Today is the day that High Speed 5G cellular service is being launched by both AT&T and Verizon based on their very expensive Spectrum Auction wins at the FCC last year. The Aviation world including the FAA, commercial airline, and private aircraft communities, airport authorities, and others have fought this roll-out for the last several months, initially focusing on safety of life issues and more recently talking about massive disruptions in airline flight schedules. This has all been headline news with claims and counterclaims between the two major cellular providers operating in the contested spectrum band and the aviation industry.
For their part, the cellular providers point to over 40 nations who have successfully deployed 5G in the so-called C-Band spectrum while the aviation community counters with the fact that these countries have significant restrictions on the use of the band which until recently did not exist in the U.S. All this has made for a very confusing and contentious situation where the lack of information and the failures of the FCC and FAA to resolve their differences in a timely fashion while the cellular carriers delay their roll-out and alter their plans on an almost weekly basis. Today (January 19) is the day when all the hoopla finally comes to a head with AT&T and Verizon beginning to deploy their high speed 5G service minus any deployments within a few miles of a major U.S. airport.
That is the top-level state of play, but is there really a problem and going forward what should be done now? First, the unfortunate truth is that there is a real problem, but it is what can be described as an “edge case” problem, that is, a problem that only occurs in unusual circumstances. So, what is the problem? Fundamentally, the problem is a design issue with the aviation industry’s radio or radar altimeters. These are the devices that sense how high the aircraft is above the ground and especially in bad weather when ground visibility is limited, this is a crucial component of an aircraft’s ability to safely land.
Understanding the Technical Challenges
Now to get a little technical. The altimeters are supposed to operate in their assigned spectrum band between 4.2 and 4.4 GHz. Unfortunately, when these devices were originally designed, they had very low power neighbors, i.e., satellites beaming their information to the Earth from very distant orbits. Since the altimeters operate on a radar principle, looking for a signal reflected from the ground their receivers couldn’t detect the very low power neighboring satellite signals. This led the early designers of the altimeters to decide they really could ignore their assigned spectrum boundaries and as a result they allow transmitted energy far outside their band into the receiver. For decades this was not an issue given their quiet neighborhood, but with new neighbors now moving in (AT&T and Verizon), the spectral space that they were allowing into the receiver is now a potential problem.
Adding a little more technical information to the mix, AT&T and Verizon have now (as of 19 January 2022) commenced operation in the spectrum range from 3.7 GHz to 3.8 GHz, i.e., 400 MHz away from the altimeter band. To put this in perspective, the whole FM radio band (all stations) is only 20 MHz wide, so the spectral separation between the new 5G cellular band and the altimeter band is very, very large. The FCC for its part when granting the use of the band (which will ultimately be expanded to cover 3.7 to 3.98 GHz) determined that there shouldn’t be an issue because of the vast separation between the 5G cellular use of the new spectrum and the altimeter spectrum allocation. Unfortunately, this is not the case for old, technically “wide open” altimeters. These radar altimeters may send out a signal and be unable to discern the reflected signal because of energy from the far away 5G towers entering the receiver, causing the radar altimeter to either fail to function or possibly provide a false reading.
To make matters worse, though the altimeters were once only a standalone instrument that had an altitude indicator on the pilot’s panel, today the altimeter is highly integrated into the avionics for modern aircraft. If, for instance, the altimeter says the aircraft is still in the air when it has actually landed, it will cause the reverse thrusters and spoilers that normally create a rapid reduction in the airplane’s speed on the ground to not operate. I am told that in icy runway conditions the lack of reverse thrusters and spoilers could increase the landing distance by as much as four times, which for short runway airports (e.g., Chicago’s Midway Airport) with the potential for poor landing and runway conditions could be an enormous problem.
How Do We Get Out of This Mess?
First, most of the time the situation is not nearly as bad as the dire challenge the worst-case scenarios would suggest. Modern altimeters are well designed and do not have the problem of looking far outside their assigned band. The addition of a very low-cost component, historically a small piece of ceramic (called a filter) at the antenna input to the altimeter receiver, eliminates the issue of looking outside the altimeter’s assigned band.
Of course, retrofitting and certifying a new radar altimeter in an aircraft is a non-trivial expense in both time and dollars. Happily, most modern altimeters have filters and will not experience any 5G interference problem. The FAA is currently determining the nature of the altimeters on various aircraft and certifying those that have more robust designs. Those that don’t should be required to replace their altimeter or suffer a significant reduction in the weather conditions in which they are allowed to fly.
Given this small, but critical, step, the aviation world can be returned to a safe environment in the presence of 5G technology and AT&T and Verizon can be fully deploy their new C-Band systems including deployments around airports. As an important aside, while all of this turmoil has been proceeding, it should be noted that T-Mobile’s deployment of high speed 5G is currently unimpeded by these concerns since it operates in spectrum that is even further away from the altimeter band at 2.5 GHz.
Hopefully this article will help many of you to unravel this high-profile issue. I will appreciate hearing any questions or comments you may have on this important issue.
