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Pilots rely on radio altimeters to provide accurate measurements of an aircraft’s distance above the Earth’s surface, especially during the approach and landing phases of a flight. Data received from altimeters is especially critical for safely executing a low-visibility landing.
If an aircraft were to lose a signal or receive erroneous data from a radio altimeter, there are several different scenarios that may occur depending on the type of aircraft, the landing requirements of the airport and the weather. At the lower end of the risk spectrum, the autopilot may need to be disabled and the pilot(s) would need to manually land the aircraft. Some airports or certain weather conditions at other airports may not allow for landing of certain aircraft without altimeter data. This may mean that if visibility is poor and an altimeter is not working, the aircraft may need to wait until the weather is better or it may need to land at a different airport. Perhaps the biggest potential risk is if radio altimeter signal interference occurs during the final stages of landing as this could lead to a hazardous or catastrophic situation with the landing.
For decades in the US, the chance of signal interference occurring between an aircraft’s altimeter and other RF signals was negligible because altimeters, and the systems that rely on these devices, operate in the 4.2GHz to 4.4GHz range with the closest operational frequency, 3.7GHz to 3.98GHz, dedicated to low power satellite communications (satcom). The 220MHz of guard band between these two applications was considered sufficient because the satcom applications operating in the nearby spectrum were transmitting low power signals that were not likely to interfere with altimeter signals.
In recent years, this has changed as this spectrum was reallocated in the US from satcom transmissions to 5G telecommunications.
Concerns of reallocating C band spectrum
As the demand for higher bandwidth communication is rapidly accelerating, there is a lot of pressure on the US Federal Communications Committee (FCC) to allow 5G communications to occur in additional portions of the RF spectrum. From late 2020 into early 2021, the FCC held Auction 107 to reallocate the dedicated satcom bandwidth between 3.7GHz to 3.98GHz to mobile telecommunications operations. It is important to note that this portion of the spectrum is known as the C band by the FCC and the telecom industry, but this can be slightly 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.
As the FCC prepared for this auction, organisations ranging from the US Federal Aviation Administration (FAA), to pilots’ unions and individual airlines voiced concerns to the FCC that allowing 5G telecom operations in this portion of the C band may cause interference with aircraft altimeters. This is because the signals emitted by 5G base stations near airports would be much more powerful than the low power signals used for satcom transmissions. These more powerful signals could interfere with altimeter signals and without this data available during landing, engine and braking systems may not transition to landing mode, making it difficult or impossible for an aircraft to stop on a runway. Therefore, the aviation industry felt the 220MHz of guard band in place was no longer sufficient and more evaluation was needed to determine how likely it was for aircraft altimeters to experience interference from C band 5G signals.
Likely interference
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.
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 as this device does not fall under the same standards.
Additionally, the power of that spectrum at a receiver is dependent on the distance from the transmitter to the receiver. In this instance, the receiver is a radar-based altimeter on a quickly moving airplane, which makes 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 built over the past three decades by various manufacturers in use today. 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, which means the altimeter may pick up some signals below 4.2GHz.
The launch of C band 5G in the US
In late 2021, large telecommunications companies such as Verizon Wireless and AT&T were ready to capitalise on the nearly $70bn they had spent licensing portions of the C band. Concerns around interference were communicated to the FCC by the avionics industry prior to, during and after the spectrum auction, and as the deployment dates for these 5G networks approached the aviation industry put pressure on the FCC to act.
The rollout of C band 5G was initially scheduled for December 2021, but at this point, the impact of these 5G signals on altimeters was still not clear. Therefore, the FCC worked with carriers to delay deployment to late January 2022. This gave altimeter equipment manufacturers more time to evaluate data from wireless companies regarding signals and to perform additional testing on their equipment.
Just before the new 19 January 2022 deployment date, altimeter manufacturers had determined that the majority of altimeters in use in passenger planes would not be likely to experience interference with these 5G signals. Some major international airlines were not comfortable with this and cancelled many flights to the US. Due to this apprehension and the fact that altimeter manufacturers were still testing their devices, just before their scheduled launch, Verizon and AT&T agreed to hold off on turning on 5G base stations near approximately 50 airports that commonly needed to perform low-visibility approaches until July 2022.
Filters climb high
Incorporating high performance filtering to avoid altimeter interference has not been uppermost in the mind of altimeter manufacturers for the last few decades. 5G signals occurring in the neighbouring spectrum led altimeter manufacturers to add high performance filters to devices to ensure signals below 4GHz will not be detected. The guide includes recommendations for protection levels for radio altimeters that can help prevent receiver front end overload, degradation of sensitivity and false altitude readings.
As of July 2022, some planes were found to be using altimeters that may experience interference, leading to Verizon and AT&T agreeing to a two-phase extension plan for turning on 5G base stations near airports. As part of this plan, the FAA mandated that airlines must replace altimeters that have not been cleared for safe operation with a model that has been cleared by the FAA, such as Honeywell’s ALA-52B. This is believed to be the only radar altimeter currently available to show almost no sign of 5G interference due to its bandpass RF filtering, which provides resilience to 5G interference and creates the smallest protection radius (that is, the distance between a 5G base station and an aircraft) available today, says Honeywell.
The first deadline for altimeter replacement at the end of 2022 is for regional aircraft identified as most susceptible to interference and the second deadline of July 2023 is for commercial airlines. Altimeter manufacturers are working with airlines to provide installation kits that make it fairly easy to swap the altimeters, but airlines and altimeter manufacturers are still concerned about potential lingering pandemic-induced supply-chain issues that may make it difficult to meet the latest deadlines.
Working through any potential interference issues between 5G and aircraft now is critical because demand for 5G communications – and future generations of wireless communications – will continue to expand globally.
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