Trump spectrum sale leaves airlines with $4.5B bill for altimeter do-over
- Reference: 1767882852
- News link: https://www.theregister.co.uk/2026/01/08/airlines_face_another_altimeter_upgrade/
- Source link:
On Wednesday, the US Federal Aviation Administration (FAA) issued a [1]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.
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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.
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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) [5]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 [6]Notice of Proposed Rulemaking [PDF] last year.
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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.
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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.
[9]FAA signs radar deals to drag US air traffic control out of the 1980s
[10]British Airways fears a future where AI agents pick flights and brands get ghosted
[11]Parachutists told to check software after jumper dangled from a plane
[12]Diversion to power datacenters earns Boom Supersonic a ticket to revive fast air transport
This isn't the first time this issue has arisen. Back in 2021, The Register reported that the FAA was concerned enough to [13]consider restricting flights once 5G services were up and running. European aviation regulators declared that this [14]wasn't an issue on their side of the Atlantic, however.
The solution from the FAA back then was also to [15]replace the affected RA systems , and the agency [16]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.
We approached two bodies representing airlines for comment, Airlines for America (A4A) and the International Air Transport Association (IATA), and will update if we hear anything back. ®
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[1] https://www.federalregister.gov/documents/2026/01/07/2026-00051/requirements-for-interference-tolerant-radio-altimeter-systems
[2] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/networks&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2aV_iu3_y7R55PK-AJ0Z-CwAAAMM&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0
[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/networks&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44aV_iu3_y7R55PK-AJ0Z-CwAAAMM&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[4] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/networks&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33aV_iu3_y7R55PK-AJ0Z-CwAAAMM&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[5] https://www.theregister.com/2025/07/08/trump_budget_bill_spectrum_auctions/
[6] https://docs.fcc.gov/public/attachments/DOC-415193A1.pdf
[7] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/networks&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44aV_iu3_y7R55PK-AJ0Z-CwAAAMM&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[8] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/networks&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33aV_iu3_y7R55PK-AJ0Z-CwAAAMM&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[9] https://www.theregister.com/2026/01/07/faa_radar_atc_deals/
[10] https://www.theregister.com/2025/12/13/british_airways_fears_a_future/
[11] https://www.theregister.com/2025/12/11/atsb_parachute_snagged_software/
[12] https://www.theregister.com/2025/12/10/boom_supersonic_datacenter_turbine/
[13] https://www.theregister.com/2021/12/08/aircraft_5g_interference/
[14] https://www.theregister.com/2022/01/14/5g_airliners_uk/
[15] https://www.theregister.com/2022/05/04/faa_5g_altimeters/
[16] https://www.theregister.com/2023/01/10/faa_2024_altimeter_deadline/
[17] https://whitepapers.theregister.com/
Isn't this the same position that was taken in Europe?
It's ridiculous that aircraft don't use bandpass filters on their radio equipment in this day and age.
Older designs
A lot of avionics have a pedigree from between ten and twenty years ago and there are good reasons.
An update to anything safety critical (and radalts can come under that heading) is a time consuming process as the new kit has to be thoroughly qualified and there is a rule: Don't be the first with anything . We only want to use a proven basis, which is sensible in safety critical design.
When those earlier systems were designed, the band limit filtering was easily sufficient for the operational profile required. As noted by at least one other, brick wall filters are very difficult to achieve (and we prefer to not have any software if possible).
Circuit fundamentals haven't really changed over the years although there are some newer modules that can help with the situation but I suspect part of the solution will be to detect interference and remove it. There are a number of ways to do that depending on what the interference type actually is.
So it is not that we don't use filters - we do. It is just that we need to update them to account for a new RF threat that did not previously exist.
Lazy Altimeter Design
FCC equipment rules require devices not to cause harmful interference to authorized radio services and to accept any interference received from other legal radio emissions. There is no reason that a well-engineered radar altimeter operating in the 4.2-4.4 Ghz band should be compromised by 5G cell phone signals in the 3.7-4.2 Ghz band.
Re: Lazy Altimeter Design
There is no *good* reason. There are several bad ones.
Re: Lazy Altimeter Design
Brick wall filters are hard to design and expensive. Best to have a transition zone of 50Mhz either side of the limit.
Re: Lazy Altimeter Design
Yes. Why don't we design equipment to be resistant to vulnerabilities that don't even exist yet just in case they may arise later. /s
Buffer zones are there for a reason, and it's very easy to design something that meets the design specs but also radiates (far) outside the intended frequency range. It's a lot harder to design against something radiating outside tolerances.
As an undergraduate apprentice at BAe Hatfield in 1989 I spent 6 weeks in Flight Development and one of my projects was to write a program in Fortran to help calibrate a new radio altimeter for the BAe 146 at low altitudes.
The Flight Development aircraft kitted out with multiple data recording systems would make low altitude passes (between 50ft and 250ft) along the centreline of the runway recording the measured output from the radio altimeter system every tenth of a second.
At the same time a 50fps camera would track the aircraft from a datum point on the airfield recording the pan angle of the camera and a high precision time record onto the images.
We then manually measured the distance on enlarged photos between 2 targets on the side of the aircraft and the distance of the targets from the top and bottom of the photograph. Using this data along with the pan angle and the known position of the datum point compared to the runway centreline, my program used trig to calculate the height of the aircraft above the runway in a particular photo. This data along with the time stamp was then used to compare the altitude from the photograph to the recorded altitude from the radio altimeter.
Wouldn't it be cheaper just to pay Donald Trump to change the rules?
Depends on whether an auction takes place.
Does this mean Air Force 1 and the gold-plated Trumpbo Jumbo are grounded?