First, what is a radar altimeter or “RA”? RAs are radios that emit a microwave signal downwards and receive a bounced return signal. Based on the time taken to receive the return signal, RAs can calculate the height above the surface, supplementing an aircraft’s normal barometric altimeter during low level maneuvering. RAs were given their name as they essentially work using the same principle as a search radar that emits a beam, detects the return, and calculates the range, azimuth, and vector to a target.
RAs have been around since just before World War II. However, it was only after the war when they were commonly adopted to supplement barometric altimeters for very-low instrument approaches, mainly as part of the Instrument Landing System. In civil aviation, RAs use the IEEE “C-band” between 4.2 and 4.4 GHz.
In most of the world, RAs are not affected by 5G, because 5G signals most commonly radiate in the 900MHz, 1.8, 2.3, 2.5 and 3.5 GHz bands, leaving a safe 800MHz between 5G and RA bands. However, in the USA, demand for high-speed data on cellular devices has led to the Federal Communications Commission (FCC) auctioning additional bands in the C-band range between 3.7-3.98 GHz, only 200 MHz below the RA band.
5G uses the same Orthogonal Frequency Division Multiplexing (OFDM) radio technology as 4G, with only some minor changes. These changes allow 5G to operate in higher frequency bands than before, and to occupy wider spectrum bands. It is the ability to operate in these higher, wider spectrum bands that allows 5G to offer faster speeds than 4G. (4G tops out at 2.6 GHz with 100 MHz wide bands, providing maximum theoretical speeds up to 1Gbps, but more typically around 200 Mbps). 5G can operate up to 47 GHz using occupied channel bandwidths of up to 400 MHz, which allows 5G to offer speeds of up to 3-4 Gbps. 5G operators such as Verizon and AT&T are using this increased speed to compete with fiber-to-the-home offerings.
It’s important to note that when radios are designed, they operate in the bands intended by having an internal circuit that “oscillates” or vibrates at a particular frequency, called the resonant frequency. However, simply due to physics of resonance, the circuits also resonate at other frequencies (or sub tones) as they leak on frequencies to either side of the resonant frequency. To protect adjacent bands, the FCC limits the amount of power that may radiate outside of the designated band. Radios use bandpass filters on both the transmit and receive channels to make sure that the radio emits nearly all its power inside the allocated frequency band. All 5G base stations have bandpass filters and transmit and receive inside their allocated frequency bands with very little leakage.
So, the culprit is not really 5G technology itself. Instead, the problem is a result of the intersection of two industries across a technical and political landscape. In the USA, the FCC regulates radio frequencies while the Federal Aviation Administration (FAA) regulates aviation. The FCC allocated the 3.7-3.98 GHz band for 5G deployment assuming that all radios would be designed to FCC requirements for out-of-band emissions. However, RAs are regulated and tested by the FAA, which didn’t consider out-of-band emissions to be a problem last century when cellular devices operated in the sub-2GHz band and only carried voice traffic. As a result, the FAA certified many RAs that had poor out-of-band filtering. The technical solution is to add modern bandpass filters to all aviation RAs, but it is expensive and time consuming in the highly regulated world of aviation to do so.
Today we are faced with the problem of radar altimeters that are highly integrated into the flight management systems of high-end aircraft, operating systems such as auto-brakes and thrust reversers, auto-land systems and so on. These RAs may have inadequate filtering, and therefore, the receivers may not be able to distinguish ground reflections from adjacent band 5G interference. On top of that, regulations make it difficult and expensive to update the RAs accordingly. On the flip side, wireless operators, such as AT&T and Verizon, spent billions of dollars to deploy C-band 5G, with the promise of the FCC that adjacent C-band radios would play nicely, as all other bands have done.
For now, the only current solutions impose serious costs. We could for example tell aircraft operators to not use RAs in the USA, although this may not be possible for some aircraft and will limit landings in low IFR conditions. We could tell 5G operators to turn down the power emitted underneath airport-approach paths, however this will increase costs and reduce internet speeds for wireless customers.
In the longer run the FAA should fast-track Supplemental Type Certificated (STCs) to replace older RAs with improved designs. At the same time, the FCC should fund 5G operators to operate in other frequency bands. But most importantly, we need better communications between the FCC and FAA, something which has now become essential for aviation safety – or it will all end in tears.
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