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shorter wavelengths (this has become an important consideration for cel- lular providers). The most practical demonstration of wavelength capabil- ity is the scratchy and hollow audio you receive from AM radio compared to the crisper sound produced by FM. The upshot of low-fidelity long-wave is range. Cruising at FL280 at night, you can use an ADF to tune an AM radio station from Albuquerque while over Montana. Over the middle of the Pacific, the quality of the audio is going to be lousy (HF utilizes a longer, scratchier wavelength), but it is nonetheless available.
In the early era of cellular services, relatively long wavelengths reigned supreme. Pricey cellular towers could cover a large area of customers who were hardly consuming any data. Early cell phones were essentially walkie-talkies connected to tradition- al phone services via cellular towers. Flash forward a couple of decades and the device in your pocket is a mini computer more capable than the million-dollar mainframes which got us to the moon. The breathtak- ing advance in electronic capability has rapidly increased the thirst for faster download speeds. Millimeter wavelengths suddenly became worth billions. Before all of this happened, there was quite a bit of elbow room around the short wavelength. Radio altimeters had a nice little monopoly.
With cellular customers in the bil- lions worldwide, there are two limita- tions to data speeds. The first is that only so much total data can be trans- mitted over the range of spectrum available to cellular providers. The second is that interference between cellular towers determines how many customers will have to share a spec- trum in a certain location. Here is where short wavelength technology produces a neat trick. Not only can more data be transmitted per second, but the limited range of the millime- ter spectrum also means that fewer consumers are fighting for data per cell tower. And this is how aviation and telecommunications finally man- aged to butt heads.
5G spectrum is now the shortest wavelength being offered to cellular
customers. It represented $81 billion worth of purchases by cellular provid- ers in the auction last year. Corpora- tions do not drop that sort of mint to sit on an asset. As such, 5G is being rapidly deployed by cellular provid- ers worldwide. The spectrum closely abuts the wavelength that radio al- timeters use. The separation is tight enough that the FAA became con- cerned (somewhat at the last minute) that 5G cellular towers would induce false radio altimeter readings in air- craft. If you have flown much over the past few months, you have undoubt- edly come across NOTAMs limiting the use of certain ILS procedures as a result of the still uncertain potential for 5G to corrupt aircraft systems.
This has mostly created a headache for approaches that determine mini- mums via the radio altimeter (such as CAT II and III). But on many aircraft the radio altimeter is utilized by other systems as well. Enhanced Ground Proximity Warning Systems (EGPWS) routinely utilize the radio altimeter to trigger terrain and wind-shear alerts. In some aircraft, gear and flap warnings also require a function- ing radio altimeter. Traffic Collision Avoidance Systems (TCAS) can be impacted as well. Many of the devic- es aboard technologically advanced aircraft are now compromised to an unknown degree by frequency shar- ing with 5G. In bygone eras, aviation safety would have taken precedence around airports. Yet when it comes to cellular services, the pure force of hundreds of billions in spectrum auc- tions has placed the supremacy of avi- ation behind that of 5G deployment.
Procedural Dilemma
Threat and error management (TEM) combined with a forward- looking briefing is a good technique for pilots to utilize during a flight. An airport with a 5G NOTAM indi- cates a location where radio altimeters may be compromised (if your aircraft does not have a radar altimeter, the presence of 5G interference is large- ly inconsequential). In an aircraft equipped with radio altimeters, spu- rious terrain alerts may be encoun- tered. Windshear detection may not be
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August 2022 / TWIN & TURBINE • 5