Airlines operating in the US may have to upgrade their aircraft radio altimeters again at a cost of billions of dollars, to avoid potential interference with cell networks following the Trump administration's decision last year to auction off additional spectrum to bidders.

On Wednesday, the US Federal Aviation Administration (FAA) issued a notice of proposed new regulations. If implemented, these will require all radio altimeters to meet specific minimum performance requirements.

Those requirements are for altimeters to able to withstand interference from wireless signals in neighboring spectrum bands, while continuing to provide accurate altitude readings to both pilots and integrated aircraft safety systems.

And the cost for all this? The FAA estimates the total bill to retrofit aircraft with interference-tolerant altimeters will be $4.49 billion, or $424 million annualized at a 7 percent discount rate over 20 years.

According to the FAA, the blame for this move lies with the Trump administration and its US budget bill signed into law last year that directed the Federal Communications Commission (FCC) to open up more radio frequencies to be auctioned off for commercial use.

This sell-off must include at least 100 MHz and possibly as much as 180 MHz of the 3.98 to 4.2 GHz band (Upper C-band), which the FCC intends to make available for terrestrial wireless flexible use, as disclosed in a Notice of Proposed Rulemaking [PDF] last year.

However, aircraft radio altimeter (RA) systems operate in the adjacent 4.2 to 4.4-GHz band, and the FAA is clearly worried about the possibility of interference if cellular network operators start using the Upper C-band to deliver services.

"FAA expects future wireless services in the Upper C-band aligned with service rules in the Lower C-band to cause interference to current RA systems. Existing RA systems are not compatible with this envisioned use, and airworthiness directives issued by FAA in 2023 are insufficient to address the unsafe condition that will result from wireless services in the Upper C-band," the agency says in its proposal document.

Wireless service providers have already taken voluntary measures in the proximity of airports, the FAA says, but these commitments will hit a sunset date at the start of 2028 unless extended by mutual agreement, and long-term compatibility between Lower C-band wireless services and RA systems has not been resolved beyond that date.

It believes that a single retrofit of RA systems will be able to address compatibility issues with wireless services in both the Lower and Upper C-band, and says the aviation industry has been working to develop standards for next-generation RA systems for several years now.

The deadline for making these changes is proposed to coincide with the FCC's date authorizing the initiation of new wireless services in the Upper C-band, which the FAA currently expects to be sometime between 2029 and 2032.

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This isn't the first time this issue has arisen. Back in 2021, The Register reported that the FAA was concerned enough to consider restricting flights once 5G services were up and running. European aviation regulators declared that this wasn't an issue on their side of the Atlantic, however.

The solution from the FAA back then was also to replace the affected RA systems, and the agency set a deadline of February 1, 2024 for all updates to be carried out. It later declared that, as of the end of September 2023, the entire US airline fleet had upgraded their equipment and the risk of 5G interference had been mitigated.

Until last year, of course, when the newly installed Trump administration decided to start re-allocating spectrum in order to generate more revenue from license fees.

The FAA's notice of proposed rulemaking (NPRM) document is open for comments for 60 days, a period that ends on March 9.

A spokesperson for Airlines for America - a trade association for US passenger and cargo airlines - sent a statement to The Register:

"We have been working collaboratively with the telecommunications industry, the FAA and the FCC to identify solutions that ensure our nation's airspace remains safe while allowing the spectrum to be used.

"We were expecting this NPRM, which was published to fulfill requirements included in the One Big Beautiful Bill, and are currently evaluating all of the details in the proposal and will be providing comments."

The Reg also asked International Air Transport Association (IATA) for comment. ®