized internationally, for primary use in radio altimeters. As noted in the linked paper, the band was selected based on benefits of the frequency range being able to penetrate clouds, and due to it being free from surrounding interference at the time it was selected (the paper references 1997, other data indicates it was in use before that, but in all cases, it was allocated and put to use long before cellular data was as prevalent as it is now). Additionally, the high frequnecy range can provide very precise measurements, and permits the equipment to be relatively compact and lightweight, a key consideration for aviation electronics.

Radio altimeters are a common, if not mandated, piece of equipment on multiple types and classes of aircraft, including commercial passenger planes. The devices work well and provide critical data to pilots and to other aircraft systems that ensure safe flight.

5G cellular data is allocated the spectrum range of 3700-3980 MHz. As it is clearly apparent, there is no overlap in allocated spectrum between 5G and the radio altimeter band, in fact there is a fairly wide 20MHz gap between the upper end of the 5G spectrum and the lower end of the altimeter spectrum. 5G cellular data utilizes a frequency range previously assigned to C-band satellite communications (3.7-4.2GHz), which was reallocated as part of the FCC's Auction 107 - 3.7GHz Service. The auction summary notes that the 3980-4000 MHz range would not be auctioned, and would instead be a guard band, a process used to minimize interference between different types of services or applications using frequencies in close proximity to each other. 20MHz is essentially the entire spectrum of the FM radio allocation (87.5-108 MHz) in the US, though there are many differences between the relatively low frequency FM signals and the higher frequency 5G spectrum that make it hard to directly compare the two, but the main point is that there is a decent gap between the 5G and radio altimeter allocations.

At this point you may be wondering then how two non-overlapping RF spectrum bands are interfering with each other in the first place. First, as various statements from the FAA show, not all aircraft altimeters are affected, only roughly 10-25% of altimiters are prone to 5G interference. Second, the interference is mostly only present in low-altitudes near locations that have deployed, or proposed to deploy, higher power output 5G equipment, frequently in more rural areas. Thus, this is not a scenario of 5G equipment universally impacting aircraft, but more of a case of some equipment affecting some aircraft in particular areas.

A major contributor to the problem is that many of the altimeters in use were designed and deployed well before the anticipation of ground broascast of frequencies in nearby RF ranges. When the 5G spectrum was utilized for C-Band satellites, the RF signals came from satellites far about the aircraft, transmitting a relatively low power signal down to earth. This arrangement provided some natural shielding of the C-Band singals relative to the downward-facing altimiter antennae, which helped to reduce interference. Now, with that spectrum re-allocated to ground-based transmitters, the aircraft are dealing with signals coming more directly into the altimeter receiver, and while those signals are outside of the allocated frequencies for altimeter usage, the affected altimeters do not appear to have adequate noise rejection capabilites to prevent those signals from overwhelming their receiver circuits, which causes bad data.

Who is at fault for these issues? Depending on your perspective, the fault could lie with the FAA, the altimiter designers, or the 5G equipment manufacturers and cellular customers. Or, it could be nobodies fault specifically and more of a scenario of small things all stacking up to be a big problem, which is probably the more realistic perspective.

Finally, what does this mean for the 5G rollout, particularly as it relates to security applications? From what we can see so far, the answer to this question is "not much". For the most part, the FAA, the airlines, and the companies deploying 5G equipment are working together to minimize the impacts, and to make their respective equipment better suited to co-operate. Low-power 5G equipment has not been cited as a source of interference to the altimeters, which is the bulk of the equipment being deployed, particularly in the more dense areas. Areas that do not have large airports in their vicinity are also relatively unaffected, as low-altitude flight in these areas is not common. Also, aircraft with legacy altimeter equipment are being slowly upgraded with newer units that can better reject frequencies outside of their allocated RF spectrum, which is a good idea, as these devices probably should not be able to be easily disabled by unexpected interference.

If your organization is looking at utilizing 5G for security communications, there are certainly advantages, and drawbacks, to this somewhat new technology, but concern over FAA or FCC actions suddenly halting or delaying 5G rollouts should not be a factor in consideration.